Heathrow Expansion PRELIMINARY ENVIRONMENT INFORMATION ... · CONTENTS 21. Water Environment 21.1 21.1 Introduction 21.1 21.2 Relevant legislation, policy and other important and

Heathrow Expansion PRELIMINARY ENVIRONMENT INFORMATION REPORT: Chapter 21: Water Environment

21 © Heathrow Airport Limited 2019

Volume 1, Chapter 21

Water environment



CONTENTS

21. Water Environment 21.1

21.1 Introduction 21.1

21.2 Relevant legislation, policy and other important and relevant matters 21.3 Introduction 21.3 Legislation and national planning policy 21.3 Regional and local planning policy 21.7 Other important and relevant matters 21.7

21.3 Scoping and engagement 21.8 Overview 21.8 Scoping Opinion 21.8 Technical engagement 21.13

21.4 Scope of the assessment 21.17 Overview 21.17 Spatial scope and study area 21.17 Temporal scope 21.18 Receptors 21.19 Identification of potential effects 21.22 Effects no longer being considered 21.29

21.5 Embedded environmental measures 21.30 Overview 21.30 Measures embedded into permanent infrastructure 21.31 Measures incorporated into the construction phase 21.37

21.6 Methodology for baseline data gathering 21.47 Overview 21.47 Desk study 21.47 Survey work 21.49

21.7 Assessment methodology for PEIR 21.54 Significance evaluation methodology 21.54

21.8 Assumptions and limitations of this PEIR 21.63 Overview 21.63

21.9 Overall baseline 21.66 Overview of the study areas 21.66 Surface water features 21.67 Water supply 21.71 Drainage 21.71 Surface water quality 21.73 WFD water bodies 21.74 Flood risk 21.79 Protected sites 21.83 Abstractions and discharges 21.83 Future baseline 21.84

21.10 Assessment of water environment effects 21.86 Overview 21.86 Introduction to the receptor-based assessments 21.88 WE1 Horton Brook 21.88 WE2 Colne Brook 21.90 WE3 Wraysbury River 21.93 WE4 River Colne 21.96



WE5 Duke of Northumberland’s River and Longford River 21.99 WE6 Portlane Brook 21.102 WE7 River Crane 21.103 WE8 and WE9 River Thames (freshwater and tidal) 21.105 WE10 Upper River Colne 21.107 WE11 Reservoirs 21.107 WE12 Lower Thames Gravels 21.108 WE14 Local Water Supply Network 21.113 WE15 Foul Drainage Infrastructure 21.114 WE16 Flood Risk Receptors 21.115

21.11 Preliminary assessment of significance 21.119 Overview 21.119

21.12 Assessment of cumulative effects 21.120 Overview 21.120

21.13 Consideration of additional environmental measures or compensation 21.127 Overview 21.127

21.14 Next Steps 21.129 Overview 21.129 Baseline 21.129 Assessment 21.130 Consultation and engagement 21.130 Environmental measures 21.130

TABLE OF TABLES

Table 21.1: Legislation relevant to the water environment 21.3 Table 21.2: National planning policy relevant to the water environment 21.5 Table 21.3: PINS Scoping Opinion consultation 21.8 Table 21.4: Water environment technical engagement January 2017 to May 2019 21.13 Table 21.5: Temporal scope of water environment assessments 21.19 Table 21.6: Receptors requiring assessment for water environment 21.21 Table 21.7: Likely significant effects on water environment receptors scoped in for further assessment 21.22 Table 21.8: Effects no longer considered in this PEIR 21.30 Table 21.9: Summary of measures relating to the water environment 21.31 Table 21.10: Summary of the embedded environmental measures in the design and how these influence the water environment assessment 21.40 Table 21.11: Summary of the good practice environmental measures and how these influence the Water Environment assessment Source: draft CoCP 21.44 Table 21.12: Data sources used to inform the water environment assessment 21.47 Table 21.13: Survey work undertaken 21.50 Table 21.14: Definitions of receptor sensitivity 21.54 Table 21.15: Definitions of magnitude of change 21.56 Table 21.16: Significance evaluation matrix 21.59 Table 21.17: Assessment methodology for the PEIR and ES 21.60 Table 21.18: Environment Agency aquifer designations for bedrock strata in the GWSA 21.70 Table 21.19: Environment Agency aquifer designations for superficial strata in the GWSA 21.71 Table 21.20 Second Cycle Status of WFD waterbodies 21.76 Table 21.21: Assessment of individual effects for WE1 Horton Brook 21.89 Table 21.22: Assessment of individual effects for WE2 Colne Brook 21.91 Table 21.23: Assessment of individual effects for WE3 Wraysbury River 21.94 Table 21.24: Assessment of individual effects for WE4 River Colne 21.97



Table 21.25: Assessment of individual effects for WE5 Duke of Northumberland's River and Longford River 21.99 Table 21.26: Assessment of individual effects for WE6 Portlane Brook 21.102 Table 21.27: Assessment of individual effects for WE7 River Crane 21.103 Table 21.28: Assessment of individual effects for WE8 and WE9 River Thames (freshwater and tidal) 21.106 Table 21.29: Assessment of individual effects for WE10 Upper River Colne 21.107 Table 21.30: Assessment of individual effects for WE11 Reservoirs 21.108 Table 21.31 Assessment of individual effects for WE12 Lower Thames Gravels 21.108 Table 21.32: Assessment of individual effects for WE13 Bedrock Aquifers 21.111 Table 21.33: Assessment of individual effects for WE14 Local Water Supply Network 21.113 Table 21.34: Assessment of individual effects for WE15 Local Foul Drainage Network 21.114 Table 21.35 Assessment of effects for WE16 flood risk receptors 21.116 Table 21.36: Summary of significant positive and negative effects 21.119 Table 21.37 Phase 1 CEA of DCO Project effects, together with ‘other developments’ unrelated to the DCO Project 21.124 Table 21.38: Summary of additional environmental measures or compensation and how these influence the water environment assessment 21.128 Table 21.39:Further baseline work to support the ES 21.129 Table 21.40: Further development of environmental measures 21.131

APPENDICES Appendix 21.1 Surface Water Quality Assessment

Appendix 21.2 Groundwater Modelling Assessment

Appendix 21.3 Preliminary WFD Risk Assessment

Appendix 21.4 Flood Risk Assessment

Appendix 21.5 Drainage Impact Assessment

Appendix 21.6 Screening of Abstractions and Discharges

Appendix 21.7 Colne Hydraulic Modelling Report

Appendix 21.8 Crane Hydraulic Modelling Report


21.1 © Heathrow Airport Limited 2019

21. WATER ENVIRONMENT

21.1 Introduction

21.1.1 This chapter of the Preliminary Environmental Information Report (PEIR) presents the initial results of the assessment of the likely significant effects of the DCO Project with respect to the water environment, including effects on water quality, water quantity (such as changes to river flow or groundwater level), flood risk, drainage and water resources. It should be read in conjunction with the DCO Project description provided in Chapter 6: DCO Project description and the relevant parts of the following chapters:

1. Chapter 8: Biodiversity - in relation to water dependent ecological features

2. Chapter 10: Climate change - in relation to effects of future changes in climate on the water environment

3. Chapter 13: Historic environment - in relation to the effects of changes in groundwater levels on buried historic assets

4. Chapter 14: Land quality – in relation to the mobilisation of potential contaminants already present on the site

5. Chapter 16: Major accidents and disasters - in relation to accidents or disasters either relating to the water environment or that could have an effect on the water environment.

21.1.2 This chapter describes:

1. The legislation, planning policy and other documentation that has informed the assessment (Section 21.2: Relevant legislation, policy and other important and relevant matters)

2. The outcome of consultation and external engagement that has been undertaken, including how matters on the water environment within the Scoping Opinion adopted in July 2018 have been addressed (Section 21.3: Scoping and engagement)

3. The scope of the assessment for the water environment (Section 21.4: Scope of the assessment)

4. Embedded and good practice mitigation relevant to the water environment (Section 21.5: Embedded environmental measures)

5. The methods used for the baseline data gathering (Section 21.6: Methodology for baseline data gathering)



6. The assessment methods used for the PEIR (Section 21.7: Assessment methodology)

7. The assumptions and limitations of the PEIR assessment (Section 21.8: Assumptions and limitations of this assessment)

8. The overall baseline (Section 21.9: Overall baseline)

9. The assessment of effects on the water environment (Section 21.10: Assessment of effects)

10. A summary of positive and negative water environment effects (Section 21.11: Preliminary assessment of significance)

11. The assessment of cumulative effects (Section 21.12: Cumulative effects assessment)

12. Consideration of any additional environmental measures or compensation required (Section 21.13: Consideration of additional environmental measures or compensation)

13. An outline of further work to be undertaken for the Environmental Statement (ES) (Section 21.14: Next steps)

14. Chapter 23: Bibliography

15. Glossary of terms and list of abbreviations.

21.1.3 In-combination effects are addressed in Chapter 22: In-combination effects.

21.1.4 The assessments in this water environment chapter are supported by the following appendices:

1. Appendix 21.1 Surface water quality assessment, Volume 3

2. Appendix 21.2 Groundwater modelling assessment, Volume 3

3. Appendix 21.3 Preliminary Water Framework Directive risk assessment, Volume 3

4. Appendix 21.4 Flood Risk Assessment, Volume 3

5. Appendix 21.5 Drainage Impact Assessment, Volume 3

6. Appendix 21.6: Screening of abstractions and discharges, Volume 3

7. Appendix 21.7: Colne Hydraulic Modelling Report, Volume 3

8. Appendix 21.8: Crane Hydraulic Modelling Report, Volume 3



21.2 Relevant legislation, policy and other important and relevant matters

Introduction 21.2.1 This section identifies the legislation, policy and other documentation that has

informed the assessment of effects presented in the water environment. Further information on policies relevant to the EIA and their status is provided in Chapter 2: Legislative and policy overview of this PEIR.

Legislation and national planning policy 21.2.2 Table 21.1 lists the legislation relevant to the assessment of the effects on water

environment receptors.

Table 21.1: Legislation relevant to the water environment

Legislation description Relevance to assessment

The EU Water Framework Directive (2000/60/EC) (WFD), as enacted into domestic law by the Water Environment (Water Framework Directive) (England and Wales) Regulations 2017

The aim of the WFD is for all water bodies to achieve Good Status (comprised of scores for Ecological Status and Chemical Status) by 2021 or 2027 as appropriate, and to ensure no deterioration from current status.

The development has the potential to have an effect on surface water bodies (rivers and lakes), transitional water bodies (estuaries) and groundwater bodies, during construction and operation. A draft WFD Compliance Assessment is provided in Appendix 21.3 and will be developed to accompany the Application to demonstrate the effect on WFD water bodies and describe the appropriate environmental measures.

Directive on Environmental Quality Standards (2008/105/EC)

Provides environmental quality standards for priority substances set out in Article 16 of the Water Framework Directive 2000/60/EC

The development has the potential to release in to, or otherwise affect the concentrations of priority substances in water. Effects relating to priority substances are assessed in Section 21.10 and in more detail in Appendix 21.1. Effects associated with mobilisation of contaminants in groundwater are considered in Chapter 14.

The EU Groundwater Directive (2006/118/EC)

The aim of the directive is to protect groundwater against pollution caused by dangerous substances.

The development has the potential to have an effect on groundwater bodies through the introduction of dangerous substances, during the construction and operation phase. Appropriate environmental measures have been put forward in Section 21.5 of this chapter and in Chapter 14 to help ensure the protection of groundwater.




Flood and Water Management Act 2010

The Flood and Water Management Act sets out the Government’s proposals to improve flood risk management, water quality and ensure water supplies are more secure.

Appropriate flood and water management must be incorporated through construction and operation, to protect local populations, maintain/improve water quality and mitigate the risk of flooding. Flooding and drainage issues are addressed in the Appendix 21.4 and Appendix 21.5, and embedded environmental measures are identified in Section 21.5 in this chapter.

Environment Act 1995

The Environment Act 1995 established the Environment Agency and gave it responsibility for environmental protection and flood defence.

The Act empowers the Environment Agency to be the primary regulator for this assessment with respect to compliance with national legislation and policy for flood defence on main rivers.

Land Drainage Act 1991

The Land Drainage Act 1991 & 1994 places responsibility for maintaining flows in watercourses on landowners. The Acts give the Local Authorities powers to serve a notice on landowners to ensure works are carried out to maintain flow of watercourses.

This Act regulates activities that interfere with ordinary watercourses, requiring consent of local authorities for such activities and requiring riparian landowners to ensure that no obstructions to flow are introduced.

Water Resources Act 1991, Water Act 2003 The Environmental Permitting (England and Wales) Regulations 2016

The Water Resources Act 1991 states that it is an offence to cause or knowingly permit polluting, noxious, poisonous or any solid waste matter to enter controlled waters. The Act was revised by the Water Act 2003, which sets out regulatory controls for water abstraction, water impoundment and protection of water resources. Important for the Project is the requirement to obtain a licence for dewatering of engineering works and to ensure that any impact on the environment can be mitigated. Provisions for the regulation of water discharges to controlled waters are set out in the Environmental Permitting (England and Wales) Regulations 2016, these have replaced provisions in the earlier Acts.

This Act sets out the permitting and compliance framework which will regulate all site emissions, water abstractions and discharges with the potential to interact with the water environment.

Environmental Protection Act 1990

The Environmental Protection Act 1990 makes provision for the improved control of pollution arising from certain industrial and other

This Act empowered the Environment Agency to be the primary regulator for works associated with this assessment that could have effects on surface water




processes; to re-enact the provisions of the Control of Pollution Act 1974 relating to waste on land with modifications as respects the functions of the regulatory and other authorities concerned in the collection and disposal of waste and to make further provision in relation to such waste.

and ground quality. This gave responsibility to the Environment Agency for holding all abstraction licenses and discharges.

21.2.3 Table 21.2 lists the national planning policy relevant to the assessment of the effects on water environment receptors.

Table 21.2: National planning policy relevant to the water environment

Policy description Relevance to assessment

Airports National Policy Statement (ANPS) (Department for Transport 2018)

Chapter 2: Legislative and policy overview provides an explanation of the relevance of the ANPS to the DCO Project in general terms. The ANPS is the primary basis for decision making on the DCO Project. The ANPS includes dedicated sections on flood risk (paragraphs 5.147 to 5.171) and water quality and resources (5.172 to 5.186) which set out requirements for the DCO Project for the management of Flood risk and Water quality and resources. It sets out the expectations for the scope of assessment, referencing the requirements of the:

1. National Planning Policy Framework (NPPF) (5.148)

2. UK Climate Change Risk Assessment (5.157)

3. Flood and Water Management Act 2010 (5.170)

4. The Water Framework Directive (5.184) and its daughter directives (Priority Substances and Groundwater) (5.172)

It states that “in terms of Water Framework Directive compliance, the overall aim of the development is to prevent deterioration in status of water bodies and to support the objectives in the Thames River Basin Management Plan, and not to jeopardise the future achievement of good status for any affected water bodies. If the development is considered likely to cause deterioration or water body status or good potential status or potential compliance with Article 4.7 must be demonstrated”. Specifically, it requests that “in preparing a Flood Risk Assessment the applicant should (5.154):

1. Consider the risk of all forms of flooding arising from the development, comprised in the preferred scheme, in addition to the risk of flooding to the project, and demonstrate how those

The ANPS references the requirements of legislation and policy relating to the water environment, which thus informs the assessment methodologies and hence the conclusions on the level of significance associated with predicted effects. Appendix 21.3 considers the physical characteristics of the water environment and WFD bodies which could be affected by the DCO Project. It also considers all relevant effects on water bodies and protected areas under the WFD. The ANPS requirement for a Flood Risk Assessment (FRA) has been addressed within Appendix 21.4 and Appendix 21.5, and within the assessment sections of this chapter. This assessment has taken into




risks will be managed and, where relevant, mitigated, so that the development remains safe throughout its lifetime

2. Take into account the impacts of climate change, clearly stating the development lifetime over which the assessment has been made

3. Consider safe access and exit arrangements

4. Include the assessment of residual risk after risk reduction measures have been taken into account, and demonstrate that this is acceptable for the development

5. Consider if there is a need to remain operational during a worst-case flood event over the scheme’s lifetime

6. Provide evidence for the Secretary of State to apply the Sequential Test and Exception Test, as appropriate”.

In relation to water quality and resources, it specifies that “any environmental assessment should describe (5.176):

1. The existing quality of water affected by the proposed Project

2. Existing water resources affected by the Project and the impacts of the proposed Project on water resources

3. Existing physical characteristics of the water environment (including quantity and dynamics of flow) affected by the project, and any impact of physical modifications to these characteristics

4. Any impacts of the proposed project on water bodies or protected areas under the Water Framework Directive and source protection zones around potable groundwater abstractions

5. Any cumulative effects”.

It also specifies that the applicant should “assess the effects on the surrounding water and wastewater treatment network in cooperation with the relevant water and sewerage undertaker(s) and address any future water infrastructure needed for the preferred scheme, including for supplied and sewerage treatment, and the effects on the surrounding water and wastewater treatment network” (5.177).

account projections of climate change in agreement with the Environment Agency. An assessment has been carried out in relation to water quality and water resources receptors. Appendix 21.1 and Appendix 21.2 have considered potential effects from the DCO Project on the existing quality receptors. Appendix 21.2 also considers potential effects on water resources receptors (including source protection zones), along with a report on the screening of abstractions and discharges (Appendix 21.6). Section 21.10 of this chapter considers the existing water resources and waste water treatment networks that could be affected by the DCO Project. A preliminary assessment of the effects on the surrounding water and wastewater treatment network has been carried out in Section 21.10 of this chapter. This chapter summarises the information from these appendices, along with environmental measures to carry out an assessment of effects for key receptors.

National Networks National Policy Statement, (NN NPS) (Department for Transport 2014)




Chapter 2 provides an explanation of the relevance of the NN NPS to the DCO Project in general terms. The expectations and requirements are comparable to those of the ANPS, with no additional assessment requirements compared to ANPS.

The NN NPS references the requirements of legislation and policy relating to the water environment, which thus informs the assessment methodologies and hence the conclusions on the level of significance associated with predicted effects.

National Planning Policy Framework (NPPF) (Communities and Local Government 2019)

The NPPF sets out planning policy for England and places a general presumption in favour of sustainable development. The policies relating to planning and flood risk are set out in NPPF paragraphs 155 to 165. Paragraph 5 of the NPPF states that the framework “does not contain specific policies for Nationally Significant Infrastructure Projects (NSIPs)”. However, it states that “these are determined in accordance with the decision making framework and relevant national policy statements for infrastructure, as well as any other matters that are relevant (which may include the NPPF)”. The requirements of the NPPF are mirrored in paragraph 5.154 of the ANPS described above.

The NPPF requirements are captured within the FRA (Appendix 21.4), which presents information on the sequential and exception tests as well as demonstrating that the development will not result in an increase in flood risk from any source of flooding. This assessment includes consideration of climate change in line with NPPF requirements, as agreed with the Environment Agency.

Regional and local planning policy 21.2.4 Appendix 2.1: Regional and local planning policy and other important and

relevant matters, Volume 3 presents details of the local planning policy relevant to the assessment relating to flood risk, surface water runoff management, water use and water quality. Relevant local policy covers the boroughs, district and county councils within the study area (including the London Borough of Hounslow, London Borough of Hillingdon, Slough, Spelthorne, South Bucks, Windsor and Maidenhead, Richmond upon Thames, Runnymede, Chiltern and South Bucks Joint Strategic Council, and the Three Rivers District Council).

Other important and relevant matters 21.2.5 A summary of other technical documentation relevant to the assessment

undertaken for the water environment is provided within Appendix 2.1.



21.3 Scoping and engagement

Overview 21.3.1 This section describes the outcome of, and response to, the Scoping Opinion in

relation to water environment assessment and also provides details of the ongoing technical engagement that has been undertaken with stakeholders and individuals and is relevant to this PEIR. An overview of engagement undertaken can be found in Section 1.5 of Chapter 1: Introduction.

21.3.2 Engagement has taken the form of discussions, meetings, workshops and sharing of draft technical documents and is summarised in the following sections.

Scoping Opinion 21.3.3 A Scoping Report requesting a Scoping Opinion was submitted to the Secretary of

State, administered by the Planning Inspectorate (PINS) on behalf of the Secretary of State, on 21 May 2018. The Scoping Report set out the proposed water environment assessment methodologies, outlined the baseline data collected to date and that proposed for the ES and set out the scope of the assessment.

21.3.4 A Scoping Opinion was adopted by PINS on behalf of the Secretary of State on 2 July 2018. Table 21.3 sets out the comments in Section 4 of the Scoping Opinion (‘Aspect based scoping tables’) for the water environment (Table 4.10 of the Scoping Opinion) and how they have been addressed in this PEIR. A full list of the PINS Scoping Opinion comments and responses is provided in Appendix 5.1: Response to the Scoping Opinion, Volume 3. The information provided in the PEIR is preliminary and therefore not all of the Scoping Opinion comments have been able to be addressed at this stage. All Scoping Opinion comments will be addressed within the ES.

Table 21.3: PINS Scoping Opinion consultation

PINS ID number Scoping Opinion comment How is this addressed?

189 Risk of tidal flooding to and from the DCO Project is ruled out in Table 18.9. Paragraph 18.6.32 makes no reference to the increased risk of tidal flooding elsewhere due to the project and Table 18.11 states that the flood risk assessment will cover all sources of flood risk including tidal. The Inspectorate does not agree that this matter can be scoped out because the Scoping Report lacks a robust justification and does not quantify the discharges from

The approach to the FRA assessment was shared with the Environment Agency for review during the consultation process and is set out within Appendix 21.4. On the basis of the information presented in that appendix it is concluded that for the range of extreme event and climate change being assessed there is no risk to the DCO Project from tidal flood risk. Further explanation for this decision is provided in Section 21.4, Table 21.8 of this chapter.




the Proposed Development. The Applicant should make efforts to agree the need for a tidal flood risk assessment with the relevant consultation bodies.

The assessment in this chapter includes consideration of effects of discharges from the development on downstream runoff and river flow which, as identified by PINS, could in theory manifest as effects on tidal flooding in the tidal reaches of the Thames. Effects on river flows remain scoped in to the assessment and are considered in detail in Appendix 21.4 and Appendix 21:5.

190 Table 18.3.1 identifies several matters that could be scoped out of the groundwater modelling. These matters are not identified as scoped out within section 18.8 of the Scoping Report. In the absence of justification within the main Scoping Report, these matters are not scoped out.

This comment related to the Surface Water Quality modelling proposals rather than the groundwater model. Appendix 18.3 of the Scoping Report on surface water quality modelling did not propose scoping out effects altogether, but suggested that numerical assessment may not be required in relation to some potential effects. Further clarifications on the approach to assessment have been discussed with the Environment Agency and are included in the revised methodology for the surface water quality monitoring (Appendix 21.1).

191 Portable toilet facilities: The Inspectorate considers that assessment of such facilities during construction may be scoped out from further assessment on the basis that a discharge is not required.

Potential effects from portable toilet facilities that do not require a discharge have been scoped out of the PEIR assessments.

192 No accretion or detailed groundwater surveys are planned for the Crane/Thames catchments: The Applicant should ensure that the assessment in the ES is underpinned by sufficient baseline survey information. Effort should be made to agree the need for more detailed baseline survey information with relevant consultation bodies. If no further survey effort is conducted the ES should include a robust justification to support the exclusion of such surveys from the identified catchments.

Information regarding the proposed approach to groundwater and accretion surveys, as well as all other monitoring, has been included in Section 21.6. The approach to monitoring and surveys has been shared with the Environment Agency.

The scope of monitoring includes the Crane catchment. No additional monitoring has been carried out in the River Thames itself, with the focus having been on the tributaries of the Thames.

The full monitoring suite to support the Application will be reported in the ES and utilised in the assessment.

193 Due to the proposed increase in impermeable area during operation, the Inspectorate considers that increased sediment loading to surface water during operation could result in a likely significant effect and should be assessed.

The PEIR is supported by a draft Drainage Impact Assessment (DIA) (Appendix 21.5) and surface water quality assessment (Appendix 21.1), which describe how surface water run-off would be controlled and treated in both the construction and operation phases. This includes management of sediment in runoff.




194 Groundwater study area: The study area is defined only with respect to the Lower Thames Gravels WFD groundwater body. Paragraph 14.4 of the land quality chapter identifies the potential for effects on the underlying chalk aquifer to arise from construction works including basements or piled foundations that extend below the base of the London Clay and Table 18.7 and Appendix 18.2 highlight the need to consider effects on the Cretaceous chalk aquifer. The Applicant should ensure that the extent of the model is sufficient to address effects on all aquifers likely to be affected."

The Chalk is included in the numerical groundwater model, as discussed in Appendix 21.2. The model is of sufficient spatial size to simulate all interventions in the Chalk due to both construction and operation.

The Chalk is considered as a receptor in Appendix 21.2 and within Section 21.4 and Section 21.10 of this chapter. A final groundwater impact assessment will be included with the ES.

195 The Inspectorate considers that further justification is required for the additional surveys to be undertaken. Currently the listed surveys lack detail regarding their extent, timing, duration, detailed methodology or reference to recognised survey standards. Considering the Environment Agency’s comments in relation to the lack of flow gauging and monitoring, particular consideration should be given to justifying any flow monitoring surveys. Effort should be made to agree the detailed scope of surveys with the relevant consultation bodies e.g. Environment Agency and Lead Local Flood Authorities. The baseline data should include Local Authority data regarding the extent of functional floodplains and from the Catchment Data Explorer, strategic flood risk assessments and relevant water quality assessments (for example as referenced in the South Bucks, Buckinghamshire and Surrey CC responses).

Baseline data includes those sources listed in Section 21.6. This includes floodplain extent information from the Catchment Data Explorer website and strategic flood risk assessments contained within the FRA (Appendix 21.4) and DIA (Appendix 21.5).

The approach to monitoring and surveys has been shared with the Environment Agency.

The approach to monitoring agreed for the PEIR will be carried forward to inform the ES.

196 The receptor WE10: Upper River Colne should be included in the change in land use and river diversion and linked effect changes to channel morphology in operation, where relevant.

Although the DCO Project does not extend into WE10 Upper River Colne there may be opportunities for WFD mitigation along channels associated with the receptor. Therefore, it is included in the assessment sections of this chapter. The Preliminary WFD Risk Assessment in Appendix 21.3 also screens in the Upper River Colne but does not further assess it at the PEIR stage. The effects of WFD mitigation on the Upper River Colne will be assessed in more detail in the




ES.

197 Assessment scenarios: The ES should also consider the potential for likely significant water quality effects to arise during the early ATM uplift scenario due to increased pollutant deposition due to increased aircraft and ground vehicle activity.

The PEIR provides an initial assessment of the potential effects of increased pollutant deposition on water quality, within the range of likely scenarios and is reported in Section 21.10 and Appendix 21.1.

Based on the outcome of the PEIR assessment, which has shown a negligible effect (see Section 21.10), no further detailed assessment is proposed for the ES.

198 Receptor sensitivity and magnitude of effect criteria: Whilst there is no industry standard methodological approach to undertake water environment assessments, the Scoping Report makes no reference to existing methodological approaches that are commonly used to assess water environment effects. The source of the proposed sensitivity and magnitude criteria are not set out in Table 18.14 and Table 18.15, although they appear to be loosely based on DMRB Volume 11 Section 3 HD45/09 Road Drainage and the Water Environment (or WebTAG) and Table 18.2 refers to DMRB requirements. The criteria reference designated areas but do not reference specific ecological elements of affected waterbodies (e.g. fish) that contribute to the sensitivity of the waterbodies. The Inspectorate recommends that effort is made to agree the elements to be considered as part of the criteria with the relevant consultation bodies.

The definitions of sensitivity and magnitude described in Section 21.7, Table 21.15 and Table 21.16 are both defined and applied using professional judgement, but are based on recognised approaches to classification relevant to the receptor types, including WFD and NPPF. They have a basis in DMRB guidance but have been adapted to be more widely applicable to other types of infrastructure and development. Further detail on this has been provided in Section 21.8.

Ecological receptors and ecological WFD classification elements are assessed in Chapter 8.

Heathrow are in the process of consulting the Environment Agency and HSPG on the definitions and justification of the criteria. The definitions applied at PEIR will be utilised for the ES.

199 The CoCP is referenced as an example of a ‘possible’ plan, whereas section 5.10 of the Scoping Report states that a CoCP will be produced. The Inspectorate expects a CoCP to be submitted as part of the ES. The applicability of the CoCP to deliver environmental management of maintenance activities should be considered. The Inspectorate expects the ES to contain details of the proposed operational

A draft CoCP has been produced.

Outline drainage principles have been developed and are presented in Appendix 21.5.

A draft CoCP and DIA including reference to the operational drainage strategy will be submitted with the ES.




drainage strategy, in order to understand the potential effects of the development on the water environment and the effectiveness of any mitigation proposed.

200 The appendices refer to the potential need for derogation under the WFD, with specific reference to Article 4.7 of the WFD. Any requirements under Articles 4.8 and 4.9 of the WFD should also be considered and addressed through the assessment.

This has been reflected in the revised method statement and accounted for in the preliminary WFD risk assessment (Appendix 21.3).

201 The interim WFD objectives for the Thames RBMP, which are due to be released in 2019, should be used to inform the ES and WFD assessment where relevant.

Due to the reporting timescales relative to the release of the interim WFD objectives, the PEIR has utilised the current (2016) data.

The methodologies and assessment in the ES will be updated to utilise the interim WFD objectives, assuming they are available in time for publication.

202 The screening assessment should take into account the interim WFD objectives for the Thames RBMP in case of changes in the status of waterbodies subject to assessment.

Due to the reporting timescales relative to the release of the interim WFD objectives, the PEIR has utilised the current (2016) data.

The methodologies and assessment in the ES will be updated to utilise the interim WFD objectives, if they are available, and will consider the implications of any changes in baseline water body status.

