Recommendations to Update Non-Statutory Technical

Recommendations to Update

Non-Statutory Technical

Standards for Sustainable

Drainage Systems (SuDS)

Final Report

February 2021

© Crown copyright 2021

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Contents

Glossary of key terms................................................................................................................ 8

Acronyms ................................................................................................................................... 9

Summary .................................................................................................................................. 11

1 Introduction ....................................................................................................................... 13

PART 1 – PROJECT REPORT

2 Project description............................................................................................................ 14

2.1 Aim and objectives..................................................................................................... 14

2.2 Scope of work ............................................................................................................ 15

3 Overview of technical approach ...................................................................................... 16

3.1 Task 1 – Clarify the current situation ........................................................................ 18

3.2 Task 2 – Evaluate hydraulic design methods........................................................... 21

3.3 Task 3 – Define the delivery barriers and enablers ................................................. 23

3.4 Task 4 – Move forward with multiple benefit SuDS ................................................. 26

4 Research findings ............................................................................................................ 29

4.1 Summary of findings .................................................................................................. 29

4.2 Task 1 research questions ........................................................................................ 33



5 Recommendations ........................................................................................................... 66

5.1 Updating the standards ............................................................................................. 66

5.2 Alignment with other national policy and guidance .................................................. 68

5.3 Further research and development........................................................................... 70

PART 2 – UPDATED STANDARDS

6 Application of the standards ............................................................................................ 72

6.1 Overview .................................................................................................................... 72

6.2 Use of ‘a SuDS Approach’......................................................................................... 73

6.3 Early and integrated design ...................................................................................... 73

6.4 Embedding SuDS within the development planning process.................................. 74

7 Standard 1 – Runoff destinations .................................................................................... 74

7.1 Objectives................................................................................................................... 74

7.2 Evidence base............................................................................................................ 75

7.3 Proposed standard .................................................................................................... 76

7.4 Accompanying clarifications ...................................................................................... 77

7.5 Additional recommendations ..................................................................................... 79

8 Standard 2 – Everyday rainfall ........................................................................................ 79

8.1 Objectives................................................................................................................... 79

8.2 Evidence base............................................................................................................ 80




9 Standard 3 – Extreme rainfall .......................................................................................... 83

9.1 Objectives................................................................................................................... 83

9.2 Evidence base............................................................................................................ 83




10 Standard 4 – Water quality ........................................................................................... 89

10.1 Objectives ............................................................................................................... 89

10.2 Evidence base ........................................................................................................ 89

10.3 Proposed standard ................................................................................................. 91

10.4 Accompanying clarifications .................................................................................. 91

10.5 Additional recommendations ................................................................................. 93

11 Standard 5 – Amenity.................................................................................................... 93

11.1 Objectives ............................................................................................................... 93

11.2 Evidence base ........................................................................................................ 94

11.3 Proposed standard ................................................................................................. 95

11.4 Accompanying clarifications .................................................................................. 95

11.5 Additional recommendations ................................................................................. 96

12 Standard 6 – Biodiversity .............................................................................................. 97

12.1 Objectives ............................................................................................................... 97

12.2 Evidence base ........................................................................................................ 98

12.3 Proposed standard ............................................................................................... 100

12.4 Accompanying clarifications ................................................................................ 100

12.5 Additional recommendations ............................................................................... 101

13 References .................................................................................................................. 102

Figures

Figure 1 Relationship between project tasks, research questions and stakeholder

engagement ............................................................................................................................. 16

Figure 2 Project timeline ......................................................................................................... 28

Figure 3 The proportion of survey respondents requiring or delivering various design

criteria....................................................................................................................................... 34

Figure 4 Number of sites including different SuDS component types from a survey of 18

planning applications (UKWIR, 2021) .................................................................................... 35

Figure 5 The specific issues considered by survey respondents to pose a significant

influence on drainage design .................................................................................................. 36

Figure 6 The proportion of approvers requiring the use of specific hydraulic criteria and the

proportion of practitioners using those criteria for over 50% of their applications ............... 37

Figure 7 Survey of adopted flow rate criteria for 18 planning applications for greenfield and

previously developed sites (UKWIR, 2021) ........................................................................... 37

Figure 8 The proportion of approvers requiring the use of specific runoff estimation

methods and the proportion of practitioners using those methods for over 50% of their

applications (for both greenfield and previously developed sites) ........................................ 38

Figure 9 The proportion of approvers requiring the use of specific rainfall models and the

proportion of practitioners using those models for over 50% of their applications (for both

greenfield and previously developed sites) ............................................................................ 38

Figure 10 The perceived key influences on inconsistent estimation of runoff rates and

volumes .................................................................................................................................... 39

Figure 11 The perceived compliance of planning applications with local policy and

guidance requirements............................................................................................................ 40

Figure 12 The perceived influence of policies/guidance on multiple benefit SuDS design

outcomes.................................................................................................................................. 41

Figure 13 The perceived influences on the delivery of multiple benefit SuDS..................... 42

Figure 14 Constraints to the delivery of multiple benefit SuDS, as identified by approvers 43

Figure 15 Constraints to the delivery of multiple benefit SuDS, as identified by practitioners

.................................................................................................................................................. 43

Figure 16 Constraints to the delivery of multiple benefit SuDS, as identified by other

stakeholders............................................................................................................................. 44

Figure 17 Peak allowable discharge rate for a reviewed range of planning applications

(UKWIR, 2021) ........................................................................................................................ 47

Figure 18 Attenuation storage volume provided on a number of surveyed planning

application sites (UKWIR, 2021)............................................................................................. 48

Figure 19 Ranges of estimated discharge rates for 5 case study site for relatively high

permeability soils (Soil type 2) and relatively poor permeability soils (Soil type 4) .............. 49

Figure 20 Previously developed runoff rates (reduced by 50%) compared to greenfield

peak runoff rates estimated using the IH124 method............................................................ 50

Figure 21 Greenfield runoff volume estimates from the site model testing .......................... 51

Tables

Table 1 Research questions ................................................................................................... 17

Table 2 Additional evidence of constraints to the delivery of multiple benefit SuDS ........... 44

Table 3 Site areas for which the minimum flow control size is relevant and the equivalent

rate of discharge ...................................................................................................................... 53

Table 4 Summary of feedback from Welsh stakeholders on individual standards .............. 58

Table 5 Where reference is made to SuDS as a delivery mechanism for wider government

policy and regulatory objectives.............................................................................................. 59

Table 6 Planning legislative enablers and barriers................................................................ 61

Table 7 Other legislative enablers and barriers ..................................................................... 61

Table 8 Policy and guidance drivers/enablers and barriers .................................................. 63

Table 9 Resources enablers and barriers .............................................................................. 65

8

Glossary of key terms

Approving body is the organisation responsible for approving the surface water drainage

arrangements for the site. In England this is the local planning authority, as advised by the

lead local flood authority. In Wales this is the SuDS Approving Body (SAB).

Attenuation storage volume is the volume in which runoff is stored when the inflow to the

storage is greater than the controlled outflow.

Everyday rainfall describes rainfall events with a total depth of up to 5 mm (i.e. the

smallest rainfall events that happen very often).

Extreme rainfall describes rainfall events with a return period of 1 in 1 year (i.e. an annual

probability of exceedance of 100%) or greater (up to and including a 1 in 100 years return

period).

Greenfield runoff rate is the rate of surface water runoff expected from a site before

development.

Greenfield runoff volume is the volume of surface water runoff expected from a site

before development.

Interception is the retention on site of the first 5 mm of the majority of rainfall events

through the year.

Minimum flow control size is the smallest permitted flow control size specified to

manage risks of blockage associated with small orifices.

Multiple benefit SuDS are SuDS designed to deliver more than one of the four types of

SuDS benefits: water quantity, water quality, amenity and biodiversity.

Peak allowable discharge rate is the largest permitted discharge rate from a site after

development.

Receiving water or sewer refers to the waterbody downstream of the development site

that receives the controlled discharge from the site. A development site might have more

than one receiving water or sewer.

Soil type is the commonly used term for the Winter Rain Acceptance Potential of different

surface soils, as used by the Flood Studies Report (NERC, 1975).

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Acronyms

AMP Asset Management Plan

AQMAs Air Quality Management Area

ASA Association of SuDS Authorities

B£ST Benefits Estimation Tool

BBBBC Building Better, Building Beautiful Commission

BGI Blue Green Infrastructure

BNG Biodiversity Net Gain

CCC Committee on Climate Change

CIC Construction Industry Council

CIEEM Chartered Institute of Ecology and Environmental Management

CIHT Chartered Institute of Highways and Transportation

CIRIA Construction Industry Research and Information Association

CIWEM Chartered Institution of Water and Environmental Management

CSO Combined Sewer Overflow

DCG Design and Construction Guidance

DWMP Drainage and Wastewater Management Plan

ENG Environmental Net Gain

ETF European Training Foundation

FEH Flood Estimation Handbook

FSR Flood Studies Report

FWMA Flood and Water Management Act (2010)

GI Green Infrastructure

HA Highway Authority

HBF Home Builders Federation

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ICE Institute of Civil Engineers

IEMA Institute of Environmental Management and Assessment

IH124 Institute of Hydrology report 124

LASOO Local Authority SuDS Officer Organisation

LI Landscape Institute

LLFA Lead Local Flood Authority

LPA Local Planning Authority

MHCLG Ministry of Housing, Communities and Local Government

NDG National Design Guide

NE LLFAs North East Lead Local Flood Authorities

NERC Natural Environment Research Council

NPPF National Planning Policy Framework

NSTS Non-Statutory Technical Standards

POS Public Open Space

PPG Planning Practice Guidance

Qbar Mean annual flood

Qmed Median annual maximum flood

RBMP River Basin Management Plan

ReFH2 Revitalised Flood Hydrograph method 2

RQ Research Question

SIA Simple Index Approach

SPD Supplementary Planning Document

SuDS Sustainable Drainage System

UKWIR UK Water Industry Research

WaSC Water and Sewerage Company

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Summary

The aim of this research project is to report whether updating the current Non-Statutory

Technical Standards (NSTS) (Defra, 2015a) could help provide for multiple benefit (also

referred to as multi-functional) sustainable drainage systems (SuDS), and if so, what those

updates should be.

The research included the following:

• An investigation into the understanding, interpretation and application of the current

NSTS. This included a review of existing literature and an online survey with

stakeholders.

• An evaluation of current hydraulic design methods and investigation into how the

current NSTS could be updated to improve consistency and effectiveness of

delivery of the water quantity standards.

• A review of current and evolving drivers, barriers and enablers for delivering

multiple benefit SuDS. This included a review of the implementation of Schedule 3

of the Flood and Water Management Act 2010 in Wales.

Based on the findings from this research, recommendations have been made to replace

S1 to S6 in the current standards with a new suite of six standards. Although it is

recognised by stakeholders that S7 to S14 in the current NSTS would also benefit from a

review, this was outside the scope of this project.

The new standards cover the following:

• Standard 1: Runoff destinations

• Standard 2: Everyday rainfall

• Standard 3: Extreme rainfall

• Standard 4: Water quality

• Standard 5: Amenity

• Standard 6: Biodiversity

Each standard is accompanied by a set of clarifications that provide guidelines to support

the interpretation, delivery and evaluation of the standard. In addition to the standard and

clarifications, it is also recommended that three key principles are applied when planning

and designing schemes to meet the new standards. These are:

1. Use of ‘a SuDS approach’

2. Early and integrated design

3. Embedding SuDS within the development planning process.

12

Draft versions of the standards and clarifications were shared with stakeholders via a

focus group workshop and an online survey. The response received was positive and the

feedback was used in preparation of the final versions, as presented in this report.

The successful implementation of these new standards will require:

• Appropriate supporting guidance and tools (including new metrics for e.g. amenity)

• Alignment with and referencing within other national policy and guidance to

maximise outcome value;

• Processes that facilitate the design, approval and adoption of integrated and

multiple benefit SuDS that meet the new suite of standards; and

• Increased awareness and skill development of those designing and approving

SuDS.

The research approach and findings, plus the resultant recommendations from the

Contractor are summarised in Part 1 of this report. The recommended updates to the

NSTS are provided in Part 2 of this report.

This report is accompanied by eight separate annexes that present in detail the findings

from the research and the evidence base for the recommendations.

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1 Introduction

This document is the Final Report for the research project Recommendations to Update

Non-Statutory Technical Standards for Sustainable Drainage Systems (SuDS).

This report has two parts:

• Part 1 – Project Report – This describes the objectives of the project, the technical

approach, the research findings and recommendations.

• Part 2 – Updated Standards – This presents six new standards that are proposed to

replace the existing standards 1 to 6 in the current Non-Statutory Technical

Standards (NSTS) for SuDS (Defra, 2015a).

This report is also accompanied by eight separate annexes, which are referenced

throughout this report. These provide the evidence base for the recommendations for

updating the standards, these being:

• Annex A – Review of existing evidence and drivers for multiple benefit SuDS

• Annex B – Review of criteria and methods for delivering hydraulic standards

• Annex C – Review of the application of the current NSTS (online survey 1)

• Annex D – Review of the draft updated standards (online survey 2)

• Annex E – Feedback from focus groups

• Annex F – Review of the implementation of Schedule 3 in Wales

• Annex G – Case studies

• Annex H – Review of the Planning White Paper.

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PART 1 – PROJECT REPORT

2 Project description

2.1 Aim and objectives

The aim of this research project is to report whether updating the current NSTS could help

provide for multiple benefit (also referred to as multi-functional) SuDS, and if so, what are

the requirements to update the standards for the integration of high-quality multiple benefit

SuDS.

The objectives of this research project are:

1) To build evidence on how the existing standards for SuDS are understood and

applied.

2) To determine what is needed to improve consistency in the application of current

standards to enable SuDS to deliver better surface water flood management.

3) Test any potential changes to the standards and understand the impacts of these

changes before they are implemented.

4) To review the latest approaches to calculating runoff estimations and advise on

appropriate methods to calculate peak flow and volumes which can be referenced

in the current standards.

5) To determine how multiple benefit criteria could be brought into the standards to

help deliver the National Planning Policy Framework (NPPF) (MHCLG, 2019a)

requirements for delivering multiple benefits (including contributing to biodiversity

net gain, BNG1) through SuDS.

6) To define the constraints, disadvantages and conflicts that could arise when

delivering any additional multiple benefit criteria.

7) To guide and engage with the stakeholder community, who will apply and use the

standards, to incorporate sustainable drainage approaches within new

developments.

8) To recommend updates to the SuDS standards.

1 Biodiversity net gain is development that leaves the environment in a better state than before (CIEEM, CIRIA, IEMA, 2016)

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2.2 Scope of work

The scope of work included the following technical tasks:

• Task 1 – Investigation into the understanding, interpretation and application of the

current NSTS

• Task 2 – Recommendations to improve the consistency and effectiveness of SuDS

water quantity standards

• Task 3 – Definition of the current barriers to and implications of requiring the

delivery of multiple benefit SuDS, evolving drivers, and likely enablers

• Task 4 – Drafting of proposed updated industry-endorsed NSTS that will help drive

and improve the delivery of multiple benefit SuDS.

All four technical tasks are described in this report.

Key aspects of the scope to note include the following:

• The stakeholder engagement activities for Tasks 1, 3 and 4 were revised in light of

current social distancing guidance from the UK Government, with all engagement

taking place online.

• Any guidance or standards published by Government may be considered statutory

unless they state otherwise. Therefore, it is not possible to change the title of the

NSTS unless its status also changes. This project was carried out without any

expectation regarding the status of the NSTS changing (i.e. from non-statutory to

statutory). Stakeholders were not asked to comment on this. However, some

stakeholders chose to share their views regarding whether the standards should be

non-statutory or statutory, and such detailed have been reported.

• This project focused on the content of the NSTS within the existing planning context

for England – i.e. the current NPPF (MHCLG, 2019a) and Planning Practice

Guidance (PPG) (MHCLG, 2020a). It is not within the scope of the project to

account for any potential changes to that context. However, it is recognised that

findings from the project might include discussion regarding that context. A review

of the White Paper Planning for the Future (MHCLG, 2020b), which was published

during this project, has been added to the scope of work for this project in order to

help provide this context (Annex H).

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3 Overview of technical approach

There were four technical tasks as part of this project as shown in Figure 1.

Figure 1 Relationship between project tasks, research questions and stakeholder

engagement

Tasks 1 to 3 reviewed evidence in order to answer ten research questions (Table 1), in

order to provide recommendations regarding the existing NSTS (Task 4). All tasks were

supported by extensive stakeholder engagement. The review process and resultant

recommendations were also supported by modelling of example sites and testing of

proposed amendments to the NSTS.

Two different sets of case study sites were used as part of the evidence review:

a) Task 1 case study sites provided examples of how ‘key issues’ on development

sites can be addressed (summarised in Annex A, Section 3 and described more

fully in Annex G); and

b) Task 2 case study sites supported hydraulic ‘scenario testing’ (as described in

Annex B).

Detailed descriptions of the approach taken for each of the Tasks 1 to 4 (including the

research questions and stakeholder engagement activities) can be found in Sections 3.1 to

3.4.

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Table 1 Research questions

Task Related research questions

Task 1 – Clarify the current

situation

RQ1 - How are the NSTS interpreted and applied

currently and what influences any variation?

RQ2 - To what extent does local guidance and policy

influence the outcomes?

RQ3 - What currently drives or constrains the

requirements for and delivery of multiple benefit SuDS

(over and above hydraulic control)?

Task 2 – Evaluate hydraulic

design methods

RQ4 - What are the implications of the range of methods

and design approaches required by LLFAs and/or used

by designers to meet the current criteria?

RQ5 - How could new guidance be used to deliver a more

consistent design approach and more consistent

outcomes?

RQ6 - Is there new evidence that supports modifying the

current criteria from a hydraulic perspective?

RQ7 - To what extent would modifications to existing

hydraulic standards drive the delivery of multiple

benefits?

Task 3 – Define the deliver

barriers and enablers

RG8 - What lessons can we learn from implementation of

Schedule 3 of the Flood and Water Management Act

(2010) in Wales?

RQ9 - How could a requirement for multiple benefit SuDS

deliver wider government policy and regulatory

objectives?

RQ10 - What are the barriers, enablers and drivers for the

inclusion and delivery of multiple benefit SuDS criteria?

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3.1 Task 1 – Clarify the current situation

The overall aim of Task 1 was an investigation into the understanding, interpretation and

application of the current NSTS.

The research questions (RQs) specific to Task 1 were:

• RQ1 - How are the NSTS interpreted and applied currently and what influences any

variation?

• RQ2 - To what extent does local guidance and policy influence the outcomes?

• RQ3 - What currently drives or constrains the requirements for and delivery of

multiple benefit SuDS (over and above hydraulic control)?

These questions were addressed by carrying out:

• A review of existing evidence;

• Stakeholder engagement, in the form of an online survey and focus groups;

• A review of case studies to demonstrate key issues.

3.1.1 Review of evidence

Evidence reviewed as part of this task included the following:

National, published or ongoing studies

• CIWEM Big SuDS survey (2017) and A Place for SuDS (2018)

• MHCLG SuDS review (2018)

• Achieving SuDS – A review of delivery by Lead Local Flood Authorities (LI / CIC

Review, 2019)

• ICE Route Maps (2018)

• CIRIA’s B£ST outputs (2015, 2019)

• Surface Water Drainage from Developments (UKWIR, 2021)

Local studies

• Surface Water Drainage from Developments (UKWIR, 2021)

• Review of Developer SuDS Schemes in Somerset (Somerset County Council,

2018)

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• Review of Policy and Practice in North East LLFAs (North East LLFAs, 2019)

These pieces of evidence were identified in the original project specification and proposal,

and were confirmed by the Project Steering Group (PSG) as being the only suitable and

available pieces of evidence.

3.1.2 Stakeholder engagement and consultation

Details of the stakeholder engagement and consultation undertaken as part of this task are

provided below.

3.1.2.1 Engagement with Association of SuDS Authorities members

A focussed initial engagement meeting with key Association of SuDS Authorities (ASA)

members took place in April 2020, before the first full PSG meeting. The purpose of this

meeting was to gauge initial thoughts on the project approach and the opportunities and

challenges of potentially revising the NSTS.

Representatives from the ASA are also members of the PSG, which held a meeting in May

2020, providing further opportunity for engagement.

3.1.2.2 Online survey 1

An online survey was carried out between 29 May 2020 and 26 June 2020.

Responses were requested from different stakeholders that included:

• Approvers, i.e. local authorities, Lead Local Flood Authorities (LLFAs), Local

Planning Authorities (LPAs) and Water and Sewerage Companies (WaSCs)

• Practitioners/designers, e.g. engineers, landscape architects

• Developers, i.e. those commissioning SuDS

• Others, e.g. supply chain members, regulators, academics

The survey was developed to help clarify the current situation with regards to compliance

with the NSTS and the potential enablers and challenges to SuDS delivery. The objectives

of the survey were to:

• Understand approaches to applying the current NSTS.

