Flood Risk Management (Scotland) Act 2009

Methods to Screen and Quantify Natural Flood Management Effects

Scottish Environment Protection Agency & Forestry Commission Scotland, May 2012

Version history

Issue Date Author

Draft issue for comment 16/03/12 Neil Nutt CEng MICE Halcrow Group Ltd

16 Abercromby Place, Edinburgh, EH3 6LB

Finalised report 11/05/12 Neil Nutt CEng MICE Halcrow Group Ltd

16 Abercromby Place, Edinburgh, EH3 6LB

Acknowledgements

Section 20 Technical Advisory Group

Andrea Johnstonova (Chair) SEPA

Drew Aitken SEPA

Heather Forbes SEPA

Julia Garritt Forestry Commission Scotland

Lorna Harris SEPA

Kirsty Jack SEPA

Richard Jefferies SEPA

Roy Richardson SEPA

David Scott SEPA

Nadeem Shah Forest Research

Mark Williams SEPA

Dominic Habron SEPA Corresponding Member

Mark McLaughlin SEPA Corresponding Member

External Review by CREW

CREW (Centre of Expertise for Waters) is a hub which ensures that water research

and expertise is available and accessible to the Scottish Government and its

agencies. This is designed to ensure that existing and new research and expertise

can feed into the development of water related policy in Scotland in a timely and

effective manner.

The Author and Technical Advisory Group would like to thank Prof Alan Werritty

(Dundee University), Dr Scott Arthur (Heriot-Watt University) and Dr Tom Ball

(Dundee University) for their detailed review and subsequent guidance within a tight

timescale during the development of this report.

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Executive summary

The Flood Risk Management (Scotland) Act 20091 (the FRM Act) introduces a new sustainable approach to managing flood risk and places new duties on SEPA and Responsible Authorities. One responsibility placed on SEPA is to assess and consider the role which Natural Flood Management can play in reducing flood risk. Natural Flood Management was defined by SAIFF (2011)2 as:

Natural Flood Management can be defined as those techniques that aim to work with natural hydrological and morphological processes, features and characteristics to manage the sources and pathways of flood waters. These techniques include the restoration, enhancement and alteration of natural features and characteristics, but exclude traditional flood defence engineering that works against or disrupts these natural processes.

The aim of this study is to advance development of the methodology for assessing the contribution that Natural Flood Management can make to managing flood risk. This report builds on previous work undertaken by SEPA, Jacobs and others in the field of Natural Flood Management. The main tasks have included:

Review the previous work by Jacobs and the proposed GIS method for assessing Natural Flood Management;

Collate and review additional literature on Natural Flood Management, focusing on non-fluvial Natural Flood Management techniques;

Collate and review existing methods of assessing Natural Flood Management techniques; and

Propose a way forward for development of methods to identify opportunities and appraise the contribution of range of Natural Flood Management techniques.

As part of the delivery of Section 20 of the FRM Act SEPA appointed Jacobs to develop a methodology to assess Natural Flood Management measures (Jacobs, 2011)3. The Jacobs method was founded on a comprehensive and up-to-date literature review (Werritty et al., 2012)4. Unfortunately this methodology is associated with licensing issues, technical difficulties and a catchment based approach which did not allow national identification of opportunities led to this tool not being adopted by SEPA.

In this study SEPA has sought to build on the Jacobs literature review by extending it to include all flood mechanisms (pluvial, fluvial, groundwater, coastal and urban) and by expanding it to include a wider literature base. The extended literature review has formed the foundation for the development of more robust methods for screening and assessing Natural Flood Management measures.

Section 4 of the report proposes methods for identifying and appraising Natural Flood Management opportunities. The methods will provide an overview of the effect of Natural Flood Management measures that will inform the appraisal process. It is crucial that these predictions are kept in context. At no point should these estimates be taken as the correct answer, rather they should be seen as an approximate guide to what potentially might happen within the bounds of the reported uncertainty.

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It is proposed that the identification and appraisal of Natural Flood Management measures will be undertaken using screening and assessment tools as summarised in Figure 1.

Figure 1: Outline process for the delivery of Natural Flood Management requirements under the FRM Act

It is proposed that the identification and appraisal of Natural Flood Management will include three phases of work:

1. Identification phase: A national screening process to identify opportunities for Natural Flood Management measures. This screening process will identify areas within catchments with Potentially Vulnerable Areas that have natural flood management opportunities but will make no consideration of constraints or other benefits. The screening process will not directly recommend which specific measure should be implemented where, nor will the screening facilitate the quantification of the flood risk management benefits of undertaking a specific natural flood management activity. However the process will facilitate the identification of areas that are worth further investigations at a later stage. The main output of the screening process will be six maps showing:

2015 16

The relevant

Responsible Authority

2013 - 2014

SEPA / Responsible

Authorities

2012

SEPA

Section 20 Identification of potential

Running of Screening Tools

Output: Maps showing the potential for Natural Flood Management in catchments with

Potentially Vulnerable Area

Output: Section 20 maps

Section 28 Appraisals

Using output of Screening in combination with catchment characteristics and further

information to develop a long list of measures and refinement to short list. More detailed

appraisal producing a list of preffered measures.