203 Temporary effects: the assessment relies on the European Commission, Common Implementation Strategy for the Water Framework Directive (2000/60/EC), Guidance Document No. 20, 2009 and states that impacts are considered to be temporary, and therefore not to constitute deterioration of WFD status if the water body would “Recover without the need for any mitigation”. The EC guidance actually uses the term “restoration” rather than mitigation. The ES should ensure consistent use of terminology between assessments and guidance. The duration of temporary effects should be fully defined.

The terminology has been reviewed and revised as necessary through Appendix 21.3 and its annexes and this chapter.

The Environment Agency has been consulted to confirm the correct application of the terminology, as described in the Technical Engagement section of Section 21.3.

204 Climate change: Limited reference is made within the surface and groundwater modelling appendices to climate change. The ES should ensure that the modelling process takes account of future climate change scenarios and clearly cross

Climate change has been accounted for in the methodologies and assessments as appropriate, and in line with the assessments in Chapter 10.

- The FRA (Appendix 21.4) and DIA (Appendix 21.5) take account of NPPF requirements with




references to the findings of the climate and climate change assessment.

regards to climate change.

- The groundwater assessment (Appendix 21.2) and surface water quality assessment (Appendix 21.1) do not include climate change assessments at this stage, but it will be considered in the ES assessments as appropriate.

The Environment Agency has been consulted regarding the proposed approaches (refer to the Technical Engagement section of Section 21.3).

205 Pluvial flood risk: Paragraph 6.2.2 suggests that existing publicly available Environment Agency surface water flood risk mapping will be sufficient to inform the flood risk assessment, but that modelling may also be required. Efforts should be made to agree the requirement for pluvial flood risk modelling with the relevant consultation bodies e.g. Environment Agency and Lead Local Flood Authorities.

Requirements for pluvial flood modelling are set out in Appendix 21.4, including Annex B. The conclusions of the PEIR assessment do not identify the need for pluvial flood modelling. The Environment Agency and Lead Local Flood Authorities will have the opportunity to review and comment on this position as presented in the PEIR.

The assessment of pluvial flood risk for the ES will be in line with the conclusions of Appendix 21.4, subject to further consultation and development of the approach as appropriate.

206 If during design development the Proposed Development approaches within 100m of a relevant waterway, the Canal and River Trust, should be consulted.

The Canal and River Trust has been consulted in relation to flood risk associated with the canal, as assessed in Appendix 21.4 Annex C.

Technical engagement 21.3.5 Technical engagement has been ongoing with a number of prescribed and non-

prescribed consultation bodies and local authorities in relation to the water environment. A summary of the engagement undertaken that is relevant to this PEIR is outlined in this section.

21.3.6 Table 21.4 provides a summary of the technical engagement up until the finalisation of the PEIR, and how any comments received have or will be addressed.

Table 21.4: Water environment technical engagement January 2017 to May 2019

Subject of Engagement No. meetings Stakeholders Water Framework Directive Assessment

4 Environment Agency Natural England

Groundwater Baseline, Modelling and Assessment 3

Environment Agency Natural England



Subject of Engagement No. meetings Stakeholders Surface water quality baseline and Assessment 2 Environment Agency FRA 1 Environment Agency Masterplanning Briefings

4

Environment Agency Natural England HSPG

WFD Additional Mitigation Sites consultation

1

Environment Agency Colne Valley Park Community Interest Company (CVP CIC) Royal Parks Friends of the River Crane (FORCE) Crane Valley Partnership Affinity Water

Twin River Management Group

3

Environment Agency Royal Parks Colne Valley Park Community Interest Company (CVP CIC) Friends of the River Crane (FORCE) Green Corridor Crane Valley Partnership

CRC Design Meeting 1

Environment Agency Natural England

FRA, Hydraulic Modelling and Hydrology 3 Environment Agency Initial Permitting Discussion 1 Environment Agency Surface Water Drainage Design & Assessment 1 HSPG Water Environment Update (Assessments and approaches) 2 HSPG Interaction with Thames Water Assets (Foul Drainage) 10 Thames Water Interaction with Affinity Water Assets (Public Water Supply) 9 Affinity Water

Environment Agency 21.3.7 Engagement with the Environment Agency, as regards the water environment, has

been ongoing since May 2017 in the form of meetings/workshops (see Table 21.4) and the sharing of draft technical documentation. In addition to the engagement described below with regard to the water environment, the Environment Agency has provided advice on other aspects of the DCO Project including Biodiversity (Chapter 8), Land quality (Chapter 14) and Waste (Chapter 20).

21.3.8 The Environment Agency has provided advice on a range of subjects relevant to the assessment. These can be categorised as follows:

1. Review of assessment approaches and methodologies for groundwater modelling, surface water quality, flood risk and WFD. The WFD engagement



has included the approach to quantitative and qualitative assessment of surface and groundwaters

2. The approach to the design of the water environment, including the choices around the diversion of rivers and the design of the Covered River Corridor (CRC)

3. Principles to underpin the approach to permitting

4. Review comments on the following baseline reports, which will be presented in more detail in the ES:

a. Groundwater Baseline Report

b. FRA baseline (within Appendix 21.4)

c. Low flows hydrology report

d. Drainage Baseline (within Appendix 21.5)

e. Engagement on the evidence base and the outline design of the CRC (within Annex 21.3A to Appendix 21.3)

5. Review of the draft Code of Construction Practice (CoCP).

Natural England 21.3.9 Engagement with Natural England, as regards the water environment, has been

ongoing since May 2017 in the form of technical meetings/workshops held jointly with the EA (see Table 21.4) and the sharing of draft technical documentation. In addition to the engagement described below with regards to the water environment, Natural England has provided advice on other aspects of the DCO Project including the Biodiversity assessment (Chapter 8), Land quality (Chapter 14) and Landscape and Visual Assessment (Chapter 15).

21.3.10 Natural England has provided advice on a wide range of subjects relevant to the assessment of the water environment. These can be categorised as follows:

1. Review of assessment approaches and methodologies for groundwater modelling and WFD. The WFD engagement has included the approach to quantitative and qualitative assessment of surface and groundwaters

2. The approach to the design of the water environment, including the choices around the diversion of rivers and the design of the CRC

3. Review comments on the Groundwater Baseline Report.



Colne Valley Park Community Interest Company (CVP CIC) 21.3.11 Engagement with CVP CIC, as regards the water environment, has been ongoing

since 2017 in the form of meetings/workshops (see Table 21.4). In addition to the engagement described below with regard to the water environment, CVP CIC has provided advice on other aspects of the DCO Project including Biodiversity (Chapter 8), Recreation (within Chapter 11: Community), and Landscape and Visual Assessment (Chapter 15).

21.3.12 CVP CIC has provided advice on the options for river diversions and flood storage in the Colne catchment, as well as information on potential sites for WFD additional environmental measures.

Heathrow Strategic Planning Group (HSPG)

21.3.13 Engagement with HSPG, as regards the water environment, has been ongoing since 2017 in the form of meetings (see Table 21.4) and the sharing of draft technical documentation.

21.3.14 The HSPG has also been consulted on the following:

1. The approach to the design of the water environment and on the approach to surface water drainage assessment, in their role as Lead Local Flood Authorities (LLFAs)

2. The draft CoCP.

Affinity Water 21.3.15 Engagement with Affinity Water, as regards the water environment, has been

ongoing since 2017 in the form of meetings (see Table 21.4). This has covered water supply to the DCO Project, impacts on Affinity Water Assets and the potential for WFD additional environmental measures in the Colne catchment.

Thames Water 21.3.16 Engagement with Thames Water, as regards the water environment, has been

ongoing since 2017. This has covered the use of Thames Waters foul drainage infrastructure including connections to Mogden Waste water treatment works and Iver North waste water treatment plant, impacts on Thames Water assets within the draft DCO limits and the residual flood risk from Thames Water’s reservoirs.

Friends of the River Crane (FORCE)

21.3.17 FORCE is part of the Crane Valley Partnership (CVP). Engagement with FORCE, as regards the water environment, has been ongoing since 2017. This has covered the evolving rivers design, through the Twin Rivers Management Group,



and the potential for WFD additional environmental measures along the Duke of Northumberland’s River and the River Crane.

Royal Parks

21.3.18 Engagement with Royal Parks, as regards the water environment, has been ongoing since 2017. This has covered the evolving rivers design, through the Twin Rivers Management Group, and the potential for WFD additional environmental measures along the Longford River.

21.4 Scope of the assessment

Overview 21.4.1 This section describes the spatial and temporal scope for the assessment as it

applies to the Water Environment and outlines the receptors on which assessment has been undertaken.

21.4.2 This scope has been developed as the DCO Project has evolved and responds to feedback received to date as detailed in Section 21.3. The information presented in the PEIR is by its nature preliminary and should not be considered a ‘draft’ ES (in accordance with PINS Advice Note Seven). Further scope refinement may be required to take full account of the preferred DCO Project design and subsequent engagement.

Spatial scope and study area 21.4.3 The spatial scope of the assessment of the water environment covers the area of

the draft DCO limits (the Site) together with the study areas. The study areas for the water environment have been defined by the relevant surface and ground water body extents defined in the Thames River Basin Management Plan (RBMP). The following study areas are shown in Figure 21.1, Volume 2.

1. Local Surface Water Study Area (LSA). Includes the catchments of the WFD surface water bodies within the Colne and Crane operational catchments (as defined in the Thames RBMP) that are intersected by elements of the DCO Project

2. Wider Surface Water Study Area (WSA): Incorporates the wider catchment extent of the Colne and Crane operational catchments beyond the LSA to cover the Colne and Crane catchments from their source to the Thames and allow consideration of any effects propagating either upstream or downstream. It also includes the catchments of the River Thames WFD water bodies into which the Horton Brook, River Colne and River Crane discharge



3. Groundwater Study Area (GWSA). Includes the full lateral extent of the Lower Thames Gravels WFD groundwater body, and underlying strata.

21.4.4 These are the same as the study areas set out in the Scoping Report. No changes to the study areas were suggested in the Scoping Opinion.

21.4.5 The appropriate scale of baseline data gathering and assessment varies for different aspects of the water environment, and these variations have been accounted for and identified in the methodology sections (Section 21.6 and Section 21.7).

Temporal scope 21.4.6 The DCO Project will be developed in a phased approach, meaning that in some

periods and/or some locations both construction and operational activities will take place in parallel. The effects of these activities are also felt at different receptors, at different times. Three phases have been identified in Chapter 5: Approach to the EIA to broadly correspond with the most prevalent activities that will arise as a result of the DCO Project.

21.4.7 Phase 1 would run from 2022 to runway opening which is currently anticipated to be 2026 and includes major earthworks associated with airfield expansion, river diversions, changes to the M25, A4 and other roads, construction of rail facilities and utility diversions. Phase 2 would run from late c.2026 to 2033 and would include further work to the road network, airfield and other infrastructure. Phase 3 would run from c.2034 to 2050 and includes relatively low levels of development, with the majority focused on changes within the airfield to build capacity.

21.4.8 As outlined in Chapter 5, in order to provide appropriate flexibility in the design of the DCO Project post grant of the DCO and at the same time maintain a rigorous EIA process, a reasonable worst-case approach has been undertaken for each aspect assessment. For the purposes of defining a reasonable worst case, all assessments of the water environment have considered the permanent land use changes at the end state of the DCO Project, i.e. the end of Phase 3 in 2050. In addition, the assessments have considered effects relating to construction activities, for which the majority of effects will commence in Phase 1, with some continuing through Phases 2 and 3.

21.4.9 The assessments consider historical hydrological conditions and the frequency and distribution of hydrological events, as appropriate to the type of effect and relevant policy requirements. Climate change is considered explicitly in the floods and drainage assessments for the PEIR. It will be considered, where appropriate, in the other assessments for the ES.

21.4.10 Further details of the temporal scope of each assessment, within the framework of the DCO Project phasing, are presented in Table 21.5.



Table 21.5: Temporal scope of water environment assessments

Assessment Timing of effects Assessment approach

Surface water quality assessment

Surface water quality is mainly influenced by the timing of river diversions and installation of airfield drainage (Phase 1), with those effects then persisting through Phases 2 and 3.

The assessment for PEIR considers the effects of the DCO Project as a whole (i.e. based on the permanent extent in 2050), assessed against recent hydrological conditions.

Groundwater modelling assessment

The groundwater regime is influenced throughout Phases 1 to 3 by changes to the land surface and sub-surface, including both construction activities (e.g. borrow pits) and permanent infrastructure (e.g. land raising, tunnels and basements).

The groundwater model scenarios assess the permanent extent of the development in 2050, as well a reasonable worst case during construction. These scenarios are assessed against historical hydrological conditions from 1995 to 2015.

Preliminary WFD risk assessment

The WFD assessment is mainly influenced by the timing of river diversions (Phase 1), although activities in Phases 2 and 3 (particularly those with the potential to affect groundwater) are also relevant.

The assessment for PEIR considers the effects of the DCO Project as a whole (i.e. based on the permanent extent in 2050) in relation to the current and target status of water bodies.

FRA The FRA is mainly influenced by the timing of river diversions and provision of flood storage areas, and the installation of airfield drainage (Phase 1), with those effects then persisting through Phases 2 and 3.

The PEIR FRA and DIA assess the effects of the permanent extent of the development in 2050. They account for the effects of climate change up to 2115, in accordance with the relevant guidance. Shorter periods of climate change will be applied for construction scenarios, as appropriate, in the ES.

DIA The DIA is mainly influenced by the installation of airfield drainage (Phase 1), with those effects then persisting through Phases 2 and 3.

Receptors 21.4.11 The spatial and temporal scope of the assessment enables the identification of

receptors which may experience a change as a result of the DCO Project.

21.4.12 Table 21.6 presents the receptor groups that may potentially be affected by the DCO Project and hence are assessed in Section 21.11. The table outlines the individual receptors that are included within each group. The receptor groups are shown in Figure 21.2, Volume 2. The list of receptors will be kept under review as the EIA process continues.



21.4.13 Sixteen receptor groups have been identified for the water environment assessment. They have been categorised using the acronym WE and numbered 1 to 16 as follows.

21.4.14 Receptor groups WE1 to WE10 have been defined based on WFD water body catchments, with other receptors within those catchment areas (any abstractions, water-dependent designated sites, non-designated water-dependent habitats and discharges) included within the group. This approach was taken because in many cases it is possible to apply the assessment of effects for the watercourse to the other receptors, but it also allows for individual receptors within the group to be assessed separately where necessary.

21.4.15 Reservoirs are included in a separate receptor group (WE11) as they are not hydraulically connected (except by pumping) to either the river network or to groundwater, and hence will not necessarily be subject to the same effects as the catchment within which they are geographically located.

21.4.16 The groundwater environment is covered by two receptor groups, WE12 (Lower Thames Gravels) and WE13 (bedrock aquifers beneath the gravels, including the Chalk). The separation between the two groups allows a distinction in the assessment of effects between the two, while recognising the potential for pathways to the Chalk. Although the surface water and groundwater regime are connected, the groundwater regime has been placed in separate receptor groups from surface water to allow effects on groundwater to be assessed in their own right and at an appropriate spatial scale. The assessment of effects on the water environment in Section 21.10 takes into account the potential for hydraulic continuity between groundwater and surface water.

21.4.17 Local water supply and waste water infrastructure have been placed in their own receptor groups (WE14 and WE15 respectively) to reflect the differing nature of effects, which do not necessarily conform to either surface water or groundwater catchment boundaries.

21.4.18 Flood risk receptors, i.e. people, property or infrastructure who could be affected by flooding, are included in a separate group, WE16. Although the sources of flooding lie within other receptor groups (e.g. fluvial in WE1-WE9, groundwater in WE12-WE13), the assessment approach differs such that it is more straightforward to group them together and allow more direct referencing of the FRA.



Table 21.6: Receptors requiring assessment for water environment

Receptor group Receptors included within group

WE1: Horton Brook Horton Brook and its tributaries. Surface water abstractions from the Horton Brook, located within and downstream of the Site.

WE2: Colne Brook

Colne Brook and its tributaries. Poyle Channel. Surface water abstractions from the Colne Brook, located within and downstream of the Site. Surface water discharges within the Site in the vicinity of proposed infrastructure.

WE3: Wraysbury River

Wraysbury River and its tributaries, including the Bigley Ditch. Surface water abstractions from the Wraysbury River, located within and downstream of the Site. Surface water discharges within the Site in the vicinity of proposed infrastructure.

WE4: River Colne

The extent of the River Colne and its tributaries located within the Colne (Confluence of the Chess to the River Thames) water body. Surrey Ash water body. Surface water abstractions along the River Colne, located within and downstream of the SIte. Surface water discharges within the draft DCO Project limits in the vicinity of proposed infrastructure.

WE5: Duke of Northumberland’s River and Longford River

The Duke of Northumberland’s River from its bifurcation from the Colne River to its confluence with the River Crane. The Longford River from its bifurcation from the River Colne to the confluence with the River Thames. Surface water abstractions along the Duke of Northumberland’s River (upstream of the River Crane) and Longford River, located downstream of the Site.

WE6: Portlane Brook The extent of the Portlane Brook waterbody and its tributaries, including Felthamhill Brook.

WE7: River Crane

Yeading Brook WFD water body and its tributaries. The River Crane WFD water body and its tributaries. The Lower Duke of Northumberland’s River from its bifurcation from the Crane to its confluence with the River Thames.

WE8: River Thames (freshwater)

River Thames reaches included in the water bodies Thames (Cookham to Egham) and Thames (Egham to Teddington). Surface water abstractions along the River Thames, located downstream of the Site.

WE9: River Thames (tidal)

The uppermost stretch of the tidal Thames, extending from Teddington Weir to downstream of the confluence with the River Crane. Surface water abstractions along this extent of the River Thames.

WE10: Upper River Colne

The full upstream extent of the River Colne and its tributaries upstream of the Chess confluence (WE4).



Receptor group Receptors included within group

WE11: Reservoirs

The Queen Mother Reservoir Wraysbury Reservoir King George VI’s Reservoir Staines North Reservoir Staines South Reservoir

WE12: Lower Thames Gravels

Full lateral extent of the Lower Thames Gravels groundwater body. Lake habitats in direct connectivity with the Lower Thames Gravels, including WFD lakes (Wraysbury Lake, Wraysbury No. 2 Gravel Pit, Heron Lake) and non WFD lakes (including to the northwest of the site Saxon Lake, Old Slade Lake, Colnbrook North, Orlitts Lake, Colnbrook West, Swan Lake). Groundwater abstractions from the Lower Thames Gravels.

WE13: Bedrock Aquifers

Aquifers beneath the London Clay, including the Chalk and Lambeth Group, within the lateral extent of the GWSA. Groundwater abstractions from the Chalk aquifer within the lateral extent of the GWSA.

WE14: Local Water Supply

Affinity Water-maintained assets around the boundaries of the airfield and Heathrow-maintained assets on the airfield.

WE15: Foul Drainage Infrastructure

Thames Water maintained assets around the boundaries of the airfield and Heathrow-maintained assets on the airfield.

WE16: Flood Risk Receptors

People, property or infrastructure at risk of flooding from any source.

21.4.19 The list of receptors will be kept under review during the EIA as more detailed information is obtained during baseline surveys and other forms of data collection by other aspects and will be reflected in the final ES.

Identification of potential effects 21.4.20 The water environment receptors that have been scoped in for assessment are

summarised in Table 21.7.

Table 21.7: Likely significant effects on water environment receptors scoped in for further assessment

Receptor Activity Effect Likely effects on surface waters (WE1-WE11) Construction phase WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE6 Portlane Brook

Construction activities 1. River diversions 2. Construction working

areas and haul routes

Changes to channel geomorphology New channels will be created for the diversions. The geomorphology of the existing or new channels could be affected by the process of transferring flows between channels.



Receptor Activity Effect Channel geomorphology could also be affected by temporary road crossings over watercourses.

WE1 Horton Brook WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE6 Portlane Brook WE7 River Crane WE8 River Thames (freshwater) WE9: River Thames (tidal)

Construction activities 1. Dewatering 2. Construction working

areas and haul routes 3. Construction of river

diversions 4. Construction of flood

storage areas 5. Altered landforms,

including excavations and stockpiling

Changes to river flow Potential for altered river flow regimes, as a result of:

1. Reduced baseflow as a result of dewatering, and increased flows due to discharge of dewatered groundwater to surface waters. The locations of these two effects may not coincide, for instance, the discharge could be released to a different river or reach from that experiencing reduced baseflow

2. Changes to surface runoff as a result of increased hardstanding and compacted, un-vegetated surfaces

3. Changes to local catchment areas as a result of earthworks

4. Redistribution of flows to new river channels

WE1 Horton Brook WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE6 Portlane Brook WE7 River Crane WE8 River Thames (freshwater) WE9: River Thames (tidal)

Construction activities 1. Excavations and infilling 2. Dewatering 3. Construction working


Increased sediment loading to surface water could result from ground disturbance and sediment mobilisation during the construction of new river channels, and the process of diverting flows in those channels. Increased sediment loading to surface water could also occur as a result of unmanaged runoff and dewatering from construction areas.

WE1 Horton Brook WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE6 Portlane Brook WE7 River Crane

Construction activities 1. Construction working

areas and haul routes 2. Excavations 3. Dewatering 4. Piling through landfills

Introduction of pollutants to surface waters Surface water quality could be affected by the accidental spillage or leakage of fuels, lubricants or other chemicals and runoff from construction areas. Surface water quality could also be affected from the mobilisation of contaminants in landfill areas during excavation and piling activities, and subsequent release during dewatering.



Receptor Activity Effect Permanent land use changes WE1 Horton Brook WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE6 Portlane Brook WE7 River Crane

Permanent land use changes 1. River diversions

Changes to channel geomorphology There are proposed modifications to the existing routes of the Colne Brook, Wraysbury River, River Colne, Duke of Northumberland's River and Longford River, with new channels being created. The Colne Brook would be diverted around the western boundary of the Airport. The Wraysbury River, River Colne, Duke of Northumberland’s River and Longford River would pass beneath the runway in a CRC. Diversions could lead to altered flow conveyance as a result of alterations to existing channel flow paths, or modifications to cross-sectional capacity. The creation of new channels, alterations to channel form and length, and associated changes to the flow regime, could result in changes to erosion and deposition processes over time, altering channel morphology. The modification of channel routes and form could alter the connectivity with the floodplain, and this could affect the associated extent and function of active floodplain.

WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE7 River Crane WE10 Upper River Colne

Permanent land use changes 1. WFD additional

environmental measures

Changes to channel geomorphology Additional environmental measures will be implemented in the local and wider study area, providing improvements to channel geomorphology in those areas.

WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River WE10 Upper River Colne

Permanent land use changes 1. River diversions

Loss of ecological connectivity The river diversions and CRC could result in a loss of ecological connectivity between the lower and upper reaches of the River Colne and its tributaries. This is assessed in Chapter 8: Biodiversity

WE1 Horton Brook WE2 Colne Brook WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland’s River and Longford River

Permanent land use changes and operational activities

1. River diversions 2. Flood storage areas 3. New sub-surface

features (basements,

Changes to river flow Potential for altered river flow regimes, which could be associated with:

1. Changes to groundwater-surface water interactions in diverted reaches, due to their location or



Receptor Activity Effect WE6 Portlane Brook WE7 River Crane WE8 River Thames (freshwater) WE9: River Thames (tidal)

tunnels) 4. Altered landforms,

including the new runway and modified roads and junctions

5. Increased areas of hardstanding, and associated drainage networks

6. Abstractions and discharges from rivers or groundwater

the river bed being lined; 2. Changes to groundwater flow or

levels associated with sub-surface structures;

3. Changes to the extent and/or distribution of floodplain storage;

4. Changes to surface runoff as a result of increased hardstanding;

5. Changes to local catchment areas and drainage networks;

6. Flow not being redistributed downstream of river diversions according to baseline conditions;

7. Abstractions resulting in reduced flow in rivers.


Permanent land use changes 1. River diversions 2. Abstractions and

discharge from rivers or groundwater

Effects on water quality associated with changes to mixing and dilution Changes to mixing and dilution could occur due to the transfer of water between different surface water bodies, associated with channel diversions and combining multiple channels. There could be changes to in-channel mixing processes associated with flow through a CRC, and changes to dilution capacity resulting from changes to baseflow.


Operational activities 1. Use of chemicals and

other substances 2. Operation of the

drainage network

Introduction of pollutants to surface waters Surface water quality could be affected by the accidental spillage or leakage of fuels, lubricants or other chemicals required for operation. Surface water quality could be affected by discharge of runoff from the Airport, including from areas where de-icing is carried out. Discharges associated with the thermal strategy could potentially have a higher temperature than the receiving water.


Operational activities 1. Increased traffic

movements, with associated emissions

Increased atmospheric deposition of pollutants to surface waters Changes to air quality associated with increased emissions from aircraft and land-based vehicular traffic could lead to the increased atmospheric deposition of pollutants into surface waters.



Receptor Activity Effect WE8 River Thames (freshwater) WE9: River Thames (tidal) WE11: Reservoirs WE2 Colne Brook WE3 Wraysbury River WE4 River Colne

Permanent land use changes 1. Altered landforms,

including the new runway and modified roads and junctions, and areas of hardstanding

2. New sub-surface features (basements, tunnels)

3. Flood storage areas 4. Diversions

Loss of abstractions or discharges Permanent changes to land use could result in the physical removal of existing abstractions and discharges.

Likely effects on groundwater receptors (WE12-13) Construction phase WE12 Lower Thames Gravels WE13 Bedrock Aquifers

Construction activities 1. Excavations and infilling 2. Dewatering 3. Piling

Changes to groundwater regime Construction activities such as the use of groundwater cut off controls, dewatering, extraction of gravels, and changes to the permeability of subsurface fill material could result in groundwater level changes and/or changes to lake or wetland levels in connectivity with the gravels.

WE12 Lower Thames Gravels WE13 Bedrock Aquifers



Introduction of pollutants Changes to groundwater quality could arise from the accidental spillage or leakage of fuels, lubricants or other chemicals at the surface, with infiltration to the superficial aquifer. Where pathways exist or are created, there is some potential for contamination to reach the Lambeth Group/Chalk.




Mobilisation of contaminants Changes to groundwater quality could arise from mobilisation of contaminants in groundwater, for example as a result of disturbing contaminated ground, creating new pathways, through excavating and piling, and dewatering, as assessed in Chapter 14.

Permanent land use changes and operational phase Permanent land use changes


Reduced recharge Reduced and/or locally displaced recharge to the superficial gravel aquifer due to increased impermeable surfaces and collection of rainfall runoff in drainage systems, with subsequent release to surface waters.



Receptor Activity Effect WE12 Lower Thames Gravels WE13 Bedrock Aquifers

Permanent land use changes 1. River diversions 2. Flood storage areas 3. New sub-surface

features (basements, tunnels)

4. Altered landforms, including the new runway

5. Abstractions from groundwater or lakes

Changes to groundwater regime The influence of new or relocated permanent engineered structures, e.g. re-designed and/ or re-purposed landfill sites, basements, tunnels, pipelines and any other sub-surface structures (e.g. an airfield drainage network) could affect the groundwater regime. The realignment of rivers and other surface water bodies such as flood storage areas, including restricted interactions with surface water as a result of bed lining, could affect the groundwater regime. This could include changes in lake or wetland levels for those lakes and wetlands in connectivity with the gravels. Changes to the groundwater regime and/or lake levels could also occur as a result of new or increased abstractions.


Operational activities 1. Use of chemicals and

other substances 2. Operation of the

drainage network

Introduction of pollutants Changes to groundwater quality could arise from the accidental spillage or leakage of fuels, lubricants or other chemicals required for operation at the surface, with infiltration to the superficial aquifer. Where pathways exist or are created, there is some potential for contamination to reach the Lambeth Group/Chalk.

Likely effects on water supply and foul drainage infrastructure (WE14 and WE15) Construction phase WE14 Local Water Supply Water use during

construction

Effects on the local public water supply network Increases in water demand from the Airport associated with construction activities could exceed the capacity of the local public water supply network or affect the sustainability of supply in the local water resource zone.

WE15 Foul Drainage Network Discharge to sewers during construction

Effects on the foul drainage network Increases in the discharge of drainage associated with construction activities could lead to exceedance of capacity in the foul drainage network



Receptor Activity Effect Operational phase WE14 Local Water Supply Water use during operation

Effects on the local public water supply network Increases in water demand from the Airport could exceed the capacity of the local public water supply network or affect the sustainability of supply in the local water resource zone.

WE15 Foul Drainage Network Operation of drainage system

Effects on the foul drainage network Increases in the discharge of foul drainage from the Airport could lead to an exceedance of down pipe capacity

Likely effects on flood risk receptors (WE16) WE16 Flood Risk Receptors Construction Activities

1. Excavations 2. Land raising and

levelling 3. Dewatering 4. Construction working

areas and haul routes 5. Construction of river

diversions 6. Construction of Flood

Storage Areas

Change to the risk of fluvial flooding Changes to fluvial flood risk could occur as a result of:

1. The permanent changes listed above for the operational phase

2. Loss of floodplain storage associated with construction activities including excavations, stock-piling, areas of hardstanding, compacted surfaces for construction compounds and working areas

3. Discharge of dewatered groundwater dewatered to surface waters

4. Release of runoff from drainage systems into a different catchment; and

5. Changes in the surface water run-off regime of contributing sub-catchments

WE16 Flood Risk Receptors Permanent land use changes and operational activities

1. River diversions 2. Flood storage areas 3. Altered landforms,

including the new runway and modified roads and junctions


Change to the risk of fluvial flooding Changes to fluvial flood risk could occur as a result of:

1. Changes to the extent of the fluvial floodplain, associated with development on the floodplain and diverting rivers to new locations

2. Changes to flow conveyance 3. Changes to river flows; and 4. Changes in the surface water run-

off regime of contributing sub-catchments

Construction activities 1. Land raising and

levelling 2. Construction working


Change to the risk of pluvial flooding Changes to pluvial flood risk could occur as a result of: an increase in areas of hardstanding and other unvegetated surfaces in construction working areas, or changes to surface flow paths as a result



Receptor Activity Effect of changes to landform (such as from stockpiling)

WE16 Flood Risk Receptors Permanent land use changes and operational activities

1. Altered landforms, including the new runway and modified roads and junctions

2. - Increased areas of hardstanding, and associated drainage networks

Change to the risk of pluvial flooding Changes to pluvial flood risk could occur as a result of:

1. An increase in areas of impermeable surfaces, and associated increased potential for runoff

2. Changes to surface flow paths

WE16 Flood Risk Receptors Construction and operational activities

1. Excavations and infilling 2. Dewatering 3. Piling 4. River diversions 5. Flood storage areas 6. New sub-surface

features (basements, tunnels)

7. Altered landforms, including the new runway

Change to the risk of groundwater flooding Changes to groundwater flood risk could occur due to:

1. Reduced recharge from an increase in impermeable surfaces

2. Changes to groundwater flow and/or levels from construction activities and permanent below ground engineering structures

3. Increased infiltration/ recharge causing reduced groundwater flood risk in specific areas


1. Altered landforms 2. Drainage networks

Change to the risk of flooding from infrastructure Changes to the risk of flooding could arise from modifications to water levels or the banks of infrastructure such as reservoirs, canals and new raised water bodies (including surface water drainage attenuation basins, flow lagoons and treatment areas.)