• Understand (where possible) how the NSTS are/can be applied for SuDS on sites

with different contexts.

• Identify enablers and challenges to good practice in achieving the outcomes

intended from the application of the NSTS.

20

• Understand what other multiple benefits from SuDS are desired and delivered in

practice, and what the enablers or challenges are to this.

• Explore opportunities to use an updated version of the NSTS to deliver multiple

benefits.

• Understand how maintenance and operational requirements can influence the

desired outcomes and final designs.

A full account of the survey can be found in Annex C.

3.1.2.3 Focus groups

The original face-to-face workshop that was included in the project proposal, was replaced

with three online workshops in July 2020 using focus groups. The objective of the focus

groups were to understand the particular opportunities and challenges in setting

requirements to deliver multiple benefits. This would complement the information obtained

from the survey.

Participants were invited by the project team to represent lead local flood authorities, local

planning authorities, developers, consultants (including drainage engineers and landscape

architects) and representatives from the WaSCs.

A full account of the online workshops can be found in Annex E.

3.1.3 Case studies to demonstrate ‘key issues’

Selected case studies available in existing evidence were reviewed together with case

studies supplied by respondents to the first online survey, with the purpose of helping to

demonstrate:

• How the current standards are interpreted and implemented;

• The range of barriers, enablers and opportunities;

• The influences on delivery of best practice;

• The impacts of adoption options and maintenance obligations.

Case studies were drawn from the following sources:

• Submitted via the online survey

• Susdrain awards (2018 and 2020)

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• Susdrain case studies2

• Project team members.

The review focussed on private residential housing and commercial development. It was

considered that ‘public realm’ developments (schools, libraries, community housing

projects, etc.) tend to provide easier opportunities for the delivery of multiple benefits and

do not represent the majority of developments where improvements in delivery of multiple

benefits are needed.

A total of seven case studies were selected to demonstrate how it is possible to deliver

multiple benefits successfully, showing how issues were overcome and the benefits

delivered to a ‘high standard’.

Case studies were selected on the basis of good and consistent information regarding the

site, the components, the delivery mechanisms and process. This requirement limited the

suitable sites to those already available as Susdrain case studies or those submitted for

Susdrain awards. However, information on enablers and constraints sourced from all case

study material provided by survey responders was also reviewed.

Sites were selected that deliver one or more of the following to a ‘high standard’:

1. Interception

2. Rainwater harvesting

3. Maximising infiltration

4. Water quality management

5. Biodiversity

6. Amenity3

A summary of the case studies is provided in Annex A, Section 3. Further information

about the case studies can be found in Annex G.

3.2 Task 2 – Evaluate hydraulic design methods

The overall aim of Task 2 was to evaluate current hydraulic design methods with a view to

providing recommendations to modifying the current standards within the NSTS related to

2 https://www.susdrain.org/case-studies/ 3 Ideally as part of blue-green infrastructure and a focus on climate resilience that goes beyond flood risk, e.g. Urban Heat Island effect, air quality, etc.

https://www.susdrain.org/case-studies/

22

the hydraulics (or water quantity management) for a site. Full details are provided in

Annex B.

The research questions specific to this task were:

• RQ4 - What are the implications of the range of methods required by LLFAs and/or

used by designers to meet the current criteria?

• RQ5 - How could new guidance be used to deliver a more consistent approach?

• RQ6 - Is there new evidence that supports modifying the current criteria from a

hydraulic perspective?

• RQ7 - To what extent would such modifications drive the delivery of multiple

benefits?

The task was broken down into the following sub-tasks:

• Description and suitability evaluation of current methods and design approaches

• Review of the findings from the first online survey (see Section 3.1.2.2) related to

hydraulic design methods

• Site model testing of different design criteria and methods

• Critical review of current standards and guidance, and recommendations for

modifications of these.

3.2.1 Evaluation of current methods and design approaches

In order to set the technical context for the site model testing, the key design inputs and

design approaches to setting SuDS flow controls and estimating required storage volumes

were summarised and reviewed.

Evidence was drawn from the UKWIR research into surface water management for

developments (UKWIR, 2021) and technical documentation on individual methods (as

detailed in Annex B).

3.2.2 Review of the online survey

The responses from both approvers and practitioners regarding the hydraulic questions

included in the online survey (see Task 1) were summarised and evaluated.

The findings from the Task 1 review of existing evidence relating to hydraulic design

methods and application of standards were also cross-checked to ensure the evaluation of

the survey results was robust and comprehensive.

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3.2.3 Site model testing

The aim of the site model testing was to demonstrate how different design criteria and

methods affect runoff flow and volume estimates, and the storage volumes required as a

result.

A series of tests were carried out using very simple hydrodynamic models of selected real-

life sites. This enabled a quantitative assessment of the impact of a range of

methodologies and design approaches on allowable discharge rates and subsequent

required attenuation storage volumes.

The results from all elements of the testing are presented in Annex B.

3.2.4 Critical review of current standards and recommended modifications

The current standards were critically reviewed, with reference to the results from the site

model testing; the findings from the evaluation of current methods and design approaches

and the results from the online survey.

Recommendations were then made for modifications to these standards, with a view to

addressing existing implementation challenges and improving the delivery of multiple

benefits.

3.3 Task 3 – Define the delivery barriers and enablers

The overall aim of Task 3 was to review of current and evolving drivers, barriers and

enablers for delivering multiple benefit SuDS. Details are provided in Annex F.

The research questions specific to this task were:

• RG8 - What lessons can we learn from implementation of Schedule 3 of the Flood

and Water Management Act (FWMA) in Wales?

• RQ9 - How could a requirement for multiple benefit SuDS deliver wider government

policy and regulatory objectives?

• RQ10 - What are the barriers, enablers and drivers for the inclusion and delivery of

multi benefit criteria?

These questions were addressed by carrying out:

• A review of the Welsh SuDS process

• A review of existing and evolving drivers for multiple benefit SuDS

• A review of evidence and consultations in context of barriers and enablers.

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3.3.1 Review of the Welsh SuDS process

The review of the implementation of Schedule 3 in Wales was in two parts:

a) A review of the implementation process, resulting in the Welsh SuDS Standards by

Phil Chatfield - the Welsh Government Water Policy Project Manager during the

period of implementation of Schedule 3 of the FWMA responsible for managing the

policy development, consultations, secondary legislation and related stakeholder

engagement.

b) Interviews with Welsh stakeholders to discuss the application of the Standards to

date (see below).

During July 2020 seven interviews were undertaken with Welsh stakeholders involved in

the delivery of their Statutory standards for SuDS. Interviews were held with:

1. Welsh Government and Welsh Local Government Association

2. Home Builders Federation (HBF) with representatives from Wates and Llanmoor

Homes

3. Caerphilly County Borough Council

4. Swansea Council

5. Vale of Glamorgan

6. Flintshire County Council

7. Cardiff City Council

Representatives from the SuDS Approval Body (SAB) were interviewed from five local

authorities (LAs). These were selected following discussion with Welsh Government and

other SAB officers to ensure a variety of different LA and catchment areas were covered.

The interviews were undertaken to obtain opinion on how the Welsh SuDS standards have

been applied, as well as how the individual standards are evaluated, the challenges and

enablers, as well as explore suggestions for potential changes to secure better outcomes.

The feedback obtained via the interviews and comprehensive notes from individual

interviews can be found in Annex F.

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3.3.2 Review of current and evolving drivers for multiple benefit SuDS, including barriers and enablers

This review considered existing and evolving drivers for multiple benefit SuDS, including

identification of perceived and actual barriers and enablers to widen the scope of the

NSTS. The review took account of drivers for England as of June 20204.

The structure of the review was:

• Current national drivers for multiple benefit SuDS

• Evolving national drivers for multiple benefit SuDS

These were followed by a review of drivers, enablers, barriers, opportunities and

challenges for key benefits. These categories were established as representing the most

significant benefits from SuDS, as described in CIRIA (2015) and CIRIA (2019). The

benefit categories selected were:

1) Landscape and amenity

2) Biodiversity

3) Water quality

4) Integrated Water Management

5) Climate Change

6) Air quality

7) Highways

8) Health and wellbeing

Each review identified:

• Evolving drivers for SuDS, including any recognition and definition of these in new

policy, regulation and relevant industry guidance documents

• Recognised existing enablers and challenges established through previous industry

reviews and consultations, and those undertaken as part of this project

4 This pre-dates the publication of the Planning White Paper Planning for the Future (MHCLG, 2020d). However, subsequent to this, a review of the Planning White Paper was carried out and details can be found in Annex H.

26

• Possible opportunities and challenges arising from a requirement to deliver the

benefit in any revised standards for SuDS (reflecting on the mechanisms and

processes for delivering multiple policy objectives)

• Key linkages and alignment needs to ensure that the messaging regarding the

requirements for multiple benefit SuDS is consistent across policy and guidance.

The methodology for the review is based on the approach outlined in European Training

Foundation (ETF) Guide to Policy Analysis (ETF, 2018) which is derived from academic

research into policy analysis. Each driver was analysed for:

• Content Analysis: Does the driver identify direct or indirect challenges and

enablers to multiple benefit SuDS?

• Evidence and Findings: Does the driver contain quantitative or qualitative

evidence or findings about the barriers and enablers of multiple benefit SuDS?

• Recommendations: Does the driver contain recommendations for multiple benefit

SuDS and, if so, what are they?

3.4 Task 4 – Move forward with multiple benefit SuDS

The overall aim of Task 4 was to provide recommendations regarding how best to move

forward with the SuDS design to maximise the delivery of multiple benefits. This had two

aspects: updating the standards and providing other recommendations related to the

successful implementation of the standards.

3.4.1 Stakeholder engagement and consultation

A key aspect of this task was further stakeholder engagement. Details of the stakeholder

engagement and consultation undertaken as part of this task are provided below.

3.4.1.1 Focus group

The original face-to-face workshop that was included in the project proposal, was replaced

with an online workshop in October 2020 using a focus group. The objective of the focus

group was to enable stakeholders (mostly those who had not been previously involved in

the project) to review an early draft of the updated standards.

Participants were invited by the project team to represent lead local flood authorities, local

planning authorities, developers, consultants (including drainage engineers and landscape

architects) and representatives from the WaSCs.

A full account of the online workshops can be found in Annex E.

27

3.4.1.2 Online survey 2

An online survey was carried out between 23 October and 23 November 2020.

Responses were requested from the same range of stakeholders as the first online survey

(see Section 3.1.2.2).

The survey was undertaken to enable stakeholders to provide feedback on the draft

updated standards. The objectives of the survey were to:

• Explore whether the standards and clarification statements were clear

• Obtain specific feedback on the wording of the proposed standards, i.e. are they

easy to understand, relevant?

• Determine whether the standards could be delivered and assessed

• Understand what additional guidance or tools are needed to deliver and/or assess

the standards.

A full account of the survey can be found in Annex D.

3.4.2 Stages of development of updated standards

Development of the updated standards went through the following steps:

1. Preparation of ‘initial draft’ updated standards and clarifications

2. Review of these standards and clarifications by the PSG at a meeting in September

2020

3. Revision of these updated standards and clarifications based on feedback from the

PSG for sharing with the Focus Group (Section 3.4.1.1)

4. Completion of an online workshop with the Focus Group in October 2020 to review

these ‘interim draft’ updated standards and clarifications (feedback from the focus

group is summarised in Annex E)

5. A further revision of these ‘interim draft’ updated standards and clarifications for

sharing with the wider stakeholder group via an online survey (Section 3.4.1.2)

6. Completion of the online survey during October and November 2020 (survey results

are provided in Annex D)

7. Further revision of these updated standards and clarifications based on feedback

from the online survey to be issued to the PSG as ‘final draft’

8. Review of these ‘final draft’ standards and clarifications by the PSG at a meeting in

December 2020

9. Further revision of these updated standards and clarifications and inclusion in

Part 2 of the draft version of this report

10. Peer review of the draft Final Report

11. Final amendments to the updated standards and clarifications as presented in

Part 2 of this final version of the Final Report.

These steps are summarised in the project timeline shown in Figure 2.

28

Figure 2 Project timeline

3.4.3 Amendments to updated standards and clarifications based on feedback

The draft updated standards and clarifications, developed on the basis of the research

findings presented in Section 4, were refined a further five times based on the feedback

received from a series of stakeholder engagement activities plus the peer review, as

described in Section 3.4.2.

The key modifications resulting from the feedback received included:

1. Definition of ‘a SuDS approach’:

• Substantial re-wording to simplify, improve clarity and reduce sentence length

• Re-focus on objectives of the approach rather than the mechanisms for delivery

• Removal of reference to underground components

2. Standard 1 – Runoff destination

• Simplification of hierarchy statements

• Improved clarification on rainwater harvesting

• Inclusion of statements relating to long term management and maintenance

3. Standard 2 – Everyday rainfall

• Clarification that this refers to discharges to surface waters only

• Clarifications on terms used (e.g. ‘majority’ and ‘frequent’)

4. Standard 3 – Extreme rainfall

• Re-organisation of wording to simplify and improve clarity

• Improved clarity over definition of fixed threshold/default flow rates and removal of

perceived additional obligations for setting these rates locally

29

• Additional clarifications (e.g. urban creep allowance expectations)

5. Standard 4 – Water quality

• Improved clarity over risk assessment and groundwater protection

6. Standard 5 – Amenity

• Inclusion of reference to multi-functionality

7. Standard 6 – Biodiversity

• Removal of reference to habitat in the standard

A significant proportion of the feedback received via the online survey related to the need

for guidance on definition, delivery mechanisms, and compliance assessment frameworks.

This has been reflected in the additional recommendations in this report (Section 5.3).

4 Research findings

Section 3 describes the research questions and the approach taken for addressing each of

these questions. This section presents the Contractor’s findings, provided in increasing

levels of detail.

• Section 4.1 provides a brief overview of the answers to the research questions

(RQs) for each of the Tasks 1 to 3.

• Sections 4.2 to 4.4 provide more detailed answers to the research questions.

• Full details of the research findings from this project are documented in the annexes

that accompany this report (listed in Section 1). References to these annexes are

provided throughout sections 4.2 to 4.4.

4.1 Summary of findings

4.1.1 Task 1 research questions

4.1.1.1 How are the NSTS interpreted and applied currently and what influences any

variation? (RQ1)

For the majority of new developments, the rate of runoff from the site is currently limited

using flow controls and storage. A large proportion of these sites collect runoff using

gullies and pipes, and deliver storage in underground systems. Where the design attempts

to deliver multiple benefits, this is often a pond or basin, downstream of a piped network.

With the focus of the current NSTS being on hydraulic control (i.e. reducing peak flow

rates and volumes only), there is little incentive to design SuDS that deliver multiple

benefits. Site characteristics (in particular soils) and the scale and type of the new

development are key influences on SuDS design. Design criteria, calculation methods and

design approaches all contribute to variability in design outcomes.

30

A more detailed answer to this research question is provided in Section 4.2.1.

4.1.1.2 To what extent does local guidance and policy influence the outcomes?

(RQ2)

Adopted local SuDS policy and guidance is important for supporting engagement between

approving bodies and developers, and facilitating the delivery of multiple benefit SuDS.

However, compliance with local SuDS policy is less likely than more established and

documented flood risk management policy.


4.1.1.3 What currently drives or constrains the requirements for and delivery of

multiple benefit SuDS (over and above hydraulic control)? (RQ3)

There are a significant number of legal, regulatory, and policy drivers for multiple benefit

SuDS that are described in the Annex A and summarised within the evidence base for the

individual standards, set out in Sections 7 to 12 of this report. In particular, the NPPF

requires new development, where possible, to include ‘multi-functional’ sustainable

drainage systems (with multi-functional in this context meaning SuDS that have been

designed to maximise the benefits that can be achieved following design best practice).

However, these high-level drivers are not often recognised by, nor act as a direct influence

on those approving and delivering SuDS on a day-to-day basis. In addition, the NSTS

currently only specify water quantity design requirements, falling short of SuDS best

practice.

The greatest influence on multiple benefit outcomes is perceived by stakeholders to be the

expertise of those involved in the commissioning, design and approval process. It is

evident that considering SuDS early in the development process and embedding SuDS

principles through pre-application discussions also plays an important role. Perceived

constraints (in addition to the NSTS focus on only hydraulic control) include land

allocation, developer timelines, and poor engagement between those approving and

delivering schemes.



4.1.2.1 What are the implications of the range of methods and design approaches

required by LLFAs and/or used by designers to meet the current criteria?

(RQ4)

The range of methods and design approaches currently used leads to variability in

estimated peak allowable discharge rates, greenfield runoff volumes and attenuation

storage volumes. However, the variations in estimated runoff rates using different methods

are less significant than the impact of using an incorrect soil type. Any of the methods

currently in use are sufficient for delivering the objectives of the standards. The impact of

31

the runoff rate estimation method on required storage volumes is limited. For small sites,

the minimum flow control size (e.g. 5 l/s) will dictate the required storage volume instead.

For other sites, storage volumes may be determined by the flow rate specified by the local

authority as the lowest necessary discharge rate that ensures flood risk in the receiving

water or sewer is not exacerbated by the development.

A more detailed answer to this research question is provided in Sections 4.3.1 to 4.3.4.

4.1.2.2 How could new guidance be used to deliver a more consistent design

approach and more consistent outcomes? (RQ5)

Guidance is needed to support implementation of the NSTS to ensure consistent design

approaches are adopted, see Section 4.1.2.3.

New, consistent guidance is needed for the following:

• Acceptable runoff estimation methods and their application

• Areas to be used for pre and post runoff estimation

• Minimum flow control sizes and blockage risk management strategies

• Percentage runoff values for impervious and vegetated areas

• Design rainfall, climate change uplift factors, urban creep.

Although there is existing guidance for each of these topics, it can be found in different

places, it is not consistent and it needs updating to reflect the recommended updates to

the NSTS.

At a national level, this guidance can be found in:

• The SuDS Manual (Woods Ballard et al., 2015)

• Non-statutory technical standards for sustainable drainage: Practice Guidance

(LASOO, 2016)

• Design and Construction Guidance (Water UK, 2020a)

• Flood risk assessment: climate change allowances (Environment Agency, 2020a).

There is also often local guidance available on many of these issues.


4.1.2.3 Is there new evidence that supports modifying the current criteria from a

hydraulic perspective? (RQ6)

There is evidence that the current NSTS do not make the intent of the hydraulic standards

clear and this leads to misinterpretation, so they should be re-drafted. There are drivers

32

and stakeholder support for an additional standard that is focussed on the management of

‘everyday’ rainfall.


4.1.2.4 To what extent would modifications to existing hydraulic standards drive the

delivery of multiple benefits? (RQ7)

The inclusion of a standard focussed on the management of ‘everyday’ rainfall through the

requirement of Interception (i.e. the retention on site of the first 5 mm of the majority of

rainfall events through the year) will encourage the use of more SuDS components that

are on the surface and vegetated, which in turn will drive the delivery of multiple benefits.



4.1.3.1 What lessons can we learn from implementation of Schedule 3 of the Flood

and Water Management Act (2010) in Wales? (RQ8)

Designing to comply with the statutory standards for SuDS in Wales is perceived by the

stakeholders consulted to be relatively straightforward. The Interception, amenity and

biodiversity standards are considered effective at driving multiple benefit SuDS. Important

supporting procedures for delivering multiple benefits include pre-application discussions,

‘deemed to comply’ approaches, alignment of SuDS with highway drainage approval

processes, guidance on data and SuDS strategies, education and training.


4.1.3.2 How could a requirement for multiple benefit SuDS deliver wider

government policy and regulatory objectives? (RQ9)

Multiple benefit SuDS are referenced within multiple government, industry and regulatory

documents as a key mechanism for delivering policy objectives, including:

• The 25 year Environment Plan (HM Government, 2018)

• National Planning Policy Framework (MHCLG, 2019a)

• Planning Practice Guidance (MHCLG, 2020a)

• Climate Change National Adaptation Programme (Defra, 2018)

• National Flood and Coastal Erosion Risk Management Strategy for England

(Environment Agency, 2020b)

• National Design Guide (MHCLG, 2019b)

33

• Design and Construction Guidance (Water UK, 2020a).

These policy and guidance documents in turn act as enablers for the delivery of multiple

benefit SuDS (see Section 4.1.3.3).


4.1.3.3 What are the barriers, enablers and drivers for the inclusion and delivery of

multiple benefit SuDS criteria? (RQ10)

There are wide ranging enablers for the delivery of multiple benefit SuDS through the

legislative framework of the planning system (e.g. local plans, permitted development).