Output: A list of preferred FRM measures for inclusion in FRM Strategies

Section 34 Local FRM Plans

Development of the preferred measure using appropriate modelling and assessment tools (not exclusively the proposed hydrological

assessment tool)

Output: Local FRM Plans

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Areas of high runoff generation;

Areas of floodplain storage potential;

Opportunities to remove hydraulic constrictions;

Areas of heightened hydromorphological activity;

Areas of estuarine surge attenuation potential; and

Wave energy attenuation potential.

It is proposed that these maps, will meet the requirements of Section 20 of the FRM Act and they will be made publically available along with appropriate supporting descriptions and guidance.

2. Appraisal phase: The second phase will seek to bring together the outputs from the screening process with further information about catchment characteristics in order to develop better understanding of what may be achievable where within the catchment. Using the findings of the screening process in combination with other catchment information the measures will be presented as options to manage the sources and pathways of floodwaters, as summarised in the Natural Flood Management Summary Table (Appendix B). This information together will be used to appraise natural flood management measures alongside other flood risk management measures. Over the course of this stage the long list will be reduced to a short list and the short list appraised to identify a basket of the most sustainable measures. The process will give due consideration to environmental, economic and social costs and benefits. The end result of this phase is a short list of options that will be agreed by all responsible authorities.

3. Further development through local FRMPs: The third phase will include more detailed assessment of the agreed, measures using existing and new assessment approaches. This more detailed assessment will be carried out by the relevant responsible authority as part of a local flood risk management plan.

The literature review has identified that some measures can be assessed using existing tools and methods. It is therefore proposed that where possible, this further assessment is carried out using existing assessment tools. However, it is also proposed that a new hydrological assessment method is developed to facilitate the quantification of fluvial and pluvial Natural Flood Management measures. This study has investigated how this assessment could be conducted and the report presents a proposed method for how this could be accomplished.

The proposed hydrological assessment methodology is a single event spatially distributed model which builds on the uniformly distributed PDM (Probability Distributed Model) runoff generation model. In the absence of viable alternatives many elements of the PDM configuration are based on the ReFH (Revitalised Flood Hydrograph). The effect of land use change is included within the PDM runoff generation model through the inclusion of a canopy interception moisture store and variation of the antecedent moisture condition based on the vegetations moisture demand. There is also provision to assess the effect of soil degradation on the soil moisture capacity as a consequence of intensive land management practices.

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Flow routing within the hydrological model is based on a time to outlet grid which is generated for a representative bankfull flood event. This grid summarises the travel time for runoff to travel from any location to the catchment outlet. It facilitates the generation of a hydrograph at the catchment outlet by the summation of the runoff generated for the array of PDMs covering the catchment. This approach allows the effect of any number of diffuse land management changes to be accumulated over a wide range of catchment sizes. The 1D kinematic wave hydraulic routine within the flow routing engine permits the quantification of land cover roughness changes prior to flood water reaching the watercourses in addition to in channel modifications such as the increase in riparian vegetation or restoration of a watercourses planform. Although not included within the core of the proposed flow routing method, there is sufficiently flexible to allow the time to outlet grid to be calculated using an alternative method where appropriate. This flexibility permits the use of more detailed 1D-2D hydraulic models to test the time lag effects of floodplain interventions such as the planting of trees on floodplains to increase roughness.

There is currently no software available to assist in the application of the fluvial hydrological assessment methodology proposed by this study. Testing undertaken as part of this study has identified that it would be feasible to construct it as a Toolbox in ArcGIS or similar. If or when a tool is developed it will be necessary to undertake a comprehensive calibration and verification process using a large selection of catchments across Britain. It is essential that the calibration and verification process considers catchments with a wide range of land use, soils, rainfall, potential evaporation, size, topography and network shapes. The calibration process should not solely focus on recreating the reported effects at a small number of Natural Flood Management test catchments which are likely to only represent the conditions found within a small proportion of catchments which the methodology might ultimately be applied to.