Changes to the risk of sewer flooding as a result of: Changes in rates and volumes of surface water runoff being discharged into existing sewer systems during the construction or operation of the airport.

Effects no longer being considered 21.4.21 The following effects have been scoped out of the assessment for the reasons

summarised in Table 21.8.



Table 21.8: Effects no longer considered in this PEIR

Receptor Activity Effect Justification

People, property or infrastructure at risk of flooding

All construction and operation activities

Potential effects of the DCO Project on tidal flood risk, or risks from tidal flooding to the DCO Project

The DCO Project is located upstream of Teddington Lock (the tidal limit of the Thames), on a tributary of the Thames. The DCO Project has a minimum elevation of approximately 15m Above Ordnance Datum (mAOD), compared to the highest recorded tidal flood level at Teddington of 5.62mAOD. Given the location and elevation of the DCO Project there is considered to be no risk to the development from tidal flooding, as set out in Appendix 21.4. The PINS Scoping Opinion (PINS ID 189, as set out in Table 21.3) considered that insufficient evidence had been provided regarding the quantification of discharges from the proposed development. The effects of discharges from the development on downstream runoff and river flows remain scoped in to the assessment and are considered in detail in Appendix 21.4 and Appendix 21:5. The approach to the FRA, including tidal flood risk and management of surface water, was shared with the Environment Agency for review during the consultation process.

21.5 Embedded environmental measures

Overview 21.5.1 The DCO Project will consider a number of environmental measures to avoid or

minimise likely significant effects. This approach is described in Chapter 5. Some of the environmental measures have been embedded into the DCO Project design and are outlined in Chapter 6: DCO Project description. Those embedded environmental measures that have been taken in to account in the assessment of the water environment are introduced in Table 21.9.

21.5.2 Good practice environmental measures would occur with or without input from the EIA feeding into the design process. They include actions that would be undertaken to meet other existing legislative requirements, or that are considered to be standard practices. Those good practice environmental measures that have been taken in to account in the assessment of the water environment are introduced in Table 21.9.



21.5.3 Each measure, or set of measures, is then discussed in more detail, before being summarised in Table 21.10 and Table 21.11.

Table 21.9: Summary of measures relating to the water environment

Measures embedded into the DCO Project design (outlined in Chapter 6: DCO Project description)

Measures incorporated into construction activities (outlined in the draft CoCP)

1. River diversion design 2. Design of CRC 3. Design of road crossings of rivers 4. Provision of, and design of, Compensatory

Flood Storage 5. Management of on-airfield surface water

drainage 6. Management of off-airfield surface water

drainage 7. Management of subsurface flow pathways 8. Pollution Control Strategy

1. Measures relating to dewatering activities 2. Management of surface water run-off 3. Measures relating to piling activities 4. Measures relating to construction of new

river channels 5. Measures relating to backfilling of old river

channels 6. Measures relating to construction within the

flood plain 7. Measures relating to the provision of

alternative water supplies 21.5.4 It should be noted that all environmental measures with respect to water resources

use can be found in Section 3: Water Resources of Appendix 20.1: Draft Resource Management Plan, Volume 3.

Measures embedded into permanent infrastructure

River diversion design 21.5.5 The third runway will transect the Colne Valley, crossing reaches of the Colne

Brook, Wraysbury River, River Colne, Duke of Northumberland’s River and the Longford River. Figure 21.3 provides an overview of the proposed changes to the water environment, including river diversions, that will be required as a result. The routes of the diversions are described in Chapter 6.

21.5.6 The new channels will be designed to appropriately accommodate flows (including flood flows), providing spatially variable aquatic habitat and with connectivity to a riparian zone. The following design principles will be embedded in the design of the new rivers:

1. For any river that is diverted, space will be provided, where reasonably practicable, for riparian corridors between a minimum of 8m and up to 20m on each side of the bank top. Where reasonably practicable, permanent above ground infrastructure will be moved away from these corridors (this includes road overpasses), and appropriate vegetation will be provided to maximise the ecological connection between the floodplain and the channel, once matured



2. As far as reasonably practicable channel planform and sinuosity of any new or modified river channels will be designed appropriately, making use of expert geomorphological and aquatic ecology knowledge and best practice

3. Preliminary estimates of the requirement for erosion protection have indicated the suitability of options that are commonly used in low stream power / low hydraulic loading environments, to protect the new channels and their banks. This creates opportunities for a number of different bio-engineering ‘soft’ erosion protection options, which are currently being developed. The requirement for any soft engineering erosion control / bank stabilisation measures will be consulted upon with the Environment Agency prior to the Application

4. Where reasonably practicable, river bed sediment for the new river diversion channels will be reused from the old river channels, as long as it is of appropriate type and quality. If this is not practicable then sediment will be sourced that is of an equivalent lithology and is of an appropriate size and shape in the context of the newly designed channels and the typical natural characteristics of watercourses in this area (noting that this may be subtly different to the baseline conditions and especially where flows are being combined)

5. Any structures installed on the diverted channels will be designed to enable fish passage within the river channels. All fish passage solutions will be designed in consultation with the Environment Agency

6. Heathrow will engage with the Environment Agency on the method of the diversion of rivers prior to the Application, with the objective of ensuring compliance with the requirements of the Water Framework Directive

7. Where reasonably practicable, riparian bankside vegetation will be planted to achieve the best ratio between open and shaded areas, once matured. Any planting of bank vegetation will include only native species

8. Where reasonably practicable new channels’ morphological features will be designed to provide an appropriate amount of variability in channel width and depth and appropriate substrate. This includes the provision of suitable areas for fish spawning, nursing and feeding. The WFD hydromorphology quality elements have been used as a basis for informing the channel design. Conceptual channel designs are shown in the Appendix 21.3: WFD Assessment, Figure 21.3.11

9. Channels will be designed to the required design flood conveyance requirements. Where it is appropriate to connect the channel and its wider floodplain, this will be included in the design. Where it is not appropriate, multi-



stage channels will be designed to facilitate natural processes at low, moderate and high flows contained within the channel, as well as flood events that would exceed bankfull. Any required flood defences would be set back from the bank top

10. Where the underlying sediment is contaminated and a liner is required, environmental measures will be considered to simulate free movement of water in the hyporheic zone. Where underlying sediment is not contaminated, the design will allow for free movement of water in the hyporheic zone and therefore exchange of water between the new channel and the ground

11. Where the sediment and/or groundwater underlying the new channels is contaminated, natural river bed substrate will form an appropriate thickness of material above the material used to line the channel. This will create minimal or no local exchange of water between the channel and its underlying substrate. The nature of this lining will take into account the risk of signal crayfish burrows.1

12. Any baseline river channels that will receive altered flow relative to their baseline flow regime will need to have their cross-sectional capacity and form modified to suit the new flow regime.

Design of CRC 21.5.7 The CRC is the term that refers to the main diversion of the River Colne,

Wraysbury River, Duke of Northumberland’s River and Longford River, where they pass beneath the northwest runway and associated taxiways. The CRC will be designed to the key principles of diversions and will incorporate natural light. Space will be provided for riparian buffer corridors either side of channels to facilitate ecological connectivity, and channels will only be lined where the new channel flows over contaminated land, to minimise disruption to river baseflow and groundwater connectivity.

Channel cross-section 21.5.8 The concept design includes appropriate channel asymmetry and low flow channel

wetted width in order to locally increase the unit stream, power whilst still providing the required conveyance during large floods (Appendix 21.3, Figure 21.3.11). An illustration of the proposed planform and asymmetrical channel cross – sections is provided in the CRC Concept Design Report presented in Annex 21.3A.

1 Signal Crayfish are an invasive species known to be present in the River Colne and associated rivers (see Chapter 8: Biodiversity). Crayfish can burrow some distance through clay, hence the liner design will need to take into account this risk and mitigate it by the choice of liner thickness and/or material.



Ecological Zones and lighting

21.5.9 A key part of the design will be the provision of lighting along the length of the CRC, using a combination of artificial and natural lighting “ecological island” concept. The concept would provide different types of ecological zone, including ecological islands and connecting zones. There will be a lighting strategy for the zones and the transition between them will provide lighting tailored to its specific use, creating a river corridor that has a combination of natural and artificial light. These ecological and lighting zones are described in the CRC Concept Design Report in Annex 21.3A to Appendix 21.3 and in Chapter 6.

Other design considerations

21.5.10 The CRC concept also includes other design considerations as follows:

1. Cover/lid thickness and type: The potential to “open up” parts of the CRC through the use of re-enforced grills, and natural light ingress will be maximised as much as possible and where airport safety constraints allow

2. Terrestrial ecological connectivity: Provision of features for bats, otters and other mammals

3. Airflow and temperature regulation

4. Access/maintenance and security requirements

21.5.11 All proposed embedded design measures are described in more detail within Appendix 21.3.

21.5.12 Further work will be undertaken to develop the CRC concept prior to the Application. This will include lighting trials to refine the ecological zone concept and develop the understanding of the range and duration of light intensities needed for sustainable plant growth.

Design of road crossings of rivers 21.5.13 Permanent road watercourse crossings will be designed to convey 1% AEP fluvial

flood flows, including a 35% allowance for climate change. To facilitate ecological connectivity new road crossings > approximately 25 m will contain CRC-type design interventions where reasonably practicable, as appropriate for the crossing type, location and length.

Provision and design of compensatory flood storage 21.5.14 New upstream flood storage will be required, prior to the removal of the flood

plains, as described in Section 14.6 of the draft CoCP and Appendix 21.4. Appropriate phasing of movement of rivers and re-provision of flood storage areas has been embedded into the design to ensure no increase in flood risk at any



stage during construction. Locations for the flood storage areas within the Colne Valley have been identified and each involve a similar conceptual design, which comprises:

1. A gated offtake control structure, to regulate the timing and filling of the storage area

2. An inlet weir

3. An outfall structure to allow the draining of the storage area during a flood event

4. Excavation of the reservoir basin and construction of perimeter embankments to form the storage area

5. An appropriate lining solution where the storage area is located on landfill and on permeable ground such as natural gravels.

21.5.15 Structures will be passive where reasonably practicable (draft CoCP section 14.9). The designs of the flood storage areas will continue to be tested and refined through to ES. Their effectiveness in providing the required flood storage is assessed using hydraulic modelling, as set out in Appendix 21.8 and reported in Appendix 21.4.

21.5.16 Off-airfield development will satisfy the requirements of the NPPF Sequential Test. If any off-airfield development is required within the floodplain (as discussed in Appendix 21.4), it will satisfy the Exception Test and provide any necessary compensatory flood storage requirements.

Management of on-airfield surface water drainage 21.5.17 The DCO Project will result in an increased impermeable area compared to the

existing. This will be accommodated by new and expanded drainage networks.

21.5.18 Current airfield discharges are to the Crane, Portlane Brook and groundwater. There are currently two options for the drainage network for the DCO Project, which are described in Appendix 21.5. The options would involve different discharge points, which have not yet been precisely defined, but would include discharges to one or more of the Colne Brook, Wraysbury River, River Colne and Duke of Northumberland’s River (in addition to the current discharge locations).

21.5.19 The design of surface water attenuation and storage for on-airfield drainage will be developed to manage the quantity and rates of runoff in compliance with the ANPS and with due regard for national guidance and good practice. Surface water will be captured on site during all intensity events up to 1% Annual Exceedance Probability (AEP) plus climate change rainfall event. Rates and volumes of runoff will be set between two bookends: a lower bookend of greenfield runoff rates and



an upper bookend of ‘no worse than existing’. Appropriate allowances for increases in peak rainfall intensity as a result of climate change will be applied. For changes in climate beyond the factor of safety incorporated into the design (likely to be longer term changes in climate), Heathrow will employ an adaptive management approach, whereby the treatment capacity could be improved in future.

21.5.20 Runoff will be treated prior to discharge if required, and discharges will be subject to the need for environmental permits, both in relation to the rate of runoff and water quality. Airfield runoff will be routed to dedicated attenuation and treatment facilities for each airfield catchment. The attenuation areas will include forebays as appropriate to reduce sediment loading. Flows will have on – line monitoring to determine de – icer load. Water containing de-icer contaminants above a threshold value would be directed to dedicated reed bed treatment facilities (vertical aerated gravel reed beds) via flow balancing as necessary. Flows below the threshold value and treated flows will be discharged to nearby watercourses, via additional SuDS if necessary. An overview of the on-airfield surface water drainage is provided in Appendix 21.5.

Management of off-airfield surface water drainage 21.5.21 Off-airfield land parcels will be self-mitigating in terms of managing surface water

runoff. The design of surface water attenuation and storage for off-airfield drainage, which result from increased impermeable areas at off-airfield developments and in relation to road drainage, will be developed to adequately manage the quality, quantity and rates of runoff within those land parcels. A sustainable drainage systems (SuDS) approach (which usually involves manging water at the surface, in open channels and basins for example), will be employed, unless there is clear evidence that this would be inappropriate. Other measures may be required and will be developed on a site – specific basis.

21.5.22 An overview of off-airfield and roads drainage is provided in Appendix 21.5.

Management of subsurface flow pathways

21.5.23 The design of the voids for permanent baseline and tunnel installations will protect against groundwater ingress. Other engineering solutions and appropriate management solutions for groundwater management will also be integrated into the design, including to facilitate groundwater flow around new subsurface features, if required.

Pollution control strategy

21.5.24 Heathrow will develop a pollution control strategy which will include for the provision of a range of facilities to manage spills or other unintended releases.



This could include petrol interceptors, measures to capture spilt de-icer, and fuel storage bunding as appropriate in order to protect the water environment against spillages or other incidents.

Measures incorporated into the construction phase

Dewatering activities 21.5.25 The earthworks will require substantial dewatering during excavation of borrow

pits, potential excavation of landfills, infilling of lakes and construction of subsurface features or foundations. Section 14.6 of the draft CoCP sets out good practice measures to reduce effects on groundwater levels (and potential associated changes to river flows), to avoid mobilisation of historic contaminants through changes to local groundwater flow pathways, to protect surface water quality and to ensure no increase in flood risk. The following sections provide more detail on the measures expected to be embedded into the design.

Dewatering of landfill voids 21.5.26 This activity will be focused around the areas of land fill excavation. There will be a

requirement to treat the leachate pumped from these areas in leachate treatment plants prior to discharge and leachate treatment facilities will be deployed as necessary.

Dewatering of non-landfill voids and lakes 21.5.27 Dewatering of excavated non-landfill voids will take place in a number of locations,

associated with both lake excavation and excavation of new voids. As set out in Section 14.6 of the draft CoCP, groundwater cut-off measures will be used as required to protect receptors from dewatering effects. This may include the protection of groundwater flows into the Colne Brook and avoidance of mobilisation of historic contaminants through changes to local groundwater flow pathways.

21.5.28 As set out Section 14.4 of the draft CoCP, water will be attenuated prior to discharge to ensure no increase in flood risk associated with excessive discharges to nearby watercourses, and to allow for the removal of sediments and any other treatment. This will require attenuation ponds located to service each construction site. The location of these ponds may need to change as the construction work develops.

21.5.29 Discharge could be to a number of sources, including the local river systems, Thames Water sewers or, in some instances, to ground. All discharges will be agreed with the Environment Agency or Thames Water (as appropriate) prior to discharge through the environmental permitting and consent application process.



21.5.30 Construction discharges will be reactive to flood events and discharges will be ceased at set trigger flows to ensure no increase in downstream flood risk. This could require some flooding (through groundwater ingress or containment of surface water run-off) of the construction voids to store water on-site.

21.5.31 Discharges from dewatering will be appropriately controlled and treated, in line with industry good practice measures as set out the draft CoCP, Section 14.2.

Management of surface water run-off 21.5.32 Consideration will need to be given to the control of water from all construction

sites and the interaction with the on-site surface water drainage network. The surface water run-off footprint of the site will change through the construction phase as earthworks and construction change the land cover and topography. As the new operational drainage and treatment network is brought on line it will be possible to use this to attenuate some aspects of construction site surface water run-off, however, there will still be a requirement to attenuate and store surface water run-off on construction sites. Rates and volumes of runoff will be managed in compliance with the ANPS and with due regard for national guidance and good practice. Relevant measures are set out in Section 14.7 of the draft CoCP.

21.5.33 Measures will be implemented to limit and manage sediment entrainment, transport and deposition associated with construction sites. Sediment settlement or other treatment facilities will be provided as required, as set out in Section 14.7 of the draft CoCP.

Construction of new river channels 21.5.34 The detailed approach to the diversions is still being developed, but it will involve a

phased approach taking account of seasonal requirements (particularly fish spawning) and habitat establishment. The following measures will be embedded into the construction phasing:

1. The new channel will be constructed whilst all flow remains in the existing river channel

2. The phased movement of rivers (including relocation of fish from old river channels) will be sensitive to fish spawning seasons

3. Fish subject to removal from old river channels will be placed upstream and downstream of the construction site in reaches where habitat will not be disturbed by construction activities from the DCO Project

4. The removal of old river corridors will only take place once alternative terrestrial ecological corridors are established, including the establishment of riparian areas where these are vital for ecological connectivity.



Backfilling of old river channels

21.5.35 The old “decommissioned” channels of the Colne Brook, River Colne and Wraysbury River will be infilled with suitable infill materials.

Construction within the flood plain 21.5.36 The new up-stream flood storage areas will be required to be provided before the

loss of any floodplain is incurred as a result of the DCO Project (Section 14.9 of the draft CoCP). Appropriate phasing will be implemented in relation to the movement of rivers and re-provision of flood storage lost during construction to ensure that flood risk will not be increased anywhere during any phase.

21.5.37 Temporary haul roads for construction, including any temporary watercourse crossings, will be designed to ensure an appropriate level of flood conveyance to ensure they do not increase flood risk to off-site receptors.

21.5.38 Construction activities will be avoided in the floodplain where reasonably practicable, as set out in the draft CoCP. Fuel and chemical storage and concrete batching will take place in Flood Zone 1 (Section 14.2 of the draft CoCP). Stand-off distances from watercourses will be applied (Section 14.4 of the draft CoCP). Where the works phasing or site conditions dictate that stockpiles, accommodation, temporary facilities, machinery and plant are located within Flood Zone 2 or 3, or areas at significant risk of flooding from other sources including groundwater, temporary floodplain compensation will be required (Section 14.9 of the draft CoCP).

Provision of alternative water supplies

21.5.39 The above measures embedded into the permanent infrastructure and the construction phase will be carried out to avoid effects on licensed and non-licensed water supplies as far as reasonably practicable. Where adverse effects on water quality or availability are identified as a result of works associated with the DCO Project, appropriate measures will be put in place, including provision of an alternative water supply, as appropriate.

21.5.40 Table 21.10 summarises the relevant water environment embedded measures which have been incorporated into the DCO Project design, and how this influences the assessment in Section 21.10.



Table 21.10: Summary of the embedded environmental measures in the design and how these influence the water environment assessment

Receptor Changes and effects Embedded measures and influence on assessment (receptors influenced)

Surface WFD water bodies Horton Brook (WE1) Colne Brook (WE2) Wraysbury River (WE3) River Colne (WE4) Duke of Northumberland’s River and Longford River (WE5) Portlane Brook (WE6) River Crane (WE7) River Thames (freshwater and tidal) (WE8 – WE9) Upper River Colne (WE10)

Permanent changes to land use, river diversions and drainage networks resulting in: Changes to channel geomorphology; Changes to river flow; Loss of ecological connectivity (assessed in Appendix 21.3 and Chapter 8 Effects on water quality from changes to mixing and dilution; Introduction of pollutants to surface waters

Measures embedded into the permanent infrastructure design:

1. Design of CRC (WE3 - 5) 2. Road Crossings of Rivers 3. Compensatory Flood Storage (WE2, WE4) 4. River Diversion Design (WE2 – 5) 5. On airfield surface water drainage (WE2 - WE7) 6. Off airfield surface water drainage (WE2- 4); and 7. Management of subsurface flow pathways (WE2 –

4, & WE7) 8. Pollution Control Strategy (WE2 – 4, & WE7)

These embedded measures would avoid or reduce effects on channel geomorphology river flow, and water quality.

Surface WFD water bodies Horton Brook (WE1) Colne Brook (WE2) Wraysbury River (WE3) River Colne (WE4) Duke of Northumberland’s River and Longford River (WE5)

Construction activities (e.g. establishing river diversions, dewatering, other construction activities) resulting in: Changes to geomorphology; Changes to river flow;

Measures embedded within the construction phase relating to:

1. Dewatering Activities (WE2, WE4, WE5) 2. Management of Surface water run-off (WE2 - WE4,

WE7) 3. Backfilling of old river channels (WE2 – WE5) 4. Construction of new river channels (WE2 – 5) 5. Construction within the flood plain (WE1 – 4, and

WE7) and




Portlane Brook (WE6) River Crane (WE7) River Thames (freshwater and tidal) (WE8 – WE9) Upper River Colne (WE10)

Increased sediment loading to surface water; and Introduction of pollutants to surface waters

6. Provision of alternative water supplies (WE2, WE4). These measures would help avoid or reduce effects on channel geomorphology river flow, and water quality during the construction phase.

Groundwater Bodies Lower Thames Gravels (WE12) Bedrock Aquifers (WE13)

Permanent changes to land use, river diversions and drainage networks resulting in: Reduced recharge; Changes to groundwater flow and/or levels; and Introduction of pollutants during operation.

Measures embedded into the permanent infrastructure design:

1. Compensatory Flood Storage 2. River Diversion Design 3. On airfield surface water drainage 4. Off airfield surface water drainage and 5. Management of subsurface flow pathways

These embedded measures would help avoid or reduce effects on groundwater quantity and quality during the operation phase.


Construction sites and associated activities (including dewatering) resulting in: Changes to groundwater flow/and or levels; and introduction of pollutants during construction

Measures embedded within the construction phase: 1. Dewatering activities 2. Backfilling of new river channels and 3. Construction of new river channels 4. Provision of alternative water supplies

These measures would help avoid or reduce effects on the groundwater body quantity and quality during the construction phase.




Water Users Local Water Supply (WE14) Foul Drainage Infrastructure (WE15)

Effects on the capacity of the local public water supply network Effects on the capacity of the foul drainage network

Measures embedded within the permanent infrastructure design:

1. On airfield surface water drainage 2. Off airfield surface water drainage

Flood Risk Receptors Flood Risk Receptors (WE16)

Permanent changes to land use, river diversions and drainage networks resulting in: Changes to fluvial and pluvial flood risk Changes to groundwater flooding Changes to flood risk from infrastructure

Measures embedded within the permanent infrastructure design:

1. Design of CRC 2. Compensatory Flood Storage 3. River Diversion Design 4. On airfield surface water drainage 5. Off airfield surface water drainage and 6. Management of subsurface flow pathways

The embedded measures would avoid increases in flood risk elsewhere from permanent land use changes associated with the DCO Project.

Flood Risk Receptors Flood Risk Receptors (WE16)

Construction sites and associated activities resulting in: Changes to fluvial and pluvial flood risk Changes to groundwater flooding Changes to flood risk from infrastructure

Measures embedded within the construction phase: 1. Dewatering Activities 2. Management of Surface Water Runoff 3. Backfilling of old river channels 4. Construction of new river channels and 5. Construction within the floodplain

These embedded measures would avoid increases in flood risk elsewhere during the construction phase.



21.5.41 Table 21.11 summarises the types of good practice environmental measures that would be implemented during construction as part of the draft CoCP, as considered within the assessments in Section 21.10. These measures are cross referenced to the relevant draft CoCP section as appropriate.



Table 21.11: Summary of the good practice environmental measures and how these influence the Water Environment assessment Source: draft CoCP

Receptor Changes and effects Good practice measures and influence on assessment Draft CoCP Section reference

Surface WFD water bodies Horton Brook (WE1) Colne Brook (WE2) Wraysbury River (WE3) River Colne (WE4) Duke of Northumberland’s River and Longford River (WE5) Portlane Brook (WE6) River Crane (WE7) River Thames (freshwater and tidal) (WE8 – WE9) Upper River Colne (WE10)

Construction activities (e.g. establishing river diversions, dewatering, other construction activities) resulting in: Changes to geomorphology; Changes to river flow; Increased sediment loading to surface water; and Introduction of pollutants to surface waters

1. Sediment Control Measures; 2. Chemical Spillage Prevention and Control Measures 3. Environmental Permitting and Discharge Consents 4. Piling Control Measures; 5. Dewatering control measures; and 6. Surface water quantity/quality monitoring measures; 7. Targeted risk assessment for abstraction points and emergency

contingency measures including provision of alternative supplies. These good practice measures would help avoid or reduce effects on channel geomorphology river flow, and water quality during the construction phase.

Draft CoCP Sections 14.2 – 14.4, and 14.6 – 14.8





Construction sites and associated activities (including dewatering) resulting in: Changes to groundwater flow/and or levels Introduction of pollutants during construction

1. Chemical Spillage Prevention and Control Measures 2. Environmental Permitting and Discharge Consents 3. Piling Control Measure; 4. Dewatering and control measures 5. Groundwater quantity/quality monitoring measures 6. Targeted risk assessment for abstraction points and emergency

contingency measures. These good practices would help to avoid or reduce effects on the groundwater bodies quantity and quality during the construction phase.

Draft CoCP Sections 14.2, 14.4, and 14.6 – 14.8




Water Users Local Water Supply (WE14) Foul Drainage Infrastructure (WE15)

Effects on the capacity of the local public water supply network Effects on the capacity of the foul drainage network

Utilities infrastructure control measures including: 1. Protection of buried infrastructure measures 2. Control and management of foul drainage measures.

These good practice measures would avoid or reduce the effects on the local public supply network and the foul drainage network.

Draft CoCP Section14.3, and 14.8

Flood Risk Receptors (WE16)

Construction sites and associated activities resulting in: Changes to fluvial and pluvial flood risk Changes to groundwater flooding Changes to flood risk from infrastructure.

Flood risk management measures including:

1. Emergency flood warning and response procedures 2. Flood risk plan preparation 3. Avoidance of construction activities (including temporary material

storage) in areas at risk of flooding. These measures would avoid or reduce the risk of flooding elsewhere during the construction phase.

Draft CoCP Section 14.9



21.6 Methodology for baseline data gathering

Overview 21.6.1 Baseline data collection has been undertaken to obtain information over the study

areas described in Section 21.4. The current baseline conditions presented in Section 21.9 sets out currently available information from the study areas.

21.6.2 The overall baseline has been informed by:

1. Desk-based data collation, as summarised in this section;

2. Baseline surveys, as summarised in this section;

3. Quantitative baseline characterisation using models for flood risk, groundwater flow and surface water quality. These are discussed in Section 21.7 as part of the assessment methodology.

Desk study 21.6.3 The desk study has utilised data from a number of organisations, through a

combination of data requests and data freely available on the internet. A summary of the organisations that have supplied data, together with the nature of that data, is outlined in Table 21.12.

Table 21.12: Data sources used to inform the water environment assessment

Organisation Data provided Data time period

Date received

WFD Environment Agency catchment data explorer

River Basin Management Plan water body classification data

2009 - 2016 October 2017

Gov.uk open data Shapefiles of rivers and all WFD water bodies

N/A August - October 2017

Environment Agency Thames River Basin Catchment Plan in combination with WFD catchment plans

2015 – 2017 October 2017

Flood Risk Gov.uk open data Statutory and non-statutory designated

nature conservation sites Flood Zones 2 and 3, flood defences and flood storage areas Surface water flood risk mapping Susceptibility to groundwater flooding mapping LiDAR topographic data

N/A August - October 2017

Environment Agency Hydraulic models: Lower Colne; Colne N/A 2017 – May 2018




Date received

upstream of Denham; Crane

Lead Local Flood Authorities (Hillingdon Preliminary Flood Risk Assessment)

Sewer flooding information

Variable - Latest 2008

August 2017

Canal and River Trust

Canal flooding records 1974 - 2011 2014

FEH Web Service Catchment boundaries and descriptors N/A October 2017

Abstractions and discharges

Gov.uk open data Consented discharges Variable Earliest 1976 – 2016

October 2017

Environment Agency Licensed abstractions and consented discharges

Variable Earliest 1966 – present

2017 – May 2018

Local Authorities Private water supplies from:1. Chiltern and South Buckinghamshire District council 2. Spelthorne Borough Council 3. Runnymede Borough Council

N/A August 2018

Water Quality Environment Agency Water quality monitoring locations and

data 2010 - 2016 October 2017

Heathrow Water Quality Monitoring Data and

Discharge Rates Dec 2017 – 2018

December 2018

Thames Water Lake water quality data

January 2015 – May 2018

August 2018

Gov.uk open data Record of Historical Pollution Incidents Variable 2001 – present

October 2017

Hydrology National River Flows Archive (NRFA) and Environment Agency

River flows and levels at gauged locations

Variable. Earliest 1952-present

June 2017

National River Flow Archive

Daily average river level and flow data at gauging stations

Variable Earliest 1984 – 2017

November 2017

Environment Agency Ratings for key gauging stations Variable Earliest 1932 – 2010

June 2017




Date received

Geology and Soils Environment Agency Landfill Locations and Types N/A 2017 British Geological Survey

Borehole record scans from on-line Geo-Index 1:50,000 digital geology London Basin Lithoframe 3D geological model

Variable Earliest 1979 - 2002

March 2014

Quod 2015/ Wood 2018 Mineral Sites Plan

Mineral Extraction Sites N/A

MAGIC website Designated sites Soilscape Aquifer designations

N/A January 2018

Hydrogeology Environment Agency Groundwater level and quality data

Groundwater vulnerability mapping Landfill locations and types

Variable Earliest 1966 – present

2017 – May 2018

Topography

Ordnance Survey 1:50,000 and 1:25,000 mapping 50m topography grid (OS Terrain 50) Mastermap (properties & infrastructure and rivers network layers)

N/A May – November 2017

Weather/ Climate Centre of Ecology and Hydrology

Gridded estimates of daily and monthly rainfall Evapotranspiration data

1890 - 2015 November 2017

Metrological Office Metrological Office Rainfall and Evaporation Calculation System (MORECS version 2.0)

N/A November 2017

Environment Agency/ Metrological Office

Rainfall: tipping bucket rain gauge 15 minute and daily data for nine sites

Variable Earliest 1991 -present

November 2017

Survey work 21.6.4 Surveys of groundwaters and surface waters have been undertaken within the

study areas. Groundwater monitoring has been focussed mainly in the vicinity of the Site, to supplement data provided by other parties (as identified in the desk study section) for the GWSA. Rivers and lakes have been monitored, where relevant, across the LSA to better understand local conditions which could be affected by the diversion of rivers, discharges, temporary earthworks or other activities. No monitoring of reservoirs has been carried out, since regular



monitoring data were available from Thames Water. The monitoring network in the LSA is also considered sufficient to inform an understanding of hydrological interactions with the WSA, by providing data upstream and downstream of the Site.