However, many of these also pose barriers, where requirements for multiple benefit SuDS

are inconsistent or the current scope does not explicitly include multiple benefits. There

are many drivers for a requirement for multiple benefit SuDS within environmental

legislation. These drivers relate to flood risk management, water quality, climate change

management, biodiversity net gain and highway drainage. Biodiversity and highway

drainage requirements currently have the potential to pose barriers where multiple benefit

SuDS are not aligned with legislative requirements. There are a large number of policy and

guidance documents that either contain direct references to multiple benefit SuDS, or

contain ‘hooks’ or encouragement for equivalent outcomes through the delivery of green

and blue infrastructure. Some of those documents have the potential to pose barriers

where requirements are conditioned in some way or where principles are not backed up by

robust industry guidance. There are a range of barriers to the delivery of multiple benefit

SuDS relating to skills, capacities, perceived economic impacts and challenges associated

with the approval and adoption process.


4.2 Task 1 research questions

4.2.1 How are the NSTS interpreted and applied currently and what influences any variation? (RQ1)

MHCLG (2018) reports that for a sample of approved planning applications, 87% explicitly

stated that SuDS would feature in the proposed development (whether proposed by the

applicant, or conditioned by the local planning authority). Although the characteristics of

schemes classified as including SuDS is not defined in the report it is likely that the

inclusion of flow control and storage (as required by the NSTS) will have formed the basis

of the assessment. LI/CIC (2019) reports that half of County Councils (38% of Unitary

Authorities and Metropolitan/London Boroughs) considered that their approach to

managing surface water had changed “significantly” as a result of the 2015 changes to

national planning policy requiring SuDS in all new (major) development.

34

Although the evidence is showing a shift towards the use of SuDS, this shift is related

primarily to the inclusion of runoff control only and not the other multiple benefits offered by

best practice SuDS design. Evidence for this has been found in the results of the online

survey of current practices undertaken for this project (Annex C, Sections 4.1, 7.1 and

10.2) as summarised in Figure 3. This figure shows that runoff control in the form of ‘peak

flow control’ only is the most common requirement, with peak flow and volume control’

being the second most common requirement. It should be noted that respondents to the

survey were given the options of ‘never required’ and ‘don’t know’ as well as ‘always

required’ and ‘sometimes required’. ‘Others’ as shown in this figure and all similar figures

in this report refers to other stakeholders, rather than those designing SuDS schemes

(‘Practitioners’) and those responsible for approving SuDS schemes as part of planning

applications (‘Approvers’).

Similar evidence can also be found in CIWEM (2017), Somerset CC (2018), LI/CIC (2019)

and UKWIR (2021). All of these reports present evidence that piped networks together

with subsurface attenuation tanks or end of pipe basins or ponds are the most common

solutions to delivering current requirements as set out in the NSTS. The reports highlight

that, although there is an understanding that gullies, pipes and tanks are very unlikely to

deliver a sustainable solutions to runoff management, these types of solutions still

proliferate and standard approaches to road drainage (dictated by highway authorities

approval and maintenance requirements) often lead to dual or poorly integrated systems.

Figure 3 The proportion of survey respondents requiring or delivering various design

criteria

Key supporting comments from the CIWEM (2017) report are presented in Box 1, and

summary results from UKWIR (2021) are presented in Figure 4. The characteristics of the

sites reviewed are described in Section 2.6.2.2 of Annex A.

0

10

20

30

40

50

60

70

80

90

100

Amenity Biodiversity Water quality Peak flow &volume

Peak flow Compliance withNSTS

% r

esp

on

den

ts

Approvers: Always require Approvers: Sometimes require Practitioners: Always include

Practitioners: Sometimes include Others: Always require Others: Sometimes require

35

Box 1 Evidence that underground systems and end of pipe systems are the most common outcomes for drainage systems designed to meet the current NSTS (CIWEM, 2017)

• “a conventionally piped surface water system with attenuation via oversized pipes and a

restricted discharge may be defined as sustainable drainage under the current non-

statutory guidance”;

• where higher quality SuDS were aspired to, schemes often adopted pipe-to-pond

systems that offer few benefits and can be problematic to manage.

• Only 8% of survey respondents believed that the NSTS were driving installation of high

quality and effective SuDS in England

• “The NSTS are effective in delivering high quality SuDS … only with respect to flood risk

(management)”

• “The standards generally result in piped or tanked attenuation systems, which will be

poorly maintained and could result in increased flood risk”

• The standards are likely to encourage more hard, ‘grey’ solutions and can be

implemented with conventional drainage solutions.

Figure 4 Number of sites including different SuDS component types from a survey of 18

planning applications (UKWIR, 2021)

At a high level, the key influences on the variations in approach and outcome for the SuDS

design are summarised in Figure 5 (see also Annex C, Sections 4.2 and 7.2). This figure

shows that, with respect to the characteristics of the site considered to have a high

36

influence on design approach and outcome, the level of flood risk was selected by the

highest proportion of respondents, followed by the scale of the development and the site

ground conditions. It should be noted that respondents to the survey were also given the

option of ‘no influence’.

Figure 5 The specific issues considered by survey respondents to pose a significant

influence on drainage design

More specifically, acknowledging that the current NSTS contain only hydraulic

requirements, the online survey of current practices (3.1.2.2) asked respondents to

indicate:

• The range of criteria used to specify or deliver schemes;

• The runoff estimation method used to compute the limiting discharge rates;

• The rainfall model used for the design; and

• The top five factors believed to lead to inconsistent calculation of rates and volumes

of runoff.

The results from the online survey show the variations in the hydraulic criteria required

and/or used to determine the allowable runoff rates (and volumes) from development sites

(Annex C, Sections 5.1-5.2 and 8.1-8.2) as summarised in Figure 6. Similar supporting

evidence from UKWIR (2021) is presented in Figure 7.

37

Figure 6 The proportion of approvers requiring the use of specific hydraulic criteria and the

proportion of practitioners using those criteria for over 50% of their applications

(i) Greenfield sites (ii) Previously developed sites

Figure 7 Survey of adopted flow rate criteria for 18 planning applications for greenfield and

previously developed sites (UKWIR, 2021)

The results from the online survey show the variations in greenfield runoff estimation

methods required and/or used to calculate the allowable discharge rate for the site

(Annex C, Sections 5.5 and 8.5), as summarised in Figure 8.

38

Figure 8 The proportion of approvers requiring the use of specific runoff estimation

methods and the proportion of practitioners using those methods for over 50% of their

applications (for both greenfield and previously developed sites)

The results from the online survey show the variations in the rainfall model required and/or

used for the drainage design (Annex C, Sections 5.7 and 8.7), as summarised in Figure 9.

It appears that practitioners are more aware of and likely to use the most up to date (and

therefore suitable) FEH rainfall model.

Figure 9 The proportion of approvers requiring the use of specific rainfall models and the

proportion of practitioners using those models for over 50% of their applications (for both

greenfield and previously developed sites)

39

The results from the online survey show the perceived key influences on the lack of

consistency in calculating rates and volumes of runoff (Annex C, Sections 5.6 and 8.6), as

summarised in Figure 10.

Figure 10 The perceived key influences on inconsistent estimation of runoff rates and

volumes

The results from the online survey show that the current recommended climate change

40% uplift factor was required by 61% of approvers (with 13% requiring a 30% uplift), and

was frequently or always used by 86% of practitioners (with 31% frequently or always

using a 20% uplift and 18% frequently or always using a 30% uplift) (Annex C, Sections

5.8 and 8.8).

The results from the online survey show that a 10% urban creep uplift factor was required

by 45% of approvers (with 22% not requiring such a factor), and was frequently or always

applied by 50% of practitioners (with 37% frequently or always not applying such a factor

at all) (Annex C, Sections 5.9 and 8.9).

4.2.2 To what extent does local guidance and policy influence the outcomes? (RQ2)

According to CIWEM (2017), strong local plan policy and guidance is considered important

in influencing multiple benefit outcomes from SuDS. The results from the online survey

support this industry view that local authority requirements (planning policy and guidance)

can (together with other drivers described in Section 4.2.3) directly influence their level of

engagement with developers and the inclusion of multiple benefits in drainage

submissions (Annex C, Section 9.2).

Additional feedback from the online survey is summarised in Figure 11.

40

This figure shows that planning application compliance with local plan policy on drainage

and flood risk is most likely to be required and complied with, followed closely by

compliance with any SuDS Supplementary Planning Document. Compliance with the

NSTS and local plan policy on green infrastructure or biodiversity were considered less

likely. It should be noted that respondents to the survey were given the options of ‘never

required’ and ‘don’t know’ as well as ‘always required’ and ‘sometimes required’ (Annex C,

Sections 4.1, 7.1 and 10.2).

Figure 11 The perceived compliance of planning applications with local policy and guidance

requirements

The results from the online survey show that, amongst approvers and practitioners, just

below 50% of respondents felt that poor drainage submissions (on > 50% of occasions)

were due to the lack of or poor Local Plan policies on SuDS (Annex C, Sections 4.5 and

7.4). Other factors were considered more influential in contributing to poor drainage

submissions.

The results from the online survey show that, SuDS-specific local guidance or

Supplementary Planning Documents were considered influential in delivering SuDS with

multiple benefits alongside local authority green infrastructure, biodiversity, drainage, flood

risk, and local plan policy (Annex C, Sections 6.2, 9.2 and 11.2). The guidance of greatest

influence was considered to be The SuDS Manual (Woods Ballard et al., 2015). The NSTS

were considered the weakest influence compared to the other policy or guidance

considered here. The evaluation, summarised in Figure 12, is based on respondents

41

giving the policy/guidance an influence score or 4 or 5 out of 5, with 5 representing the

highest level of influence.

Figure 12 The perceived influence of policies/guidance on multiple benefit SuDS design

outcomes

Two further pieces of evidence that help contextualise the online survey outcomes are:

• The LI/CIC (2019) and MHCLG (2018) reports which suggest that development

applicants often have a poor understanding of local SuDS policies, and that this

contributes to poor drainage design submissions and outcomes.

• Despite the increasing representation of SuDS guidance within local policy, Local

Authorities feel they have little to support them should they wish to contest designs

that do not provide amenity or biodiversity benefits (CIWEM, 2017) and that these

aspects (together with source control and management train principles) are not

often incorporated in designs.

4.2.3 What currently drives or constrains the requirement for and delivery of multiple benefit SuDS (over and above hydraulic control)? (RQ3)

There are a significant number of legal, regulatory, and policy drivers for multiple benefit

SuDS that are described in the Annex A and summarised within the evidence base for the

individual standards, set out in Sections 7 to 12 of this report. However, these high-level

drivers are not often recognised by, nor act as a direct influence on those approving and

delivering SuDS on a day-to-day basis.

42

The results from the online survey show that the appreciation, technical skills and

expertise of those involved in the commissioning, design and approval process were of

greatest influence on multiple benefit outcomes, together with considering SuDS early in

the development process and embedding principles through pre-application discussions

(Annex C, Sections 6.2, 9.2 and 11.2 and the case studies (Annex G) as described in

Annex A Section 3). The evaluation is based on respondents giving these skills and

process aspects an influence score of 4 or 5 out of 5, with 5 representing the highest level

of influence. This is summarised in Figure 13.

Figure 13 The perceived influences on the delivery of multiple benefit SuDS

The results from the online survey show that the constraints to the delivery of multiple

benefit SuDS that were perceived as being of greatest influence varied between

approvers, practitioners and other stakeholders, as presented in Figure 14 to Figure 16.

Approvers considered that the focus on flow rates and volumes was the most significant

constraint to the delivery of multiple benefit SuDS, with all suggested constraints being

acknowledged as relevant by more than 80% of respondents (Annex C, Sections 4.5-4.6)

(Figure 14). Practitioners considered that site characteristics were the most significant

constraint, with ground conditions and development timelines also important (Annex C,

Sections 7.4-7.5) (Figure 15). All suggested constraints were acknowledged as relevant by

more than 80% of respondents from other stakeholders, with poor engagement with

approvers and developer timelines considered to be the greatest influence (Annex C,

Sections 10.3-10.4) (Figure 16).

43

Figure 14 Constraints to the delivery of multiple benefit SuDS, as identified by approvers

Figure 15 Constraints to the delivery of multiple benefit SuDS, as identified by practitioners

44

Figure 16 Constraints to the delivery of multiple benefit SuDS, as identified by other

stakeholders

Additional constraints to the delivery of multiple benefit SuDS that were raised by

respondents to the online survey that have also been documented in previous evidence

reviews are summarised in Table 2.

Table 2 Additional evidence of constraints to the delivery of multiple benefit SuDS

Evidence of constraint Details

Clarity regarding the

definition of SuDS (CIWEM,

2017), (LI/CIC, 2019)

• SuDS definitions encompass a range of components

from underground tanks to a number of vegetated

SuDS in series

• The NSTS support the perception that drainage

systems without multiple benefits can deliver the

principle of SuDS

Application requirements

and exceptions (CIWEM,

2017)

• The suggestion in the NPPF (clause 163) that SuDS

are of particular importance for development in

floodplains

• The stated NPPF (clause 165) requirement for SuDS

in major development (without reference to smaller

scale development)

45

Evidence of constraint Details

• The opt-out clauses of inappropriateness (NPPF,

clause 154), and reasonable practicable (NSTS)

• The focus of local planning policy and drainage

strategies on local flood risk management

Breadth of current

standards (LI/CIC, 2019),

(CIRIA, 2013)

• Difficulties for local authorities to uphold a requirement

for multiple benefit SuDS at a planning inquiry when

the NSTS only require water quantity controls

• Without the management of everyday rainfall as a

requirement (i.e. not just extreme rainfall), multiple

benefits are difficult to secure

Skills, awareness,

resources, responsibilities

(CIWEM, 2017), (MHCLG,

2018), (LI/CIC, 2019), Case

Studies (Annex A, Section

3)

• Lack of consistent views within local authorities on the

acceptability and value of SuDS located within public

open space

• Low confidence across all industry stakeholders with

respect to long-term SuDS maintenance needs.

Precautionary approaches may mean lower benefit

outcomes

• Lack of technical skills and expertise with respect to

SuDS means there are risks of poor SuDS design and

approval of those poor designs

Adoption and funding

(LI/CIC, 2019), (NE LLFAs,

2019), Case Studies (Annex

A, Section 3)

• Lack of clear adoption routes for integrated, multiple

benefit systems

• Inconsistent approvals and adoption criteria between

LPAs and highways authorities

Development process

(MHCLG, 2018), (LI/CIC,

2019), Case Studies (Annex

A, Section 3)

• Risk of multiple benefits being proposed initially but

then dropped during later stages of planning to save

costs

• SuDS are not designed until development proposals

are largely complete, which reduces the opportunities

for multiple benefits

• Where SuDS are approved as conditions of planning,

LPAs have limited resources to subsequently enforce

these conditions

46


4.3.1 What are the implications of the range of methods and design approaches required by LLFAs and/or used by designers to meet the current criteria? (RQ4)

Different approving bodies will require or allow designers to use different methods and

design approaches when designing systems to meet the current NSTS. The availability of

multiple methods and approaches has implications for both the process of design and

approval, and the outcomes. In terms of the process, the implications are:

• The availability of a range of methods and design approaches leads to uncertainty

over which are ‘correct’ (for both designers and approvers).

• The availability of a range of methods and design approaches means there is

flexibility in the outcomes that can be secured i.e. allowing more or less

conservative designs. This is relevant for both designers and approvers.

• The need for expert guidance, skills and data to apply the latest (Environment

Agency recommended) Flood Estimation Handbook methods for greenfield runoff

estimation (CEH, 1999) means older, simpler methods, e.g. IH124 (Marshall and

Bayliss, 1994), are often used instead.

In terms of outcome, the method selection will influence:

• The estimated peak allowable discharge rate from the site (this may be

representative of greenfield or previously developed conditions);

• The estimated allowable discharge volume from the site (noting that controlling the

runoff volume to the greenfield volume for the 1 in 100 year, 6 hour duration event

is rarely required or implemented).

The main influence on the required attenuation storage volume will be the rate at which

runoff is allowed to discharge from the site. This rate is not always the estimated peak

allowable discharge rate as there are two flow ‘thresholds’ that may over-ride that

estimated rate. These are:

a) A minimum flow control size, specified by the approving body in conjunction with the

proposed adoption organisation, in order to minimise the risk of blockage of the flow

control occurring. This is normally set at 1, 2 or 5 l/s; and

b) A lower limit flow rate, specified by the approving body as the lowest peak allowable

rate necessary to ensure flood risk in the receiving water or sewer is not

exacerbated by development. This is normally set at 2 l/s/ha.

The peak allowable flow rate will also depend on which design return period is used to set

the criteria, which may vary if the requirements of the current NSTS are not being followed

explicitly.

47

The required attenuation storage volume will also depend on the estimated rates and

volumes of runoff from the development itself. These will depend on the proportion of

runoff assumed from specific surfaces, and on the area of the site that is assumed to

contribute flow to the drainage system.

The range of methods for estimating greenfield or previously developed runoff rates,

together with the range of criteria applied (e.g. 1 in 1 year, 1 in 2 year, 1 in 100 year return

period) lead to a range of peak allowable discharge rates, as demonstrated in Figure 17.

Figure 17 Peak allowable discharge rate for a reviewed range of planning applications

(UKWIR, 2021)

The impacts on outcome are described in UKWIR (2021) and were evaluated as part of

Task 2 of this project (Annex B). These are summarised in the following sections.

It should be noted that as the most significant influence on the magnitude of runoff rates

and volumes will be the area of the site, all values presented here are given as rates or

volumes per hectare.

The range of methods and design approaches lead to a range of attenuation storage

volumes. Figure 18 shows the storage volumes (per ha) provided for a set of planning

applications for both greenfield and previously developed sites. The limits of the vertical

lines indicate the minimum and maximum storage volumes provided, and the horizontal

line in the box represents the median (or most common) value. A higher proportion of the

sites lie within the range indicated by the boxes.

48

Figure 18 Attenuation storage volume provided on a number of surveyed planning

application sites (UKWIR, 2021)

The following sub-sections evaluate:

1. The influence of method on estimated peak allowable discharge rate

2. The influence of method on estimated allowable discharge volume

3. The influence of method and design approach on required attenuation storage

volumes

4.3.2 The influence of method on estimated greenfield peak allowable discharge rates (RQ4)

The impact of using alternative methods, based on the outcomes of the review of

estimation methods and site model testing carried out as part of Task 2 and reported in

Annex B, is summarised below for (a) greenfield estimates; and (b) previously developed

estimates.

4.3.2.1 Estimated greenfield peak runoff rates

Development peak runoff rates without the use of SuDS to control runoff rates are an order

of magnitude greater than greenfield peak runoff rates. As the proportion of development

in a catchment increases, the extent of impermeable surfacing will increase and runoff

rates into the receiving waterbody or sewer will rise - potentially leading to an increased

risk of flooding. The aim of the peak runoff rate standard is to ensure that development

runoff is reduced to appropriate levels (i.e. levels that are similar to greenfield rates or

better than from the previously developed site) through the use of a flow control.

None of the methods currently used for greenfield runoff rate estimation, i.e. the Institute of

Hydrology report 124 (IH124) (Marshall and Bayliss, 1994); the FEH13 statistical method

49

(Kjeldsen et al., 2008); or the Revitalised Flood Hydrograph Method 2 (ReFH2) (Kjeldsen,

2007), give consistently higher or lower results. This was demonstrated with the five sites

included in the site model testing carried out as part of Task 2 (Annex B, Section 5). The

range of estimates for different return period events are strongly influenced by Soil type

and each site test was undertaken assuming the site soils were a Soil type 2 (relatively

permeable) and a Soil type 4 (relatively impermeable). The results are presented in Figure

19.

Figure 19 Ranges of estimated discharge rates for 5 case study site for relatively high

permeability soils (Soil type 2) and relatively poor permeability soils (Soil type 4)

None of the greenfield runoff rate estimation methods have been validated against

development plot scale runoff measurements. Therefore, there is no robust evidence

confirming which method is most ‘correct’ and the uncertainty associated with any of the

estimates is likely to be high.

In many circumstances, the soil type will be more critical in determining greenfield runoff

estimates than the estimation method used. This highlights the need to undertake

appropriate soil testing to ensure that the estimation of peak runoff rates is realistic.

4.3.2.2 The influence of method on estimated previously developed peak allowable

discharge rates

The standards currently allow previously developed sites to discharge at previously

developed peak runoff rates, if achieving greenfield rates is not considered ‘reasonably

practicable’. An estimate of the peak runoff rates for the previously developed site,

therefore, provides the peak allowable discharge rate from any new development on a

50

previously developed site. In practice, approving bodies generally specify that the new

development has a 30-50% reduction (or betterment) in runoff rate compared to the

estimated rate for the previously developed site.