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Table of Contents

Executive summary .................................................................................................. i

1 Introduction ..................................................................................................... 1

2 Review of Jacobs Methodology..................................................................... 5

2.1 Introduction ................................................................................................ 5

2.2 Review of Jacobs Section 20 Review (CREW, 2012) ................................. 5

2.2.1 Comments relating to literature review ............................................... 6

2.2.2 Comments relating to Section 20 assessment tool ............................. 6

2.3 Natural Flood Management Implementation Learning from Practice Workshop Report (SNIFFER & SEPA, 2011) ........................................................ 7

2.4 Additional comments .................................................................................. 7

2.5 Summary ................................................................................................. 12

3 Literature review ............................................................................................ 13

3.1 Introduction .............................................................................................. 13

3.2 Review of previous literature reviews on Natural Flood Management ...... 13

3.3 Science relating to a range of Natural Flood Management types ............. 29

3.3.2 Summary.......................................................................................... 72

3.4 Review of Natural Flood Management screening and assessment tools .. 74

3.5 Literature review summary ....................................................................... 86

4 Proposed methodology for assessing and considering the contribution of Natural Flood Management .................................................................................. 87

4.1 Introduction .............................................................................................. 87

Identification of Natural Flood Management measures ........................................ 91

4.2 Screening for Natural Flood Management opportunities .......................... 92

4.2.1 Introduction ...................................................................................... 92

4.2.2 Screening methods .......................................................................... 92

4.2.3 Summary........................................................................................ 108

4.3 Quantifying the flood risk management benefits of Natural Flood Management measures ..................................................................................... 110

4.3.1 Introduction .................................................................................... 110

4.3.2 Specification of proposed S20 hydrological assessment tool.......... 113

4.3.3 Data requirements .......................................................................... 137

4.3.4 The capabilities of the assessment tool .......................................... 139

4.3.5 The inclusion of elements of the Refh ............................................ 140

4.3.6 Calibration, verification and improvement ....................................... 140

4.3.7 Flooding and the role of forestry ..................................................... 142

4.3.8 Summary........................................................................................ 143

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5 Summary of key recommendations ........................................................... 144

References ........................................................................................................... 146

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Table of Figures

Figure 1: Outline process for the delivery of Natural Flood Management requirements under the FRM Act ..................................................................................................... ii

Figure 2: Natural Flood Management techniques (SAIFF, 2011) .............................. 2

Figure 3: Example of the differences between the catchment delineations used within the Jacobs S20 tool ................................................................................................... 8

Figure 4: Example of a riparian intervention that generated a 0.64% reduction in the 1 in 200yr flood event without any of the proposed riparian woodland being positioned in the riparian zone. (Proposed riparian woodland shown in pink). ......................... 8

Figure 5: Example of the output from the proposed Jacobs S20 assessment tool showing the hydrological benefits of restoring the floodplain area (purple) to facilitate an increase in flood depths of 300mm with a 50% efficiency factor. ........................ 10

Figure 6: Derivation of the risk of compaction classes (Holman et al, 2001) (GLU Grazing Livestock Unit) ........................................................................................... 33

Figure 7: Mass infiltration rates (Holtan and Kirkpatrick, 1950) ................................ 34

Figure 8: Monitored soil moisture at two adjacent sites in the Allt AMharcaidh catchment in the Cairngorms: a) mature natural Scots Pine woodland, b) wet heath moorland. (Dots = averaged field measurement, Unbroken line = HYLUC model simulation) ............................................................................................................... 42

Figure 9: Simulated average seasonal soil moisture deficit for upland grass, heather and coniferous woodland for the River Calder catchment, Renfrewshire. (Period simulated: 1983 1993) .......................................................................................... 43

Figure 10: Predicted attenuation of: a) winter, b) summer flood peaks due to a hypothetical 100% conversion of the Calder catchment (Renfrewshire) from upland grass to various types of woodland\forest or lowland grass (Price et al, 2000). Given as a percentage of the peak flows predicted for upland grass. (Prediction uncertainty band reflects the range of values attributed to the model parameters coming from different field experiments). ..................................................................................... 43

Figure 11: Modelled potential reduction of flood peaks on the Kamp following afforestation (increase in pine forest from 47% to 86%), plus the predicted increase of flood peaks following deforestation of the existing forest (from 47% to 0% forest cover). [Source: Frances et al (2008)] ..................................................................... 45

Figure 12: Modelled potential reduction of flood peaks on the Iller following a 29% increase in pine forest cover. [Source: Frances et al (2008)] Note the 25 50 year event is estimated to be approximately 800 cumecs................................................ 46