21.6.5 A summary of the surveys that have been carried out to date, and those planned in future, are provided in Table 21.13.

Table 21.13: Survey work undertaken

Survey type Date of surveys / monitoring completed

Dates of future surveys / monitoring

Monitoring Locations

Surface water quality monitoring

Monthly sampling from December 2017 through 2018.

Ongoing (monthly).

Water quality data is collected by the Environment Agency (various locations, Heathrow (Eastern Balancing Reservoir, Mayfields Farm, Spout Lane Lagoon) and Thames Water (their reservoirs). These existing locations have been supplemented by 111 additional surface water monitoring locations, along rivers, streams, ponds and lakes within the LSA. These include locations on the Main Rivers (River Colne, Wraysbury River, Colne Brook, Horton Brook, Duke of Northumberland’s River, Longford River, River Crane, and Portlane Brook, lakes, Heathrow’s surface water management treatment locations (Mayfield’s Farm, Clockhouse Lane Pit, the Eastern Balancing Reservoir, Spout Lane Lagoon), and various smaller drains and ditches that drain into the wider river network (identified in Table 21.8). Locations are shown in Figure 21.4.

Flow gauging

Surveys on existing Environment Agency gauges (for spot gauging data): approximately bi-weekly from 13 August 2018 through to end 2018.

Programme for additional surveys on existing Environment Agency gauges (for spot gauging data) to be assessed in early 2019.

The Environment Agency collects data at eight existing gauges within the LSA. Thirteen additional temporary flow monitoring locations have been selected as part of initial proposals for the flow monitoring programme. These locations are situated along the Duke of Northumberland’s, River Colne, Horton Brook, Longford River, and River Crane.






Installation of additional temporary level/flow gauging stations: November 2018-early 2019. Surveys on additional temporary gauges (for spot gauging data): bi-weekly during 2019. Continuous water level monitoring: through 2019.

For two locations along the Horton Brook and River Crane south of the Airport work is still ongoing to identify a suitable location. Locations are shown in Figure 21.4. Additional flow gauging has been located upstream and downstream of the Site to enable any effects related to the development to be identified.

Geomorphology surveys

September 2017 February 2018 May 2018.

One further tranche of survey work (date to be confirmed).

The locations of watercourses for geomorphology surveys include the Horton Brook, Colne Brook, Poyle Channel, River Wraysbury, Fray’s River, River Colne, Longford River and Upper Duke of Northumberland’s River.

River topographic survey

March 2018 August 2018.

Tranche 3 to be completed in future, date to be confirmed.

Existing Environment Agency Lower Colne and Crane models will be supplemented by 590 channel and survey locations along the River Cole, Horton Brook, Colne Brook, River Wraysbury, Duke of Northumberland’s River and River Crane.

Bathymetric surveys

May-June 2018 (six lakes).

None planned at present.

Locations for bathymetric and sediment profiling surveys include Colnbrook West Lake, Ortlitts Lake an unnamed water body between Ortlitts Lake and Old Slade Lake, Old Slade Lake, Saxon Lake, and Thorney Lake.

Lake level monitoring (and associated groundwater level and river stage monitoring)

None to date.

Quarterly dips and continuous loggers to be installed, date to be confirmed.

24 lake locations have been selected for lake level monitoring including Thorney Mill Road Lake, Saxon Lake, Eric Mortimer Rayner Memorial Lakes, Quarry Lakes in Staines Moor and Clockhouse Lane Pit.






River accretion spot flow surveys

27-29 November 2017 16-18 April 2018 17-19 September 2018.

March 2019.

The spot flow gauging locations discussed above have been formulated to provide 65 locations for the accretion profiles upstream and downstream of the Site.

Groundwater level monitoring

Monthly monitoring February 2018 to October 2018 Continuous level loggers installed from mid-2018.

Ongoing (monthly) Additional continuous level loggers planned for installation in late 2018.

Approximately 700 ground investigation boreholes in various strata including made ground, superficial gravels, in high density around the Site. 38 boreholes drilled across superficial gravels, Lambeth Group or Chalk across the GSA.

Groundwater quality monitoring

Monthly monitoring February 2018 to end 2018.

Ongoing (monthly).

The borehole locations for the groundwater quality monitoring network are the same as the ground investigation groundwater level monitoring network.

Field permeability testing

A number of open borehole tests and slug tests in OBHs have been carried out over 2018.

Under review, including the option of carrying out pumping tests in future.

A range of hydraulic permeability test locations proposed across the Site to characterise alluvium, and superficial gravels

21.6.6 Both groundwater level and groundwater quality monitoring form part of the ground investigation surveys. The locations of all groundwater quality monitoring locations are shown in Figure 14.4 supporting Chapter 14. These locations were predominantly designed to assess the nature and extent of ground contamination but are also used to improve the conceptual understanding of the groundwater baseline environment including the analysis of temporal and spatial patterns of groundwater levels, flow directions, zones of groundwater discharge, and areas with potential for groundwater flooding.

21.6.7 Surface water quality monitoring commenced in December 2017 to establish baseline conditions for the LSA, in order to characterise the surface water quality of waterbodies (rivers, streams and lakes), for a wide range of water quality parameters. The locations of all surface water quality monitoring locations are shown in Figure 21.4. The baseline monitoring data are being periodically reviewed to ensure relevant information is being collected through the pre-development period.

21.6.8 The main surface water reservoirs situated south and west of Heathrow are managed and routinely monitored by Thames Water, with surface water quality



data being made available through data request (see Table 21.12). Therefore, no additional surface water quality monitoring is being undertaken at those locations.

21.6.9 A programme of continuous flow gauging is underway for the watercourses in the vicinity of Heathrow Airport. The locations of the flow gauging survey stations are shown in Figure 21.4. The monitoring network comprises existing Environment Agency water level gauges (for which stage-discharge rating curves are being developed to calculate flow data), and additional temporary flow gauges at which continuous water level and flow data will be collected to understand and monitor flow. These are being used to collect continuously recorded flow data, over a range of flows. Bi-weekly spot flow monitoring is also being undertaken in parallel, to develop stage-discharge rating curves at all gauges, taking into account the range of seasonal flows that occur during the period of monitoring. This monitoring will continue through 2019 to include as a minimum a “dry” season (summer) and a “wet” season (autumn and winter) to inform the ES assessment.

21.6.10 A series of river spot flow surveys are being undertaken to deliver ‘snapshots’ of the typical pattern of gain (or loss) in flow within the network of watercourses near the Site, and how this varies seasonally. Accretion profiles will be constructed that will improve the conceptual understanding of the dependency of river flows on the contribution from (or losses to) the underlying alluvial deposits and gravel aquifer, during both high flow and low flow periods.

21.6.11 Geomorphology field surveys are being undertaken to measure the physical characteristics of the rivers in the local surface water study area. Due to access limitations, the field surveys have, to date, only been undertaken at targeted sections within water bodies. A Fluvial Audit approach, adapted to the specific requirements of the Site and the DCO Project, has been and will be used to survey rivers within the vicinity of the Site, as well as upstream, downstream and adjacent reaches. The surveys aim to classify the rivers in terms of their morphology and processes, in order to understand their geomorphological and hydrological characteristics and behaviour to define the baseline condition of the watercourses.

21.6.12 Topographic surveys of watercourses are being carried out, including channel cross-sections, bank profiles, structures as-built design (including bridges, culverts and weirs), outfall locations and water levels. These are required to further develop the flood models used for the FRA.

21.6.13 Lake bathymetric and sediment profiling surveys have been undertaken for a number of lakes, primarily to inform the earthworks strategy and potential for other uses of the lake water. However, the data also inform both the baseline conceptual model of the water environment and the calibration of the numerical groundwater model. The surveys include the measurement of lake depths over the entire lake area and also the lake water level elevation.



21.6.14 Hydraulic field permeability testing is being carried out to characterise permeabilities of the alluvium deposits, the Langley Silt Member and the river terrace gravels. The resulting permeability distribution will inform both the baseline conceptual model of the water environment and the calibration of the numerical groundwater model.

21.6.15 A programme of lake level monitoring is proposed at a number of lakes to improve the understanding of the current relationship between lake levels and the adjacent groundwater levels in the superficial gravel aquifer. Associated groundwater level and river stage monitoring will be undertaken adjacent to lakes either at existing or new monitoring locations as appropriate.

21.6.16 Further detail regarding the use of the data resulting from the surveys is presented in the relevant technical appendices.

21.7 Assessment methodology for PEIR

Significance evaluation methodology 21.7.1 The assessment of significance will be undertaken as set out in Chapter 5. The

significance level attributed to each effect has been assessed based on the ‘sensitivity’ (value) of the affected receptor and the magnitude of change resulting from the development. The level of significance is then determined by the combination of sensitivity and magnitude.

21.7.2 Sensitivity is assessed on a scale of high, medium, low and very low, whilst magnitude is assessed on a scale of high, medium, low and negligible. The criteria for defining sensitivity and magnitude can be found in Table 21.14 and Table 21.15, along with example applications. These criteria are defined and applied based on professional judgement, using recognised approaches to classification relevant to the receptor types, including WFD, the NPPF and DMRB.

Sensitivity of receptor 21.7.3 Definitions of receptor sensitivity used in the assessment are provided in Table

21.14, with examples of receptors that would be placed in each class.

Table 21.14: Definitions of receptor sensitivity

Sensitivity Criteria Examples

High Features with a high yield, quality or rarity, with little potential for substitution.

Conditions supporting a site with an international conservation designation (Special Area of Conservation (SAC), Special Protection Area (SPA), Ramsar site),




where the designation is based specifically on aquatic features.

Water use supporting human health and economic activity at a regional scale.

Regionally important public surface water or groundwater supply (and associated catchment/SPZ/Groundwater Management Unit (GWMU)).

Features with a high vulnerability to flooding.

Land use type defined as ‘Essential Infrastructure’ (i.e. critical national infrastructure, such as essential transport and utility infrastructure) and ‘Highly Vulnerable’ (e.g. police/ambulance stations that are required to operate during flooding, mobile homes intended for permanent residential use) in the NPPF flood risk vulnerability classification.

Medium Features with a medium yield, quality or rarity, with a limited potential for substitution.

Conditions supporting a site with a national conservation designation (e.g. Site of Special Scientific Interest (SSSI), National Nature Reserve (NNR)), where the designation is based specifically on aquatic features. Water quality and hydro-morphology elements of WFD surface water body (or part thereof).

Water use supporting human health and economic activity at a local scale.

Local public surface water and groundwater supply (and associated catchment/GWMU) or permitted discharge. Licensed non-public surface water and groundwater supply abstraction (and associated groundwater catchment) which is relatively large relative to available resource, or where raw water quality is a critical issue, e.g. industrial process water, or permitted discharge.

Features with a moderate vulnerability to flooding.

Land use type defined as ‘More Vulnerable’ in the NPPF flood risk vulnerability classification (e.g. hospitals and health centres, educational institutions, most types of residential development).

Low Features with a low yield, quality or rarity, with some potential for substitution.

Conditions supporting a site with a local conservation designation (e.g. Local Nature Reserve (LNR), County Wildlife Site (CWS)), where the designation is based specifically on aquatic features, or an undesignated but highly/moderately water-dependent ecosystem, including a Local Wildlife Site (LWS) and a Groundwater Dependent Terrestrial Ecosystem (GWDTE).

Water use supporting human health and economic activity at household/individual business scale.

Licensed non-public surface water and groundwater supply abstraction (and associated catchment/ GWMU), which is relatively small relative to available resource, or where raw water quality is not critical, e.g. cooling water,




spray irrigation, mineral washing or permitted discharge. Unlicensed potable surface water and groundwater abstraction (and associated catchment) e.g. private domestic water supply, well, spring or permitted discharge.

Features with a low vulnerability to flooding.

Land use type defined as ‘Less Vulnerable’ in the NPPF flood risk vulnerability classification (e.g. most types of business premises, including land and buildings used for agriculture).

Very Low Commonplace features with very low yield or quality with good potential for substitution.

Conditions supporting undesignated ecosystems or those with low sensitivity to water supply, such as a LWS, undesignated GWDTE or pond.

Water use does not support human health, and of only limited economic benefit.

Unlicensed non-potable surface water and groundwater abstraction (and associated catchment) e.g. livestock supply.

Features that are resilient to flooding.

Land use type defined as ‘Water-compatible development’ in the NPPF flood risk vulnerability classification (e.g. flood control infrastructure; water transmission infrastructure), and undeveloped land.

Magnitude of change 21.7.4 The magnitude of change from baseline conditions includes a consideration of the

duration and reversibility of the change, and relevant legislation, policy standards and guidance. Table 21.15 provides examples of how various magnitudes of change could be determined with respect to water features.

21.7.5 Magnitude of change may be either positive or negative. The criteria and examples in Table 21.15 focus on negative changes, but positive changes may also occur and will be considered on a case by case basis as required.

Table 21.15: Definitions of magnitude of change

Magnitude Criteria Examples

High Results in complete loss or major change to feature, of sufficient magnitude to affect its use/integrity.

Deterioration in river flow regime, morphology or water quality, leading to sustained, permanent or long-term breach of relevant conservation objectives (COs) or non-temporary downgrading (deterioration) of WFD surface water body status (including




downgrading of individual WFD elements), or resulting in the inability of the surface water body to attain Good status by the relevant deadline in line with the measures identified in the RBMP. Deterioration in groundwater levels, flows or water quality, leading to non-temporary downgrading of WFD groundwater body status, or the inability of the groundwater body to attain Good status in line with the measures identified in the RBMP. Complete or severely reduced water availability and/or quality, compromising the ability of water users to abstract. Change in flood risk resulting in potential loss of life or major damage to the property or infrastructure.

Medium Results in partial loss or noticeable change to feature, of sufficient magnitude to affect its use/integrity in some circumstances.

Deterioration in river flow regime, morphology or water quality, leading to periodic, short-term and reversible breaches of relevant COs, or potential temporary downgrading of surface water body status (including potential temporary downgrading of individual WFD elements), although not affecting the ability of the surface water body to achieve future WFD objectives. Deterioration in groundwater levels, flows or water quality, leading to potential temporary downgrading of WFD groundwater body status, although not affecting the ability of the groundwater body to achieve future WFD objectives. Moderate reduction in water availability and/or quality, which may compromise the ability of the water user to abstract on a temporary basis or for limited periods, with no longer-term impact on the purpose for which the water is used. Change in flood risk resulting in potential for moderate damage to the property or infrastructure.

Low Results in minor change to feature, with insufficient magnitude to affect its use/integrity in most circumstances.

Measurable effect on river flow regime, morphology or water quality, but remaining generally within COs, and with no short-term or permanent change to WFD surface water body status (of overall status or element status).




Measurable effect on groundwater levels, flows or water quality, but with no short-term or permanent downgrading of WFD groundwater body status. Minor reduction in water availability and/or quality, but unlikely to affect the ability of a water user to abstract. Change in flood risk resulting in potential for minor damage to property or infrastructure.

Negligible Results in little or no change to feature, with insufficient magnitude to affect its use/integrity

No measurable effect on river flow regime, morphology or water quality, and no consequences in terms of COs or surface water body status. No measurable effect on groundwater levels, flows or water quality, and no consequences in terms of WFD groundwater body status. No measurable change in water availability or quality and no change in ability of the water user to exercise licensed rights. Increased frequency of flood flows, but which does not pose an increased risk to property or infrastructure.

21.7.6 During the assessment of effects for each identified receptor the sensitivity value in Table 21.14 will be combined with the magnitude of change from Table 21.15 to produce an overall significance rating based on the evaluation matrix shown in Table 21.16. A ‘Significant’ effect is assessed as Major rating whereas a Moderate rating will be considered to be potentially significant at this stage of the EIA process. The latter will be subject to further investigation as part of the ES following refinement of design information. This approach will be based on professional judgement and carried out on a precautionary basis.



Table 21.16: Significance evaluation matrix

Magnitude of change

High Medium Low Negligible

Se

ns

itiv

ity

High Major

(Significant)

Major

(Significant)

Moderate

(Significant/ Not

Significant)

Minor

(Not significant)

Medium Major

(Significant)

Moderate

(Significant/ Not

Significant)

Minor

(Not significant)

Negligible

(Not significant)

Low

Moderate

(Significant/ Not

Significant)

Moderate

(Significant/ Not

Significant)

Negligible

(Not significant)

Negligible

(Not significant)

Very Low

Moderate

(Significant/ Not

Significant)

Negligible

(Not significant)

Negligible

(Not significant)

Negligible

(Not significant)

Assessment methodology evolution

21.7.7 At this stage of the EIA, the DCO Project design is still under development and is

the subject of statutory consultation. The likely significant effects are presented at

this preliminary stage. Furthermore, detailed assessment work will be undertaken

between PEIR and preparation of the ES on the final DCO Project design.

21.7.8 The methodology for the EIA assessment may therefore develop further from that

used for the PEIR. Anticipated changes in the assessment methodology are

summarised in Table 21.17, with reasons for any likely amendments detailed. The

table is set out by assessment type, with a summary of which likely significant

effects are addressed by each assessment.

21.7.9 Detailed methodologies covering each of the assessment approaches are

presented in Appendix 21.1 to Appendix 21.5.

21.7.10 The following effects related to the water environment are assessed in other

chapters:

1. Hydrological effects on relevant designated sites and other ecological

receptors (e.g. Groundwater Dependent Terrestrial Ecosystems) are assessed

in Chapter 8. This allows the significance of the hydrological effects to be

assessed in relation to the sensitivity of the ecological features, alongside any

other potential effects on those features



2. Mobilisation of potential contaminants, and resulting potential effects on groundwaters and surface waters, are assessed in Chapter 15. However, the assessment of effects on groundwater quality are also reported in relation to WFD water body status in Appendix 21.3.

Table 21.17: Assessment methodology for the PEIR and ES

Assessment (relevant likely effects)

Assessment methodology used for this PEIR

Assessment methodology to be used for the ES

Surface Water Quality Assessment (Appendix 21.1) (Increased sediment loading to surface water, reduction in surface water quality from in-channel changes, introduction of pollutants to surface waters, increased atmospheric deposition to surface waters)

The water quality assessment identifies parameters that could be influenced by the DCO Project and provides an initial assessment in relation to the requirements of the WFD. For the PEIR, a subset of key parameters has been assessed, based on an understanding of the potential effects and review of the available baseline data. The focus is on the quality of airfield discharges, and the relevant parameters include BOD, Orthophosphate, PFOS and PAHs. Of these, a numerical model (adapted from the Environment Agency’s SIMCAT model) has been used for BOD.

In addition, for the reservoirs, mass balance calculations have been used to assess potential changes to water quality resulting from increased atmospheric deposition of NOx.

The assessment of surface water quality will use similar approaches to those applied at PEIR, but will utilise a more extensive period of baseline data. This may identify further parameters of interest, and will further inform modelling and assessment of the PEIR suite of parameters. Effects associated with both construction and operational activities will be assessed. Where relevant, combined effects from multiple discharges will be considered. In relation to road drainage, assessment will be undertaken using the Highways Agency Water Risks Assessment Tool (HARWAT) as appropriate. For the ES, a dedicated groundwater quality assessment will be produced. This will identify potential sources of contamination of groundwater in all phases of development. It will provide evidence to support both the Water Environment and Land Quality assessments.

Groundwater modelling Appendix 21.2 (changes to river flow, reduced recharge, changes to groundwater flow and levels)

The assessment presents preliminary output from a suite of indicative groundwater modelling scenarios, including:

1. Future baseline 2. Operational scenario 3. Construction scenario, reflecting

a reasonable worst case for dewatering requirements. Two variants of this scenario have been modelled, with or without

The numerical modelling will be refined and enhanced to incorporate additional baseline data being collected, and to account for:

1. Ongoing refinements to the DCO Project design and construction phasing

2. Further calibration of the baseline in the numerical model

3. Underlying uncertainty in hydrogeological input






the use of low permeability barriers surrounding dewatering zones. This is described in more detail in Appendix 21.2.

parameters 4. The effects of climate change.

The model will be used to assess the effectiveness of environmental measures, including those embedded in to permanent infrastructure and measures relating to construction activities. The latter will focus on key time slices, including dry/wet years.

Preliminary WFD risk assessment Appendix 21.3 (changes to channel geomorphology, changes to river flow, changes to water quality, changes to groundwater quantity or quality)

The WFD assessment for PEIR provides a preliminary assessment of the risk posed by the DCO Project in relation to the WFD, based on the design information that is currently available. It considers the effects of the DCO Project on the objectives of the WFD for water bodies within the wider surface water study area and the groundwater study area and includes effects on overall status as well as individual element classification (including hydromorphology, biology, physico-chemical parameters, chemical status, and quality and quantity tests for groundwater bodies). These are presented in terms of risk of failure, in advance of the details of additional environmental measures (see Section 21.13) having been identified in detail. This gives a ‘worst case’ assessment for PEIR. The assessment hence draws from both this chapter and Chapter 8: Biodiversity. All WFD tests except the biological elements are relevant to this chapter, with the latter being discussed within Chapter 8: Biodiversity. The WFD assessment draws together all relevant conclusions from both chapters.

For the ES the assessment will be extended to produce a formal WFD compliance assessment, including assessment of the effectiveness of additional environmental measures to be implemented in the Colne and/or Crane catchments (se Section 21.13). In terms of the assessment methods relevant to this chapter, the final assessment will:

1. Develop the assessment using additional design information, revised baseline data and further information about sites for additional environmental measures. An assessment of the construction phase will also be included.

2. Agree relevant principles with the Environment Agency, in relation to no deterioration for water quality, temporary deterioration and approach to undertaking an Article 4.7 assessment.

3. Provide more detail on the location and nature of additional environmental measures.

An assessment of total flows will be undertaken to quantify effects on non-flood flows, to support conclusions on WFD compliance.

Flood Risk Assessment Appendix 21.4 (loss of floodplain storage,

The PEIR FRA provides a preliminary assessment of all potential sources of flood risk, including fluvial, surface water, groundwater and artificial sources. It

The process of design refinement and design-assessment feedback loop will continue until the ES is prepared. When complete, the Application FRA will






increased runoff, changes to river flow, changes to flood extents/depths, changes to groundwater levels)

presents the key elements of an ANPS and NPPF compliant FRA, providing a proof of concept for the DCO Project design.

Fluvial: The assessment is informed by hydraulic modelling of the fluvial flood risk baseline and the modelling of an indicative representation of the DCO Project. This involved the refinement of existing EA hydraulic models for the Lower River Colne and Lower River Crane. A single ‘with development’ model scenario has been simulated for the PEIR, representing the 1% AEP event plus 35% increase in flows to account for climate change.

Pluvial: The Environment Agency surface water flood mapping has been used to identify areas potentially at risk from the DCO Project.

Groundwater: The groundwater model developed for the groundwater impact assessment (as introduced above and detailed in Appendix 21.4) has been used to inform an initial assessment of the risk of changes to groundwater at the surface. Artificial sources: The FRA assesses residual risks from: raised reservoirs (using Defra mapping); canals and their associated infrastructure (using CRT’s own methodology); sewer (using Thames Water recorded sewer flooding incidents); and DCO Project-related water management infrastructure. Surface water: This assessment is based on the outcome of the DIA, which is reported in Appendix 21.5.

include the presentation of a range of fluvial flood event scenarios, including climate change scenarios. Blockage and infrastructure failure scenarios will also be undertaken, and alternative climate change allowances will be simulated as part of a comprehensive range of additional sensitivity tests. Assessment of the construction period will also be undertaken. The Sequential Test will be presented for both airfield and off-airfield developments. More detailed fluvial flood resilience/defence requirements will be presents for any off-airfield developments that would require mitigation to pass the NPFF Exception Test. The Application FRA will provide further assessment of groundwater flood risk, utilising the numerical groundwater model in more detail. The assessment of surface water runoff will be developed in line with the DIA (Appendix 21.5). Further assessment of pluvial (surface water run-on) flood risk will be carried out for any localised areas at risk at ES. More detailed assessment of artificial sources of flood risk at ES stage is not considered necessary based on the current DCO Project design (see Section 21.10), however this will be reviewed as the DCO Project design develops to application for development consent.

Drainage Impact Assessment Appendix 21.5 (Increased runoff,

The PEIR assessment methodology comprises a semi-qualitative assessment of the potential effects in terms of changes to contributing catchment areas

The DIA will provide a factual account of the proposed changes to the drainage regime across the entire Site. It is envisaged that the DIA will be






changes to river flow, introduction of pollutants to surface or groundwater)

associated with those elements for which sufficient detail has been developed in the design at this stage, i.e. the airfield and the river diversions. The intent is to provide an overview of the potential configuration of drainage areas and discharge locations that could become the final design presented at DCO, and to provide an overview of the possible effects associated with each.

supported by Preliminary Drainage Strategies for the key elements of the expansion proposals, supplemented by outline drainage design assumptions for other elements of the scheme. A quantitative assessment of the proposed drainage regime will be included, taking into account anticipated discharge characteristics and discharge locations, and suitable allowable runoff rates and volumes. Construction-phase discharges will also be assessed.

Screening of abstractions and discharges Appendix 21.6 (effects on other water users)

The assessment for PEIR has involved the screening of abstractions and discharges for potential effects. The screening is based on the location and nature of the abstraction or discharge, and its potential be impacted by the DCO Project.

Individual risk assessments will be undertaken, where required, to identify potential effects for each relevant abstractor and any requirements for further environmental measures.

21.8 Assumptions and limitations of this PEIR

Overview 21.8.1 The assessments presented in this chapter use initial quantitative data and

modelling to provide an informed view of the likely significant effects on the water environment as a result of the DCO Project, taking into consideration the environmental measures that have been incorporated in to the DCO Project, or can reasonably be expected to have been incorporated by the time of the application for development consent. Key assumptions and limitations that apply to the assessments are discussed in this section.

21.8.2 The baseline monitoring programme is ongoing, as described in Section 21.6. The timing of the development of the PEIR assessments in relation to the monitoring programme is such that limited data from the monitoring programme was available to inform the PEIR assessments. For the PEIR this means that the baseline assessments have been largely built on historical desk study information, with some resultant gaps in conceptualisation and limitations to model calibration (as identified in the relevant technical appendices). As described in Section 21.6, a more extensive period of data will be available to support the application for development consent.



21.8.3 The quantitative assessments that have been carried out for the PEIR are presented in Appendices 21.1 to 21.5 and are representative of the DCO Project, however, some aspects differ slightly as a result of ongoing design modification alongside the quantitative assessments. This is relevant to the Appendix 21.2 and Appendix 21.4. The changes have been summarised in the bullet points below and are discussed in further detail in the appendices. The assessments are sufficiently representative of the DCO Project to allow reasonable conclusions regarding the effectiveness of environmental measures and significance of effect on the water environment. Key assumptions, limitations and simplifications relating to each of the technical appendices include:

1. Appendix 21.1 focusses on potential changes of discharges from a sub-set of parameters as a result of the DCO Project (see Table 21.7). The parameters were identified based on a review of available baseline water quality data (noting that baseline data collection is ongoing) and an initial screening of likely significant effect. There are uncertainties around some aspects of the existing drainage system and the parameters identified in baseline monitoring of that system, as a result of which conclusions regarding significance have not been reached for all parameters in this PEIR assessment. The precise locations of new discharges associated with the DCO Project are not yet known, so assumptions have been made for the purposes of this PEIR assessment.

2. The scenarios presented in Appendix 21.2 are precautionary in terms of their effects on the water environment. They do not incorporate any embedded measures intended to maintain the continuity of groundwater levels and flow paths around impermeable structures. The construction scenarios provide a simplification of relevant construction activities, based on a reasonable worst-case scenario. Two variants of the construction scenario have been developed (with and without low permeability barriers surrounding dewatering zones), to inform more detailed design of dewatering strategies (see next steps in Section 21.14): these are with or without the use of low permeability barriers surrounding dewatering zones.

3. The assessment in Appendix 21.3 does not form a formal compliance assessment, due to the level of detail currently available regarding embedded and other environmental measures. Instead, the assessment for PEIR focusses on a preliminary assessment of the risks posed by the DCO Project to WFD status of water bodies. The approach to the PEIR assessment currently provides effectively a worst-case view of the DCO Project, which will be improved in the application for development WFD Compliance Assessment through full recognition of all embedded and additional environmental measures.



4. Appendix 21.4 uses a slightly earlier version of the DCO Project design compared to that presented in Chapter 6. The process of design, modelling and assessment is such that modelling and assessment lag behind the design slightly. The small differences between the design presented in Chapter 6 and that in the hydraulic model (which are discussed in Appendix 21.7) relate to the precise locations of the river channels and the exact footprints of the proposed FSAs. However, the differences are minor and do not affect the overall conclusions of the assessment of flood risk. The results of the modelling allow consideration of the effectiveness of the proposed compensatory storage areas, the new river channels and the flood storage provided within the new river corridors.