Piped networks are normally designed to BS EN 752:2017, which recommends a standard

design rainfall intensity for pipe networks. Industry practice is to apply this intensity to all

impervious surfaces on the site, with a 100% runoff factor. The recommended intensity

was increased from 35mm/hr to 50 mm/hr in the latest edition of the standard, to reflect

climate change impacts. However, as previously developed sites would have been

designed to older standards, the original 35 mm/hr is considered suitable. Alternatively, if a

hydrodynamic model is being applied, then a 2 year, 30 minute rainfall design event could

be used. Both approaches were considered as part of the site model testing.

Figure 20 illustrates that, even with a 50% betterment factor applied, estimates of

previously developed peak runoff rates are 10-20 times greater than Qbar (mean annual

flood) calculated for the greenfield site and are, therefore, still likely to pose a significant

contribution to flood risk in the receiving waterbody or sewer (Annex B, Section 5.12).

Figure 20 Previously developed runoff rates (reduced by 50%) compared to greenfield peak

runoff rates estimated using the IH124 method

51

4.3.3 The influence of method on estimated allowable discharge volume (RQ4)

The aim of the volume control standard is to encourage the reduction of runoff volumes (as

well as of runoff rates) from the developed site, recognising that high volumes of runoff

compared to greenfield can also contribute to flood risk in the receiving waters or sewer,

particularly where the capacity is already constrained. If interpreted correctly, the standard

allows the design to make use of a 1 in 100 year peak allowable discharge rate, only

where it is demonstrated that the development runoff volume is no greater than the

greenfield runoff volume for the 1 in 100 year, 6 hour event.

Greenfield runoff volumes can be estimated by applying the 1 in 100 year, 6 hour design

rainfall to the contributing site area, using a percentage runoff factor that is representative

of the soil type on the site. They can also be estimated using the ReFH2 modelling

software which computes a runoff hydrograph, from which the volume can be calculated.

The results for each of the sites used as part of the testing for both these methods are

presented in Figure 21 (Annex B, Section 5.3).

Figure 21 Greenfield runoff volume estimates from the site model testing

52

It should be noted that the site model testing showed that use of ReFH2 method, with

baseflow, can lead to runoff volumes that are greater than for the development area runoff,

so baseflow was excluded.

The uncertainty associated with any estimate of greenfield runoff volume will be high.

However, the objective of the standard is to check that a suitable level of volume reduction

has been achieved by the SuDS design.

In many circumstances, the soil type will be more critical in determining greenfield runoff

estimates than the estimation method used. This highlights the need to undertake

appropriate soil testing to ensure that the estimation of greenfield runoff volumes is

realistic.

4.3.4 The influence of method and design approach on the required attenuation storage volume (RQ4)

The size of the attenuation storage volume that is needed in order to accommodate the

design runoff from the development without flooding occurring from the drainage system

will be strongly influenced by the rate at which flow is allowed to discharge from the

storage volume. This control rate is not always the estimated peak allowable discharge

rate as there are two flow ‘thresholds’ that may over-ride that estimated rate. These are:

a) A minimum flow control size, specified by the approving body in conjunction with the

proposed adoption organisation, in order to minimise the risk of blockage of the flow

control occurring. This is normally specified as an equivalent flow rate of 1, 2 or

5 l/s; and

b) A lower limit flow rate, specified by the approving body as the lowest peak allowable

rate necessary for any drainage system to comply with in order to ensure flood risk

in the receiving water or sewer is not exacerbated by development. This is currently

normally set at 2 l/s/ha but may be varied depending on the level of flood risk

associated with the receiving water or sewer.

In these scenarios, the calculation method used for estimating peak allowable discharge

rates becomes irrelevant.

Table 3 presents the site areas below which alternative minimum flow control sizes come

into effect, assuming that otherwise the flow is controlled to 2 l/s/ha. This shows, for

example, that if the minimum flow control size is defined by a flow rate of 2 l/s, all sites

below 1 ha (i.e. the majority of planning applications) will default to this value rather than

any method-dependent peak allowable runoff rate estimate. If the minimum flow control

size is defined by a flow rate of 5 l/s, then this will over-ride estimated rates for all sites

below 2.5 ha.

Table 3 also presents the equivalent runoff rate per hectare for small (0.1 ha and 0.25 ha)

sites, when these minimum flow control sizes are dictating the allowable discharge rate

from the site. For example, a limiting discharge rate of 5 l/s will equate to a rate of 20 l/s/ha

53

from a 0.25 ha site and 50 l/s/ha from a 0.1ha site. These are an order of magnitude

greater than greenfield runoff rates and are more similar to development runoff rates.

Cumulatively (i.e. when applied for a large number of small developments), such

discharges could have a significant impact on downstream flood risk.

Table 3 Site areas for which the minimum flow control size is relevant and the equivalent

rate of discharge

Minimum flow

control size (defined

as an equivalent

flow rate) to prevent

blockage

Site area below

which the minimum

flow control size is

adopted

Equivalent rate of

discharge (per ha)

for a 0.1 ha

development site

Equivalent rate of

discharge (per ha)

for a 0.25 ha

development site

1 l/s All sites ≤ 0.5 ha 10 l/s/ha 4 l/s/ha

2 l/s All sites ≤ 1 ha 20 l/s/ha 8 l/s/ha

5 l/s All sites ≤ 2.5 ha 50 l/s/ha 20 l/s/ha

The NSTS require that, if runoff volumes from the development site are not reduced to the

greenfield runoff volume for the 1 in 100 year, 6 hour event, then the peak allowable

discharge rate should be limited to Qbar (or the 2 year return period greenfield flow rate).

This criterion was adopted for the majority of the site model testing scenarios undertaken

as part of Task 2 (Annex B, Section 5). The results of the site model testing determined

that:

• For small sites, the storage will depend on the minimum f low control size set by the

adopting body to protect the control from blockage risks (e.g. defined by equivalent

flow rates of 1 l/s, 2 l/s or 5 l/s), in which case the required volume of storage will be

independent of the estimation method. As the minimum flow control size is lowered,

the required storage volume will rise. Site model testing indicated increases in

storage of approximately 20% might be expected if the rate was lowered from 2 to 1

l/s, with a similar reduction if the rate was relaxed to 5 l/s.

• For Soil type 2 sites (i.e. sites with relatively permeable soils), Qbar will usually be

less than or close to 2 l/s/ha, the lower limit flow rate value set by the approving

body as the minimum rate necessary for effective flood risk protection. Therefore, 2

l/s/ha will be the allowable peak discharge rate and the required volume of storage

will generally be independent of the estimation method.

• For Soil type 4 sites (i.e. sites with relatively impermeable soils), Qbar will tend to

exceed 2 l/s/ha. Therefore, the required volume of storage will depend on the runoff

rate estimation method. However, the results from the site model testing showed

that the estimation method could result in a variation in storage volume of less than

10%, which is not significant.

54

The results from the site model testing showed that, if high peak runoff rates based on the

previously developed site are adopted as the control, then the required storage volume is

approximately 50% less than that required if the control rate was determined by the

greenfield Qbar rate.

The results from the site model testing showed that use of a 1 in 100 year return period

estimated peak runoff rate as the peak allowable discharge rate (instead of a Qbar runoff

rate estimate) reduced storage requirements by approximately 10% for Soil type 2 sites

and 25% for Soil type 4 sites. This indicates the benefit of reducing runoff volumes from

the development to greenfield runoff volumes for the 1 in 100 year, 6 hour event) and then

being able to make use of the higher return period discharge rate.

Confusion has arisen over the years about the use of suitable percentage runoff factors for

estimating the runoff from a developed site in response to an extreme rainfall event. That

confusion has been related to misunderstandings about the appropriate use of the

volumetric coefficient, Cv, intended for use with the Modified Rational Method for

designing piped networks. This has led to runoff factors of 0.75 or 0.84 being applied to

impermeable surfaces. But using these factors in this way leads to an underestimation of

runoff volumes during extreme events. During extreme events it is more appropriate to

assume 100% runoff from impermeable surfaces and up to say 30% runoff (reflective of

greenfield runoff rates) from pervious surfaces, where these are landscaped in a way that

means any runoff that is unable to infiltrate into the ground passes into the drainage

system.

The results from the site model testing showed that:

• Using these runoff factors in design could lead to an underestimation of the required

storage volume of 30%.

• Using the impermeable surface area only in greenfield runoff estimation methods

(to compensate for the fact that the drainage system for the developed site has

been designed on the basis of runoff from only the impermeable surfaces) still

means volumes are significantly underestimated.

• Adding inflows to the drainage system to represent pervious surface runoff (to

compensate for the fact that the drainage system for the developed site has been

designed on the basis of runoff from only the impermeable surfaces) still means

volumes are significantly underestimated.

4.3.5 How could new guidance be used to deliver a more consistent design approach and more consistent outcomes? (RQ5)

Approver responses to the online survey (Annex C, section 5.10) indicated that ‘lack of

consistent guidance on runoff estimation’ (41%) and ‘complexity and lack of understanding

of the hydraulic Standards’ (39%) were considered the two key constraints to delivering

consistent hydraulic design outcomes. Many approvers and practitioners also felt that the

NSTS are poorly understood and poorly applied (due to lack of clarity and supporting

guidance) and that local policy often ignores volume control.

55

For practitioner and developer respondents to the online survey (Annex C, Section 8.10)

the most commonly suggested constraints to delivering consistent hydraulic design

outcomes were the ‘lack of consistent guidance’ on approaches (46%), ‘the inability to

deliver volume control’ (38%), and ‘the challenges of delivering storage to meet low

discharge rates’ (36%). Comments regarding constraints referred to the experience of

approvers and designers interpreting and applying requirements differently. Poor industry

knowledge and understanding, and the lack of clarity and simplicity were also cited as

specific constraints.

This provides evidence that a re-statement of the hydraulic requirements of the Standard,

together with supporting clarifications and guidance is needed in order to support the

delivery of a more consistent approach.

4.3.6 Is there new evidence that supports modifying the current criteria from a hydraulic perspective? (RQ6)

The current criteria cover both peak runoff rate and runoff volume control. However,

volume control is not often implemented or the requirements for volume control and

designs follow inconsistent approaches. Over a quarter of respondents to the online

survey suggested that the NSTS should change and over half of respondents suggested

changes should be considered (Annex C, Sections 5.12 and 8.12). Many of the comments

received related to the need for strengthening, clarifying and extending the Standards.

As part of this study, the opportunity has been taken to review the industry position of

using of a fixed discharge rate of 2 l/s/ha instead of an estimated greenfield runoff rate

where reductions in runoff volumes to greenfield equivalents are not achievable (or

attempted). The review included the origins of this fixed discharge rate; the impact of this

fixed discharge rate on SuDS designs; and the likely effectiveness and implications of

amending this rate. Key outcomes include mis-application of 2 l/s/ha as a suitable

minimum discharge rate for all runoff, the comparability of 3 l/s/ha to 2 year greenfield

runoff rates for relatively permeable soil types, the constrained impact of any change due

to the limit set for minimum flow control sizes to minimise blockage risks that will over-ride

the proposed figure, and the impact of low discharge control rates on extending the critical

duration storm for the attenuation storage (potentially to much longer durations than will be

relevant for the flood risk associated with the receiving watercourse). This review (Annex

B, Appendix C) has therefore concluded that a fixed discharge rate of 3 l/s/ha is more

appropriate, particularly if coupled with a new requirement for delivering Interception (see

below), and should be included in the updated Standards.

Evidence suggests that the inclusion of a requirement for delivery of Interception (design

approaches that retain runoff from the first 5 mm of rainfall on site) is important for

protecting receiving environments (Annex A, Section 2.6, and The SuDS Manual (Woods

Ballard et al., 2015)) and sewer capacity (UKWIR, Phase 2, 2021) and driving multiple

benefits. Interception was considered by online survey respondents to be a valuable

addition to a revised set of Standards (Annex A, Section 6.4) and comments included that

56

policy in this area should be strengthened. The focus groups from Task 1 showed a

significant level of support for a new standard that addressed the management of

everyday rainfall (Annex E). Feedback from Wales, where an Interception requirement is

included within their statutory standards, suggested that the standard was welcomed as an

important driver of higher quality SuDS and that it was relatively straightforward to assess

and deliver using ‘deemed to comply’ rules (Annex F).

4.3.7 To what extent would modifications to existing hydraulic standards drive the delivery of multiple benefits? (RQ7)

Rainwater harvesting and infiltration are both integral to the suite of multiple benefits

sought from SuDS designs. A clearer focus on volume control that links to the requirement

to prioritise rainwater harvesting and infiltration will result in reduced runoff from sites.

Half of those responding to the online survey considered an Interception requirement

would support the delivery of multiple benefit SuDS (Annex C, Sections 5.11 and 8.11).

Many respondents suggested the NSTS need to include both Interception and multiple

benefit criteria.

There was recognition from the focus groups for Task 1 (Annex E, Section 3.5) that

managing everyday rainfall using Interception is necessary in addition to the management

of larger rainfall events.


4.4.1 What lessons can we learn from implementation of Schedule 3 of the Flood and Water Management Act (2010) in Wales? (RQ8)

Key issues drawn from the development of and enacting process for the statutory SuDS

standards (that include multiple benefits) for SuDS in Wales (Annex F, Section 2) of

relevance to a potential update of the NSTS in England include:

• The requirement for multiple benefit SuDS standards was identified as important to

help address a number of policy drivers including meeting WFD (water quality),

Welsh Government Water Strategy (Welsh Government, 2015a) (sustainable water

management), and Well-being of Future Generations (Wales) Act 2015 (amenity,

community resilience, ecology) objectives.

• Standards considered necessary to meet the required objectives included runoff

destination, hydraulics (including Interception), water quality, amenity and

biodiversity. Text setting out the ‘principles’ of SuDS design and detailed

compliance guidance for each Standard were identified as important to support

implementation.

• Particular concerns were expressed through consultation regarding the strength of

preference for above ground, vegetated systems and reassurance was required

57

that manufactured products would have a place in supporting an integrated design

approach that maximised benefits.

• Consultation identified a clear need for common frameworks and application forms

to ensure consistency of interpretation and implementation.

• Challenges with respect to securing and maintaining appropriate expertise within

local authorities meant training and resource sharing were found to be important.

Feedback from Welsh stakeholders (Annex F, Section 3) on the design and approval

processes since implementation of the Act includes:

• The SABs are encouraging pre-application discussions as this helps obtain better

outcomes.

• Designing SuDS to comply with the Welsh Standards has proved relatively

straightforward.

• The SABs welcomed the statutory Standards with requirements for Interception,

amenity and biodiversity, as this gave them greater ability to require multiple

benefits and deliver SuDS on the surface that are more aligned with the benefits of

green infrastructure.

• Welsh Government considers the statutory standards have driven improved SuDS

with respect to the delivery of multiple benefits. However it is considered that there

is more work to be done to deliver high-quality SuDS, particularly with smaller

developers on smaller sites. There was general agreement that it is easier to get

good outcomes (particularly for multiple benefits) on larger sites.

• Improving interactions of the SAB approval process with other local authority

functions, primarily planning and highways, is still a challenge. For the future it is

hoped that there might be some synergies with the planning application process

that can be exploited to assist with the evaluation of the requirements for amenity

and biodiversity during the planning and SAB processes.

• For future improvements to the process, stakeholders felt it may be beneficial for

the language and processes for determining compliance to be made more

prescriptive e.g. additional ‘deemed to comply’ rules. It was also suggested that

further awareness and education may be required.

• Applicants need to be encouraged to consider the Standards as a whole suite of

requirements, rather than in isolation.

• Ideally applicants should provide a cohesive commentary on the drainage strategy

and overall approach as this makes the application and approval process more

straightforward.

Feedback from Welsh stakeholders (Annex F, Section 3) on individual standards is

summarised in Table 4.

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Table 4 Summary of feedback from Welsh stakeholders on individual standards

Standard Key feedback

Destination hierarchy • Compliance is relatively easy to assess

• Clear guidance is required on assessment of ground

conditions for infiltration

Hydraulic control (including

Interception) • Compliance is relatively easy to assess

• There are challenges with delivery of this standard

for small sites

• Interception is assessed using ‘deemed to comply’

rules

• Interception is recognised as a key driver for delivery

of amenity and biodiversity benefits

Water quality • Smaller sites need ‘deemed to comply’ rules

• Larger sites use the SuDS manual Simple Index

Approach (SIA)

• Additional guidance is needed on component sizing

and higher pollution hazard sites

Amenity • The term ‘maximise’ is considered helpful to allow

flexibility, but challenging as it also allows subjectivity

• Landscape expertise within approval bodies is

required, but is often not available

Biodiversity • The term ‘maximise’ is considered helpful to allow

flexibility, but challenging as it also allows subjectivity

• A more quantitative compliance approach may result

in better outcomes in the future.

• Ecologist expertise within approval bodies is

required, but is often not available

Feedback from Welsh stakeholders (Annex F, Section 3) on the perceived challenges or

constraints to the delivery of multiple benefit SuDS included:

• Lack of awareness of the standards

• Lack of skills (covering all standards requirements, especially amenity and

biodiversity)

• Lack of understanding of the SuDS approach (e.g. source control, management

train)

• Challenging site characteristics e.g. small site with low infiltration potential (need for

‘deemed to comply’ rules)

59

• Inconsistency of approach between different approving organisations

• Poor pre-application consultation (e.g. inefficient or lack of alignment and

integration with planning).

Feedback from Welsh stakeholders (Annex F, Section 3) on the perceived enablers for the

delivery of multiple benefit SuDS included:

• Pre-application discussions

• Active approving body engagement during the design process

• Inclusion of Interception promotes on the surface, vegetated systems

• ‘Deemed to comply’ approaches

• Local policy, guidance and proformas used for approval.

4.4.2 How could a requirement for multiple benefit SuDS deliver wider government policy and regulatory objectives? (RQ9)

Multiple benefit SuDS are referenced within a number of recent government, industry and

regulatory policy and guidance documents as a key mechanism for delivering on policy

objectives. Such documents are themselves drivers for the delivery of multiple benefit

SuDS. However, without aligned standards that re-state these requirements, opportunities

are being missed. Standards requiring SuDS that improve development and infrastructure

resilience, manage water quality and provide valuable amenity (including mental health)

and biodiversity value – in addition to flood risk management, would ensure SuDS policy is

aligned with and is facilitating wider policy delivery.

Table 5 highlights key policy and guidance documents that make reference to the

opportunities and value provided by multiple benefit SuDS.

Table 5 Where reference is made to SuDS as a delivery mechanism for wider government

policy and regulatory objectives

National policy and

guidance

Use of SuDS for…

Amenity5 Biodiversity

Climate

resilience and

adaptation

Water quality

management

Flood risk

management

25 year Environment Plan (HM Government, 2018)

✓ ✓ ✓ ✓ ✓

National Planning Policy Framework (MHCLG, 2019a)

Use of SuDS to deliver ‘multiple benefits’ ✓

5 Including health and wellbeing

60

National policy and

guidance

Use of SuDS for…

Amenity5 Biodiversity

Climate

resilience and

adaptation

Water quality

management

Flood risk

management

Planning Practice Guidance (MHCLG, 2020c)

✓ ✓ ✓ ✓ ✓

Climate change national adaptation programme (Committee on Climate Change, 2019)

Use of SuDS to deliver ‘co-benefits’ ✓

National FCERM strategy (Environment Agency, 2020b)

✓ ✓ ✓ ✓

National Design Guide (MHCLG, 2019b)

✓ ✓ ✓ ✓

Design and Construction Guidance (Water UK, 2020a)

✓ ✓ ✓ ✓

4.4.3 What are the barriers, enablers and drivers for the inclusion and delivery of multiple benefit SuDS criteria? (RQ10)

These two questions have been answered together in the context of an evaluation of the

current enablers and barriers to the requirement and delivery of multiple benefit SuDS.

The following tables, drawn from Annex A (Section 5), summarise the enablers and

barriers relating to planning legislation (Table 6), other legislation (Table 7), policy and

guidance (Table 8), and resources (Table 9).

Table 6 demonstrates that there are wide ranging enablers for the delivery of multiple

benefit SuDS through the legislative framework of the planning system (e.g. local plans,

permitted development). However, many of these also pose barriers, where requirements

for multiple benefit SuDS are inconsistent or the current scope does not explicitly include

multiple benefits.

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Table 6 Planning legislative enablers and barriers

Enabler Barrier Comment

General ✓ Decisions must be made in accordance

with up-to-date local plan and other

material considerations.

Local plan consultations ✓ ✓ County Councils and neighbouring LAs

have to be consulted, but LLFAs not

named specifically

Local plans ✓ ✓ SuDS within scope of ‘Strategic

Priorities’, new requirement for review

every 5 years and could be included in

Statements of Common Ground.

However, infrequent and delayed

update means that requirements for

multiple benefit SuDS are slow to be

reflected in Local Plans.

Consultations before granting

planning permission and prior

approval

✓ ✓ LLFAs have to be consulted before

granting planning permission for major

developments, but no requirement for

‘non-major’ developments. Environment

Agency consultee remit for SuDS has

been unclear.

Permitted development ✓ ✓ Planning permission needed for large

hard surfaces, although SuDS outside

scope of operational development

associated with permitted development.

Future changes outlined in

Planning for the Future ✓ A more ‘standardised’ delivery model

could include national and local design

codes for multiple benefit SuDS

Table 7 demonstrates that there are many drivers for a requirement for multiple benefit

SuDS within environmental legislation. Amenity provision through the delivery of SuDS is

not specifically addressed. These drivers relate to flood risk management, water quality,

climate change management, biodiversity net gain and highway drainage. Biodiversity and

highway drainage requirements currently have the potential to pose barriers where

multiple benefit SuDS are not aligned with legislative requirements.

Table 7 Other legislative enablers and barriers

Enabler Barrier Comments

Flooding ✓ ✓ FWMA established LLFAs, but not

implemented in full and scope does not

extend to multiple benefits

62

Enabler Barrier Comments

Climate Change ✓ Climate Change Act requires LPAs to

have a strategy for adaptation and

mitigation. Section 19 of the 2004

Planning & Compulsory Purchase Act

(2004) requires local authorities to have

local plan policies that contribute to

climate change mitigation and adaption

Biodiversity ✓ ✓ The requirement for Biodiversity Net

Gain in the Environment Bill (and

amended NPPF) is seen by some as an

enabler. But some stakeholders

consider it as a potential barrier if it is

too onerous or if it is applied poorly.

Section 40 of the NERC Act places a

duty to conserve biodiversity on public

authorities in England. 'Conserving

biodiversity' may include enhancing,

restoring or protecting a population or a

habitat.

Air Quality ✓ Multiple benefit SuDS can contribute to

reducing exposure to air pollution in Air

Quality Management Areas (AQMAs)

Water Quality ✓ LAs have to have regard to RBMPs

which can refer to multiple benefit SuDS

Water Quantity ✓ Right to connect to public sewer has

disincentivised the use of surface SuDS

Highways ✓ ✓ Highway Authorities will only adopt

SuDS draining highways, although

some highway authority guidance does

include some multiple benefit SuDS

Table 8 demonstrates that there are a large number of policy and guidance documents

that either contain direct references to multiple benefit SuDS, or contain ‘hooks’ or

encouragement for equivalent outcomes through the delivery of green and blue

infrastructure. Some of those documents have the potential to pose barriers where

requirements are conditioned in some way or where principles are not backed up by robust

industry guidance.

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Table 8 Policy and guidance drivers/enablers and barriers

Driver/

Enabler

Barrier Comments

25 Year Environment Plan (HM

Government, 2018) ✓ Multiple references to importance of GI,

SuDS, delivering multiple benefits,

commitment to environmental net gain

National Planning Policy

Framework (MHCLG, 2019a) ✓ ✓ Requirement for multiple function

SuDS, but only specifically for ‘major’

developments and diluted by ‘where

possible’

Support for ‘net gain’ principle

throughout

Planning Practice Guidance

(MHCLG, 2020c) ✓ ✓ Potential role of multiple benefit SuDS

recognised in several sections.

PPG includes ‘get-out’ clauses that can

undermine delivery.

PPG will need amending if role of SuDS

to deliver multiple functions is to be

realised (e.g. BNG)

National Design Guide

(MHCLG, 2019b) ✓ Contains ‘hooks’ for multiple benefit

SuDS and opportunity to include in

National Model Design Code and local

design codes

Living with Beauty (BBBBC,

2020) ✓ Contains encouragement for promotion

of green infrastructure and long-term

stewardship

Climate Change National

Adaptation Programme and

Committee on Climate Change

annual progress report

(Committee on Climate

Change, 2019)

✓ The Committee on Climate Change

emphasises the potential role of ‘green’

SuDS to help deliver climate change

adaptation

River Basin Management Plans

(Defra, 2015b) ✓ Refers to multi-functional SuDS –

although usually as specific projects

rather than as ‘development as normal’

Design and Construction

Guidance (Water UK, 2020a) ✓ ✓ Water UK’s national guidance for

WaSCs mandates a good practice

approach to the SuDS design process

and to achieving multiple-benefit

outcomes.

The exclusion of some common

multiple benefit SuDS components from

the suite of adoptable SuDS (e.g.

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Driver/

Enabler

Barrier Comments

pervious pavements) and inability of

WaSCs to adopt components that only

drain highway runoff are likely to be

barriers.

Drainage and Wastewater

Management Plans (Water UK,

2020b)

✓ Potentially important drivers for WaSCs

to increase construction of, adoption of

and cooperative working with others on

multiple benefit SuDS

Mainstreaming Green

Infrastructure6 ✓ Comprehensive NERC-funded

guidance to help deliver GI (including

SuDS) through local plans

Delivering better water

management through the

planning system (Bide and

Coleman, 2019)

✓ CIRIA planning policy and project

guidance that includes multiple benefit

SuDS to help deliver integrated water

management

BNG Good Practice Principles

(CIEEM, CIRIA and IEMA,

2016)

✓ ✓ These Principles could provide the

building blocks for SuDS design for

wildlife. But without clarity on good

practice regarding SuDS and BNG,

there is a risk of perverse outcomes.

Biodiversity Metric (version 2.0) ✓ ✓ Having this metric provides a means by

which delivering biodiversity benefit can

be measured. However, SuDS score

quite poorly in the 2019 version of the

Biodiversity Metric. This is being

updated, but how much the scoring will

improve is uncertain.

Fields in Trust guidelines

(Fields in Trust, 2015) ✓ Accepted standard for provision of

public open space

Manual for Streets (CIHT, 2020) ✓ The planned revision of this guidance

will include a section on SuDS

integration into highways

Improving access to

greenspace (Public Health

England, 2020)

✓ Looks at how greenspace can be

formally valued, local decision-making

and making greenspace available and

accessible to all

National Framework of Green

Infrastructure Standards ✓ Recognises the importance of green

space close to where people live, aims

to mainstream GI as essential

6 https://mainstreaminggreeninfrastructure.com/

https://mainstreaminggreeninfrastructure.com/

65

Driver/

Enabler

Barrier Comments

(Natural England, currently

unpublished)

infrastructure and support LAs in

assessing GI

Designing Blue Green

Infrastructure for water

management, human health

and wellbeing: Summary of the

evidence and principles for

design' (Choe et al., 2019)

✓ Lays out evidence for different health

benefits arising from SUDS and

highlights design principles through

which these can be achieved. Includes

the potential to help address health

inequalities

Table 9 demonstrates that there are a range of barriers to the delivery of multiple benefit

SuDS relating to skills, capacities, perceived economic impacts and challenges associated

with the approval and adoption process.

Table 9 Resources enablers and barriers


Effect on financial

viability of

developments

✓ ✓ Local plan viability assessments for SuDS policies

indicate that there is mixed evidence of any

additional cost as SuDS should be factored in at

design stage.

However, there is a perception among practitioners

that SuDS pose a challenge to financial viability (as

a result of land take costs and additional

assessment and design costs).

Competencies ✓ Concerns of the knowledge and skills of SuDS

design and evaluation of those in LLFAs and LPAs.

Knowledge and skills of design teams is also

recognised as a challenge.

Capacity ✓ There are concerns relating to the workload and

capacity of planners and flood risk managers for the

current NSTS.

There is a concern this will be exacerbated with the

introduction of requirements for multiple benefit

SuDS.

Inconsistencies ✓ Inconsistent approaches between LPAs and LLFAs

is recognised as a challenge.

Allocating

maintenance

✓ Challenge of allocating responsibilities for

maintenance and the reliance on private

management companies.

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Funding

maintenance

✓ Varied and inconsistent approaches to secure a

sustainable source of funding for maintenance, e.g.

commuted sums.

Multiple benefit

SuDS design

✓ Little involvement of landscape architects, designers

or ecologists in SuDS design or in LLFAs and LPAs.

Compliance ✓ Shortage of LLFA and LPA staff to check

compliance with SuDS design.

5 Recommendations

5.1 Updating the standards

5.1.1 Overall objectives for the standards

Based on the evidence reviews and stakeholder engagement undertaken as part of this

project, the overarching objectives for an amended suite of SuDS standards should be as

follows:

1) Be clear and easily understood (with accompanying guidance where needed)

2) Allow consistent interpretation and implementation

3) Be robust7

4) Promote and support the delivery of more sustainable, higher quality, climate

resilient development

5) Maximise the opportunities for biodiversity enhancements from SuDS (such as

through biodiversity net gain)

6) Minimise the risk of perverse outcomes.

5.1.2 Content

It is recommended that existing standards S1 to S6 are replaced with a new suite of six

standards. The recommended updates to the NSTS are presented in Part 2 of this report

(Sections 7 to 12).

7 Not ‘future-proofed’ - the future regarding policy is too uncertain, instead recommend a firm programme of evaluation and feedback to update the standards as policy and practice evolves over time.

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The new standards cover the following:

• Standard 1: Runoff destinations

• Standard 2: Everyday rainfall

• Standard 3: Extreme rainfall

• Standard 4: Water quality

• Standard 5: Amenity

• Standard 6: Biodiversity

Each standard is accompanied by a set of clarifications that provide guidelines to support

the interpretation, delivery and evaluation of the standard.

In addition to the standards and clarifications, it is also recommended that three key

principles are included as over-arching statements with the new standards, so that these

are applied when planning and designing schemes to meet the new standards (Section 6).

These principles were highlighted by stakeholders (Annex D, Annex E and Annex F) as

key requirements for ensuring good SuDS design submissions that deliver multiple

benefits. These principles are:




This project has focused on providing recommendations regarding promoting multiple

benefit SuDS, which means that it has not looked at changes to existing standards S7 to

S14 that cover for the following aspects of SuDS design:

• Flood risk within the development

• Structural integrity

• Designing for maintenance considerations

• Construction.

• However, based on feedback received from the Project Steering Group, the focus

groups and comments received from respondents to the second of this project’s

online surveys on the updated Standards (Annex D Online Survey 2)), it is

recommended that long term maintenance needs and maintainability should be

robustly considered in the design of all SuDS, in order to ensure that the system

delivers each of the standards over the design life of the development. This

requirement would logically be covered as part of existing standards by expanding

Standard S12 (‘Designing for maintenance considerations’). Standard S12 currently

only includes a statement related to the use of pumping.

• One or more additional standards are included to cover (i) requirements for effective

construction management planning to ensure both SuDS and development

construction processes protect and do not compromise the functionality of the

drainage system; and (ii) the design of SuDS to prevent pollution during the

construction process.

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One further recommendation, proposed and endorsed by the PSG at the final PSG

meeting, is that the updated standards should be reviewed in 12-24 months’ time,

depending on the progress of implementation of the Environment Bill and Planning White

Paper proposals.

5.1.3 Objectives specific to each standard

On the recommendation of the PSG, each new standard presented in Sections 7 to 12 has

its own set of objectives. These objectives represent:

• Best practice as set out in The SuDS Manual (Woods Ballard et al., 2015)

• The key drivers as identified in the evidence review (Sections 4.2.3, 4.4.2 and 4.4.3)

• Key requirements as defined by the PSG or identified by stakeholders via the Focus

Groups (Annex E) or the online surveys (Annexes C and D).

5.1.4 Evidence base specific to each standard

The evidence base presented alongside each new standard gives an overview of key

evidence identified and reviewed as part of this project. Each piece of evidence cited

supports either the Contractor’s recommendation that a standard is required or the

Contractor’s recommendations for further research or guidance.

This evidence has been identified from the following sources:

• Specific legislative and policy drivers, as detailed in Annex A

• The evaluation of hydraulic design methods, as detailed in Annex B

• Key requirements identified by stakeholders, as detailed in Annexes C, D and E

• Lessons from the implementation of Schedule 3 of the Flood and Water

Management Act (2010) in Wales, as detailed in Annex F

• The case studies presented in Annex G and summarised in Annex A, Section 3.

5.2 Alignment with other national policy and guidance

Based on the review of evidence (Annex A), the case studies (Annex D), and the feedback

received from the stakeholder engagement activities (Annexes C, D, E and F), the

Contractor recommends that the following actions, in addition to updating the NSTS, are

taken to help support and embed requirements for multiple benefit SuDS within

development:

• The SuDS Manual (Woods Ballard et al., 2015) should be updated to reflect the

proposed new standards. Potentially this could also address much of the need for

new or improved guidance at the national level.

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• MHCLG and Natural England should include requirements for multiple benefit SuDS

in the National Model Design Code and the Green Infrastructure Standards

respectively. Any requirements should be aligned with the updated standards and

best practice guidance as set out in The SuDS Manual. Alternatively The SuDS

Manual could be recognised as the SuDS component of the National Model Design

Code.

• MHCLG should consider revising the NPPF (MHCLG, 2019a) and PPG (MHCLG,

2020a) and Permitted Development Guidance for Householders (MHCLG, 2019c) in

order to:

o Deter LPAs from allowing widescale implementation of single function SuDS

o Remove inference that SuDS are only of value in flood risk areas, explaining

that SuDS are of greatest value upstream of flood risk areas in order to

mitigate the impact of development on downstream flood risk

o Reference the contribution and value of multiple benefit SuDS in other

sections (e.g. 11 - Making effective used of land; and 12 - achieving well-

designed places)

o Minimise the likelihood of multiple benefit SuDS not being implemented on

the basis of cost, inappropriateness, viability or reasonable practicability by

requiring a robust evidence base justifying that they cannot be delivered

o Require the consideration of SuDS through pre-application consultation, and

in sufficient detail at early stages of the planning process to ensure all

opportunities are taken to maximise benefits and minimise risks

o Inform householders of hard surfacing permitted development rights and how

to comply with them

• MHCLG and Defra should review the statutory consultee and SuDS approval roles

of LLFAs.

• MHCLG and Defra should work with the water industry to review the continuing

suitability of the current right of connection to public sewers (set out in Section 106

of the Water Industry Act, 1991).

• MHCLG and Defra should work with the industry on developing new adoption codes

for SuDS in private curtilage, highways, and public open space to ensure:

a) Options exist for the adoption of all elements of multiple benefit SuDS for

all land types; and

b) Risks associated with SuDS adoption by private property management

companies are managed appropriately.

70

• MHCLG and Defra should put in place appropriate national level guidance,

templates and checklists to ensure that the proposals set out in the Planning White

Paper Planning for the Future (MHCLG, 2020b), if implemented, promote and not

put at risk the delivery of multiple benefit SuDS. As part of these measures, it is

recommended that LPAs develop new or adopt existing local guidance as local

design codes in areas for ‘growth’, ‘renewal’ and ‘protection’. A detailed review of

the implications of the Planning White Paper for the delivery of multiple benefit

SuDS has been undertaken and is presented as Annex H.

• MHCLG and Defra should review how BNG and the delivery of multiple benefit

SuDS can be more complementary and develop appropriate/combined guidance.

• Defra should investigate adapting London’s Urban Greening Factor model (Greater

London Authority, 2017) for the NSTS in close co-operation with those developing

BNG and ENG.

• Defra should consider the need for further evidence to counter perceptions of cost

and viability concerns, noting that current published evidence (Defra, 2011a) does

not take land take into account.

• MHCLG, Defra and Department for Transport should increase resources to LPAs,

LLFAs and highways authorities to understand, train for and implement multiple

benefit SuDS, including to embed local SuDS policy and guidance within Local

Plans or SPDs in order to strengthen the requirements, and to develop, enforce and

monitor the discharge of suitable pre-commencement planning conditions that cover

SuDS design and delivery.

• CIHT should include multiple benefit SuDS that deliver on all of the proposed

standards in the revised Manual for Streets (CIHT, 2020).

• A central repository for SuDS best practice guidance should be retained to support

planners, drainage designers and adoption bodies, that aligns with the proposed

revised standards and the outcomes of any additional actions taken to deliver

multiple benefit SuDS.

5.3 Further research and development

There is a raft of existing and emerging policy and guidance with the same objectives as

the proposed updated Standards set out in Part 2 of this report, and there are existing

metrics and tools that cover some (but not all) of the standards. However, for the

standards to be delivered and approved consistently, there is a need to:

a) Ensure the most valuable outcomes with respect to alignment with other policies

(e.g. BNG);

b) Develop new metrics (e.g. amenity); and

71

c) Improve existing tools to support implementation of the new standards (e.g. water

quality).

Recommendations made by the Contractor linked to each of the individual standards are

set out under each standard in Part 2.

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PART 2 – UPDATED STANDARDS

6 Application of the standards

6.1 Overview

The requirements for SuDS for all new development on greenfield sites and re-

development sites have been defined through a series of standards. There are two types

of standard. Standard 1 is a Hierarchy Standard while Standards 2 to 6 are Fixed

Standards.

▪ The Hierarchy Standard gives criteria for prioritising the choice of runoff destination

(Standard 1);

▪ Fixed Standards (Standards 2 to 6) state the criteria that all SuDS designs should

satisfy.

Each standard is accompanied by a set of ‘Clarifications’. These provide supporting

principles to guide the interpretation, delivery and evaluation of each standard.

All six standards should be met for all new development on greenfield sites and re-

development sites. The standards are complementary and the delivery of each standard

should support and not compromise delivery of the other standards. A SuDS design that

delivers multiple benefits will include a range of components; each of these components

should contribute to the delivery of several if not all of the standards.

If there are specific circumstances that mean it is not possible to deliver one or more of the

standards, the standard should be met as much as possible and justification of this should

be developed in consultation and agreed with the approving body.

The standards should be applied using three key principles:




These principles follow best practice as defined in The SuDS Manual (Woods Ballard et

al., 2015) and are described in Sections 6.2 to 6.4.

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6.2 Use of ‘a SuDS Approach’

All surface water management schemes should be designed using ‘a SuDS approach’, as

defined below:

Definition of ‘a SuDS approach’

A SuDS approach:

• Mimics natural drainage systems - delivering surface water management that

recognises the value of rainfall and runoff as a resource;

• Manages surface water flooding and the increased rates and volumes of runoff

from developments;

• Captures and treats the pollutants in urban runoff;

• Uses components in combination as a ‘management train’ – integrating these

throughout the development and its landscape to help create healthy and resilient

spaces for people and habitats for wildlife;

• Manages runoff close to its source, and prioritises components that lie on the

surface and incorporate vegetation.

• Meets the requirements for delivering multiple benefit SuDS over the lifetime of

the development, by planning for a changing climate and provision of appropriate

long-term maintenance.

6.3 Early and integrated design

Surface water management should be considered at the very earliest stages of site

appraisal, planning and design in order to support and be integrated with:

• The water supply strategy;

• The layout of the roads, buildings and public open spaces;

• Any biodiversity, amenity and green infrastructure delivery strategy, and

• Climate resilience options for the development.

Planning of a new site layout should be informed by the topography and the requirements

of surface water management systems to both effectively drain and treat the runoff. Any

existing watercourses, ditches and other drainage features, both within and adjoining the

site, should help inform proposals. By doing so, biodiversity, amenity and cost-

effectiveness can be maximised through using areas of land throughout the development

for a range of multifunctional purposes in addition to surface water management (e.g.

landscaping, car parking, recreational areas, rainwater harvesting).

The design process should include professional planning, urban design, landscape and

ecology expertise in order to deliver the greatest amenity and biodiversity outcomes from

these standards.

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Where local authorities have adopted guidance on SuDS, this should also be taken into

account in any development proposal.

Infiltration testing should be undertaken at the earliest possible stage to inform the

conceptual and outline design of the scheme.

Where a development is phased, the design of the SuDS scheme should ensure that each

of the standards will be delivered for each phase of the development. The proposals

should consider the effects of each stage of site development, as well the performance of

the surface water drainage system for the development as a whole, to ensure risks are

mitigated and both short-term and long-term benefits are maximised.

6.4 Embedding SuDS within the development planning process

Developers should demonstrate that ‘a SuDS approach’ to surface water management that

delivers compliance with each of the standards has been developed from the earliest

stages of strategic planning and integrated throughout the development and its landscape

design.

Evidence of compliance should be provided at the site allocations stage, pre-application

stage, and in outline design and full planning application submissions.

The SuDS approach and compliance with the standards should be integrated with and

embedded within the Flood Risk Assessment, the Drainage Strategy, the Landscape

Strategy and Ecology Plan, and the Design and Access Statement for the development.