5. Appendix 21.5. The content of the PEIR DIA is a function of the extent to which the approach to drainage for the various elements of the DCO Project had been developed at the time of the assessment. The preliminary proposals range from conceptual approaches, with indicative spatial allowances for key drainage infrastructure (the airfield drainage for example), to preliminary design basis parameter assumptions (other off-airfield sites). A range of potential approaches to drainage remain under consideration at this stage. As a result, it is not appropriate or possible to undertake a definitive assessment of the effects associated with any one potential approach for PEIR. Instead, the currently preferred approaches to drainage have been set out in the DIA. This includes a preliminary semi-qualitative assessment of the potential effects from changes in contributing catchment areas associated with those elements for which sufficient detail has been developed in the design at this stage (i.e. the airfield expansion and the river diversions). Finally, it should be acknowledged that the plans provided for consultation indicate spatial conflicts with proposed drainage infrastructure and other proposed land uses in some locations. Any spatial conflicts will be resolved subsequent to PEIR as part of the on-going development of the expansion proposals. Further information is provided in Appendix 21.5.

21.8.4 Further detailed technical assumptions applying to the technical assessments have been set out as relevant in Appendices 21.1 to 21.5. This includes assumptions relating to the detail of environmental measures that have been identified in Section 21.5. Where appropriate, these assumptions will be addressed through increased design certainty prior to the ES.

21.8.5 Discussions with regulators and other stakeholders on the scope of the additional environmental measures associated with off-site river restoration works are ongoing. A list of potential sites for off-site works has been developed (as discussed in Section 21.13 and Appendix 21.3), and the extent of the identified sites is shown in Figure 6.3, Volume 2. This list of sites, and the measures



proposed for each site, will be further refined as the Application WFD Compliance Assessment progresses. This will be undertaken in consultation with third party landowners and stakeholders where required.

21.8.6 PEIR (Appendix 21.5) indicates that there are sufficient sites within the affected WFD waterbodies to provide adequate additional environmental measures, and the assessment of effects assumes that this will be the case, although agreements for works at individual sites have not yet been obtained.

21.8.7 It has been assumed in this assessment that agreements can be reached on permitted conditions for water abstractions and discharges, and that those permits will not fundamentally alter the approach to the activities set out in the PEIR.

21.9 Overall baseline

Overview of the study areas 21.9.1 The DCO Project is located in the lower reaches of the River Thames catchment

within the direct catchments of several tributaries of the Thames, notably the Rivers Colne and Crane as well as a number of smaller watercourses. The LSA is predominantly a low-lying area with a large urban and suburban extent. Moving out to the north of the WSA, the wider Colne catchment extends north into the Chiltern Hills and includes several Chalk streams in its headwaters, while in the south of the WSA the River Thames flows into its tidal reaches at Teddington in London.

21.9.2 Within the LSA, the land use is largely urbanised, but also contains large areas of open space including reservoirs, current and former gravel workings, Staines Moor, and areas of rough grassland and scrub. The land use surrounding Heathrow is described in more detail in earlier chapters including Chapter 4, Chapter 8, Chapter 14 and Chapter 15.

21.9.3 There is an overall topographic gradient from north to south, with all watercourses ultimately draining to the Thames. The rivers are described in more detail in the next section.

21.9.4 The geology of the GWSA comprises the Chalk at depth beneath the clay, silt and sands of the Lambeth Group, which are overlain by the London Clay. The younger superficial sediments that overlay the London Clay are dominated by Pleistocene river gravels. Patchy younger deposits also occur comprising alluvium along river channels (clay, silt, sand and gravel), head, peat and made ground. Historically, the river gravels have been heavily exploited within the GWSA, with many current and former extraction locations in the river valleys. The former extraction areas are either now bodies of open water or landfills. The geology of the GWSA is described in more detail in Chapter 14.



Surface water features

Rivers 21.9.5 The rivers across the WSA are shown in Figure 21.5, Volume 2 and Figures 21.6

to 21.9. The WSA includes tributaries of the Thames, across the River Colne and River Crane catchments, as well as the lower reaches of the Thames itself. In the LSA, from west to east the main channels comprise: Horton Brook; Colne Brook; Poyle Channel; Bigley Ditch; Wraysbury River; River Colne; River Ash, Portlane Brook, Longford River; The Duke of Northumberland’s River and the River Crane. These are described in more detail in Paragraphs 21.9.6 to 21.9.13. There are also numerous other smaller channels, which are tributaries of those named here.

21.9.6 Horton Brook is a tributary of the Colne Brook. It flows from north to south and approximately parallel to the Colne Brook, and converges with the Colne Brook approximately 400m before its confluence with the Thames in Staines.

21.9.7 The Colne Brook originates from a bifurcation of the River Colne to the west of Uxbridge, approximately 5.5km upstream of the M4. It passes under the M25 to the immediate north of the M4/M25 interchange, then under the M4 to the immediate west of the M4/M25 interchange, then flows to the west of Poyle (supplemented by flows from the Poyle channel) and Wraysbury Reservoir before discharging to the River Thames at Egham.

21.9.8 Bigley Ditch flows from the River Colne upstream of the M4, passes beneath the M4 at its junction with the M25, and flows adjacent to the M25 until it joins the Wraysbury River towards the southern end of Harmondsworth Moor.

21.9.9 The Wraysbury River splits from the River Colne upstream of the M4. It flows south through Harmondsworth Moor, then follows for a distance along the east side of the M25 before, at Poyle, it flows under the M25. At that point it splits, with the Poyle channel taking a portion of flow westwards to the Colne Brook. The remaining flow in the Wraysbury River continues south, to flow alongside the eastern side of the Wraysbury Reservoir. North of Staines Moor Site of SSSI the Wraysbury River flows back under the M25 and continues to the south, to converge with the River Colne 150m upstream of the confluence with the Thames. Flow along the lower reaches of the Wraysbury River is managed by a series of flow control structures operated by the Environment Agency.

21.9.10 The River Colne rises in a series of Chalk fed streams and flows south through the Colne Valley Regional Park, Uxbridge and on to the west of Heathrow, ultimately to converge with the Thames at Staines. The river has bifurcations near the Site, including to the Wraysbury River, Duke of Northumberland’s River and the Longford River (as described in Paragraphs 21.9.8 and 21.9.12).



21.9.11 The River Ash (referred to as the Surrey Ash in the River Thames RBMP) originates from a bifurcation of the River Colne to the south of the King George VI reservoir, from where it flows southeast. It converges with Stanwell Brook (which originates close to the southern boundary of Heathrow and flows southwards to the east of Staines Reservoirs) in Ashford, from where it takes a winding south-easterly route to the Thames to the west of Sunbury.

21.9.12 Portlane Brook and its western tributary Felthamhill Brook originate in Feltham, to the south of Heathrow, and drain approximately south-eastwards to the Thames to the east of Sunbury.

21.9.13 The Duke of Northumberland’s River and Longford River originate from bifurcations of the Colne approximately 200m downstream of the M4 in Harmondsworth Moor and 375m downstream of the A4 in Longford respectively. They are historic artificial diversion channels that, in their present state, comprise concrete cross-sections. The Duke of Northumberland’s River conveys water into the River Crane and then eastwards towards Syon Park and the River Thames at Isleworth. The Longford River conveys water around the existing Airport then southeast through Bushy Park, Hampton Court and discharges into the River Thames. Both rivers were diverted around the existing Airport boundary as part of the Terminal 5 development and currently form the ‘Twin Rivers’.

21.9.14 The River Crane is located to the east of Heathrow, with much of the existing Heathrow site lying within its catchment. The headwaters of the River Crane lie to the north of the Grand Union Canal. It flows from north to south past Heathrow, then turns to the east through Twickenham and north into Isleworth, where it converges with the tidal River Thames.

Lakes and reservoirs 21.9.15 There are a number of lakes and reservoirs within the LSA, predominantly situated

to the west and south of Heathrow. The reservoirs are artificial, embanked water bodies that are supplied by water abstracted from the River Thames and used by Thames Water for public water supply. There are also a large number of lakes, which mainly result from former gravel extraction sites.

21.9.16 The Queen Mother Reservoir is located between Datchet and Colnbrook, south of the M4, and has a surface area of 1.91km2. The Wraysbury Reservoir has a surface area of 1.83km2 and is situated south south-west of Poyle and west of the M25. Both reservoirs are fed by the abstractions from the Thames at Sunnymeads.

21.9.17 The King George VI (1.38km2) and Staines Reservoirs North (0.7km2) and South (1km2) are situated to the east of the M25, to the north of Staines upon-Thames. The King George VI Reservoir is located to the west of the Stanwell Moor Road,



whilst the Staines Reservoirs are to the east. These reservoirs are fed by abstractions from the Thames near Egham.

21.9.18 There are numerous lakes within the local surface water study area, the majority of which are disused gravel pits. They are typically off-line with the main rivers, but in hydraulic continuity through the gravels. Most are concentrated in the southwest of the local surface water study area between the village of Horton and the River Thames, including Wraysbury Gravel pit No.2. There are also a number of lakes near the M4/M25 junction, including Saxon Lake, Old Slade Lake, Colnbrook North, Orlitts Lake, Colnbrook West and Swan Lake.

Hydrogeology 21.9.19 The GWSA lies on the north-western limb of the London Basin Syncline, between

the Chilterns and the North-West Downs.

21.9.20 The geological sequence of superficial and bedrock strata beneath the GWSA is introduced in Table 21.18 (bedrock2) and Table 21.19: (superficial3). The Environment Agency aquifer designations for these strata are presented in the same tables. The respective geological sequences are shown in Figures 21.10 and 21.11.

21.9.21 The Chalk lies at depth beneath the GWSA. It is overlain by, and likely to be in some degree of hydraulic continuity, with, the clayey sediments of the Lambeth Group. The Chalk and Lambeth Group are confined across much of the GWSA by the London Clay aquitard, which may be up to 80m thick beneath Heathrow and hydraulically separates the bedrock aquifers from the overlying superficial aquifers. The main outcrop of the Chalk and Lambeth Group lies to the northwest, dipping below the London Clay along the northwest boundary of the GWSA. A ‘window’ exposing the Chalk also occurs west of the Site, where the Chalk is directly overlain by alluvium and the Shepperton Gravel, allowing hydraulic interaction.

21.9.22 Deep buried hollows have been documented in the London Clay, infilled with alluvium and river gravels. Thinning or even absence of the London Clay in these areas may lead to locally increased hydraulic continuity between the superficial aquifers and with the underlying bedrock aquifers. Two such buried hollows have been documented near Heathrow: one at Old Slade Lake, and one close to the

2 A term used for the main mass of rocks forming the Earth and present everywhere, whether exposed at the surface in outcrops or concealed beneath superficial deposits or water. http://www.bgs.ac.uk/products/digitalmaps/digmapgb_solid.html 3 These are more recent unconsolidated sediments including gravels, sands and clays commonly associated with glacial deposition or current and former river channels and their floodplains.

http://www.bgs.ac.uk/products/digitalmaps/digmapgb_solid.html



M25/M4 interchange, north west of Junction 4 of the M4 (Jacobs, 2015). Other unknown scour hollows may also exist.

21.9.23 The London Clay is overlain by younger Pleistocene gravels. These consist of six gravel members: Shepperton Gravel Member, Kempton Park Gravel Member, Lynch Hill Gravel Member, Boyn Hill Gravel and Black Park Gravel Member. The oldest two members of the river terrace gravels are designated as a Secondary A Aquifer and the younger four members are designated as a Principal Aquifer (Table 21.19:): this is based on their use in providing public water supply on a strategic scale. The river terrace gravels across the entire GWSA are classified by the Environment Agency as a Drinking Water Protected Area.

21.9.24 Each of the gravel members is typically 2-8 m thick and similar in lithology; sand and gravel with local lenses of silt, clay and peat. In some areas Langley silt members lie on the sediments, as well as patchy Holocene alluvium deposits.

21.9.25 Historically the Pleistocene gravels and Langley silt have been heavily exploited through extraction. The extraction areas have typically been infilled with waste, generating a significant amount of landfill area. Although some remain active, the majority are now closed. Some other extraction areas, as discussed above, have formed lakes.

Table 21.18: Environment Agency aquifer designations for bedrock strata in the GWSA

Formation Environment Agency designation Hydrogeological characteristics

London Clay Unproductive4 Aquitard

Lambeth Group

Secondary A Aquifer5 Aquifer. Clay, silt and sands, variable hydraulic conductivity, likely to be in hydraulic continuity with Chalk to some extent

Chalk Principal Aquifer6 White Chalk- Fractured limestone aquifer with flints and hard nodular beds Grey Chalk- Low permeability marl-rich limestone aquitard, glauconitic at base

4 Unproductive - low permeability rocks with negligible significance for water supply. 5 Secondary A Aquifer - presenting a range of permeability and storage capacity. The amount of water available from such an aquifer is usually limited, and water supply stays at a local scale, in some cases forming an important source of base flow to rivers. 6 Principal Aquifer - composed of rocks with a high intergranular and/or fracture permeability. These aquifers provide a high level of water storage, and a significant amount of water that can support water supply, baseflow rivers, lakes of wetlands on a strategic scale.



Table 21.19: Environment Agency aquifer designations for superficial strata in the GWSA

Formation Environment Agency designation Hydrogeological characteristics

Alluvium Secondary A Aquifer Variable hydraulic conductivity clay, silt, sand and gravel aquifer

Langley silt Unproductive Aquitard. Low permeability clays and silts

Pleistocene river terrace deposits: 1. Shepperton Gravel

Member 2. Kempton Park

Gravel Member 3. Taplow Gravel

Member 4. Lynch Hill Gravel

Member

Principal Aquifer Sand and gravel aquifer, with variable hydraulic conductivity depending on lenses of clay and silt. Hydraulic continuity between adjacent terraces may be limited by vertical and lateral heterogeneity

Pleistocene river terrace deposits: 1. Boyn Hill Gravel

Member 2. Black Park Gravel

Member

Secondary A Aquifer Sand and gravel aquifer, with variable hydraulic conductivity depending on lenses of clay and silt. Largely hydraulically disconnected from younger river terrace deposits due to physical separation

Water supply 21.9.26 Water supply to Heathrow is currently provided by Affinity Water. Foul drainage

from the current site is to the Thames Water operated drainage network.

21.9.27 The Airport has three existing groundwater borehole abstractions (a,b, and c) which have an annual license of 1.9Ml/d for general use relating to evaporative cooling.

Drainage 21.9.28 Surface water run-off from the existing operational airfield is currently attenuated

and treated for the removal of de-icing chemicals as well as hydrocarbons and other pollutants before discharge. The current site is divided into four catchments for runoff attenuation and treatment. These are summarised below, displayed in Figure 21.23, Volume 2, and discussed in more detail within Appendix 21.5

North – Western catchment 21.9.29 The North-Western Catchment for the existing airfield drains to the Wraysbury

River, via the North West Balancing Reservoir, and a small ditch which is culverted under the M25.



21.9.30 The North-Western Catchment covers a relatively small proportion of Heathrow’s drainage system and mostly serves landside development located outside of the airfield itself. Surface water drains to the North-West Balancing Pond and is used as a source of water for part of the Airport fire-fighting system around Terminal 5. When the pond top water level is exceeded, water is drained out under the Duke of Northumberland’s River into a ditch and then through a culvert under the M25, prior to discharge to the Wraysbury River under gravity.

21.9.31 The hydrological catchment to Portlane Brook includes significant sections of the drainage system serving the existing airfield. Two of the three main existing airfield catchments drain to the Portlane Brook catchment within the southern and western catchments.

Southern catchment 21.9.32 The Southern Catchment drains by gravity to the Southern Balancing Reservoir

(also known as Clockhouse Lane Pit or CLP), which discharges into Feltham Relief Sewer and then, in turn, Portlane Brook.

21.9.33 The CLP lake system provides the required attenuation for the Southern Catchment runoff. Treatment of surface water runoff from the Southern Catchment is primarily provided in the Heathrow Constructed Wetlands Facility at Mayfield Farm. The quality of the surface water runoff is tested at a diversion chamber, and when exceeding the quality thresholds, is diverted into the wetland facility. When the thresholds are not exceeded, or there is no treatment capacity available, the water continues to CLP. The quality is tested again at the outfall to the CLP, and if exceeding the thresholds, is diverted to the Surface Water Outfall Tunnel (SWOT) pump station and transferred to the Spout Lane Lagoon for discharge to Thames Water sewers. Spout Lane Lagoon provides buffer storage to balance the high rate of transfer of contaminated flows from the SWOT pumping station, with the agreed discharge rate offsite, to the Thames Water system.

Western catchment 21.9.34 The Western Catchment also drains to CLP, and thus the Feltham Relief Sewer

and Portlane Brook. Surface water is transferred to CLP via the SWOT in the same way as has been described for the southern catchment above.

Eastern catchment 21.9.35 The Eastern Catchment drains by gravity directly into the Eastern Balancing

Reservoir (EBR). This provides attenuation of flows and treatment in the form of aeration, oil separation and settlement, before discharge to the River Crane. The EBR also acts as the primary storage lagoon for the water for firefighting at the Airport. Works have recently been completed to modify how the storage provided



by this reservoir is managed, and to improve the treatment provided prior to discharge to the River Crane.

Thames Water network to Mogden Sewage Treatment Works 21.9.36 Thames Water’s sewage treatment works at Mogden is located adjacent to the

lower Duke of Northumberland’s River, within the wider River Crane catchment.

21.9.37 Key sewer network infrastructure in the vicinity of the Site include the Bath Road sewer, which runs east-west approximately parallel with the northern boundary of the existing airfield, and the pipe network along Staines Road.

Surface water quality 21.9.38 The Environment Agency has provided surface and groundwater quality

monitoring data for rivers, lakes and aquifers within the GWSA, LSA and WSA. Data has also been reviewed from the existing Airport drainage network. Relevant information has been presented in Appendix 21.1 and will be presented in more detail for the ES.

21.9.39 Based on an understanding of the processes that may affect water quality, and from review of the available baseline water quality data, a sub-set of key parameters of interest have been identified in Appendix 21.1 for evaluation and discussion at PEIR. These include:

1. Biological Oxygen Demand (BOD): concentrations can be elevated in the airfield’s surface water runoff as a result of the use of de-icing chemicals, including glycols, formate and acetate compounds. The use of de-icer is most extensive in winter, in response to low temperatures (below 4ºC)

2. Orthophosphate: concentrations of orthophosphate in the EBR and CLP discharge may be classed in the Moderate range under the WFD classification. The sources of orthophosphate from the Airport have yet to be confirmed. Furthermore, the Environment Agency river monitoring data indicates that within the WSA orthophosphate concentrations exceeded the Environmental Quality Standards (EQSs) in the long term (2010 – 2016). This included various sample points along the Colne Brook, Horton Brook, Longford River and River Thames

3. PFOS: PFOS is a newly-classified Priority Hazardous Substance (PHS) under the WFD. PFOS has been measured in the current discharges at concentrations greater than the WFD EQS (Environmental Quality Standard) limit, which is extremely low (at 0.00065 µg/l). Monitoring of water bodies around Heathrow shows that PFOS is also present in the rivers throughout the catchment, but at concentrations that are typically lower than in the site discharge waters. Uses of PFOS are now restricted but it was previously used



for a range of purposes including Aqueous Fire Fighting Foams (AFFF), cleaning products, metal plating and aviation fluids. Its former use in AFFF in particular has the potential to have resulted in contamination of groundwater. The sources of PFOS in discharges from the Airport have yet to be confirmed, but further work to investigate and identify these sources is being undertaken

4. PAHs: these substances are the by-products of combustion of hydrocarbons, and therefore are ubiquitous in typical urban environments. Several of these substances have low WFD EQS limits. Monitoring of the airfield site discharge waters and of river waters around Heathrow have identified that several PAH substances are present at concentrations that exceed the WFD limits

5. Nitrate: Sources of nitrate in the rivers and reservoirs may include fertilisers, sewage effluent and atmospheric deposition. With respect to the DCO Project, release of nitrogen oxides from increased vehicle emissions could increase nitrate concentrations in waterbodies.

WFD water bodies 21.9.40 The aim of the WFD is for all water bodies to achieve Good Status (comprised of

scores for Ecological Status and Chemical Status) and to ensure no deterioration from its present status. In certain situations, considerations of technical infeasibility or disproportionate cost could lead to an objective of less than Good. This is documented in the relevant River Basin Management Plan (RBMP) process, which in the case of the DCO Project is the Thames RBMP. The RBMP has defined specific water bodies within the river basin.

21.9.41 Rivers, lakes and artificial water bodies can also be designated as Heavily Modified Water Bodies (HMWB) or Artificial Water Bodies (AWB). A HMWB is a water body that has been significantly modified for human use (for example for the purposes of flood defence, land drainage or urbanisation), and an AWB is an artificially created water body that is used for a specific water related purpose (such as the transfer or storage of water). Though all HMWBs/AWBs also have a target of Good, their altered nature means that the target is for Good Ecological Potential, rather than Status. Good Ecological Potential can be defined by the achievement of a set of environmental measures (appropriate to the reasons for modification) as well as those supporting elements of Good Ecological Status, which are not compromised by the reason for modification.

21.9.42 Further details of the supporting elements of Ecological Status for each water body can be found in Appendix 21.3. The following paragraphs provide a summary of the overall status of WFD water bodies situated within the LSA and WSA. The operational WFD catchments are shown in Figure 21.12 for reference.



21.9.43 There are six river water bodies identified within the LSA, all of which have been assigned ‘Moderate’ overall status with the exception of the River Crane which has ‘Poor’ overall status (using the 2016 Cycle 2 status classification data). Excluding the River Crane and Horton Brook, all the river bodies are designated HMWBs as a result purposes of urbanisation and/or flood defence.

21.9.44 The surface water transfers and lakes within the LSA are all categorised as artificial waterbodies (AWB). The four surface water transfers comprise three in the Crane catchment (the Longford River and the Upper and Lower Duke of Northumberland’s Rivers) and the Grand Union Canal. Each of these surface water transfers are at ‘Moderate Ecological Potential’. There are seven lakes located within the Colne catchment with an overall WFD status of ‘Moderate’; with Wraysbury Lake (located in the Lower Thames catchment) being assigned at ‘Poor’ Ecological Potential.

21.9.45 More details of the LSA study area waterbodies are available within Appendix 21.3, and they have been summarised below in Table 21.20.



Table 21.20 Second Cycle Status of WFD waterbodies

WFD Waterbody ID

Name Waterbody Type

Operational Catchment

Waterbody Designation

Ecological Status (and elements less than good)

Chemical Status

Overall Status

GB106039023010

Colne Brook River Colne HMWB Moderate: (Fish; Phosphate)

Good Moderate

GB106039023030

Crane River Crane Rivers and Lake

Not HMWB Poor: (Macrophytes; Fish; Invertebrates; Dissolved oxygen; Phosphate)

Good Poor

GB106039023040

Horton Brook

River Colne Not HMWB Moderate: (Invertebrates; Phosphate)

Good Moderate

GB106039023090

Colne (Confluence with Chess to River Thames)

River Colne HMWB Moderate: (Phosphate)

Good Moderate

GB106039023480

Surrey Ash River Colne HMWB Moderate: (Phosphate)

Good Moderate

GB106039023451

Portlane Brook

River Crane Rivers and Lake

HMWB Moderate: (Invertebrates; Phosphate)

Good Moderate

GB806100108

Upper Duke of Northumberland’s River

Surface Water Transfer

Crane Surface Water Transfer (SWT)

AWB Moderate: (Physical Modification) Good Moderate



WFD Waterbody ID

Name Waterbody Type




Chemical Status

Overall Status

GB806100109

Longford River


Crane SWT AWB Moderate: (Physical Modification) Good Moderate

GB806100095

Lower Duke of Northumberland’s River


Crane SWT AWB Moderate: (Physical Modification) Good Moderate

GB70610078 Grand Union Canal, Uxbridge to Hanwell Locks, Slough Arm, Paddington Arm


Brent Canals and SWT

AWB Moderate: (Phosphate) Good Moderate

GB30642334 The Queen Mother Reservoir

Lake Colne AWB Moderate: (Phosphorus) Good Moderate

GB30642417 Wraysbury Reservoir

Lake Colne AWB Moderate: (Phosphorus) Good Moderate

GB30642430 Wraysbury Lake (No.1 Gravel Pit)

Lake Thames Lower AWB Poor: (Macrophytes; Phytoplankton)

Good Poor

GB30642488 King George VI’s Reservoir

Lake Colne AWB Moderate: (Phytoplankton; Phosphorous)

Good Moderate



WFD Waterbody ID

Name Waterbody Type




Chemical Status

Overall Status

GB30642489 Wraysbury No.2 Gravel pit

Lake Colne AWB Moderate: (Phosphorous) Good Moderate

GB30642490 Staines Reservoir North

Lake Colne AWB Moderate: (Phosphorous) Good Moderate

GB30642525 Staines Reservoir South

Lake Colne AWB Moderate: (Phytoplankton, Phosphorous)

Good Moderate

GB30642538 Heron lake Lake Colne AWB Moderate: (Phosphorous) Good Moderate

GB40603G000300

Lower Thames Gravel

Groundwater

Colne GW n/a Good Good Good



21.9.46 The Colne catchment contains 12 waterbodies included within the wider surface water study area (WSA). The waterbodies are all at ‘Good’ status/potential, with three being HMWB.

21.9.47 In the Crane catchment there is only one waterbody that is included in the WSA, the Yeading Brook, which forms the upper reaches of the Crane. Whilst it is a HMWB, it has been designated at ‘Good’ Ecological Potential.

21.9.48 Within the Thames catchment there are three waterbodies included in the WSA, including two river water bodies and one transitional water. The rivers are both HMWBs with ‘good’ WFD status, whilst the transitional waterbody (Thames Upper) has been designated a ‘Fail’ under the WFD status.

21.9.49 More details of the WSA study area waterbodies are available within Appendix 21.18.

GWSA WFD Water bodies

21.9.50 The Site is underlain by the Lower Thames Gravels groundwater body, which defines the extent of the GWSA. This is defined by Pleistocene river terrace gravel deposits (Table 21.20) which extend east-west from Windsor to Richmond and north-south from Hillingdon to Walton-on-Thames. This groundwater body is currently defined as being at Good Status. Appendix 21.3 contains more detailed information on the status and objectives of this water body.

Surface water transfers 21.9.51 The three surface water transfers within the LSA are all designated AWBs, with

‘Moderate’ status. These are all located within the Crane River catchment.

Transitional water bodies 21.9.52 The Thames Upper component of the River Thames Transitional Water Body

delineates the downstream extent of the WSA and is immediately downstream of the River Crane. Appendix 21.3 contains more detailed information on the status and objectives of this water body.

21.9.53 More details of the WSA study area waterbodies are available within Appendix 21.3.

Flood risk 21.9.54 Within the LSA and WSA there are a range of sources of flood risk, including:

1. Fluvial flood risk along the courses of the rivers in the Colne and Crane catchments and the non-tidal River Thames



2. Tidal flood risk along the course of the tidal River Thames

3. Groundwater flood risk associated with area of higher groundwater levels

4. Surface water flood risk associated with accumulation of runoff in localised depressions and urbanised areas

5. Artificial sources of flood risk including impoundments (e.g. reservoirs), canals and the sewer network.

21.9.55 The risks listed above are relevant to both the LSA and WSA, except for tidal flood risk, which is relevant only to the tidal section of the River Thames included in the WSA.

21.9.56 Though risks are present throughout the WSA, the remainder of this baseline section (and Appendix 21.4) focus on the sources of flood risk to the LSA as this is the area where there is an intersection between areas of flood risk with the elements of the DCO Project (as described in Chapter 6). All relevant sources of flood risk are summarised below, based on pre-existing sources of information. Each source is discussed in further detail within Appendix 21.4, where further modelling and mapping has been undertaken as part of the assessment.

Fluvial flood risk 21.9.57 There are areas at risk of fluvial (river) flood risk, shown in Figure 21.13 to Figure

21.16, Volume 2, which indicates that the most extensive areas of fluvial risk are associated with the freshwater Rivers Thames and Colne. Flood Zones 2 and 3 are confined to narrow zones adjacent to the channels, as well as partially or wholly corresponding to lakes and ponds and their immediate surrounding areas. The fluvial flood depth mapping is shown in Figures 21.4.5.1 to 21.4.7.4 supporting Appendix 21.4.

21.9.58 In the Colne Valley there are areas of:

1. Flood Zone 3 (1% Annual Exceedance Probability (AEP)) to the west of the M25 and adjacent to the east bank of the Colne Brook. This area corresponds with the western extent of the proposed runway alignment and a number of offline lakes and ponds

2. There is a more expansive area of Flood Zone 2 (0.1% AEP) bounded by the A4 in the north, the River Colne in the west, the A3113 in the south and the existing airfield boundary in the east. As shown in Figure 21.16, a flood defence cuts across the centre of this area in Flood Zone 2, but does not appear to provide any benefit. This is indicated by the evidence that these areas are not identified on the Environment Agency dataset that outlines the areas that are protected from river flooding up to a 1% AEP flood event



3. There are large areas downstream of the main airfield expansion area which are within Flood Zone 3, which do benefit from flood defences. Land and property in this area are identified on the Environment Agency dataset in an ‘area benefitting from flood defences’, which is protected from river flooding up to the 1% AEP event.

21.9.59 The flood zones for the River Crane pass along the eastern boundary of the existing airfield. The majority of Flood Zone 3 is confined to areas outside the Site apart from within a small area to the northwest in Cranford. South from Cranford Park along the River Crane there are areas of Flood Zone 3 that largely affect open green space, but areas of Flood Zone 2 are mapped as affecting parts of Cranford and an industrial estate south of Hounslow West.

21.9.60 Between the main watercourses of the Colne and the Crane valleys, there is little fluvial flooding to the east of the Duke of Northumberland’s River beyond Saxon Lake. To the south of this, the land to the east of the Duke of Northumberland’s River is generally within Flood Zone 1.