7 Standard 1 – Runoff destinations

7.1 Objectives

The objectives for this standard are the following:

• Rainwater harvesting is robustly considered wherever it would provide a valuable

contribution to water security and a safe, cost-effective water supply option.

• Opportunities to infiltrate runoff into the ground from impervious surfaces are

maximised wherever appropriate, in order to recharge soil moisture, river baseflows

and groundwater and to reduce volumes of runoff discharged to receiving

waterbodies or sewers.

• Discharges to (closed) surface water sewers or other piped drainage systems are

considered only after infiltration and discharges to open surface water bodies, in

order to minimise future headroom (capacity) risks and enhance system resilience.

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• Discharges to combined sewers are considered as a last resort due to the risks of

sewer surcharge and combined sewer overflows resulting from surface water runoff

contributions (particularly under future climate scenarios).

7.2 Evidence base

The evidence that has been used in determining the need for and content of this standard

and clarifications can be found in:




• Annex F – Review of the implementation of Schedule 3 in Wales.

The following key evidence either supports our recommendation that a standard on

prioritising runoff destinations is included or indicates possible compliance assessment

needs or exception clauses8:

1) The National Design Guide (NDG) (MHCLG, 2019b) ‘Resources’ section identifies

rainwater harvesting as a mechanism for development to ‘Maximise Resilience’

(paragraph 149). A standard that ensures that rainwater harvesting is given

appropriate consideration will support the delivery of this guidance.

2) The National Planning Policy Framework (NPPF) (Paragraph: 080 Reference ID: 7-

080-20150323) (MHCLG, 2019a) includes a hierarchy for runoff destinations

(although this excludes rainwater harvesting - infiltration is the highest priority

destination). An equivalent standard would strengthen implementation of this

policy.

3) The Design and Construction Guidance (DCG) (Part C3, Clause 12) (Water UK,

2020a) includes a hierarchy for runoff destinations that includes rainwater

harvesting as the highest priority destination. This is a mandatory document for all

sewers submitted for adoption by sewerage undertakers. An equivalent standard

would ensure all designs (irrespective of adoption body) were following the

same design approaches.

4) Currently all new homes must be built to a water consumption standard of 125 litres

per person per day, according to Part G of The Building Regulations 2010 (HM

Government, 2016). There is also an optional Building Regulations requirement of

110 litres per person per day, which local authorities in water stressed areas can

apply where there is a clear need. Rainwater harvesting is one way to achieve

lower consumption rates. A standard that ensures that rainwater harvesting is

8 It should be noted that exception clauses are not part of the updated standards, but recommendations are made regarding the development of these for specific standards.

76

given appropriate consideration will ensure there is a link between recycling

needs for rainwater and drainage, and will help future-proof the Standards.

5) Rainwater harvesting is actively promoted in The London Plan (Mayor of London,

2016). A standard that ensures that rainwater harvesting is given appropriate

consideration will support the London Plan objectives.

6) Evidence of the cost and carbon efficiency of individual property rainwater

harvesting systems is not currently clear (Parkes et al., 2010), but they are likely to

deliver significant benefits where there is regular demand for non-potable water and

roof areas are large (e.g. industrial, commercial, agricultural/horticultural buildings).

Rainwater harvesting for multi-family occupancy properties with shared toilets may

also merit detailed consideration. Waterwise (2020) concludes that most people feel

very positive about the idea of a rainwater harvesting system in their home, but that

perceived cost and uncertainty over maintenance needs were potential barriers. A

standard that ensures that rainwater harvesting is given appropriate

consideration will support the use of runoff as a cost-effective supply option.

7) Concerns were raised by the focus groups during the workshops undertaken as part

of Task 1 (Annex E, Section 4.5.1) that prioritising infiltration could be in conflict with

requiring biodiversity gain. Guidance on compliance should consider whether a

higher value scheme should be prioritised over a scheme that delivers

infiltration only.

8) Feedback from the online survey (Annex C, Sections 7.1 and 9.1) and the focus

groups during Task 1 (Annex E, Section 3) indicated that rainwater harvesting was

likely to be challenging on the grounds of practicality and was not generally

considered as a high priority multiple benefit at present, but there was an indication

that building in climate resilience with respect to both flood risk management and

water resources (which can be delivered by rainwater harvesting) was a high

priority.

9) The equivalent Welsh statutory standard (Welsh Government, 2018 – S1. Surface

water runoff destination) was considered by the Welsh stakeholders who were

interviewed as part of this project (Annex F, Section 3) as relatively straightforward

to implement and evaluate. Interviewees indicated that the use of water butts was

often used to demonstrate compliance with the highest priority destination.

7.3 Proposed standard

STANDARD 1: DESTINATION OF RUNOFF

Apply ‘a SuDS approach’ using the following hierarchical process for determining ‘final’

runoff destinations:

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a) Where feasible, use surface water runoff as a resource for non-potable uses

(i.e. rainwater harvesting)

b) Where appropriate, maximise opportunities across the site to infiltrate runoff

into the ground

c) Discharge any remaining runoff to a surface water or above ground surface

water drainage system

d) Where (c) is not feasible, discharge any remaining runoff to a piped surface

water drainage system

e) Where (d) is not feasible, discharge any remaining runoff to a combined

sewer.

7.4 Accompanying clarifications

The following clarifications provide guidelines to support the interpretation, delivery and

evaluation of the standard.

1. The ‘final’ runoff destination represents the point of discharge from the

development site to the environment or downstream drainage system, following

management of that runoff using ‘a SuDS approach’.

2. Depending on the site characteristics, drainage from different parts of the site can

have different drainage destinations.

3. Depending on the opportunities for rainwater harvesting and infiltration, runoff

from higher return period events may need to discharge to alternative

destinations.

4. Demonstrate that rainwater harvesting (specifically designed for water supply

and, potentially, surface water management, purposes) has been considered

where:

a. There is a suitable demand for non-potable water and available roof areas

that, together, will deliver efficient water savings (e.g. industrial,

commercial, horticultural, educational, public sector and multiple-

occupancy buildings)

b. There is a need for landscape irrigation

c. The development is in an area of high water stress.

5. Water butts make a contribution to reducing stress on potable water supplies and

are a part of sustainable building design. However water butts are not designed to

meet specific supply objectives and, unless they are designed with a storage

volume that is always available for stormwater management purposes (the

discharge from which is controlled to a suitable rate), then they cannot deliver any

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guaranteed performance with respect to surface water management. It is likely

that basic water butts will be full during extreme rainfall events.

6. Evaluate infiltration opportunities and risks across the site by assessing soil

permeabilities (through infiltration testing that follows industry good practice),

ground stability, groundwater and surface water pollution risk, groundwater

flooding, and ingress to combined or foul sewers. Evaluate and manage these

risks following the guidance set out in industry published guidance9 to present

appropriate evidence that justifies the extent of infiltration adopted in the design.

7. Where infiltration components are used to discharge into the ground, ensure

groundwater quality is protected by compliance with Standard 4 (Water quality).

8. If the approving body considers an alternative scheme for managing surface

water runoff as being more appropriate and providing greater multiple benefits

than infiltration into the ground, ensure that this alternative scheme is compliant

with all other standards and demonstrate that it can deliver greater benefits

without negatively impacting groundwater recharge needs.

9. On the surface infiltration components should be preferred as they have the

capacity to deliver Standards 5 (Amenity) and 6 (Biodiversity).

10. Where discharge of runoff is proposed to any surface water body or sewer,

ensure water quality is protected by compliance with Standard 4 (Water quality).

11. Where discharge of runoff is proposed to destinations (d) or (e), provide evidence

explaining why destinations (a), (b) or (c) cannot be used.

12. The right to discharge to any proposed receiving surface water body should be

agreed with the riparian owners at the point of discharge. Where the site is not

adjacent to the receiving surface water body, access rights to the intervening land

will also be needed.

13. Any connection to a sewer will require approval by the sewerage undertaker, and

schemes will need to meet the requirements set out in the Design and

Construction Guidance (Water UK, 2020a).

14. Provide a detailed management and maintenance plan that supports the original

design objectives to ensure that all runoff discharge destination points are

maintained to ensure performance of the system in the long term.

9 This includes The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions; BRE365 (Building Research Establishment, 2016) or subsequent editions; and industry infiltration assessment checklists (e.g. The SuDS Manual Infiltration Assessment Checklist or equivalent)

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7.5 Additional recommendations

In addition to the accompanying clarification statement above, the Contractor also

recommends that:

• Guidance is provided to support developers and LPAs to identify the opportunities

for rainwater harvesting and to clarify why, when and how rainwater harvesting can

be cost-effectively delivered for new development. This should include reference to

the potential integrated use of systems to store rainwater for use and to attenuate

stormwater and the emerging active management and smart technologies with

which these systems can be optimised.

• Guidance is provided that clarifies the required considerations for moving through

the hierarchy of runoff destinations.

• Guidance is provided for determining ‘feasibility’ of discharging to a surface water

body or sewer. This should include consideration of access, distance, securing

rights to discharge and agreements with any third parties over intervening land,

inappropriateness or inadequacy of discharge point or conveyance route, health

and safety risks, pumping requirements, and long-term maintenance risks.

8 Standard 2 – Everyday rainfall

8.1 Objectives


• Replicate natural catchment response to frequent (i.e. ‘everyday’) rainfall

• Protect receiving surface waters from the damage caused by frequent, unnatural,

high velocity runoff from development following ’everyday’ rainfall

• Protect the quality of receiving waterbodies (by preventing the discharge of polluted

runoff from ’everyday’ rainfall)

• Protect sewer and drain capacities during ‘everyday’ rainfall by managing

discharges from developments

• Reduce the volume of wastewater that goes to treatment (and thus the associated

carbon cost), where discharges are to a combined sewer.

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8.2 Evidence base







managing everyday rainfall is included or indicates possible compliance assessment

needs or exception clauses:

• There are multiple national and local government guidance and plans promoting the

integration of green and blue-green infrastructure within development including the

25 year Environment Plan (HM Government, 2018), the National Design Guide

(MHCLG, 2019b), Living with Beauty (BBBBC, 2020), the Committee on Climate

Change’s latest annual progress report on adapting to climate change (CCC, 2019),

the National Framework of Green Infrastructure Standards (Natural England,

unpublished), the London Plan (Mayor of London, 2016). A standard that requires

the retention of everyday rainfall will provide a quantitative mechanism for

strengthening the requirements for ‘on the surface’, green SuDS.

• The retention of a specified rainfall depth on site is a primary surface water

management criteria internationally (UKWIR, 2021 – Phase 1). This criterion is

aimed at driving systems that better mimic a natural hydrological response (i.e.

achieve a similar volume of runoff as greenfield sites through the year, rather than

for extreme events) and preventing regular pollution events. A standard that

requires the retention of everyday rainfall will ensure England is more in line

with international best practice.

• Research is ongoing by the water industry to establish mechanisms for a shift to

zero spills from sewers by 2050 (UKWIR, Big Question 610). Provisional outcomes

from ongoing research (UKWIR, 2021 – Phase 2) indicate that the benefits to sewer

capacity and spill frequencies from the provision of Interception together with low

allowable peak flow rates is significant. A standard that requires the retention of

everyday rainfall for development sites discharging to sewers will help

protect sewer capacities.

• Approximately 50% of respondents to the online survey undertaken as part of

Task 1 considered that the NSTS should be strengthened and extended to include

Interception and that this would help deliver on multiple benefits requirements,

although there was recognition that Interception may be difficult to achieve and

10 https://ukwir.org/eng/big-questions-facing-uk-water-industry

https://ukwir.org/eng/big-questions-facing-uk-water-industry

81

could impact on development viability (Annex C, Section 5.11 and 8.11). CIRIA

(2013) is clear that the management of regular rainfall is fundamental in embedding

urban surface water management within the land use, urban design and planning

process so that maximum value can be obtained. A standard that requires the

retention of everyday rainfall will provide a quantitative mechanism for

strengthening the requirement for ‘on the surface’, green SuDS that deliver

amenity and biodiversity benefits. Clarifications will be required to ensure

that Interception is delivered to the fullest extent possible on any site and

clear guidance will be needed to facilitate design and approval for all site

characteristics.

• From the review undertaken in the UKWIR research (UKWIR, 2021 - Phase 1), it is

clear that, where local policy includes requirements for Interception, water quality or

volume control, these aspects are still rarely applied in designs. A standard that

requires the retention of everyday rainfall will strengthen local policy

requirements.

• Feedback from the Welsh stakeholders interviewed (Annex F, Section 3) suggested

that the requirement of Interception (particularly for sites where infiltration was not

an option) was also delivering benefits in terms of compliance with the water quality,

amenity and biodiversity standards, as it encouraged the use of source control

measures such as rain gardens, rain planters, etc. A standard that requires the

retention of everyday rainfall will provide consistency with the Welsh

standards (the implementation of which is considered beneficial by the Welsh

stakeholders interviewed as part of this project).


STANDARD 2: RUNOFF FROM EVERYDAY RAINFALL

Apply ‘a SuDS approach’ so that the majority of frequent rainfall events do not result in

runoff from the site to surface waters or piped drainage systems.




1. Frequent rainfall events are events with a total depth of up to 5mm (or an

alternative depth specified by the approving body).

2. Preventing runoff from the site for frequent rainfall events is called Interception.

SuDS components that usually deliver Interception include rainwater harvesting

systems, green roofs, infiltration components, pervious pavements, bioretention

82

systems, swales and dry basins. Interception is not provided by components that

retain an initial depth of runoff, but then discharge this runoff during a subsequent

event (e.g. ponds, highway gullies).

3. Interception occurs on natural catchments including those with clay soils and its

provision does not rely on soils with high infiltration rates.

4. By providing Interception, the storage volumes required to meet Standard 3

(Extreme rainfall) will reduce by the equivalent amount.

5. There will be times, following periods of prolonged wet weather, when

Interception will be limited or impossible (e.g. soils will be saturated). Therefore, it

is recommended that designs should aim to deliver Interception for approximately

80% of events during summer months and 50% of events during the winter period

(as suggested by the SuDS Manual), noting that compliance will be subject to

significant variability.

6. Provide evidence that the runoff from each impermeable surface, for at least 5mm

of rainfall, is either collected for use or infiltrated into the ground, or else captured,

conveyed and/or stored within components that will naturally absorb or retain

runoff and from which the runoff will be ‘lost’ to soils or the atmosphere.

7. Demonstrate compliance with this Standard by using industry published guidance

(including deemed to comply approaches)11 or methods/tools accepted by the

approving body.

8. If Interception cannot be delivered for every surface or for the entire 5mm rainfall

depth, deliver Interception to the greatest extent practicable for the site.

9. Provide evidence that the approach to managing runoff from ‘everyday’ rainfall

has been developed alongside and in support of the management of runoff quality

(Standard 4) and the delivery of amenity and biodiversity benefits (Standards 5

and 6).


design objectives to ensure that the performance of the SuDS in relation to the

management of ‘everyday’ rainfall is maintained in the long term.

11 This includes The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions.

83



recommends that:

• More detailed information is produced on the objectives, principles and outcomes of

an Interception design philosophy, including how it differs from infiltration and how it

can be delivered on sites where infiltration rates are low.

• More detailed guidance is provided regarding how Interception can be delivered

across a range of development types and scales. This should include ‘deemed to

comply’ rules to facilitate the design and approval processes.

• More detailed information is produced that explains why Interception is likely to be

relevant for all sites (to cover concerns over the need to have regular runoff with

which to flush receiving wastewater systems or provide regular flows in receiving

waterbodies).

• Training and awareness of Interception approaches is delivered across the industry.

• Tools are developed for when more detailed analysis is required.

• The delivery of this Standard is aligned and integrated with wider planning

objectives on Green Infrastructure and relevant links are provided in any guidance

produced.

9 Standard 3 – Extreme rainfall

9.1 Objectives


• Effectively mitigate the impact of development on receiving watercourse flood risk

• Deliver improved protection of the capacity of receiving sewers.

9.2 Evidence base




• Annex B – Review of criteria and methods for delivering hydraulic standards

• Annex C – Review of the application of the current NSTS (online survey 1).

84


extreme rainfall is included or indicates possible compliance assessment needs or

exception clauses:

• The NSTS are currently interpreted and applied in an inconsistent manner, as

determined from the online survey (Annex C, Sections 5.1-5.9 and 8.1-8.9). This is

due to their lack of clarity and simplicity, but also the range of design approaches

available with which to demonstrate compliance. This reinforces the findings from

the ongoing UKWIR research (UKWIR, 2021 - Phase 1). The current standard

should be reviewed and re-drafted to improve simplicity and consistency of

interpretation, and facilitate easier SuDS approval.

• Findings from the site model testing undertaken as part of Task 2 of this project

indicate that the runoff estimation method has limited impact on storage volume

requirements for a large number of development scenarios (Annex B, Section 5.2).

Guidance should retain the current flexibility regarding the methods that can

be used for runoff rate estimation. Suitable methods are those described in

The SuDS Manual (Woods Ballard et al., 2015).

• The original research on which the approach to both peak flow and volume control

was based (HR Wallingford, 2003) indicated that peak discharge rates should be

limited to at least 3 l/s/ha to adequately protect the receiving waterbody where

volumes of runoff are not controlled. A precautionary approach was taken and a

figure of 2 l/s/ha has subsequently been adopted within industry guidance (e.g. The

SuDS Manual (Woods Ballard et al., 2015)). However, the site model testing

undertaken in Task 2 of this project has demonstrated that using such a lower limit

rate can lead to long critical durations for the storage design, which are unlikely to

be relevant for the receiving catchment (Annex B, Section 5.10). Detailed evidence

that supports adoption of a 3 l/s/ha lower limit rate is presented in Annex B,

Appendix C.

• Provisional findings from the ongoing UKWIR research (UKWIR, 2021 - Phase 2)

suggest that the use of 50% betterment of existing previously developed runoff

rates may deliver suitable protection to receiving sewer capacity for small sites, but

that a maximum discharge rate may be needed for large sites to more effectively

mitigate their impacts. For example, the Supplementary Planning Document: Living

with Water (Hull City Council and Yorkshire Water, 2019) requires a maximum

discharge rate of 3.5 l/s with a 50% betterment on previously developed rates for

previously developed sites. Findings from the site model testing undertaken as part

of Task 2 of this project (Annex B, Section 5.12) indicates that previously developed

runoff rates, with a 50% betterment factor, may be of the order of 100-200 l/s/ha

(compared to greenfield runoff rates of the order of 1-10 l/s/ha). Use of factors

below 50% (e.g. 30% stated by the majority of survey respondents – Annex C,

Section 5.2.1) mean allowable runoff rates are even higher. This will be

exacerbating downstream flood risk issues. The current standard relating to

85

previously developed runoff rate controls may need to be reviewed, and

suitable supporting guidance provided.


STANDARD 3: RUNOFF FROM EXTREME RAINFALL

Apply ‘a SuDS approach’ that manages runoff that is discharged from the site to a

receiving surface water or sewer so that:

(A) Where the volume of runoff discharged from the development to surface waters or

piped drainage systems for the 1 in 100 year, 6 hour rainfall event is greater than

the volume of greenfield runoff for the same rainfall event, the following is satisfied:

The peak allowable discharge rate from the site for all rainfall events up to the 1 in

100 year return period, including appropriate allowances for climate change and

urban creep, is limited to:

• For greenfield sites: the 1 in 2 year greenfield runoff rate or 3 l/s/ha, whichever

is the greater

• For previously developed sites: the 1 in 2 year greenfield runoff rate or 3 l/s/ha

(whichever is the greater) or an increase of this rate by a factor specified by

the approving body in conjunction with the organisation responsible for the

downstream system

• For sites where there are specific flood risk constraints: a rate specified by the

approving body, in conjunction with the organisation responsible for the

downstream system

(B) Where the volume of runoff discharged from the development to a surface water or

piped drainage system for the 1 in 100 year, 6 hour rainfall event is less than or

equivalent to the volume of greenfield runoff for the same event, the following is

satisfied:

The peak allowable discharge rate from the site, including appropriate allowances for

climate change and urban creep, is limited to:

• For greenfield sites:

i) For rainfall up to the 1 in 2 year event: the 1 in 2 year greenfield runoff

rate or 3 l/s/ha, whichever is the greater

ii) For more extreme rainfall up to the 1 in 100 year event: the 1 in 100

year greenfield runoff rate

• For previously developed sites for all rainfall events up to the 1 in 100 year

return period: the 1 in 2 year greenfield runoff rate or 3 l/s/ha (whichever is the

86

greater) or an increase of this rate by a factor specified by the approving body

in conjunction with the organisation responsible for the downstream system

• For sites where there are specific flood risk constraints: a rate specified by the

approving body in conjunction with the organisation responsible for the

downstream system.




1. Where discharges from the site are to a surface water that can accommodate

uncontrolled surface water discharges without any associated environmental

(including flood risk) impact, e.g. the sea or a large estuary or a surface water

specifically identified by the lead local flood authority, then this standard need not

apply.

2. The 1 in 2 year greenfield runoff event can be considered as equivalent to the

estimated Qbar or Qmed for the development site.

3. The organisation responsible for the downstream system could be the

Environment Agency, highways authority, Canal & River Trust, Internal Drainage

Board, sewerage undertaker or other private organisation.

4. The provision of distributed storage, i.e. at multiple points across the site, rather

than a single storage component located prior to the point of discharge from the

site, will help deliver all of the other standards.

5. Estimate greenfield runoff rate(s) for the appropriate return periods using one of

the calculation methods set out in industry published guidance for SuDS design12.

6. When estimating the greenfield runoff rate, use the total site area that will drain to

the proposed drainage system (including both pervious and impervious surfaces).

7. Wherever a ‘relaxation factor’ is agreed and applied for previously developed

sites, provide evidence explaining why greenfield runoff rates cannot be

achieved. This relaxation factor should be no greater than 5 (i.e. the maximum

allowable discharge rate should not exceed 5 times the 1 in 2 year greenfield

runoff rate).

8. Estimate the greenfield runoff volume for the 1 in 100 year, 6 hour rainfall event

using one of the approaches set out in industry published guidance13.

12 This includes The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions. 13 This includes The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions.

87

9. Where the peak allowable discharge rate is calculated to be below 2 l/s, the flow

control for discharges from the site should be set to 2 l/s or have an orifice (or

equivalent) with a minimum diameter of 50mm, unless specifically stated

otherwise by the approving body.

10. Provide evidence that the risk of blockage of all flow controls has been managed

appropriately.

11. Provide evidence that sufficient storage is included within the proposed design so

that peak flow rates are limited to those required by the Standard.

12. Use a percentage runoff of 100% when estimating runoff from impervious

surfaces for design rainfall events.

13. Where landscaped areas contribute runoff to the drainage system during design

rainfall events, this runoff should be accounted for when estimating design

inflows.

14. Confirm that the standard percentage runoff (SPR) used in calculating runoff

rates and volumes is representative of the soil type on the site.

15. Always use the most up to date rainfall model14 for drainage design, together with

the most up to date government recommendations on climate change uplift

factors for rainfall intensities.

16. Apply an urban creep uplift factor to all impermeable surfaces, where permitted

development could occur in the future, as set out below (LASOO, 2016).

Residential development density

(dwellings / ha)

Urban creep allowance (%)

≤ 25 10

30 8

35 6

45 4

≥ 50 2

Flats and apartments 0

14 This is currently FEH13.

88

17. Provide evidence that the approach to managing runoff from extreme rainfall

through flow control and storage has been developed alongside and in support of

the delivery of each of the other Standards.

18. Provide detailed non-technical summaries of the approach, calculations

undertaken, outputs and results, and conclusions with respect to the design

alongside any hydraulic model outputs.


design objectives to ensure that the performance of the SuDS in relation to the

management of extreme rainfall is maintained in the long term.



recommends that:

• Further information is produced that covers the following:

o How compliance with this standard can support the delivery of Standard 4

(water quality management), Standard 5 (amenity) and Standard 6

(biodiversity) where the standards are considered together and an

integrated, holistic design approach is taken.

o Acceptable runoff rate estimation methods (including recommendation that

ReFH2 for plot-scale application is used without the addition of baseflow)

o How the delivery of Interception, rainwater harvesting and infiltration will

impact on the required attenuation storage volume

o How to estimate storage volumes using methods/models that have been

approved by the LPA, and use of critical duration rainfall events.

o How to manage runoff volumes from developments to the estimated

greenfield runoff volume (e.g. using infiltration, rainwater harvesting or Long

Term Storage methods)

o Suitable thresholds for rainfall durations for maximum storage volume

estimation

o Suitable maximum draw-down times

o How to meet this Standard for a range of development types and scales

o Clarification of the use of SuDS for sites in flood risk areas, and the use of

SuDS to manage existing surface water flood risk on the site.

89

• Research is undertaken that mimics the ongoing UKWIR research looking at sewer

capacity (UKWIR, 2021 - Phase 2) to assess impact of flow control strategies on

receiving waterbody flood risk.

10 Standard 4 – Water quality

10.1 Objectives


• Minimise the risk of transferring urban diffuse pollutants from developments to

receiving groundwater or surface waters (including sensitive coastal waters), either

directly or via sewers

• The quality (and associated ecology and amenity) of the receiving groundwater or

surface waters (including sensitive coastal waters) is protected and opportunities

are taken to support future improvements to that quality and value

• The quality of water in SuDS components located in accessible and visible space is

suitable.

10.2 Evidence base








managing the quality of runoff is included or indicates possible compliance assessment

needs or exception clauses:

• Local authorities are required to support the delivery of:

o Environmental quality objectives through Local Plans - NPPF, paragraphs

156 and 178 (MHCLG, 2019a)

o Water quality objectives set out in River Basin Management Plans (Defra,

2015b)

These duties are highlighted in the guidance provided by the MHCLG to local

authorities on water supply, wastewater and water quality (MHCLG, 2015, updated

2019) which advises on how planning can ensure water quality and the delivery of

adequate water and wastewater infrastructure including through requirements for

90

SuDS. A standard that requires SuDS designs to manage risks associated

with pollution in urban runoff will support local authorities in their statutory

duties.

• Using SuDS to help protect and improve water quality is highlighted as a need in

the 25 year Environment Plan (HM Government, 2018), in the NPPF (MHCLG,

2019a), in the PPG (MHCLG, 2020a), and in the NDG (MHCLG, 2019b). A

standard that requires SuDS designs to manage risks associated with

pollution in urban runoff will support the delivery of these objectives.

• Drainage and Wastewater Management Plans (DWMPs) are being developed by

the WaSCs as long-term frameworks for improving drainage performance and

environmental water quality (Water UK, 2020b). A standard that requires SuDS

designs to manage risks associated with pollution in urban runoff will support

WaSCs with alignment of DWMP options and delivery of quality objectives.

• The MHCLG SuDS Review (2018) found that, in general, there was a consensus

from local authorities that source control, water quality and provision of additional

benefits should be the fundamental objectives of a SuDS proposal and attenuation

was only one aspect.

• The DCG, Policy C1, Clause 1 (Water UK, 2020a) states that “the purpose of

surface water drainage is to carry water away from buildings and yards belonging to

buildings in a manner that manages flood risk and water quality”. A standard that

requires SuDS designs to manage risks associated with pollution in urban

runoff is required in order that SuDS are delivered in accordance with the

above definition, and there is no inconsistency in understanding and

approach between stakeholders.

• There is a raft of evidence that the discharge of urban runoff is a significant

contributor to the degraded water quality of surface waterbodies, including Defra

(2012), WCA Environment (2013), Lundy and Wade (2013), Mayor of London

(2019a) and Müller et al. (2020).

• There are risks of increased consequences of discharges of pollutants to

waterbodies under future river flow conditions driven by climate change (Brown et

al., 2017). A standard that requires SuDS designs to manage risks associated

with pollution in urban runoff will help mitigate this climate change risk.

• The management of runoff water quality is a primary surface water management

criteria internationally (UKWIR, 2021 – Phase 1). The Welsh statutory standards

(Welsh Government, 2018) include a water quality standard and, in Scotland,

91

General Binding Rule 10 of the Controlled Activities Regulations15 states that

“Developments are drained by a SUD system equipped to avoid pollution of the

water environment” – making water quality management a mandatory requirement.

A standard that requires SuDS designs to manage risks associated with

pollution in urban runoff will ensure England is more in line with international

best practice and devolved government’s standards.

• A water quality standard is the most desired addition to the existing standards,

according to stakeholders who responded to the online survey (Annex C, Sections

6.1 and 9.1) and the focus groups (Annex E, Section 4.2). This view is supported by

feedback collated by MHCLG (2018). Welsh stakeholders interviewed as part of this

project (Annex F, Section 3) considered the inclusion of a water quality standard

within the Welsh Standards (Welsh Government, 2018) as important and

deliverable. A standard that requires SuDS designs to manage risks

associated with pollution in urban runoff is supported and actively sought by

industry stakeholders.


STANDARD 4: WATER QUALITY

Apply ‘a SuDS approach’ that manages the quality of the surface water runoff to prevent

pollution; protects groundwater, surface waters and sensitive coastal waters; and

delivers suitably clean water to SuDS components in accessible and visible public

space.




1. The quality of surface water runoff should be managed to protect groundwater,

surface waters and sensitive coastal waters from both direct and indirect

pollution.

2. Any improvements to groundwater, surface waters and coastal waters should

refer to objectives and measures in the relevant River Basin Management Plan

and any other local strategy for improving waterbodies.

15 The Water Environment (Controlled Activities) (Scotland) Regulations as amended, include general binding rules as part of the authorisation process regulated by SEPA.

92

3. Provide evidence that runoff from all development surfaces is sufficiently treated

before discharges are made to groundwater, surface waters, sensitive coastal

waters or piped drainage systems.

4. Where runoff is collected for use, determine the treatment required with reference

to industry published guidance16.

5. Provide evidence of the following in order to demonstrate that the water quality

design is sufficient:

a. Appropriate components have been included (in series as a ‘management

train’ where required) and assessed using an evaluation approach set out

in industry published guidance for SuDS design17 and approved for use by

the approving body (noting that when discharging to sensitive or

designated surface waters or groundwater, additional treatment may be

required); and

b. The SuDS components used to provide treatment have been designed for

effective water quality management as well as to meet hydraulic

performance criteria, following guidance on individual component design

set out in industry published guidance18;

c. Consideration has been given to the performance of the SuDS treatment

functionality through the lifetime of the development (e.g. through the use

of drought tolerant planting); and

d. Where above ground, vegetated systems are not practicable or cannot

deliver sufficient treatment, and manufactured treatment products are

required, ensure that these have been tested and designed to an approved

industry standard and assessed for their contribution to the required

treatment using an industry approved method.

6. For sites with high pollution hazards19, provide a risk assessment for approval by

the local authority and any other relevant environmental regulators, ensuring any

requirements for an environmental permit can be met.

7. When discharging to groundwater, follow guidance set out in the Environment

Agency’s groundwater protection guidance20 and, where required, provide a risk

assessment for approval by the local authority and any other relevant

environmental regulators, ensuring any requirements for an environmental permit

can be met.

16 Such as BS EN 16941-1:2018 or subsequent editions. 17 Such as the Simple Index Approach as described in The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions. 18 This includes The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions 19 As def ined in The SuDS Manual (Woods Ballard et al., 2015) or subsequent editions 20 https://www.gov.uk/government/collections/groundwater-protection

https://www.gov.uk/government/collections/groundwater-protection

93

8. Provide evidence that the approach to managing runoff quality has been

developed alongside and in support of the management of everyday rainfall

(Standard 2) and the delivery of amenity and biodiversity benefits (Standards 5

and 6).


design objectives to ensure that the water quality performance of the SuDS is

maintained in the long term.



recommends that:

• Detailed information is produced that will enable the design of suitable treatment

‘deemed to comply’ approaches. This should include:

o A definition and explanation of industry approved standards and approaches

o Reference to the design of components for water quality treatment, as set

out in The SuDS Manual (Woods Ballard et al., 2015)

o Clarity on the use of proprietary products within any treatment system and

their integration with surface, vegetated systems.

o How this Standard can be met for a range of development types and scales

(including small and higher risk sites).

• Guidance is required on managing hydraulic performance risks related to

poor/insufficient maintenance regimes for water quality treatment components.

• The existing SIA assessment tool is updated, along with the accompanying

guidance currently provided in The SuDS Manual (Woods Ballard et al., 2015) to

improve simplicity, clarity and accessibility. There are also updates to The SuDS

Manual required with respect to new testing methods and standards. There would

be value in reviewing the data on which the method is based, as there are newer

datasets that should better reflect pollutant loadings on urban surfaces.

11 Standard 5 – Amenity

11.1 Objectives


• SuDS are designed to generate wellbeing benefits for people

94

• SuDS are designed to improve the quality, liveability and climate resilience of

developments and the public spaces within them

• SuDS design is considered and integrated throughout the development via the

masterplanning process, not only in the design of designated public open amenity

space

• The amenity value of SuDS is a design consideration for all accessible

spaces/places

• The amenity potential of the drainage system is maximised, in ways that also

support the delivery of benefits for wildlife.

11.2 Evidence base








amenity is included or indicates possible compliance assessment needs or exception

clauses:

• The NPPF (MHCLG, 2019a), Planning Practice Guidance (PPG) (MHCLG, 2020a),

National Design Guide (MHCLG, 2019b) and the 25 Year Environment Plan (HM

Government, 2018) all cite the need for SuDS and their amenity and multiple benefit

value as a key part of regeneration and urban design more widely. Key

opportunities for using SuDS to deliver high quality public open space and,

conversely, for public open space (POS) to provide opportunities for SuDS are

routinely missed (CIWEM, 2017; LI/CIC, 2019; UKWIR, 2021; Case Studies (Annex

A, Section 3)). A standard that requires the consideration and delivery of

amenity value from SuDS and that links with planning policy objectives will

reduce risks of lost opportunities, and strengthen the position of LPAs when

requesting integrated, multifunctional drainage design.

• The DCG (Policy C5, Clause 2) (Water UK, 2020a) includes a requirement for

integrated development and drainage design at masterplanning stages (including

POS and ecology delivery). A standard that supports an equivalent approach



• The use of SuDS and green infrastructure to help improve urban air quality is a

recommendation in the PPG (MHCLG, 2020a) and guidance on this has been

95

provided by the Mayor of London (Mayor of London, 2019b). Blue Green

Infrastructure (BGI) such as SuDS has been demonstrated to not only deliver

improvements in noise reduction, heat stress, and air quality, but also provide a

restorative and stress reducing environment, increasing social interaction and

cohesion, and increasing community physical activity (Choe et al., 2020). A

standard that requires the consideration and delivery of amenity value from

SuDS will improve opportunities to maximise health and wellbeing value from

public open space.

• An amenity standard is the third most desired addition to the existing standards

(after biodiversity and water quality) – with similar preference levels to ‘climate

resilient development’, according to stakeholders who responded to the online

survey (Annex C, Sections 6.1 and 9.1) and the focus groups (Annex E, Section

3.4). There is widespread recognition (e.g. CIWEM, 2017; LI/CIC, 2019) that the

focus of the current NSTS on flow control alone is limiting the capacity of LPAs to

require on the surface, green SuDS that are integrated with and add value to public

open space. A standard that requires the consideration and delivery of

amenity value from SuDS will support the delivery of local planning policy

objectives and strengthen LPA position in requiring green/blue infrastructure,

BNG and delivering Natural England’s Urban Greening Factor.

• According to the Welsh stakeholders interviewed as part of this project (Annex F,

Section 3), the Welsh standard (Welsh Government, 2018) wording of ‘maximising

amenity’ is considered suitable, but insufficiently prescriptive, which means it can be

challenging to assess compliance. A standard that clearly requires a positive

outcome and is less open to challenge (i.e. no amenity value could be argued

as the maximum that can be delivered) would be preferred.


STANDARD 5: AMENITY

Apply ‘a SuDS approach’ that generates amenity benefits through the creation of multi-

functional places and landscapes.




1. Provide evidence that:

a. Opportunities have been taken to deliver visual, recreational and wellbeing

value throughout the SuDS design to meet the needs of users;

96

b. The SuDS components contribute to place-making, and are both physically

and visually integrated into the site design, creating accessible and

attractive landscape features;

2. Provide evidence that the design of SuDS to deliver amenity value has been

developed alongside and in support of requirements to deliver biodiversity

(Standard 6).


design objectives to ensure that the amenity value of the SuDS is maintained in

the long term.



recommends that:

• Guidance is provided for LPAs so that they can:

o Develop the local mechanisms by which SuDS proposals will meet these requirements and by which delivery of amenity benefit can be assessed.

o Encourage LPAs to work closely with their LLFAs to ensure that the requirement for amenity and its role as an essential part of the site design is fully understood, and that it should be discussed with the developer at the

earliest opportunity through the pre-app process and during its evaluation.

o Encourage LPAs to produce Supplementary Planning Documents aligned with The SuDS Manual (Woods Ballard et al., 2015), but which also understand and reference local landscape character, soils and vegetation

types and integration within any Local Design Codes.

o Determine any appropriate exception clauses, based on their local circumstances.

o Understand that SuDS should be designed as a network, through a masterplanning process that allows amenity to be part of spatial planning

throughout a site in a comprehensive manner, relative to its size and scale.

o Fully understand the implications and requirements of BNG and Natural England’s Urban Greening Factor, such that SuDS amenity value can contribute positively to these outcomes.

• Updated information is produced on designing for amenity. This should include:

o The benefits of considering delivery of the required amenity and biodiversity outcomes together.

97

o Reference to The SuDS Manual (Woods Ballard et al., 2015) for guidance on safety considerations related to SuDS.

o Reference to The SuDS Manual (Woods Ballard et al., 2015) and Design and Construction Guidance (Water UK, 2020a) for guidance on the soil and

planting requirements of each specific SuDS component relative to its functionality as a tool for integrating amenity and biodiversity objectives.

o Clarity regarding where above ground SuDS do not deliver amenity and biodiversity, POS calculations should exclude these areas.

o Clarity that POS can be used for storage of water. Where water will only be stored on an infrequent basis, the POS will be able to retain its primary function for the majority of the time. Permanent water features that provide storage should be designed primarily for their amenity value and should not

occupy a disproportionate proportion of the amenity space.

o The linkages between SuDS delivery and the delivery of wider national and/or local planning policy objectives on amenity and landscape provision, and health and wellbeing.

o Links to existing information and guidance on good practice on amenity delivery provided in The SuDS Manual (Woods Ballard et al., 2015).

o How to meet this Standard for a range of development types and scales.

12 Standard 6 – Biodiversity

12.1 Objectives


• The design, implementation and long-term maintenance of SuDS generates

benefits for wildlife

• Linkages between SuDS design and the delivery of BNG are defined, and SuDS

provide a mechanism for development to achieve BNG when adhering to industry

BNG good practice (CIEEM, CIRIA, IEMA, 2016)

• SuDS designs benefit wildlife at the site and landscape level, even where it is not

possible or appropriate for SuDS to count towards BNG

• The biodiversity potential of the drainage system is maximised in ways that

contribute towards social and environmental benefits, for example enhancing

people’s wellbeing and enriching the amenity value of the development

• The climate resilience of habitats and the wildlife they support is improved through

the delivery of SuDS.

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12.2 Evidence base








biodiversity is included or indicates possible compliance assessment needs or exception

clauses:

• LPAs have a duty to conserve biodiversity and the revised NPPF requires that

planning policies and decisions should achieve net gains in biodiversity. In addition,

good practice SuDS guidance has included biodiversity objectives, but none of this

has been successful at consistently driving biodiverse surface water management

systems (LI/CIC, 2019; UKWIR, 2021). A standard that requires biodiversity

enhancements through SuDS design will strengthen current requirements.

• The NPPF (MHCLG, 2019a), PPG (MHCLG, 2020a), NDG (MHCLG, 2019b) and

the 25 Year Environment Plan (HM Government, 2018) all cite the need for SuDS

and their biodiversity value as a key part of regeneration and urban design more

widely. A standard that requires the delivery of biodiversity benefits from

SuDS with links to planning policy objectives will reduce risks of lost

opportunities, and strengthen the position of LPAs when requesting

integrated, multifunctional drainage design

• Delivering greater biodiversity in urban green spaces has been demonstrated to

provide more restorative and stress-reducing amenity benefits (Choe et al., 2020).

A standard that requires biodiversity value through SuDS design will

strengthen the delivery of amenity value (Standard 5).

• LPAs are now required to take a lead in responding to biodiversity losses by

adopting policy that requires developers to deliver BNG. SuDS design can include

meadow, grasslands, open water and wetlands, trees and more formally planted

components, thereby providing breadth of opportunities to deliver the required BNG

for a development, when meeting industry good practice. But a significant barrier is

the lack of standards and guidance on how SuDS can deliver BNG, especially with

regards to applying ‘like for like or better’ and the additionality principle21.

21 Additionality is defined in the context of the BNG Good Practice Principles as achieving nature conservation outcomes that demonstrably exceed existing obligations, i.e. do not deliver something that would occur anyway (CIEEM, CIRIA, IEMA, 2016).