Surface water flood risk7

21.9.61 The extent of surface water flooding occurs as a combination of topography, geology and interaction with other flood sources. For instance, impermeable sites are less conducive to infiltration and generate greater volumes of run-off, whilst low-lying sites with limited connectivity to rivers and drains can prevent efficient drainage. Convergence and ponding sites have been identified within and surrounding the Site. These, as well as flow paths, have been identified in the Environment Agency’s Risk of Flooding from Surface Water maps of the development area, as shown in Figure 21.17 to 21.19.

21.9.62 The flow paths on and around the Site are generally 0.0-0.3m deep for 1% Annual Exceedance Probability (AEP) events, although ponding located on the flatter areas can be up to 0.6m deep. Depths of at least 1.2m are associated with low points, an example is the underpass at the Concorde roundabout. Further details of areas at risk of surface water flooding are presented in Appendix 21.4.

Groundwater flood risk 21.9.63 There are areas with increased potential for elevated groundwater in the LSA,

which could have the potential for flooding at the surface or to sub-surface structures such as basements. The baseline information on groundwater flood risk

7 Surface water flooding occurs when rainwater does not drain away through the normal drainage systems or soak into the ground, but lies on or flows over the ground instead. Managing the risk of flooding from surface water is the responsibility of lead local flood authorities (LLFAs). (https://www.gov.uk/government/publications/flood-risk-maps-for-surface-water-how-to-use-the-map)

https://www.gov.uk/government/publications/flood-risk-maps-for-surface-water-how-to-use-the-map



is based on the British Geological Survey (BGS) ‘susceptibility to groundwater flooding’ mapping as shown in Figure 21.20. This indicates that the majority of the area within the Site is classified as having potential for groundwater flooding to occur at the surface. There are also areas within the Site which are classified for groundwater flooding to occur at below ground level. These results are presented in more detail in relation to development zones within Appendix 21.4.

21.9.64 The area around Colnbrook and Poyle in the Colne Valley is understood to be susceptible to groundwater flooding, and under periods of high rainfall groundwater levels in the alluvium and gravels can rise quickly (Slough Borough Council, 2014)8. It has been suggested that groundwater flow through this area may be impeded by a reduction in the aquifer cross-sectional area due to the presence of landfills and other developments including the raised reservoirs (Slough Borough Council, 2013)9.

Risks of flooding from reservoirs and other artificial sources

21.9.65 Flooding within the LSA could occur following breach of embankments. Hypothetical scenarios in which the reservoir embankments are breached have been mapped by the Environment Agency, which show potentially extensive flooding associated with the reservoirs in the southwest of the LSA. This mapping is shown on Figures 21.4.24 to 26 supporting Appendix 21.4. The probability of a reservoir causing flooding is dependent on the structural and geotechnical conditions of the reservoir embankments. The operation and maintenance of the reservoirs is regulated by the Reservoirs Act 1975, which ensures that the design is fit for purpose, and that maintenance, including frequent inspections of reservoir embankments, ensures the condition of the embankments. As such, the chance of them failing and giving rise to flooding is remote.

21.9.66 A further potential artificial source of flood risk is from the canal network located north of the M4, with the nearest point being the Grand Union Canal in West Drayton. If there were to be a breach of the canal structures in this area then water discharged from the breach would flow south, likely making its way into the fluvial network. The flood extents in the vicinity of the Site are likely be bounded by those already considered for fluvial flooding above.

21.9.67 Elements of the DCO Project may be at risk of potential overtopping and/or breach of the canal system at embankments and aqueduct/culverted river crossings due to increased flows in Fray’s River, River Colne, Colne Brook and the River Crane. Figure 21.4.27 supporting Appendix 21.4 shows these assets. The recorded breaches of the Canal System are relatively uncommon. There were 122 recorded

8 Slough Borough Council, Section 19 Flood Investigation: Colnbrook Flooding, 2014 9 Slough Borough Council, Local Flood Risk Management Strategy for Slough, 2013



breaches of the canal system between 1974 and 201110. Aqueduct failures are also shown to be very rare events, with only one recorded in an assessment of breaches from 1974 to 2015.

21.9.68 Information from the Thames Water DG5 register has been utilised in Appendix 21.4: Flood Risk Assessment Annex C to obtain records of sewer flooding. Records indicated that exceedance of the sewer system is prevalent during high intensity rainfall events when the volume of runoff from surrounding roads, houses and land can significantly exceed the typical 3.33 AEP (1 in 30 year) design standard for sewer networks. Thames Water recorded a total of 1,472 property and 263 non – property sewer flooding incidents 1km from the Site, south of the Great Western railway. The data shows that the vast majority of historic sewer flooding surrounding the Site is low depth and localised.

Protected sites 21.9.69 Protected sites are identified and discussed in Chapter 8. Some of those sites

contain water-dependent habitats (e.g. reservoirs, lakes and wetlands). As identified within Chapter 8, the study area across which protected sites are identified and assessed will evolve as the design phases and technical assessments progress. This will ensure that all relevant (i.e. water dependent) sites within the LSA, WSA and GWSA are considered.

Abstractions and discharges

Abstractions 21.9.70 Licensed abstractions within the LSA, downstream reaches of the WSA, and the

GWSA are listed in Appendix 21.6. These include abstractions for a range of purposes including agriculture, industry and public water supply. The largest abstractions are for public water supply from the River Thames at various intakes including Egham, Walton, Chertsey and Hampton. Figure 21.21, Volume 3 shows the respective groundwater and surface water abstractions in relation to the WSA and GWSA. In summary:

1. Thirty one licensed surface water abstractions are screened in for assessment given that they are located within or downstream of the DCO limits. Ten licensed surface water abstractions are screened out due to being upstream of the DCO limits

2. Within the DCO limits, in total, there are seven licensed surface water abstractions. This comprises of agricultural supplies within the Horton Brook

10 Dun, R., W. and Wicks, J., M. (2014) Canal breach risk assessment for improved asset management. Proceedings of the Institution of Civil Engineers - Water Management 2014 167:1, 5-16



catchment (WE1), agricultural and wetland support supplies for the Arthur Jacob Nature Reserve in the Colne Brook catchment (WE2), and flow maintenance supplies in the River Colne catchment (WE4)

3. Downstream of the DCO limits there are 24 surface water abstractions. This includes abstraction for agriculture along the Colne Brook (WE2), agriculture and environmental remediation along the River Colne (WE4), industrial treatment on the Duke of Northumberland (WE5) and industrial development, potable public surface water supplies and agriculture on the freshwater and tidal River Thames (WE8 -WE9)

4. Seventeen groundwater abstractions are within a modelled zone of potential drawdown, based on results from Appendix 21.2. 178 groundwater abstractions are screened out given that they are located outside of this zone and a 200m buffer. From the 16 screened in groundwater abstractions, nine abstract from the gravels aquifer (WE12), and eight abstract from the Chalk aquifer (WE13). The gravels abstraction uses comprise of general agriculture, industrial process water and mineral washing. The abstractions from the Chalk comprise of evaporative cooling water, and process water for industry and transport

5. There are 28 groundwater private water supplies for domestic use which have all been scoped out due to being located outside of the modelled zone of drawdown.

Discharges

21.9.71 Consented discharges within the LSA, and the GWSA are listed in Appendix 21.6 and shown on Figure 21.22, Volume 2. There are 7 trade discharge points in total within the Site, including one groundwater and six surface water discharges.

The surface water discharges are to the Colne Brook, and a Cess drain of the County Ditch (WE2), the Wraysbury River and Old Bigley Ditch (WE3), and Orlitts Lake (WE12). The groundwater discharge is received by the gravels (WE12).

Future baseline 21.9.72 The baseline conditions will be influenced over time by changes to the climate,

changes to land use (across the WSA and GWSA), and improvements to land and/or water quality as a result of legislation, policy or other drivers. These factors are considered in the remainder of this section.

Other developments

21.9.73 In the absence of the DCO Project, there is likely to be a change in baseline conditions as a result of other development being pursued at Heathrow Airport.



The Kilobox Apron Development and Runway Access Taxiway projects, described in Chapter 5, would be completed prior to the commencement of construction of the DCO Project. The groundwater model (Appendix 21.2) has incorporated the Kilobox Apron Development, which involves a sub-surface structure. Other water environment assessments at PEIR have not explicitly considered these developments as they would not materially influence other aspects of the water environment.

21.9.74 Additional Heathrow Airport supporting development may also come forward in the future (with their construction and operation phases overlapping with the construction and/or operation phases of the DCO Project). Such development is presented and assessed within Section 21.11.

21.9.75 There will also be other changes in baseline conditions in the wider area as a result of land use changes through development un-related to Heathrow Airport. Such ‘other development’ could result in new receptors or result in other cumulative effects with the DCO Project. Where these developments are built out before the construction of the DCO Project commences, or where the construction and operation phases of these developments overlap with the construction and / or operation phases of the DCO Project, they are assessed in Section 21.11.

Climate and other land use changes 21.9.76 The UKCP18 data indicates that in London there will be greater seasonality with

winters becoming wetter and summers becoming drier. Under the high emissions scenario, the central estimate of the predicted increase in winter precipitation is 10% by 2020s and 30% by the 2080s. The central estimate of the predicted decrease in summer precipitation is for -10% in the 2020s and -40% by the 2080s (Met Office, 2018). Therefore, there is potential for an increase in peak river flows during winter months, and reduced low flows during summer months.

21.9.77 The FRA (Appendix 21.4) has taken into account future baseline scenarios for peak river flow allowances during future time horizons or ‘epochs’. In the context of the DCO Project, the 2020s epoch has been used for the construction phase and the 2080s epoch has been used for the operation phase, in accordance with Environment Agency guidance (Environment Agency, 2017).

21.9.78 Changing land use, in the form of urban development, or development of major industrial sites and transport links could cause changes to the water environment over time. These changes could result in changes in patterns and rates of rainfall infiltration, changes in flow pathways and sources of sediment inputs, direct morphological alterations to water bodies, or the introduction or removal of sources of pollution.



21.9.79 WFD water bodies have an overall target of Good Status by 2027 (or earlier) unless a lower status is justified by means of technical infeasibility or disproportionate cost. For the purposes of this assessment, all WFD waterbodies have been assumed to be at Good Status by 2027.

21.10 Assessment of water environment effects

Overview 21.10.1 Table 6.1 in Chapter 6 outlines the key construction and operational activities that

will be occurring in Phases 1 to 3 of the DCO Project. The activities that are relevant to the water environment, and assessment of the effects of those activities on the water environment, are described for each Phase in the following sections.

Activities

Phase 1 21.10.2 Construction of the following permanent features of the DCO Project will be

required in Phase 1 and could affect the water environment:

1. Development of the northwest runway, taxiways and other reprofiled areas

2. Off-airfield developments, including sub-surface features as well as the M25 realignment and rail tunnels

3. Completion of the new airfield drainage system. The new drainage network includes options to discharge to the Colne Brook, Wraysbury River, River Colne and Duke of Northumberland’s River. There are also options for extensions to the existing drainage networks with discharges to Portlane Brook, to ground, and the River Crane

4. River diversions, as described in Chapter 6, including diverting the Colne Brook to the west, diverting and combining the Wraysbury River and River Colne in to a single channel, and diverting and combining the Longford River and Duke of Northumberland’s River in to a single channel. This includes development of the CRC

5. Upstream Flood Storage Areas, with associated structures to provide connection to the rivers

6. Potential new Wastewater Treatment Works

7. Modifications to roads and junctions including the M25, A4 and A3044. This includes crossings over rivers (including the Colne Brook, Wraysbury River, River Colne, Longford River and Duke of Northumberland’s River)



8. Provision of green infrastructure, including the creation and enhancement of green corridors.

21.10.3 In order to construct all of these features of the DCO Project, the following construction activities will be required and could affect the water environment:

1. Construction of the river diversions, transfer of flow to the diverted channels, and backfilling of old river channels

2. Land raising and levelling, and infilling of lakes (including Old Slade Lake, Colnbrook North Lake, Orlitts Lake, Colnbrook West Lake, Swan Lake)

3. Excavations and tunnelling works, potentially through landfill

4. Piling through landfills

5. Borrow pits, to source construction materials

6. Dewatering activities associated with excavations, borrow pits and dewatering of lakes, and associated discharges

7. Creation of construction working areas and haul routes with hardstanding or unvegetated compacted surfaces

8. Stockpiling of earthworks.

21.10.4 Following construction, operation of the Airport would involve the following activities that could affect the water environment

1. Increased aircraft and ground-based traffic movements, with associated emissions (as assessed in Chapter 7)

2. Abstraction of groundwater for non-potable supply

3. Abstraction of groundwater and/or surface water (lakes or rivers) for water-source heat pumps

4. Operation of the drainage network

5. Use of chemicals and other substances for operational activities, including the use of glycol for de-icing.

Phase 2 21.10.5 During Phase 2, further construction will continue, with works on the M25 and local

roads, and development of Terminal 2 and Terminal 5X. Construction activities including borrow pits, excavations, stockpiling and dewatering will continue, associated with those facilities. The operational activities listed against Phase 1 will continue.



Phase 3

21.10.6 During Phase 3, further construction will continue, including development of Terminal 3 and some other supporting facilities. Some construction activities including borrow pits, excavations, stockpiling and dewatering will continue to be required, associated with those facilities. The operational activities listed against Phase 1 will continue.

Introduction to the receptor-based assessments 21.10.7 The assessment is carried out for each receptor group individually, considering the

activities that are relevant to each receptor group and the effects that could occur as a result of each activity. For river receptors, the catchment area of each river (as identified in Figure 21.2) has been used to assign relevant activities. However, flood risk has been considered in its own receptor group (WE16) to allow clear presentation of the flood risk-related effects.

21.10.8 Where there is more than one receptor in the group (for example, group WE12 includes the superficial aquifer, lakes in connectivity with the aquifer, and abstractions from the aquifer), individual receptors are grouped together as far as possible for assessment but discussed individually where necessary.

21.10.9 The assessment covers the range of effects, for relevant receptors, that were identified in Table 21.7. The assessment takes account of all environmental measures identified in Section 21.5, including both embedded environmental measures (incorporating design elements relating to the water environment) and good practice measures in the draft CoCP.

21.10.10 As discussed in Section 21.4, the assessments have taken a reasonable worst case approach, with all assessments considering the permanent land use changes at the end state of the DCO Project, i.e. the end of Phase 3. In addition, the assessments have considered effects relating to construction activities, for which, as set out above, the majority of effects will commence in Phase 1, with some continuing through Phases 2 and 3. The assessments discussed below therefore cover all Phases of the development together, as relevant to the type of effect.

WE1 Horton Brook 21.10.11 The assessment of individual effects is presented in Table 21.21. The

assessments are based on the Horton Brook being a WFD water body and hence assigned Medium sensitivity. The existing licensed non-potable abstractions from the Horton Brook are defined as Low sensitivity based on their status and use, as identified in Appendix 21.6. An overall summary follows the table.



Table 21.21: Assessment of individual effects for WE1 Horton Brook

Detail of activity Assessment of effects (magnitude of change and significance of effect)

WFD water body (Sensitivity Medium)

The following construction activities are proposed within the Horton Brook catchment:

1. No dewatering would be carried out within the catchment itself, but dewatering elsewhere could potentially have an effect within the catchment

2. Construction working areas

3. Haul roads will cross the catchment, including across the Horton Brook

Changes to channel geomorphology would have a Negligible magnitude of change. The potential effects are associated with possible temporary watercourse crossing activities, which would be appropriately constructed and maintained as per the requirements of the draft CoCP. This would result in an effect that is not significant.

Changes to river flow would have a Negligible magnitude of change. No activities that would reduce river flows are intended in the vicinity of Horton Brook. Effects of dewatering were assessed using numerical groundwater modelling in Appendix 21.2 which indicate negligible changes to flows in Horton Brook. This gives an effect that is not significant.

Effects from increased sediment loading to surface water would have a Low magnitude of change. Although no river diversions or in-channel works are required for Horton Brook, there would be other construction activities in the catchment. Sediment would be managed in line with good practice, as set out in the draft CoCP. This gives an effect that is not significant effect.

Effects from the introduction of pollutants to surface waters would have a Low magnitude of change. This is based on the application of good practice pollution prevention during construction (as set out in the draft CoCP) and gives an effect that is not significant.

The draft DCO limits overlap with the catchment of the Horton Brook. The following permanent land use changes are proposed:

1. Development of the western portion of the Colne Brook Logistics centre

2. Diversion of the Colne Brook in to the eastern extent of the Horton Brook catchment

3. Diversion of the A4 4. Provision of green

infrastructure corridors and biodiversity links

Changes to channel geomorphology would have a Low magnitude of change, as a result of the Colne Brook diversion extending in to the Horton Brook catchment and removing existing drains. This effect is assessed in Appendix 21.3. This change is concluded to have an effect that is not significant.

Changes to river flow would have a Negligible magnitude of change. This is based on the conclusions of Appendix 21.2: Groundwater modelling assessment, which indicates that changes to baseflow in the Horton Brook would be negligible. As identified above, there would be a small change to the catchment area as a result of the Colne Brook diversion. No discharges from the airfield are anticipated, and any other discharges would be managed by permit. This would result in an effect that is not significant. Effects on water quality from changes to mixing and dilution would have a Negligible magnitude of change. This is due to there being negligible changes to total flow, with resulting negligible changes to dilution capacity of the receiving watercourse. This would result in an effect that is not significant.

Effects from the introduction of pollutants to surface waters would have a Negligible magnitude of change. As set out in Appendix 21.1, it is not




Aircraft and traffic movements associated with the DCO Project could affect air quality in the vicinity of this receptor, as assessed in Chapter 7.

anticipated that there would be any permanent operational discharges to the Horton Brook associated with the DCO Project. The DCO Project design includes only greenspace in the vicinity of the Brook. This gives an effect that is not significant.

Effects from the increased atmospheric deposition of pollutants would have a Negligible magnitude of change, as set out in Appendix 21.1. This would result in an effect that is not significant.

Abstractions from the Horton Brook (Sensitivity Low)

All construction and operational activities listed above in relation to the WFD water body.

Effects on abstractors from the Horton Brook are assessed as having a Low magnitude of change. Possible effects on abstractors would be managed where reasonably practicable and appropriate through good practice as set out in Section 21.5and the draft CoCP. Where effects could not be avoided, including the loss of supply through change in land use, an alternative supply would be provided as set out in Section 21.5. This gives an effect that is not significant.

21.10.12 Summary of effects relating to construction activities: During the construction phase, there would be a construction working area straddling the Horton Brook. This would have a buffer strip to prevent works in the immediate vicinity of the brook, although it is possible that a temporary watercourse crossing could be required. Works would be carried out in line with the draft CoCP. As a result, it is anticipated that there will be no significant effects on the Horton Brook during the construction phase.

21.10.13 Summary of effects relating to permanent land use changes and operational activities: There would be minor alterations to the drainage network of the Horton Brook. It is not anticipated that there would be any permanent discharges from the airfield to the Horton Brook. As such, it is anticipated that the permanent land use and operational changes associated with the DCO Project would result in no significant effects on the Horton Brook.

WE2 Colne Brook 21.10.14 The assessment of individual effects for the Colne Brook is presented in Table

21.22. The assessments are based on the Colne Brook being a WFD water body and hence assigned Medium sensitivity. The licensed, non-potable abstractions from the Colne Brook are defined as Low sensitivity based on their status and use identified in Appendix 21.6. An overall summary follows the table.



Table 21.22: Assessment of individual effects for WE2 Colne Brook


WFD Water Body (Sensitivity Medium)

The following construction activities are proposed within the Colne Brook catchment:

1. Infilling of Old Slade Lake, Colnbrook North Lake, Orlitts Lake, Colnbrook West Lake, Swan Lake

2. Earthworks stockpiling 3. Possible dewatering

associated with earthworks activities and excavations

4. Borrow pits 5. Demolition of existing

buildings 6. Construction working

areas 7. Haul routes crossing

and in proximity to the river

Changes to channel geomorphology are assessed as having a Low magnitude of change. The potential effects are associated with temporary watercourse crossings and the transfer of flow between the existing and diverted channel. These would be appropriately constructed, managed and maintained as per the requirements in the draft CoCP. This gives an effect that is not significant.

Changes to river flow are assessed as having a Low magnitude of change. As illustrated using numerical groundwater modelling in Appendix 21.2, dewatering would have some local effects on river flows. This would be minimised through measures such as use of slurry walls and permitting of associated discharges. This gives an effect that is not significant.

Effects from the increased sediment loading to surface water are assessed as having a Low magnitude of change. Sediment loading would be managed in accordance with measures set out in Section 21.5, including measures for dewatering, construction and diversion of river channels, and general construction management. As indicated in Appendix 21.1, this gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters are assessed as having a Low magnitude of change, based on the application of good practice pollution prevention during construction (as set out in the draft CoCP). This gives an effect that is not significant.

The draft DCO limits overlap with the catchment of the Colne Brook, including the river itself. The following permanent activities are proposed:

1. Development of the Colne Brook Logistics centre

2. Creation of Flood Storage Areas and flow control structures

3. Diversion of the Colne Brook (as shown in Figure 21.3)

4. Diversion of the A4 5. River crossings of the

Changes to channel geomorphology are assessed as having a Low magnitude of change. As set out in Appendix 21.3, the Colne Brook would be diverted along approximately 2km of new channel, fully replacing the length of the abandoned channel. The diverted channel would be of an enhanced channel design compared to some existing reaches, applying the principles set out in Section 21.5. River crossings are assessed as having negligible effect in Appendix 21.3. This gives an effect that is not significant.

Effects on ecological connectivity are considered in the Upper Colne (WE10) receptor group, with reference to Chapter 8. Changes to river flow are assessed as having a Low magnitude of change. Appendix 21.2 indicates that river flows could be affected at the reach scale as a result of changes to surface water-groundwater interactions, but at the waterbody scale the changes would be small. This gives an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change, as a result of low level changes to total flow overall, with negligible changes to dilution




A4 and A3044 6. Provision of green


7. Potential discharge of airfield runoff to the Colne Brook

Aircraft and traffic movements associated with the DCO Project could affect air quality, as assessed in Chapter 7: Air quality and odour.

capacity of the receiving watercourse. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters: There is the possibility of a discharge from the airfield to the Colne Brook via the drainage network, as part of the DCO Project. As set out in Appendix 21.1, this could influence water quality in Colne Brook:

1. High levels of BOD in runoff from the airfield are possible, associated with glycol use, which would be managed through treatment of runoff (as described in Section 21.5). Numerical modelling has illustrated that changes to Airfield discharges as a result of the DCO Project would not result in EQS failures for BOD, hence has a Low magnitude of change and a not significant effect.

2. Other potential pollutants are discussed in more detail in Appendix 21.1. Information around other potential pollutants is still developing, with the gathering of site monitoring data and work to understand the source-pathway-receptor linkages. Therefore it is not possible to reach a conclusion with respect to the potential significance of effects in relation to other parameters at this stage. Further work will be presented at ES.

Effects from the increased atmospheric deposition of pollutants are assessed as having a Negligible magnitude of change, as set out in Appendix 21.1. This would result in an effect that is not significant.

Abstractions from the Colne Brook (Sensitivity Low - Medium)

All activities listed above in relation to the WFD water body.

Effects on abstractors from the Colne Brook are assessed as having a Low magnitude of change. Possible effects on abstractors would be managed where reasonably practicable and appropriate through good practice as set out in Section 21.5and the draft CoCP. Where effects could not be avoided, including the loss of supply through change in land use, an alternative supply would be provided as set out in Section 21.5. This gives an effect that is not significant.

Discharges into the Colne Brook (Sensitivity Low)

All permanent activities in relation to the diversion of the Colne Brook and the A4.

Existing discharges into the Colne Brook are assessed as having a Low magnitude of change. In any cases where the current discharge location would no longer be useable due to land use changes, the discharge holder would be consulted to find an alternative solution, if required. This would result in an effect that it not significant.

21.10.15 Summary of effects relating to construction activities: This area would be subject

to extensive construction activities, including construction of the diverted channel, infilling of lakes, stockpiling, a borrow pit, concrete batching and other construction



activities. It is probable that there would be construction-related discharges to the river. These activities would be managed as set out in Section 21.5, allowing a conclusion of no significant effect to be drawn for the Colne Brook.

21.10.16 Summary of effects relating to permanent land use changes and operational activities: The diversion of the Colne Brook would involve the creation of approximately 2 km of new channel, replacing sometimes degraded existing channel, and would not introduce any new barriers to ecological connectivity. On the basis of an appropriate construction and phasing programme being agreed for the river diversion, these changes would lead to an improvement to the hydromorphology elements in the WFD assessment. Potential operational discharges to the Colne Brook could have an effect on water quality, but this would be of low magnitude for BOD, and insufficient information is available for other parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of no significant effect for the Colne Brook.

WE3 Wraysbury River 21.10.17 The assessment of individual effects for the Wraysbury River is presented in Table

21.23. The assessments are based on the Wraysbury River being one channel of a WFD water body, and hence assigned Medium sensitivity. An overall summary follows the table.



Table 21.23: Assessment of individual effects for WE3 Wraysbury River


WFD Water body (Sensitivity Medium)

The following construction activities are proposed within the Wraysbury River catchment:

1. Possible dewatering associated with earthworks activities and excavations

2. Demolition of existing buildings


4. Haul routes crossing and in proximity to the river

5. No dewatering is anticipated within the catchment itself, but dewatering elsewhere could potentially have an effect within the catchment.


Changes to river flow are assessed as having a Low magnitude of change. As illustrated using numerical groundwater modelling in Appendix 21.2, dewatering would have some local effects on river flows. This would be minimised through measures such as the use of slurry walls where required, and permitting of associated discharges. This gives an effect that is not significant.

Effects from increased sediment loading to surface water are assessed as having a Low magnitude of change. Sediment loading would be managed in accordance with measures set out in Section 21.5, including measures for dewatering, construction and diversion of river channels, and general construction management. As indicated in Appendix 21.1, this gives an effect that is not significant.


Permanent changes to land use and land form. Operational activities.

The draft DCO limits overlap with the catchment of the Wraysbury River, including the river itself. The following permanent activities are proposed:

1. Diversion of the M25, in two places, including passing beneath the northwest runway

2. Diversion of the Bigley Ditch in to the Wraysbury River (as shown in Figure 21.3)

Changes to channel geomorphology are assessed as having a High magnitude of change. As set out in Appendix 21.3, the Wraysbury River would be diverted and combined with the River Colne. It would flow through the CRC beneath the northwest runway, as well as a new crossing beneath the M25. These changes result in a net loss of river length combined between the Wraysbury River and River Colne, which would be compensated through improvements elsewhere in the catchment (see Section 21.13). While the new channels would be improved in some reaches compared to the existing, and would be designed according to the principles set out in Section 21.5, there remain uncertainties around the effectiveness of the CRC. Based on the currently available information and level of confidence, this gives a significant effect.

Effects on ecological connectivity are considered in the Upper Colne (WE10) receptor group, with reference to Chapter 8. Changes to river flow are assessed as having a Low magnitude of change.




3. Diversion of the Wraysbury River in to a new combined channel with the River Colne, including passage beneath the northwest runway in the CRC (as shown in Figure 21.3)

4. Diversion of the Wraysbury River beneath the M25 and reconnection to the Poyle Channel (as shown in Figure 21.15)

5. Modifications to junction 14 of the M25

6. Development of the western end of the northwest runway

7. Diversion of the A4 8. Provision of green


9. Possible discharge of airfield runoff to the Wraysbury River

Aircraft and traffic movements associated with the DCO Project could affect air quality, as assessed in Chapter 7.

Appendix 21.2 indicates that river flows could be affected at the reach scale as a result of changes to surface water-groundwater interactions, but at the waterbody scale the changes would be small. The diversions would be managed to ensure the same total proportion of flow passed down the combined channel as existing, with an appropriate split between the two at the downstream end. Potential use of river water for the thermal strategy would return all water to the channel. This would result in an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change. This would be due to low level changes to total flow overall, with negligible changes to dilution capacity of the receiving watercourse. This gives an effect that is not significant.

1. Effects from the introduction of pollutants to surface waters: There is the possibility of a discharge from the airfield to Wraysbury River via the drainage network, as part of the DCO Project. As set out in Appendix 21.1, this could influence water quality in the Wraysbury River:

2. High levels of BOD in runoff from the airfield are possible, associated with glycol use, which would be managed through treatment of runoff (as described in Section 21.5). Numerical modelling has illustrated that changes to Airfield discharges as a result of the DCO Project would not result in EQS failures for BOD, and hence has a Low magnitude of change and a not significant effect.

3. Other potential pollutants are discussed in more detail in Appendix 21.1. Information around other potential pollutants is still developing, with the gathering of site monitoring data and work to understand the source-pathway-receptor linkages. Therefore, it is not possible to reach a conclusion with respect to the potential significance of effects in relation to other parameters at this stage. Further work will be presented at ES.

In addition, use of river waters for the thermal strategy could affect water temperature.


Discharges into the Wraysbury River (Sensitivity Low)

All permanent activities in relation the diversion of the Wraysbury River, the A4, and the M25.

Existing discharges into the Wraysbury River are assessed as having a Low magnitude of change. In any cases where the current discharge location would no longer be useable due to land use changes, the discharge holder would be consulted to find an alternative solution, if required. This would




result in an effect that it not significant.


to extensive construction activities, including construction of the diverted channel and CRC and dewatering in close vicinity of both the current and diverted channels. It is probable that there would be construction-related discharges to the river. However, these activities would be actively managed as set out in Section 21.5, allowing a conclusion of no significant effect to be drawn for the Wraysbury River in relation to construction activities.

21.10.19 Summary of effects relating to permanent land use changes and operational activities: The Wraysbury River would be diverted and combined with the River Colne, which would involve the creation of new channel, but represents a net loss of channel length overall. The new channel would flow through the CRC as well as beneath a new crossing of the M25. Whilst the environmental measures described in Section 21.5 and Section 21.13 would provide appropriate hydromorphological variability and habitat through the CRC and the new channel reaches, the innovative design of the CRC creates uncertainty as to its effectiveness. Potential operational discharges to the Wraysbury River could have an effect on water quality, but this would be of low magnitude for BOD, and insufficient information is available for other parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of significant effect for the Wraysbury River.

WE4 River Colne 21.10.20 The assessment of individual effects for the River Colne is presented in Table

21.24 The assessments are based on the River Colne being one channel of a WFD water body, and hence assigned Medium sensitivity. The Surrey Ash, which is also a WFD water body in its own right, is considered separately, with Medium sensitivity. Abstractions from the River Colne are variously defined as Low and Medium sensitivity, depending on their status and use, as identified in Appendix 21.6. An overall summary follows the table.



Table 21.24: Assessment of individual effects for WE4 River Colne



The following construction activities are proposed within the River Colne catchment:

1. Borrow pits 2. Demolition of existing

buildings 3. Stockpiling 4. Proposed crossing of

the A3113 south of the bifurcation from the Wraysbury River


6. Haul routes crossing and in proximity to the rivers

7. Dewatering associated with earthworks activities and excavations


Changes to river flow are assessed as having a Low magnitude of change. As illustrated using numerical groundwater modelling in Appendix 21.2, dewatering would have some local effects on river flows. This would be minimised through measures such as the use of slurry walls where required, and permitting of associated discharges. This gives an effect that is not significant.

Effects from the increased sediment loading to surface water are assessed as having a Low magnitude of change. Sediment loading would be managed in accordance with measures set out in Section 21.5, including for dewatering, construction and diversion of river channels, and general construction management. As indicated in Appendix 21.1, this gives an effect that is not significant.


The draft DCO limits overlap with the catchment of the River Colne, including the river itself. The following permanent activities are proposed:

1. Creation of Flood Storage Areas

2. Diversion of the River Colne in to a new combined channel with the Wraysbury River, including passage beneath the northwest runway in the CRC (as shown in Figure 21.3)

3. Diversion of the new Colne channel to the

Changes to channel geomorphology are assessed as having a High magnitude of change for the River Colne. As set out in Appendix 21.3: WFD Assessment, the River Colne would be diverted and combined with the Wraysbury River. It would flow through the CRC beneath the northwest runway as well as having two new road crossings. These changes result in a net loss of river length combined between the Wraysbury River and River Colne, which would be compensated through improvements elsewhere in the catchment (see Section 21.13). While the new channels would be improved in some reaches compared to the existing and would be designed according to the principles set out in Section 21.5, there remain uncertainties around the effectiveness of the CRC. Based on the currently available information and level of confidence, this gives a significant effect. This conclusion does not apply to the Surrey Ash, which would not experience any direct effects on channel geomorphology.

Effects on ecological connectivity are considered in the Upper Colne (WE10) receptor group, with reference to Chapter 8. Changes to river flow are assessed as having a Low magnitude of change.




east of the M25, and reconnection to the existing channel (as shown in Figure 21.3)

4. Modifications to M25 junction

5. Diversion of the A4 6. Development of the

northwest runway, western apron, and parts of Terminal 6

7. Provision of green infrastructure including green corridors

8. New drainage networks, with possible discharge of airfield runoff to the River Colne


Appendix 21.2 indicates that river flows could be affected at the reach scale as a result of changes to surface water-groundwater interactions, but at the waterbody scale the changes would be small. The diversions would be managed to ensure the same total proportion of flow passed down the combined channel as existing, with an appropriate split between the two at the downstream end. Potential use of river water for the thermal strategy would return all water to the channel. This gives an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change. This would be a result of low level changes to total flow anticipated overall, with negligible changes to the dilution capacity of the receiving watercourse. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters: There is the possibility of a discharge from the airfield to the River Colne via the drainage network, as part of the DCO Project. As set out in Appendix 21.1, this could influence water quality in the River Colne:



3. In addition, use of river waters for the thermal strategy could affect water temperature. Other pollutants are discussed in Appendix 21.1.

Effects from the increased atmospheric deposition of pollutants are assessed to have a Negligible magnitude of change, as set out in Appendix 21.1. This would result in an effect that is not significant.

Abstractions from the River Colne (Sensitivity Low, Medium)

All activities listed above in relation to the WFD Water Body

Effects on abstractors from the River Colne are assessed as having a Low magnitude of change. Possible effects on abstractors would be managed where reasonably practicable and appropriate through good practice as set out in Section 21.5and the draft CoCP. Where effects could not be avoided, including the complete loss of supply through change in land use, an alternative supply would be provided as set out in Section 21.5. This gives an effect that is not significant.




to extensive construction activities, including construction of the diverted channel and CRC, and dewatering in close vicinity of both the current and diverted channels. It is probable that there would be construction-related discharges to the river. However, these activities would be actively managed as set out in Section 21.5, allowing a conclusion of no significant effect to be drawn for the River Colne in relation to construction activities.

21.10.22 Summary of effects relating to permanent land use changes and operational activities: The River Colne would be diverted and combined with the Wraysbury River, which would involve the creation of a new channel, but represents a net loss of channel length overall. The new channel would flow through the CRC as well as beneath new road crossings. Whilst the environmental measures described in Section 21.5 and Section 21.13 would provide appropriate hydromorphological variability and habitat through the CRC and the new channel reaches, the innovative design of the CRC creates uncertainty as to its effectiveness. Potential operational discharges to the River Colne could have an effect on water quality, but this would be of low magnitude for BOD, and insufficient information is available for other parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of significant effect for the River Colne.

WE5 Duke of Northumberland’s River and Longford River 21.10.23 The assessment of individual effects for the Duke of Northumberland’s River and

the Longford River is presented in Table 21.25. The assessments are based on both rivers being WFD water bodies, and hence assigned Medium sensitivity. Abstractions from the Duke of Northumberland’s and Longford River are defined as Low sensitivity, based on their status and use, as identified in Appendix 21.6. An overall summary follows the table.

Table 21.25: Assessment of individual effects for WE5 Duke of Northumberland's River and Longford River



The following construction activities are proposed within or in close vicinity of the rivers:

1. Demolition of

Changes to channel geomorphology are assessed as having a Low magnitude of change. The potential effects are associated with temporary watercourse crossings and the transfer of flow between the existing and diverted channel. These would be appropriately constructed, managed and maintained as per the requirements in the draft CoCP. This gives an effect




existing buildings 2. Borrow pits 3. Stockpiling 4. Construction working

areas 5. Dewatering

associated with earthworks activities and excavations

6. Haul routes crossing and in proximity to the river

that is not significant.

Changes to river flow are assessed as having a Low magnitude of change. Dewatering would be expected to have limited effect on flows in these rivers as the existing channels are lined, and it is assumed that the new channel would also be lined. This is illustrated using numerical groundwater modelling in Appendix 21.2 and gives an effect that is not significant effect.

Effects from the increased sediment loading to surface water are assessed as having a Low magnitude of change. Sediment loading would be managed in accordance with measures set out in Section 21.5, including measures for dewatering, construction and diversion of river channels, and general construction management. As indicated in Appendix 21.1, this gives an effect that is not significant.


The Site intersect the Duke of Northumberland’s River and the River Colne. The following permanent activities are proposed within or in close vicinity to the rivers:


2. Diversion of the Duke of Northumberland’s River and Longford River in to a new combined channel, including passage beneath the northwest runway in the CRC (as shown in Figure 21.3)

3. Diversion of the new Duke of Northumberland’s River around the western and southwestern edge of the Airport (as shown in Figure 21.3)

Changes to channel geomorphology are assessed as having a High magnitude of change. As set out in Appendix 21.3, the Longford River and Duke of Northumberland’s Rivers would be diverted and combined. The combined channel would flow through the CRC beneath the northwest runway and be rerouted around the southwestern corner of the Airport, including five new road crossings of the combined channel. These changes result in a net loss of river length combined between the two rivers, which would be compensated through improvements elsewhere in the catchment (see Section 21.13). While the new channels would be improved in some reaches compared to the existing and would be designed according to the principles set out in Section 21.5, there remain uncertainties around the effectiveness of the CRC. Based on the currently available information and level of confidence, this gives a significant effect.

Effects on ecological connectivity are considered in the Upper Colne (WE10) receptor group, with reference to Chapter 8. Changes to river flow are assessed as having a Low magnitude of change as set out in Appendix 21.2. The diversions would be managed to ensure the same total proportion of flow passed down the combined channel as existing, with an appropriate split between the two at the downstream end. The existing channels are lined, and the new channel is also likely to be lined, so there is likely to be little change to river flow. This gives an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change. This would be due to low level changes to total flow overall, with negligible changes to dilution capacity of the receiving watercourse. This gives an effect that is not




4. Diversion of the A4 5. Development of the

northwest runway and western apron

6. Development of the Southern Parkway

7. Provision of green infrastructure including green corridors

8. New drainage networks, with possible discharge of airfield runoff to the Duke of Northumberland’s River

Aircraft and traffic movements associated with the DCO Project could affect air quality, as assessed in Chapter 7: Air Quality and odour.

significant.

Effects from the introduction of pollutants to surface waters: There is the possibility of a discharge from the airfield to the Duke of Northumberland’s River via the drainage network, as part of the DCO Project. As set out in Appendix 21.1, this could influence water quality by:



Other pollutants are discussed in Appendix 21.1.


Abstractions from the Duke of Northumberland’s and Longford River (Sensitivity Medium)

All activities listed above in relation to the Duke of Northumberland’s or Longford River

Effects on abstractors from the Duke of Northumberland’s and Longford River are assessed as having a Negligible magnitude of change through the application of good practice as set out in Section 21.5and the draft CoCP. This gives an effect that is not significant.


to extensive construction activities, including construction of the diverted channel and CRC. It is probable that there would be construction-related discharges to the river. These activities would be managed as set out in Section 21.5, allowing a conclusion of no significant effect to be drawn for the Duke of Northumberland’s River and Longford River.

21.10.25 Summary of effects relating to permanent land use changes and operational activities: The Duke of Northumberland’s River and the Longford River would be diverted and combined, which would involve the creation of a new channel, but represents a net loss overall of channel length. The new channel would flow through the CRC as well as beneath five new road crossings. Whilst the



environmental measures described in Section 21.5 and Section 21.13 would provide appropriate hydromorphological variability and habitat through the CRC and the new channel reaches, the innovative design of the CRC creates uncertainty as to its effectiveness. Potential operational discharges to the Duke of Northumberland’s River could have an effect on water quality, but this would be of low magnitude for BOD, and insufficient information is available for other parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of significant effect for the Duke of Northumberland’s River and Longford River.

WE6 Portlane Brook 21.10.26 The assessment of individual effects for Portlane Brook is presented in Table

21.26. The assessments are based on the watercourse being a WFD water body and hence assigned Medium sensitivity. An overall summary follows the table.

Table 21.26: Assessment of individual effects for WE6 Portlane Brook


WFD Water body (Sensitivity Medium)

No construction activities are proposed in the catchment of Portlane Brook.

There are no effects associated with the construction phase in Portlane Brook.

The Site only slightly intersects the catchment of Portlane Brook. The following permanent activities are proposed within the catchment:

1. Green infrastructure and enhanced public open spaces

2. New drainage networks within the Site, with discharge of airfield runoff via Felthamhill Relief Sewer to Portlane Brook (as shown in Figure 21.3)

Aircraft and traffic

Changes to channel geomorphology are assessed as having a Negligible magnitude of change, given that there are no in-channel works or discharges with significant sediment content. This gives an effect that is not significant.

Changes to river flow are assessed as having a Low magnitude of change. As assessed in Appendix 21.5, there could be an increase in the total volume of runoff being discharged to Portlane Brook, however the rates of runoff would be appropriately managed. This gives an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change. This would be a result of low level changes to total flow overall, with negligible changes to dilution capacity of the receiving watercourse. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters: As set out in Appendix 21.1, there will be a discharge from the airfield to Portlane Brook (via Felthamhill Relief Sewer), as part of the DCO Project. Whilst a discharge already exists, the total volume of discharge and/or concentrations of pollutants could change. As set out in Appendix 21.1, this could influence water quality in Portlane Brook:




movements associated with the DCO Project could affect air quality, as assessed in Chapter 7.

1. BOD associated with glycol use. This would be managed through treatment of runoff. There are some uncertainties in the assessment of discharges to Portlane Brook in Appendix 21.1. However, it is anticipated that further assessment for the ES will conclude that the DCO Project would not result in EQS failures to this receptor, and hence has a Low magnitude of change and a not significant effect.



21.10.27 Summary of effects relating to construction activities: There would be no construction activities within the catchment of Portlane Brook. Thus, there would be no significant effects from construction activities.

21.10.28 Summary of effects relating to permanent land use changes and operational activities: The DCO Project only slightly extends in to the catchment of the Portlane Brook, and only with small extents of green infrastructure. Discharges from the existing drainage network can reach Portlane Brook via Felthamhill Relief Sewer, but insufficient information is available for any water quality parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of no significant effect for Portlane Brook.

WE7 River Crane 21.10.29 The assessment of individual effects for the River Crane is presented in Table

21.27. The assessments are based on the River Crane being a WFD water body and hence assigned Medium sensitivity. An overall summary follows the table.

Table 21.27: Assessment of individual effects for WE7 River Crane


WFD water body (Sensitivity Medium)

The following Changes to channel geomorphology are assessed as having a Negligible




construction activities are proposed within the River Crane catchment:

1. Demolition of existing buildings


No dewatering would be carried out within the catchment itself but dewatering elsewhere could potentially have an effect within the catchment.

magnitude of change due to no in-channel works or sediment-laden discharges. This gives an effect that is not significant.

Changes to river flow are assessed as having a Negligible magnitude of change. No activities that would reduce river flows are intended in the vicinity of the river. Effects of dewatering were assessed using numerical groundwater modelling in Appendix 21.2 which indicates negligible changes to flows in the River Crane. This gives an effect that is not significant.

Effects from the increased sediment loading to surface water are assessed as having a Low magnitude of change. Although no river diversions or in-channel works are required for the River Crane, there would be other construction activities in the catchment. Sediment would be managed in line with good practice, as set out in the draft CoCP. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters are assessed as having a Low magnitude of change. Based on the application of good practice pollution prevention during construction (as set out in the draft CoCP). This gives an effect that is not significant.

The Site overlaps with the catchment of the River Crane, and with the river itself. The following permanent activities are proposed:

1. Development of irport supporting facilities

2. Development of Terminal 2

3. Roads diversions 4. Provision of

green infrastructure corridors and biodiversity links and enhanced public open space.

5. New drainage networks within the Site, with discharge of airfield runoff to the River Crane

Changes to channel geomorphology are assessed as having a Negligible magnitude of change, given that there are no in-channel works. This gives an effect that is not significant.

Changes to river flow are assessed as having a Low magnitude of change. Appendix 21.2 indicates that changes to baseflow in the River Crane would be negligible. As identified in Appendix 21.5, there would be changes to the catchment area as a result of the DCO Project, which could result in an increase in the total volume of runoff being discharged to the river, but the rates of runoff would be managed. This gives an effect that is not significant.

Effects on water quality from changes to mixing and dilution are assessed as having a Negligible magnitude of change. This would be a result of negligible changes to total flow, with resulting negligible changes to dilution capacity of the receiving watercourse. This gives an effect that is not significant.

Introduction of pollutants to surface waters: As set out in Appendix 21.1, there will be a discharge from the airfield to the River Crane, as a result of the DCO Project. Whilst a discharge already exists, the total volume of discharge and/or concentrations of pollutants could change. As set out in Appendix 21.1 this could influence water quality in the River Crane:

1. BOD associated with glycol use. This would be managed through treatment of runoff. Numerical modelling has illustrated that changes to Airfield discharges as a result of the DCO Project would not result in EQS failures, and hence has a Low magnitude of change and a not significant effect.

2. Other potential pollutants are discussed in more detail in Appendix





21.1. Information around other potential pollutants is still developing, with the gathering of site monitoring data and work to understand the source-pathway-receptor linkages. Therefore, it is not possible to reach a conclusion with respect to the potential significance of effects in relation to other parameters at this stage. Further work will be presented at ES.

Effects from the increased atmospheric deposition of pollutants are assessed as having a Negligible magnitude of change, as set out in Appendix 21.1. This results in an effect that is not significant.

21.10.30 Summary of effects relating to construction activities: Construction works would be

carried out in line with the draft CoCP. As a result, it is anticipated that there would be no significant effects on the River Crane during the construction phase.

21.10.31 Summary of effects relating to permanent land use changes and operational activities: The DCO Project would not require any in-channel works or diversions for the River Crane but would involve some works within the floodplain. The river would experience an increased catchment area as a result of the DCO Project, with associated increase in total volume of runoff, but the rate of runoff would be managed. There is an existing operational permitted discharge from the Site to the Crane, which could increase as a result of the DCO Project. Resulting effects on water quality would be of low magnitude for BOD, and insufficient information is available for other parameters to draw conclusions regarding significance at this stage. Overall, the assessments undertaken result in a conclusion of no significant effect on the Crane

WE8 and WE9 River Thames (freshwater and tidal) 21.10.32 The assessment of individual effects for the River Thames is presented in Table

21.28. The assessments are based on the relevant reaches of the River Thames being WFD water bodies and hence assigned Medium sensitivity. Abstractions from the River Thames are variously defined as Low to High sensitivity, depending on their status and use, identified in Appendix 21.6. An overall summary follows the table.



Table 21.28: Assessment of individual effects for WE8 and WE9 River Thames (freshwater and tidal)



The Site does not overlap with the direct catchment of the River Thames, only with its tributaries (as set out in WE1-7).

Effects on WE1-7 could propagate downstream to the River Thames.

Changes to river flow are assessed as having a Negligible magnitude of change. Effects on river flows in WE1-7 have been concluded to be low or negligible. Any local variations to flow in those rivers due to changing groundwater-surface water interactions would be negligible at the catchment scale. Effects relating to runoff from the Site would be managed to avoid any increase in rates, and dewatering would be managed to minimise effects on flows. Any residual changes to flows would be negligible in the context of the total flow in the River Thames. This gives an effect that is not significant.

Effects from the increased sediment loading to surface water are assessed as having a Negligible magnitude of change. Effects on rivers in WE1-7 have been concluded to be low or negligible. Sediment loading would be managed in accordance with the draft CoCP and measures set out in Section 21.5. Any residual effects on sediment loading would be negligible in the context of the total flow in the Thames. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters are assessed as having a Negligible magnitude of change. Effects on rivers in WE1-7 have been concluded to be low or negligible. Good practice pollution prevention would be applied during construction, as set out in the draft CoCP. Any residual effects on water quality would be negligible in the context of the total flow in the Thames. This gives an effect that is not significant.

The Site does not overlap with the catchment of the River Thames itself, only with its tributaries (as set out in WE1-7).

Effects on WE1-7 could propagate downstream to the River Thames.


Changes to river flow are assessed as having a Negligible magnitude of change. As discussed above, effects on river flows in WE1-7 have been concluded to be low or negligible. Any local variations to flow in those rivers due to changing groundwater-surface water interactions would be negligible at the catchment scale. Effects relating to runoff from the Site would be managed to avoid any increase in rates. This gives an effect that is not significant.

Effects from the introduction of pollutants to surface waters are assessed as having a Negligible magnitude of change. While, as discussed above in relation to WE2-WE7, there would be new or altered discharges to rivers, and those rivers subsequently flow in to the River Thames, there would be substantial dilution in the River Thames, which would reduce any effects to negligible. This would result in an effect that is not significant.


Abstractions from the River Thames (Sensitivity Low, Medium and High)

All activities listed above in Effects on abstractors from the River Thames are assessed as having a




relation to the WFD Water Body.

Negligible magnitude of change, based on the effects discussed above for the river itself. This gives an effect that is not significant.

21.10.33 Summary of effects: Any effects on river flow or water quality reaching the River

Thames from upstream (i.e. from the rivers in receptor groups WE1-7) would be subject to dilution in the River Thames, further reducing any effects. This results in a conclusion of no significant effects for the River Thames receptor groups.

WE10 Upper River Colne 21.10.34 The assessment of effects for the catchment of the Upper River Colne (including

its tributaries) is presented in Table 21.29 and summarised following the table.

Table 21.29: Assessment of individual effects for WE10 Upper River Colne


The Site does not overlap with the catchment area of the Upper River Colne. The upper catchment could potentially be affected indirectly by diversion of rivers, creation of new river channels and passage through the CRC and/or other crossings.

Effects on ecological connectivity with the upper catchment are not assessed at this stage in Appendix 21.3, but will be assessed in the full compliance assessment at ES.

The ecological implications of the changes to the river channels, including the CRC, are set out in Chapter 8: Biodiversity, in terms of biological receptors (relevant fish species). The assessment does not specifically consider the spatial extent of effects: this will be developed for the ES. However, the assessment does identify potential effects on migratory capability for some fish species.

21.10.35 Summary of effects: Effects on the Upper Colne are considered in Appendix 21.3

and in Chapter 8. The latter concludes that, adopting a precautionary approach, there could be a significant effect on some fish species in relation to habitat fragmentation and connectivity.

WE11 Reservoirs 21.10.36 The assessment of individual effects for the reservoirs is presented in Table

21.30. The assessments are based on the reservoirs being of High sensitivity, due to their designation as a SPA. An overall summary follows the table.



Table 21.30: Assessment of individual effects for WE11 Reservoirs



Effects from the increased atmospheric deposition of pollutants are assessed as having a Negligible magnitude of change, as set out in Appendix 21.1. This would constitute an effect that is not significant.

21.10.37 Summary of effects: There are no activities that would result in direct physical interaction of the DCO Project with the reservoirs, but it is possible that changes to air quality (as assessed in Chapter 7: Air quality and odour) could affect the water quality of the lakes due to changes in atmospheric deposition. However, Appendix 21.1: Surface water quality assessment has shown that any such changes would have a negligible effect on the reservoirs, allowing a conclusion of there being no significant effect.

WE12 Lower Thames Gravels 21.10.38 The assessment of individual effects for the Lower Thames Gravels receptor

group is presented in Table 21.31. This includes assessments of effects on groundwater abstractions and lakes in connectivity with groundwater. The aquifer itself is assigned a medium sensitivity, since it is a WFD water body. Abstractions from the aquifer are variously defined as Low and Medium sensitivity, depending on their status and use, as identified in Appendix 21.6. The associated groundwater-fed lakes are assigned medium sensitivity where they are WFD water bodies, and low sensitivity for those that are not WFD water bodies (the ecological value of those lakes is assessed in Chapter 8: Biodiversity).

Table 21.31 Assessment of individual effects for WE12 Lower Thames Gravels


Lower Thames Gravels (Sensitivity Medium)

Construction activities relevant to the Lower Thames Gravels receptor include:

1. Borrow pits 2. Stockpiling

Changes to groundwater flow regime are assessed as having a Low magnitude of change. Appendix 21.2 show that potential changes to groundwater flow or levels as a result of dewatering, excavating and/or infilling with new materials would be minimised through the implementation of measures such as slurry walls where required, and with any discharges to ground being appropriately managed. This would constitute an effect that is




3. Dewatering of borrow pits and other excavations

4. Landfilling

not significant.

Effects from the introduction of pollutants to groundwater are assessed as having a Low magnitude of change, based on the application of good practice pollution prevention, as set out in the draft CoCP. This gives an effect that is not significant.

Effects from the mobilisation of contaminants in groundwater are assessed in Chapter 14.

This receptor extends beneath the full extent of the DCO project. Activities relevant to the Lower Thames Gravels receptor include:

1. Development of the northwest runway, taxiways and other reprofiled land areas

2. Development of drainage networks, including sub-surface features and discharges to ground

3. Development of tunnels and basements, including M25 realignment, rail tunnels, Terminal 6

4. Realignment of rivers

5. Thermal energy strategy

Effects from reduced recharge are assessed as having a Low magnitude of change. As assessed in Appendix 21.2, there may be localised changes in recharge to groundwater as a result of increased impermeable land surfaces and changes to drainage networks (including discharges to ground), which could result in some localised changes to groundwater flows and/or levels. This gives an effect that is not significant.

Changes to the groundwater flow regime are assessed as having a Low magnitude of change. As assessed in Appendix 21.2, appropriate design and alignment of subsurface features to replicate the existing groundwater regime would avoid any substantial changes to groundwater flow or levels. Infilling of old river channels would be with similar material to the surrounding ground, to avoid changes to flow paths. This gives an effect that is not significant.

Effects from the introduction of pollutants to the aquifer during operation are assessed as having a Low magnitude of change. Pollution prevention measures would be in place to prevent accidental spills from infiltrating, or inappropriate discharges to groundwater. There is potential for increased discharge from the drainage network to the superficial aquifer, through infiltration, but this would be relatively minor compared to the baseline. This gives an effect that is not significant.

Abstractions from Lower Thames Gravels (Sensitivity Low, Medium)

All activities listed above in relation to the aquifer itself

Effects on abstractors from superficial aquifers (including licensed abstractions and private water supplies) are assessed as having a Low magnitude of change. Possible effects on abstractors would be managed where reasonably practicable and appropriate, for instance




through slurry walls to limit dewatering. Where effects could not be avoided, including the complete loss of supply through change in land use, an alternative supply would be provided as set out in Section 21.5. This gives an effect that is not significant.

Discharges into the Lower Thames Gravels (Sensitivity Low)

Activities include the development of sub surface features and basements of airport operational buildings

Existing discharges to the Lower Thames Gravels are assessed as having a Low magnitude of change. In any cases where the current discharge location would no longer useable due to land use changes, the discharge holder would be consulted to find an alternative solution, if required. This would result in an effect that is not significant.

Lakes in connectivity with the Lower Thames Gravel Aquifer (Sensitivity Low for non WFD lakes and Medium for WFD Lakes)

Infilling of lakes Effects on water level regime are assessed as having a High magnitude of change: Lakes including Colnbrook North, Orlitts Lake, Colnbrook West and Swan Lake would be infilled as part of the DCO Project, and Old Slade Lake would be partially infilled. All of these lakes are non-WFD waterbodies (and therefore have assigned a Low sensitivity) which yields a significant effect.


Effects on water level regime (for non-infilled lakes) are assessed as having a Low magnitude of change. As assessed in Appendix 21.2, appropriate design and alignment of subsurface features to replicate the existing groundwater regime would avoid any substantial changes to groundwater flow or levels. Infilling of old river channels would be with similar material to the surrounding ground, to avoid changes to flow paths. Measures would be applied during construction to avoid effects from dewatering. Based on the assumption of connectivity between the lakes and groundwater, effects on lake level could be managed and this would give an effect that is not significant.

Effects on lake water quality (for non-infilled lakes) are assessed as having a Low magnitude of change. Pollution prevention measures would be in place to avoid spills or inappropriate discharges either directly to lakes or via groundwater. Effects from changes to air quality (as assessed for receptor group WE11 (reservoirs)) would have a negligible effect on the lakes. Use of water from the lakes for the thermal strategy could affect water temperature. This gives an effect that is not significant.

21.10.39 Summary of effects relating to construction activities: During the construction

phase, dewatering activities could continue for a number of years. Appropriate measures would be applied to minimise or manage any effects on groundwater or other receptors, resulting in no significant effects.



21.10.40 Summary of effects relating to permanent land use changes and operational activities: Some permanent changes to the groundwater regime would occur as a result of the introduction of sub-surface structures including; basements and tunnels, increased impermeable surfaces, changes to ground levels, and groundwater recharge from the drainage network. Appropriate design would manage the effects on groundwater levels and flows. Only minor changes to groundwater quality would be anticipated. Overall, these would be no significant effects.

21.10.41 Summary of effects on groundwater abstractors: Effects on groundwater would be managed to avoid effects on any water users. If effects on water supply to an abstractor could not be avoided, an alternative supply would be provided (as described in Section 21.5), allowing a conclusion of no significant effects.

21.10.42 Summary of effects on groundwater-fed lakes: Five lakes would be infilled as part of the DCO Project (Old Slade Lake, Colnbrook North, Orlitts Lake, Colnbrook West, Swan Lake), and for these a significant effect is concluded, i.e. loss of the lakes. For the remaining lakes, in connectivity with groundwater, the same conclusions can be applied as for the connected aquifer, i.e. no significant effects.

WE13 Bedrock Aquifers 21.10.43 The assessment of individual effects for bedrock aquifers, with focus on the Chalk

aquifer, is presented in Table 21.32. This includes assessments of effects on groundwater abstractions. The aquifer itself is assigned a medium sensitivity, since it is linked to a WFD waterbody. Abstractions from the aquifer are variously defined as Low and High sensitivity, depending on their status and use, as identified in Appendix 21.6.

Table 21.32: Assessment of individual effects for WE13 Bedrock Aquifers


Chalk aquifer (Sensitivity Medium)

Construction activities relevant to the Lower Thames Gravels receptor include:

1. Borrow pits 2. Stockpiling 3. Dewatering of borrow

pits and other excavations)

Changes to groundwater flow regime are assessed as having a negligible magnitude of change given that the Chalk aquifer is confined beneath the Site. While changes to the head gradient could result in changes within the Chalk, Appendix 21.2 concludes that any such changes would be negligible. This would constitute an effect that is not significant.

Effects from the introduction of pollutants to groundwater are assessed as having a Negligible magnitude of change. Pollution prevention measures would be in place to prevent accidental spills from infiltrating, or inappropriate




4. Landfilling discharges to groundwater. There would be few pathways for any such spills or discharges to reach the Chalk. This would constitute an effect that is not significant.

Effects from the mobilisation of contaminants in groundwater are assessed in Chapter 14.

This receptor extends beneath the full extent of the DCO Project. Activities relevant to the Lower Thames Gravels receptor include:

1. Development of the northwest runway, taxiways and other reprofiled land areas

2. Development of drainage networks, including sub-surface features

3. Development of tunnels and basements

4. Realignment of rivers

5. Thermal energy strategy

Changes to groundwater flow regime are assessed as having a Negligible magnitude of change, as assessed in Appendix 21.2. The Chalk aquifer is confined beneath the Site. There could be an increase in abstraction associated with the DCO Project, but this would be within the licence constraints of existing licences. This gives an effect that is not significant.