99

The introduction of a mandatory BNG requirement under the Environment Bill is a

win-win opportunity to incentivise the design of SuDS that generates benefits for

wildlife. But a biodiversity standard that clarifies and reinforces links between SuDS

and BNG is critical for this to happen. A standard that requires biodiversity

enhancement through SuDS design, together with supporting guidance on

links between SuDS and BNG, will provide the support and clarification

necessary to maximise opportunities and minimise risks when SuDS are

counted towards BNG.

• A biodiversity standard is clearly sought and valued by industry practitioners and

approving bodies. According to the feedback received via the online survey and the

focus groups (Annex E, Section 2), approvers considered biodiversity as the

foremost multiple benefit missing from the current NSTS, and for practitioners

biodiversity followed water quality in terms of importance (Annex C, Sections 6.1

and 9.1). There is widespread recognition (e.g. CIWEM, 2017; LI/CIC, 2019) that

the focus of the current NSTS on flow control alone is limiting the capacity of LPAs

to require on the surface, green SuDS that deliver biodiversity benefits. A standard

that requires biodiversity enhancements through SuDS design will support

the delivery of local planning policy objectives and strengthen the LPA

position in requiring green/blue infrastructure.

• The DCG (Policy C5, Clause 2) (Water UK, 2020a) includes a requirement for

integrated development and drainage design at masterplanning stages (including

POS and ecology delivery). A standard that supports an equivalent approach



• According to the Welsh stakeholders interviewed (Annex F, Section 3), the Welsh

standard (Welsh Government, 2018) wording of ‘maximising biodiversity’ is

insufficiently prescriptive, which means it can be challenging to assess compliance.

A standard that requires a positive outcome with clear means of compliance

(i.e. no biodiversity value could be argued as the maximum that can be

delivered) is needed.

100


STANDARD 6: BIODIVERSITY

Apply a ‘SuDS approach’ that generates biodiversity benefits.




1. Provide evidence that the SuDS design:

a. Creates new ecologically valuable habitat (for example the UK Priority

habitats listed by the Joint Nature Conservation Committee22) and/or

enhances and restores existing habitats across the site that, by doing so,

generates biodiversity benefits;

b. Supports the delivery of local biodiversity strategies and helps combat

biodiversity loss;

c. Helps to establish resilient ecological networks23;

d. Has been informed by the requirements of any relevant designated sites;

e. Minimises risks of introducing invasive species.

2. Provide evidence that, where SuDS components are used for the delivery of

biodiversity net gain (BNG), current BNG good practice and metrics are applied.

3. Provide evidence that the design of SuDS to deliver biodiversity benefits has

been developed alongside and in support of requirements to deliver amenity

benefits (Standard 5).


design objectives to ensure that the biodiversity value of the SuDS is maintained

in the long term.

22 https://jncc.gov.uk/our-work/uk-bap-priority-habitats/#:~:text=List%20of%20UK%20BAP%20Priority%20Habitats%20%20,New%20priority%20habitat%20%2035%20more%20rows%20 23 Ecological networks are “an interconnected system of ecological corridors” CIEEM, Guidelines for EcIA, 2018.

https://jncc.gov.uk/our-work/uk-bap-priority-habitats/#:~:text=List%20of%20UK%20BAP%20Priority%20Habitats%20%20,New%20priority%20habitat%20%2035%20more%20rows%20



101



recommends that:

• Guidance is provided for LPAs so that they can:

o Develop the local mechanisms by which SuDS proposals will meet these requirements and by which delivery of biodiversity benefit can be assessed

o Determine any appropriate exception clauses, based on their local circumstances.

• Updated information is produced on SuDS that generate biodiversity benefits. This

should include:

o Definitions of what qualifies as a biodiversity benefit from SuDS and

guidance on how these benefits are assessed, both qualitatively and

quantitatively

o Examples of planting that is, and is not, appropriate for SuDS to function as

SuDS in order to highlight the importance of designing SuDS in ways that

also generate biodiversity benefits

o The application of BNG good practice principles to SuDS design and

implementation, for example the ‘mitigation hierarchy’, especially for the

protection of existing ecological value drainage features; additionality and

‘like for like or better’

o Designing multiple benefit SuDS that deliver benefits for wildlife in ways that

also generate social and environmental benefits, including people’s

wellbeing, climate resilience and the amenity value of a development

o Examples of suitable management and maintenance regimes for the SuDS

to function as required, and generate biodiversity benefits

o Alignment with, and links to, existing and forthcoming key publications and

initiatives including: good practice on biodiversity delivery provided in The

SuDS Manual (Woods Ballard et al., 2015); the forthcoming British Standard

on BNG, the Building with Nature standards (Building with Nature, 2020),

and the existing British Standard 42020:2013; the Urban Greening Factor;

Local Nature Recovery Strategies, and the forthcoming Standards on Green

Infrastructure

o Links to national and/or local planning policy objectives on amenity and

landscape provision, and health and wellbeing

o How to meet this Standard for a range of development types and scales

including small-scale and urban development; and a range of issues such as

contaminated land.

102

13 References

AECOM (2013) Water. People. Places. A guide for master planning sustainable drainage

into developments. Prepared by the LLFAs of the South East of England. Available at

https://www.susdrain.org/files/resources/other-

guidance/water_people_places_guidance_for_master_planning_sustainable_drainage_int

o_developments.pdf

BRE (2016) Soakaway design, BRE Digest 365, Building Research Establishment,

Bracknell, UK (ISBN: 978-1-84806-918-6). Available at

https://www.brebookshop.com/details.jsp?id=327631

Brown, I., Bardgett, R., Berry, P., Crute, I, Morison, J., Morecroft, M., Pinnegar, J., Reeder,

T. and Topp, K. (2017) UK Climate Change Risk Assessment 2017: Evidence Report,

Chapter 3: Natural environment and natural assets, Committee on Climate Change,

London, UK. Available at https://www.theccc.org.uk/wp-content/uploads/2016/07/UK-

CCRA-2017-Chapter-3-Natural-environment-and-natural-assets.pdf

BS EN 752:2017 Drain and sewer systems outside buildings – sewer system

management, British Standards Institution, 2019 (ISBN 978 0 659 06895 5). Available at

https://shop.bsigroup.com/ProductDetail?pid=000000000030404653

Building Better Building Beautiful Commission (BBBBC) (2020) Living with Beauty:

Promoting health, well-being and sustainable growth Available at

https://www.gov.uk/government/publications/living-with-beauty-report-of-the-building-

better-building-beautiful-commission

Building Research Establishment (2016) Soakaway design, BRE Digest 365, Building

Research Establishment, Bracknell, UK (ISBN: 978-1-84806-918-6)

Building with Nature (2020) Building with Nature, The Standards, July 2020. Available at

https://www.buildingwithnature.org.uk/how-it-works

Choe, E.Y; Kenyon, A: Sharp,L (2020) Designing Blue Green Infrastructure (BGI) for water

management, human health, and wellbeing: summary of evidence and principles for

design, Sheffield, Sheffield University, UK. Available at

https://figshare.shef.ac.uk/articles/report/Designing_Blue_Green_Infrastructure_BGI_for_w

ater_management_human_health_and_wellbeing_summary_of_evidence_and_principles_

for_design/13049510

CIEEM, CIRIA and IEMA (2016) Biodiversity Net Gain: Good practice principles for

development. Available at https://cieem.net/resource/biodiversity-net-gain-good-practice-

principles-for-development/

CIHT (2020) Revising Manual for Streets Available at https://www.ciht.org.uk/knowledge-

resource-centre/resources/revising-manual-for-streets/

https://www.susdrain.org/files/resources/other-guidance/water_people_places_guidance_for_master_planning_sustainable_drainage_into_developments.pdf



https://www.brebookshop.com/details.jsp?id=327631

https://www.theccc.org.uk/wp-content/uploads/2016/07/UK-CCRA-2017-Chapter-3-Natural-environment-and-natural-assets.pdf

https://www.theccc.org.uk/wp-content/uploads/2016/07/UK-CCRA-2017-Chapter-3-Natural-environment-and-natural-assets.pdf

https://shop.bsigroup.com/ProductDetail?pid=000000000030404653

https://www.gov.uk/government/publications/living-with-beauty-report-of-the-building-better-building-beautiful-commission

https://www.gov.uk/government/publications/living-with-beauty-report-of-the-building-better-building-beautiful-commission

https://www.buildingwithnature.org.uk/how-it-works

https://figshare.shef.ac.uk/articles/report/Designing_Blue_Green_Infrastructure_BGI_for_water_management_human_health_and_wellbeing_summary_of_evidence_and_principles_for_design/13049510



https://cieem.net/resource/biodiversity-net-gain-good-practice-principles-for-development/

https://cieem.net/resource/biodiversity-net-gain-good-practice-principles-for-development/

https://www.ciht.org.uk/knowledge-resource-centre/resources/revising-manual-for-streets/

https://www.ciht.org.uk/knowledge-resource-centre/resources/revising-manual-for-streets/

103

CIRIA (2013) CIRIA RP993 – Demonstrating the multiple benefits of SuDS – a business

case. Draft literature review. Available at

https://www.susdrain.org/files/resources/ciria_guidance/ciria_rp993_literature_review_octo

ber_2013_.pdf

CIWEM (2017) A Place for SuDS (incorporating the outcomes of the Big SuDS survey

(CIWEM, 2016)). Available at https://www.ciwem.org/policy-reports/a-place-for-suds

Committee on Climate Change (2019) Progress in preparing for Climate change –

Progress Report to Parliament 2019 Available at

https://www.theccc.org.uk/publication/progress-in-preparing-for-climate-change-2019-

progress-report-to-parliament/

Defra (2015a) Non-Statutory Technical Standards for SuDS

https://www.gov.uk/government/publications/sustainable-drainage-systems-non-statutory-

technical-standards

Defra (2015b) River Basin Management Plans Available at

https://www.gov.uk/government/collections/river-basin-management-plans-2015

Defra (2018) The National Adaptation Programme and the Third Strategy for Climate

Adaptation Reporting, Making the country resilient to a changing climate, July 2018

Available at https://www.gov.uk/government/publications/climate-change-second-national-

adaptation-programme-2018-to-2023

Environment Agency (2020a) Flood risk assessment: climate change allowances Available

at https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances

Environment Agency (2020b) National Flood and Coastal Erosion Risk Management

Strategy for England Available at https://www.gov.uk/government/publications/national-

flood-and-coastal-erosion-risk-management-strategy-for-england--2

Greater London Authority (2017) Urban Greening Factor for London, Research Report,

Version 2.0, The Ecology Consultancy, July 2017. Available at

https://www.london.gov.uk/sites/default/files/urban_greening_factor_for_london_final_repo

rt.pdf

HM Government (2016) The Building Regulations 2010, Approved Document G,

Sanitation, hot water safety and water efficiency, Amended version 2016. Available at

https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-

efficiency-approved-document-g

HM Government (2018) A green future: our 25 year plan to improve the environment

Available at https://www.gov.uk/government/publications/25-year-environment-plan

Kjeldsen, T. (2007) The revitalised FWR/FEH rainfall-runoff model, Flood Estimation

Handbook, Supplementary Report No. 1, Centre for Ecology & Hydrology, Wallingford,

https://www.susdrain.org/files/resources/ciria_guidance/ciria_rp993_literature_review_october_2013_.pdf

https://www.susdrain.org/files/resources/ciria_guidance/ciria_rp993_literature_review_october_2013_.pdf

https://www.ciwem.org/policy-reports/a-place-for-suds

https://www.theccc.org.uk/publication/progress-in-preparing-for-climate-change-2019-progress-report-to-parliament/

https://www.theccc.org.uk/publication/progress-in-preparing-for-climate-change-2019-progress-report-to-parliament/

https://www.gov.uk/government/publications/sustainable-drainage-systems-non-statutory-technical-standards

https://www.gov.uk/government/publications/sustainable-drainage-systems-non-statutory-technical-standards

https://www.gov.uk/government/collections/river-basin-management-plans-2015

https://www.gov.uk/government/publications/climate-change-second-national-adaptation-programme-2018-to-2023

https://www.gov.uk/government/publications/climate-change-second-national-adaptation-programme-2018-to-2023

https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances

https://www.gov.uk/government/publications/national-flood-and-coastal-erosion-risk-management-strategy-for-england--2

https://www.gov.uk/government/publications/national-flood-and-coastal-erosion-risk-management-strategy-for-england--2

https://www.london.gov.uk/sites/default/files/urban_greening_factor_for_london_final_report.pdf

https://www.london.gov.uk/sites/default/files/urban_greening_factor_for_london_final_report.pdf

https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-efficiency-approved-document-g

https://www.gov.uk/government/publications/sanitation-hot-water-safety-and-water-efficiency-approved-document-g

https://www.gov.uk/government/publications/25-year-environment-plan

104

Oxfordshire, UK (ISBN: 0-90374-115-7). Available at

http://www.ceh.ac.uk/sites/default/files/FEH%20Supplementary%20Report%20hi-res.pdf

Kjeldsen, T., Jones, D. and Bayliss, A. (2008) Improving the FEH statistical procedures for

flood frequency estimation, Science Report SC050050, Environment Agency, Bristol, UK

(ISBN: 978-1-84432-920-5) Available at

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_

data/file/291096/scho0608boff-e-e.pdf

LASOO (2016) Non-statutory technical standards for sustainable drainage: Practice

Guidance, Local Authority SuDS Officer Organisation in association with the House

Builders Association and Home Builders Federation. Available at https://www.suds-

authority.org.uk/wp-content/uploads/2018/12/non-statutory-technical-standards-

guidance.pdf

LI/CIC (2019) Achieving SuDS – A review of delivery by LLFAs, January 2019. Available at

https://www.landscapeinstitute.org/publication/llfa-suds-report-2019/

Marshall, D. and Bayliss, A. (1994) Flood estimation for small catchments, Report 124,

Institute of Hydrology, Wallingford, Oxfordshire, UK (ISBN: 0-94854-062-1). Available at

http://nora.nerc.ac.uk/id/eprint/7367/1/IH_124.pdf

Mayor of London (2016) The London Plan, The spatial development strategy for London

consolidated with alterations since 2011, March 2016. Available at

https://www.london.gov.uk/what-we-do/planning/london-plan/current-london-plan

Mayor of London (2019a) Road Runoff Water Quality Study, Mayor of London, Greater

London Authority, December 2019. Available at

https://www.london.gov.uk/sites/default/files/road_runoff_water_quality_study_exec_summ

ary_dec_19_0.pdf

Mayor of London (2019b) Using Green Infrastructure to Protect People from Air Pollution

Available at https://www.london.gov.uk/WHAT-WE-DO/environment/environment-

publications/using-green-infrastructure-protect-people-air-pollution (Accessed 30th June

2020)

MHCLG (2015, updated 2019) Water supply, wastewater and water quality, Available at

https://www.gov.uk/guidance/water-supply-wastewater-and-water-quality

MHCLG (2018) A review of the application and effectiveness of planning policy for

sustainable drainage systems

MHCLG (2019a) National Planning Policy Framework (NPPF) Available at

https://www.gov.uk/government/publications/national-planning-policy-framework--2.

MHCLG (2019b) National Design Guide (NDG). Available at

https://www.gov.uk/government/publications/national-design-guide

http://www.ceh.ac.uk/sites/default/files/FEH%20Supplementary%20Report%20hi-res.pdf

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/291096/scho0608boff-e-e.pdf

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/291096/scho0608boff-e-e.pdf

https://www.suds-authority.org.uk/wp-content/uploads/2018/12/non-statutory-technical-standards-guidance.pdf



https://www.landscapeinstitute.org/publication/llfa-suds-report-2019/

http://nora.nerc.ac.uk/id/eprint/7367/1/IH_124.pdf

https://www.london.gov.uk/what-we-do/planning/london-plan/current-london-plan

https://www.london.gov.uk/sites/default/files/road_runoff_water_quality_study_exec_summary_dec_19_0.pdf

https://www.london.gov.uk/sites/default/files/road_runoff_water_quality_study_exec_summary_dec_19_0.pdf

https://www.london.gov.uk/WHAT-WE-DO/environment/environment-publications/using-green-infrastructure-protect-people-air-pollution

https://www.london.gov.uk/WHAT-WE-DO/environment/environment-publications/using-green-infrastructure-protect-people-air-pollution

https://www.gov.uk/guidance/water-supply-wastewater-and-water-quality

https://www.gov.uk/government/publications/national-planning-policy-framework--2.

https://www.gov.uk/government/publications/national-design-guide

105

MHCLG (2019c) Permitted development rights for householders: technical guidance

Available at https://www.gov.uk/government/publications/permitted-development-rights-for-

householders-technical-guidance

MHCLG (2020a) Planning Practice Guidance (PPG). Available at

https://www.gov.uk/government/collections/planning-practice-guidance

MHCLG (2020b) Planning for the Future Available at

https://www.gov.uk/government/consultations/planning-for-the-future

Natural England (unpublished) National Framework of Green Infrastructure Standards

NERC (1985) The FSR rainfall-runoff model parameter estimation equations updated,

Flood Studies Supplementary Report (FSSR) No. 16, Institute of Hydrology, Wallingford,

Oxfordshire, UK.

Parkes, C., Kershaw, H., Hart, J., Sibille, R. and Grant, Z. (2010) Energy and carbon

implications of rainwater harvesting and greywater recycling, Report SC090018,

Environment Agency, Bristol, UK. ISBN: 978-1-84911-198-0. Available at

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_

data/file/291745/scho0610bsmq-e-e.pdf

Somerset County Council (2018) Review of developer SuDS schemes in Somerset

UKWIR (2021) Surface water drainage from new developments, Phase 1 and 2,

2019/2020, UKWIR Project Reference SW/01/C/20524

Water UK (2020a) Design and Construction Guidance for foul and surface water sewers

offered for adoption under the Code for adoption agreements for water and sewerage

companies operating wholly or mainly in England ("the Code"), Approved Version 2.0, 10

March 2020. Available at https://www.water.org.uk/sewerage-sector-guidance-approved-

documents/

Water UK (2020b) Drainage and Wastewater Management Plans Available at

https://www.water.org.uk/policy-topics/managing-sewage-and-drainage/drainage-and-

wastewater-management-plans/

Waterwise (2020) How do people feel about domestic water recycling systems? Public

Perceptions of Rainwater Harvesting and Greywater Recycling Available, a study by

Waterwise in partnership with the Mayor of London and the Environment Agency.

Available at https://www.waterwise.org.uk/knowledge-base/2020-public-perceptions-of-

rainwater-harvesting-and-greywater-recycling/

Welsh Government (2018) Statutory standards for sustainable drainage systems –

designing, constructing, operating and maintaining surface water drainage systems , Digital

24 At the time of writing, publication by UKWIR of this research is imminent.

https://www.gov.uk/government/collections/planning-practice-guidance

https://www.gov.uk/government/consultations/planning-for-the-future

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/291745/scho0610bsmq-e-e.pdf

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/291745/scho0610bsmq-e-e.pdf

https://www.water.org.uk/sewerage-sector-guidance-approved-documents/

https://www.water.org.uk/sewerage-sector-guidance-approved-documents/

https://www.water.org.uk/policy-topics/managing-sewage-and-drainage/drainage-and-wastewater-management-plans/

https://www.water.org.uk/policy-topics/managing-sewage-and-drainage/drainage-and-wastewater-management-plans/

https://www.waterwise.org.uk/knowledge-base/2020-public-perceptions-of-rainwater-harvesting-and-greywater-recycling/

https://www.waterwise.org.uk/knowledge-base/2020-public-perceptions-of-rainwater-harvesting-and-greywater-recycling/

106

ISBN 978 1 78964 148 6. Available at

https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-

sustainable-drainage-systems.pdf

Woods Ballard, B., Wilson, S., Udale-Clarke, H., Illman, S., Scott, T., Ashley, R. and

Kellagher, R. (2015) The SuDS Manual, CIRIA (C753), London, UK. ISBN: 978-0-86017-

760-9. Available at

https://www.ciria.org/ItemDetail?iProductCode=C753&Category=BOOK&WebsiteKey=3f18

c87a-d62b-4eca-8ef4-9b09309c1c91

WRc (2012) Sewers for Adoption 7th Edition – A Design & Construction Guide for

Developers, WRc, Swindon, UK, ISBN 978 1 898920 65 6.

https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-sustainable-drainage-systems.pdf

https://gov.wales/sites/default/files/publications/2019-06/statutory-national-standards-for-sustainable-drainage-systems.pdf

https://www.ciria.org/ItemDetail?iProductCode=C753&Category=BOOK&WebsiteKey=3f18c87a-d62b-4eca-8ef4-9b09309c1c91

https://www.ciria.org/ItemDetail?iProductCode=C753&Category=BOOK&WebsiteKey=3f18c87a-d62b-4eca-8ef4-9b09309c1c91