Effects from the introduction of pollutants to the Chalk aquifer during operation are assessed as having a Negligible magnitude of change. Pollution prevention measures would be in place to prevent accidental spills from infiltrating, or inappropriate discharges to groundwater. There would be few pathways for any such spills or discharges to reach the Chalk. This gives an effect that is not significant.

Abstractions from the Chalk aquifer (Sensitivity between Low and High)


Effects on abstractors from the Chalk (including licensed abstractions and private water supplies) are assessed as having a Negligible magnitude of change. This gives an effect that is not significant.

21.10.44 Summary of effects relating to construction activities: Changes to the groundwater

regime in the Chalk would be negligible, constituting not significant effects.

21.10.45 Summary of effects relating to permanent land use changes and operational activities: Changes to the groundwater regime in the Chalk would be negligible, constituting not significant effects.



WE14 Local Water Supply Network 21.10.46 The assessment of individual effects on the local water supply network, which is

operated by Affinity Water, is presented in Table 21.33. A high sensitivity has been assigned to the water supply network, on the basis of its regional importance. Further information on water supply to Heathrow can be found in Appendix 20.1: Draft Resource Management Plan, Volume 3.

Table 21.33: Assessment of individual effects for WE14 Local Water Supply Network


Construction activities relevant to the local public supply network:

1. Subsurface works disturbing water supply infrastructure

2. Increased demand for water as a result of construction activities

Effects on the local public water supply network Utility surveys would be undertaken as a part of the pre-construction works and works undertaken to ensure that mains supplies are not disrupted as a part of construction. This could include the re-routing of some water mains away from the development footprint. Construction water demand from the main supply network would be minimised, and water efficiency measures put in place where appropriate. Supply rates and points of supply would be agreed with Affinity Water. Details of any changes to apparatus would be agreed pursuant to protective provisions for Affinity Water’s benefit in the DCO. As a result, it is concluded that the impact on the wider pubic supply network would be negligible, which constitutes a not significant effect.

Operational activities relevant to the local public supply network:

1. Increased demand for water from the operational site

Effects on the local public water supply network As discussed in Appendix 20.1: Draft Resource Management Plan, Volume 3, Affinity Water have stated that they have adequate resources to supply any eventuality although they would like to see good practice adopted with regards to water efficiency. The environmental measures will intially focus on currently available and proven solutions and technologies, specifically in an airport context. This ensures that these measures are not based on speculative approaches that may not meet performance standards or reduce passenger user experience. However, Heathrow will continue to seek to encourage adoption of innovative solutions as part of its overarching goals to embed leading edge water efficiency thinking. The proposed approach to managing water resources in the DCO Project is through the following measures:

1. Using new approaches and technology to eliminate water demand and improve water efficiency for all uses, both potable and non-potable

2. Proactively influence business partners’ development and operations and growth to improve water efficiency

3. Undertaking measures to minimise leakage levels in the new facilities and reduce leakage in existing facilities affected by or supporting the DCO Project and

4. Increase the use of non-potable water supply to meet the demand from all the non-potable end uses (such as WC and urinal flushing, cooling towers, etc).

Details of any protective provisions would be agreed pursuant to protective provisions for Affinity Water’s benefit in the DCO. It is concluded that the effects on the wider pubic supply network would be negligible, which constitutes a not




significant effect.

21.10.47 Summary of effects: Effects on the local public water supply network would be negligible, constituting no significant effects.

WE15 Foul Drainage Infrastructure 21.10.48 The assessment of individual effects on the local foul drainage network operated

by Thames Water, is presented in Table 21.34.

Table 21.34: Assessment of individual effects for WE15 Local Foul Drainage Network


Construction activities relevant to the local foul drainage network: 1. Subsurface works

disturbing foul drainage infrastructure

2. Use of foul drainage infrastructure for construction site discharges

Effects on the local Foul Drainage supply network Utility surveys would be undertaken as a part of the pre-construction works and works undertaken to ensure that the foul drainage network is not disrupted as a part of construction. This could include the re-routing of some sewer mains away from the development footprint. Construction discharge into the foul drainage network would be minimised. Discharge rates and points of discharge would be agreed with Thames Water. It is concluded that the impact on the wider pubic supply network would be negligible, which constitutes a not significant effect.

Operational activities relevant to the foul drainage network: 1. Increased

wastewater from the operational site

2. Use of foul drainage infrastructure for surface water drainage

Effects on the local Foul Drainage supply network As noted in Section 21.3 there have been several meetings with Thames Water and the discharge of the operational site’s foul drainage to sewer has been agreed in principle. Thames Water are currently commissioned to undertake a flow survey of their existing sewerage infrastructure. When this is complete Thames Water will be in a better position to agree additional discharges to their main sewer in the Bath Road. The option of a new waste water treatment works on site is also being considered, the final configuration of which would be agreed with Thames Water. It is concluded that the effect on the foul drainage network will be negligible, which constitutes a not significant effect.

Effects on sewer capacity that could result in increased Sewer flood Risk are addressed under Receptor WE16: Flood Risk Receptors.

21.10.49 Summary of effects: The effects on the local foul drainage network would be negligible, constituting no significant effects.



WE16 Flood Risk Receptors 21.10.50 Table 21.35 summarises the potential sources of flood risk relating to the DCO

Project. These are presented in further detail in Appendix 21.4. Individual flood risk receptors have not been identified at this stage for the PEIR, but are likely to range between High and Very Low sensitivities (based on the criteria presented in Table 21.14).



Table 21.35 Assessment of effects for WE16 flood risk receptors

Detail of Activity Magnitude of Change and Significance conclusions (WE16)

The following construction activities are proposed, as described in more detail for the above receptors (WE1-7, and WE12):


2. Dewatering associated with earthworks activities and excavations

3. Landfilling and infilling of lakes

4. Earthworks stockpiling

5. Haul route crossing and in proximity to rivers

Construction activities including earthworks activities, temporary areas of hardstanding, and management of drainage and dewatering could result in temporary increases to flood risk. This could occur in relation to surface water runoff, alteration of flow paths (‘pluvial flood risk’), increased river flows, increased discharges to sewers (e.g from leachate treatment plants required for landfill excavation) or increased groundwater levels. These risks are considered in Appendix 21.4 and Appendix 21.5. The measures set out in Section 21.5, and incorporated in to the draft CoCP, would manage effects associated with those activities. Following the implementation of these measures, the magnitude of effect on flood risk during the construction phase would be negligible and not significant.

The following permanent activities are proposed as described in more detail for the above receptors (WE1- 7, and WE12):


2. Development of impermeable hardstanding and buildings

3. Development of the third run runway, taxiways, reprofiled land areas western apron and part of terminals, the southern parkway airport supporting facilities

4. Diversion of the A4, M25 and associated junctions

5. Provision of green infrastructure corridors and biodiversity links, through the enhancement of the riparian zone

6. Diversions of rivers, creation of new river channels and passage through the CRC and/or other crossings

7. Development of new drainage networks and potential discharge

Fluvial flood risk

The DCO Project would result in the loss of floodplain along the River Colne, Wraysbury River and Colne Brook. As identified in Section 21.5, new up-stream flood storage areas would be completed prior to the removal of floodplain. This would be implemented through a phased approach to offset losses in floodplain as construction progressed.

The effects of these changes have been assessed in Annex A of Appendix 21.4. The assessment provides proof of the emerging concept for managing fluvial flood risk as part of the DCO Project, largely demonstrating the effectiveness of the proposed compensatory flood storage, the new river channels and the flood storage provided within the new river corridors. The FRA concludes that with continued refinement to the DCO Project design and subsequent updates to the underlying hydraulic model, which will be reflected in the Application FRA, the DCO Project will not increase flood risk to people or property. On this basis, a negligible magnitude of effect that is not significant has been predicted in relation to permanent land use changes.

Pluvial flood risk

There is potential for the DCO Project to result in an increase in pluvial flooding (changes to surface water flow pathways running on to the Site) as a result of changes the profile of the land surface, and changes to surface water pathways around new infrastructure. Appendix 21.4 found little potential pluvial flood risk and concluded that any such risks would be managed




of airfield and off-airfield runoff

8. Development of subsurface features

9. Development of tunnels and basements

through appropriate design and layout. On this basis, a negligible magnitude of effect that is not significant is concluded.

Groundwater flood risk

As set out in relation to WE12 (Lower Thames Gravels), groundwater levels could be affected (either increasing or decreasing) by the DCO Project. As set out in Appendix 21.4, higher groundwater levels could potentially manifest into flooding at the surface. This would be managed through appropriate design and alignment of subsurface features, with active groundwater management if required, as set out in Section 21.5. The requirement for appropriate drainage strategies, considering discharges to ground, is considered in Appendix 21.5. Taking these measures in to account, the magnitude of change from groundwater flood risk sources would be negligible and not significant during the operational phase.

Flood risk from infrastructure

Appendix 21.4 has considered risks to flooding from artificial sources including reservoirs, new raised water bodies, canal and sewers. In the case of reservoirs, the FRA concludes there would be no effect on flood risk as a result of the DCO Project.

In the case of canals, the FRA identifies potential sources of flood risk to the DCO Project associated with the Grand Union Canal, concluding a very low risk. The DCO Project would have no influence on canal flood risk. Therefore, it can be concluded that there would be a negligible magnitude of change that it not significant.

As part of the DCO Project new raised water bodies would be introduced on-site to attenuate and store flood flows and surface water, including the Flood Storage Areas, surface water drainage attenuation basins, flow lagoons and water treatment areas (as outlined in Section 21.5). All storage areas would be designed to appropriate return periods. As a result, there would be a negligible magnitude of change on flooding from new raised water bodies, and there would be no significant effects.

Sewer flood risk

Discharges from the built development towards the new site drainage system would interact with the current drainage system and hence with the Thames Water sewer network. This could cause changes to the risk of sewer flooding as a result of changes in rates and volumes of surface water runoff into the existing sewer systems. However, drainage strategies would be




developed to demonstrate sufficient attenuation prior to discharge to the sewer system, and all discharges would be agreed with Thames Water. In areas where redevelopment means that existing sewerage infrastructure would be removed or replaced, the risk is removed, or could be reduced through the removal of capacity or structural issues associated with the existing networks. These requirements are considered in Appendix 21.5 and Appendix 21.4, Annex C, from which it can be concluded that there would be a negligible magnitude of change and no significant effects on sewer flood risk.

Surface water flood risk

Appendix 21.5 sets out a number of guiding principles with respect to surface water management infrastructure, which are also summarised in Section 21.5. Additionally, a number of Drainage Strategies would be produced for key infrastructure elements which would set out in more detail those surface water management measures that would be incorporated within the emerging designs in order to achieve compliance. On this basis, it is concluded that sufficient information will be provided in the Application to ensure that there would be no increase in flood risk as a result of surface water arising from the DCO Project. This gives a negligible magnitude of change and constitutes a not significant effect.

21.10.51 Summary of effects of construction activities: Changes to flood risk from construction activities are assessed as having a Negligible magnitude of change on the basis of compliance with permitting and the incorporation of appropriate drainage strategies and taking account of the measures set out in Section 21.5 and the draft CoCP. This results in not significant effects.

21.10.52 Summary of effects of permanent land use changes and operational activities on flood risk: Table 21.35 sets out the effects that could occur as a result of permanent land use changes, of which the most pertinent include; diverting rivers, provision of new flood storage, changes to the groundwater regime and new drainage systems. Taking account of the measures embedded in to the DCO Project design, and as set out in more detail in Appendix 21.4 and Appendix 21.5, it is concluded that there would be a negligible magnitude of change on flood risk, constituting a not significant effect.



21.11 Preliminary assessment of significance

Overview 21.11.1 Based on the current level of available information, the preliminary assessment

presented in this chapter has identified significant effects remaining once all environmental measures and compensation have been taken into consideration. These effects are presented in Section 21.10. In summary, the significant effects that have been identified in the PEIR assessment are:

10. Significant Negative effects on the Wraysbury River, River Colne, Longford River and Duke of Northumberland’s River as a result of the river diversions that would be required, including passing the rivers beneath the runway in the CRC. Appendix 21.3 concludes a risk of deterioration of water body status for these water bodies, based on the level of evidence and certainty currently available about the design and effects of the diversions and CRC. Further evidence will be gathered regarding the effectiveness of measures prior to the ES, which may allow the assessment of effects to be revised

11. Significant Negative effects on five lakes associated with WE12 including Old Slade Lake, Colnbrook North, Orlitts Lake, Colnbrook West Lake and Swan Lake, which will be infilled, resulting in complete loss of the water features. The effects on biodiversity from this loss are assessed in Chapter 8.

21.11.2 All other effects are concluded to be not significant, on the basis of the measures discussed in Section 21.5 being implemented, or require further work before conclusions can be drawn. Further work will be carried out for the ES to design measures and assess their effectiveness.

Table 21.36: Summary of significant positive and negative effects

Receptor and effect Sensitivity Magnitude of change Significance Summary rationale

WE3 Wraysbury River

Phase 1, 2 and 3: Permanent land use changes and operational activities

Changes to channel geomorphology

Medium Negative High Significant Refer to Table 21.23

WE4 River Colne



Receptor and effect Sensitivity Magnitude of change Significance Summary rationale



Medium Negative High

Significant

Refer to Table 21.24

WE5 Duke of Northumberland’s River and Longford River



Negative High

Significant


WE10 Upper River Colne

Phase 1, 2 and 3: All activities

Ecological connectivity: assessed in Chapter 8: Biodiversity

- - - Refer to Table 21.29

WE12 Lakes in connectivity with the Lower Thames Gravel Aquifer

Phase 1, 2 and 3: Construction activities

Effects of infilling or partial infilling of lakes on lake water level regime

Low

Negative High

Significant


21.12 Assessment of cumulative effects

Overview 21.12.1 The cumulative effects assessment (CEA) presented in this section reflects

Stage 3 in the CEA process set out in Section 5.7 of Chapter 5.

21.12.2 The assessment of cumulative Water Environment effects is based on professional judgement, taking into account the levels of significance identified in the primary or ‘stand-alone’ assessment, and identifying whether effects could be different when ‘other development’ are considered.



21.12.3 A CEA is only undertaken for those receptors that are likely to experience effects greater than negligible in the primary assessment. This is because such effects are unlikely to result in any discernible positive or negative effects on a receptor. It is considered that they are extremely unlikely to result in a significant cumulative effect, even if multiple effects of a similar significance are considered.

21.12.4 The following sections present the assessment of the cumulative effects of the DCO Project and ‘other developments’ on the Water Environment. The ‘other developments’ to be considered in the CEA for the PEIR are those on the ‘assessment list’ provided in Chapter 5, Section 5.7. In keeping with the assessment presented in Section 21.10, the cumulative assessment covers all Phases of the development together, as relevant to the type of effect.

21.12.5 There are three Water Environment ZOIs for the CEA. These include a Surface Water ZOI and Groundwater ZOI, which were defined based on the LSA and GWSA, and a Sewer ZOI (a 1km buffer from the DCO order limits to take into consideration cumulative effects on drainage). These are shown in Figure 5.1, Volume 2. All developments on the assessment list falling outside these different Water Environment ZOIs (e.g. Groundwater, Surface Water and Sewer) are not considered in relation to those respective effects. The core and optional developments within the assessment list in Section 5.7 excluded from each relevant assessment include:

1. O109 Land at Harmondsworth, Holloway Close (outside the Surface Water ZOI)

2. O591 Rectory Lane, Cranford Lane (outside the Surface Water ZOI)

3. O601 Queen Mary Reservoir and Land West of Queen Mary Reservoir (outside the Surface Water ZOI and Sewer ZOI)

4. O608 Cemex Datchet Quarry, Land At Riding Court Farm (outside the Surface Water ZOI and Sewer ZOI)

5. O615 Southall Gas Works (outside the Surface Water ZOI and Sewer ZOI)

6. O732 Queen Mary Reservoir and Land West of Queen Mary Reservoir (outside the Surface Water ZOI)

7. O745 Land at Milton Park Farm (outside the Surface Water ZOI and Sewer ZOI)

8. O750 Land at Watersplash Farm (outside the Sewer ZOI)

9. O751 Slough Heat and Power Station (outside the Groundwater ZOI, Surface Water ZOI and Sewer ZOI)



10. O811 High Speed 2 (London - West Midlands) (outside the Groundwater ZOI and Sewer ZOI)

11. A2 T5+ (T5A) (outside the Surface Water ZOI)

12. A3 T5+ (T5B) (outside the Surface Water ZOI)

13. A4 T5+ (T5C) (outside the Surface Water ZOI)

14. A5 Perry Oaks Fuel Farm (outside the Surface Water ZOI)

21.12.6 Of the remaining core and optional developments contained within the assessment list in Section 5.7, the following are considered to be potentially of relevance to the Water Environment, and have been brought forward for assessment in the CEA:

1. O109 Land at Harmondsworth, Holloway Close (in relation to drainage and groundwater effects only)

2. O591 Rectory Lane, Cranford Lane (in relation to drainage and groundwater effects only)

3. O595 Stanwell Recycling, Stanwell Quarry

4. O596 Stanwell Recycling, Stanwell Quarry

5. O601 Queen Mary Reservoir and Land West of Queen Mary Reservoir (in relation to groundwater effects only)

6. O608 Cemex Datchet Quarry, Land At Riding Court Farm (in relation to groundwater effects only)

7. O609 Land East of Horton Road

8. O615 Southall Gas Works (in relation to groundwater effects only)

9. O732 Queen Mary Reservoir and Land West of Queen Mary Reservoir (in relation to drainage and groundwater effects only)

10. O745 Land at Milton Park Farm (in relation to groundwater effects only)

11. O750 Land at Watersplash Farm (in relation to groundwater effects only)

12. O810 M4 Junctions 3 to 12 Smart Motorway

13. O811 High Speed 2 (London - West Midlands) (in relation to surface water effects only)

14. O812 Western Rail Link to Heathrow

15. O813 Southampton to London Pipeline Project



16. A2 T5+ (T5A) (in relation to drainage and groundwater effects only)

17. A3 T5+ (T5B) (in relation to drainage and groundwater effects only)

18. A4 T5+ (T5C) (in relation to drainage and groundwater effects only)

19. A5 Perry Oaks Fuel Farm (in relation to drainage and groundwater effects only).

21.12.7 These developments have been considered in the assessment presented in Table 21.37



Table 21.37 Phase 1 CEA of DCO Project effects, together with ‘other developments’ unrelated to the DCO Project

Receptor / effect DCO Project effects Assessment of cumulative effect (DCO Project effects together with ‘other developments’)

Permanent changes to channel geomorphology at the following: WE1 (Horton Brook) WE2 (Colne Brook) WE3 (Wraysbury River) WE4 (River Colne) WE5 (Duke of Northumberland’s River and Longford River)

Minor negative (not significant) effect on WE1 and WE2. Major negative (significant) effect on WE3, WE4 (River Colne only) and WE5, due to channel diversions and the CRC

The following developments have the potential to affect the channel geomorphology of the listed receptors: O812 Western Rail Link to Heathrow: Includes major bridges for the rail intersection bridge, and a number of new culverts under the Great Western Main Line, within the Colne catchment. Horton Brook will be diverted to the west on the north side of the railway, with new culverts required. The scheme will include an extended bridge over the Horton Brook. It is expected that good practice and mitigation will be adopted to reduce any effects to not significant. The Western Rail Link to Heathrow project is not expected to result in a cumulative effect with the DCO Project, and the conclusions of the DCO Project assessment on the Horton Brook remain unchanged at minor negative (not significant). O811 High Speed 2 (London - West Midlands): There is the potential for short term effects relating to the construction of a viaduct across the Colne Valley, over the Newyears Green Bourne and the River Colne, along with their associated floodplains. A temporary jetty will be constructed which may require temporarily widening the River Colne channel, or the use of a temporary by-pass. Permanent effects include re-alignment of the River Colne (170m in length around the viaduct piers), and Newyears Green Bourne. The ES for the development concludes that good practice and mitigation will be sufficient to reduce any effects on relevant rivers to be not significant. However, the effects from the DCO Project (which are significant negative for the River Colne) require additional environmental measures elsewhere in the catchment (see Section 21.13), and there could potentially be overlap in the requirements for and/or locations of additional mitigation between the two projects. This creates the potential for a greater cumulative effect on the River Colne compared to the DCO Project alone and leads to a conclusion of major negative (significant) cumulative effects, which will be considered further in the ES. All other developments located within the Water Environment ZOI for the CEA do not have the potential to result in any cumulative effects on channel geomorphology.

Effects of infilling the Major negative All other developments located within the Water Environment ZOI for the CEA do not have the




following lakes: WE12 (connected lakes)

(significant) effect on lakes due to infilling

potential to result in any cumulative effects on lake water levels. Therefore, the conclusions of the DCO Project assessment on lakes in WE12 remain unchanged at major negative (significant).

All other not significant, non-negligible effects Effects of construction activities on:

1. river flow (WE1-5)

2. increased sediment loading to surface water (WE1-5, WE7)

3. introduction of pollutants to surface water and groundwater, including connected lakes (WE12)

Effects of land use changes on:

1. river flow (WE2-7)

2. groundwater regime, including connected lakes

Not significant (minor negative) effects

Other developments within the Water Environment ZOI have the potential to have a range of effects on the water environment, including those effects listed here in relation to the DCO Project. Such activities are generally managed through good practice, permitting, the production of a CoCP or other similar documents and, where relevant, other environmental measures. As a result, it is generally possible to confirm no significant effects on the water environment. As such, no significant cumulative effects are anticipated on any receptor from these types of effects. The conclusions of the DCO Project assessment (minor negative, not significant effects) therefore remain unchanged when other developments are considered.




(WE12) 3. recharge to

groundwater (WE12)

4. groundwater flow regime, including connected lakes (WE12)

5. introduction of pollutants to groundwater, including connected lakes (WE12)



21.13 Consideration of additional environmental measures or compensation

Overview 21.13.1 The assessment set out has concluded that it will be necessary to implement

some further additional environmental measures.

21.13.2 Further measures will be necessary in order to address the significant hydromorphological effects on receptors WE3 (Wraysbury River), WE4 (River Colne) and WE5 (Duke of Northumberland’s River and Longford River). These additional measures will be developed and included within the ES and the application for development consent. These measures will involve a programme of hydromorphological improvements in the wider catchment (focussed on the LSA but potentially extending in to the WSA). These are being identified through the iterative process of design and assessment within Appendix 21.3, and will be in addition to those outlined in Section 21.5 and assessed in Section 21.10. The next steps for the ES in development of an appropriate set of measures are:

1. Identification of a short list of locations for consultation as a part of the Airport Expansion Consultation 2

2. Further development of monitoring methodologies intended to help capture changes in WFD water body condition.

21.13.3 As identified in Section 21.5, the CRC design is still being developed, and will be of an innovative design. Uncertainties around the effectiveness of the design currently result in a conclusion of significant effect for the rivers that would pass through the CRC (WE3 Wraysbury River, WE4 River Colne and WE5 Duke of Northumberland’s River and Longford River). As part of the CRC design there will be scope to apply appropriate adaptive environmental measures, with regards to the lighting intensities and durations, in the event that the initial functioning is not as expected.

21.13.4 A programme of monitoring work will be carried out at the pre-construction, construction and operational phases to allow a comparison against the baseline conditions and to allow any changes from the baseline to be identified and duly investigated. Monitoring will include surface water quality monitoring, continuous surface water flow gauging, groundwater quality monitoring, groundwater level monitoring and geomorphology surveys. It will provide consistency with the baseline monitoring work, but the locations and frequency of monitoring may vary to ensure that all relevant activities can be appropriately monitored.



Table 21.38: Summary of additional environmental measures or compensation and how these influence the water environment assessment

Receptor Changes and effects Additional measures or compensation and influence on assessment

WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland and Longford River

Changes in geomorphology due to the loss of river length and uncertainties around the effectiveness of the CRC could lead to a permanent reduction in hydromorphology classification

River restoration works will be undertaken to offset WFD effects. These would be applied to reaches of the rivers that will be diverted or altered as part of the scheme, as well as works elsewhere in the catchment. A programme of monitoring will be undertaken to monitor the effectiveness of WFD restoration. A programme of maintenance will be implemented to ensure that the embedded mitigation continues to function as intended.

All river receptors in the LSA (WE1-WE7); groundwater (WE11, WE12)

The range of effects identified in Section 21.5 and discussed in Section 21.10.

A programme of monitoring will be devised, agreed and implemented during the pre-construction, construction and operational phases. This will cover the surface water and groundwater environment and will be used to identify any changes from the baseline.

WE3 Wraysbury River WE4 River Colne WE5 Duke of Northumberland and Longford River

The potential for adaptive mitigation will be incorporated into the CRC lighting design, with respect to the intensity and duration of lighting.



21.14 Next Steps

Overview 21.14.1 While this PEIR chapter has presented an assessment of the likely effects of the

DCO Project, the assessment at ES will develop these key themes with further quantitative and qualitative detail, supported by more site-specific information and design detail. The following sections present a summary of the work which will be carried out to support the ES, with reference to the next steps already identified in Sections 21.5 and Section 21.7.

Baseline 21.14.2 Table 21.39 presents an overview of further baseline work that will be undertaken

to support the ES assessment.

Table 21.39:Further baseline work to support the ES

Technical Area Nature of work

Surface Water Quality Baseline Baseline conditions will be characterised through review of available baseline surface water quality monitoring data, including data collected for the DCO Project, plus ongoing routine monitoring data from Heathrow and the EA.

Groundwater Quality Baseline Baseline information will be developed from the simple screening approach at PEIR (as presented in Chapter 14: Land quality) into a more comprehensive baseline using site data collected in 2019.

Groundwater baseline The baseline will be updated with site data collected in 2019. The baseline groundwater model will be updated using the available data to update the model structure (layer elevations); improve representation of the connection between gravels and lakes; improve baseflow representation; undertake sensitivity analysis; calibrate/validate against observed groundwater levels.

Hydromorphology Baseline Carry out further baseline surveys for river reaches not yet surveyed (including small watercourses), as land access permits.

Water Framework Directive The baseline WFD information in the ES will be updated to utilise interim WFD classifications and objectives. These updates will be based on the latest round of EA monitoring, and will be supplemented by data collected as part of the DCO Project. The total flow baseline will be developed, building on the hydrology assessments carried out for PEIR and utilising available site data.

Drainage baseline The baseline will be further developed by: 1. Updating the drainage baseline by refining greenfield and

baseline run-off rates and volumes (e.g. accounting for landfill) for sub-catchments accounting for specific development



Technical Area Nature of work

footprints 2. Seeking to improve the understanding of the hydrological regime

for the existing airfield catchments 3. Collecting additional existing pipe network information from

statutory undertakers to improve the understanding of existing drainage systems.

Flood Risk Baseline The flood risk baseline will be further developed by including a 0.1% AP event baseline for fluvial flood risk and adding to the understanding of the groundwater and drainage baselines as appropriate.

Assessment 21.14.3 Further assessment that will be undertaken for the ES has been set out in Table

21.18 in Section 21.7. This includes, as relevant to each assessment, taking account of further development and refinement of the DCO Project design, incorporating improved baseline information, and developing further quantitative modelling scenarios.

Consultation and engagement 21.14.4 Further consultation and engagement will be undertaken to inform the water

environment assessment to be presented within the ES. This is expected to cover the following technical areas:

1. Permitting approaches, including the use of protective provisions

2. Monitoring approaches

3. Review of technical assessments and associated models

4. Designs of relevant infrastructure (for example drainage, flood storage areas, new river channels, CRC, thermal strategy, non-potable strategy, road crossings)

5. Construction approaches (including water management in construction, approaches to river diversions, floodplain replacement).

21.14.5 These will be discussed with relevant consultees as appropriate for each topic, including the Environment Agency, Natural England, HSPG, Royal Parks, FORCE, Affinity Water, Highways England and Thames Water.

Environmental measures 21.14.6 Table 21.40 provides and overview of further work that will be carried out to

develop environmental measures prior to the ES, building on the information provided in Section 21.5.



Table 21.40: Further development of environmental measures

Embedded Environmental Measure Summary of work to support ES

River Diversion Design

Refinement of river diversion design to develop details on in channel and river corridor form and function. This will include details on the engineering design of the new corridors, substrate and liners as well as environmental design.

Design of the CRC

More details on the engineering approaches, including channel planform, substrate type, lid and lighting design. The lighting design will be informed by preliminary results from in-catchment trials, which will continue through the DCO determination process to inform detailed design.

Design of Road Crossings of Rivers Design of crossings to illustrate how flood conveyance and ecological connectivity will be integrated into new crossings.

Provision and design of Compensatory Flood Storage

Illustrative designs of flood storage areas and associated structures, including demonstration of how green infrastructure/biodiversity offsetting requirements can be integrated.

Management of On-airfield surface water drainage

Preliminary drainage strategies will support the DIA for the ES, setting out proposed approaches to managing runoff. The DIA will identify discharges rates and volumes and the river reaches in which discharges will be located. Management of Off-airfield surface

water drainage

Management of subsurface flow pathways

Sub-surface designs will incorporate management of groundwater levels and flows where appropriate and will be represented in the groundwater model.

Dewatering Activities

Outline design and phased mapping of water attenuation and treatment locations. Identify locations of leachate treatment plants (where required for dewatering) and rates and quality of discharges to sewer or surface waters.

Management of Surface water run-off during construction

Outline design and phased mapping of water attenuation locations. Identify rates and quality of discharges to sewer or surface waters.

Construction of new river channels

Heathrow will engage with the Environment Agency on the method of the diversion of rivers prior to the Application, with the objective of ensuring compliance with the requirements of the Water Framework Directive.

Backfilling of old river channels Further assessment of the potential influence of old river channels on groundwater flow, and appropriate approaches to backfilling the channels.

Construction within the floodplain

Construction phasing will be developed to provide adequate upstream flood storage prior to removal of any floodplain. Fluvial flood risk will be considered during development of the haul road network.



Embedded Environmental Measure Summary of work to support ES

Provision of Alternative Water Supplies

Further assessment will be carried out of abstractions identified as potentially at risk from the DCO Project, including potential requirements for additional environmental measures.

Documents

Heathrow Expansion PRELIMINARY ENVIRONMENT INFORMATION ... · CONTENTS 21. Water Environment 21.1 21.1 Introduction 21.1 21.2 Relevant legislation, policy and other important and