Figure 13: The changes to the speed of catchment response parameter (TP) and the percentage of storm event rainfall forming quick runoff (PR) observed at the Coalburn experimental peat catchment following intensive upland drainage. (Values calculated from data presented in Robinson et al, 1998. Speculated canopy cover estimated by Jacobs based on experience and the limited information provided by Robinson et al.) ................................................................................................................................ 52

Figure 14: Unit costs of implemented realignment plotted against size and year. Scott et al. (2011). ................................................................................................... 71

Figure 15: Sample output, River Ouse combined sensitivity (Environment Agency, 2008) ....................................................................................................................... 75

Figure 16: Sample output, National assessment combined sensitivity (Environment Agency, 2008) ......................................................................................................... 75

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Figure 17: User interface of the Land Management CFMP Tool (Environment Agency, 2008) ......................................................................................................... 76

Figure 18: High priority areas with the greatest potential for woodland planting to reduce downstream flooding (Broadmeadows & Nisbet, 2009) ............................... 78

Figure 19: Sample output from the POLYSCAPE tool for the Pontbren catchment (Pagnell, 2009). ....................................................................................................... 81

Figure 20: Example of current UK farms and likely impacts of land use management scenarios and how they can be mapped onto the decision support matrix (Hewett et al., 2006) ................................................................................................................. 82

Figure 21: Four hillslope runoff risk scenarios for the same common land unit (Hewett et al, 2006). ................................................................................................ 82

Figure 22: Example of a time map generated by OVERFLOW for the Pickering Beck catchment, in this instance a 14mm per day gross rainfall even, assuming a percentage runoff of ~55% after adjusting for evaporation and groundwater losses. The timescale is in hours to the catchment outlet. ................................................... 84

Figure 23: Outline process for the delivery of Natural Flood Management requirements under the FRM Act ............................................................................. 88

Figure 24: Flow chart showing the inclusion of Natural Flood Management within the FRM Act .................................................................................................................. 89

Figure 25: Variation in percentage runoff with a changing leaf area index using typical Scottish catchment descriptors ..................................................................... 95

Figure 26: Variation in PROPWET across northern Britain (Bayliss, 1999) ............. 96

Figure 27: Variation of PROPWET with SAAR (Data for HiFlows catchments) ....... 97

Figure 28: Potential means of displaying sub-catchment desynchronisation information .............................................................................................................101

Figure 29: Example output from ST:REAM for the Taff catchment in Wales (Parker, in press) .....................................................................................................................103

Figure 30: Sample output for the Surge Attenuation Potential ...............................104

Figure 31: Summary of the European Environment Agency wave climate data . Error! Bookmark not defined.

Figure 32: Pictorial overview of the proposed Natural Flood Management assessment model .................................................................................................114

Figure 33: Schematic representation of the proposed Natural Flood Management assessment model .................................................................................................115

Figure 34: Design rainfall profiles for summer and winter, drawn as normalised hyetographs (Kjeldsen, 2007) ................................................................................116

Figure 35: A comparison of the variation in canopy interception capacity with leaf area index (LAI) using MORECS (Hough and Jones, 1997) and Hoyningen-Huene (1981) ....................................................................................................................118

Figure 36: Equal water content (C ) across stores of different capacity (Kjeldsen, 2007) ......................................................................................................................119

Figure 37: Plot comparing the regression based estimate of PROPWET with the FEH CD PROPWET ...............................................................................................124

Figure 38: Plot comparing the regression based estimate of PROPWET with the FEH CD PROPWET ...............................................................................................125

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Figure 39: An example of a time to outlet grid generated for a sample Scottish catchment as part of this study ...............................................................................128

Figure 40: Roughness values recommended by USDA (2010) for shallow surface runoff for flow depths of less than 30mm. ...............................................................129

Figure 41: Example of how the time lag effects of reservoirs, lochs and floodplains could be included within the assessment method ...................................................132

Figure 42: Summary of the proposed flow routing methodology ............................133

Figure 43: Example of a five band time to outlet grid for a sample Scottish catchment ...............................................................................................................................135

Figure 44: Example hydrograph using five runoff bands for a generalised catchment ...............................................................................................................................135

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Table of Tables

Table 1: Priority research questions ...................................................................... 20

Table 2: Questions requiring further research......................................................... 20

Table 3: Summary of catchment scale estimates (from monitoring or modelling) of the effect of afforestation on peak flood flows (Jacobs, 2011). ...................................... 25

Table 4: Summary of riparian woodland studies (Jacobs, 2011) ............................. 26

Table 5: Summary of floodplain woodland studies (Jacobs, 2011) ......................... 26

Table 6: Summary of annual transpiration and interception data for a range of land cover types (Nisbet, 2005)....................................................................................... 37

Table 7: Wave run-up friction factors (CIRIA, 2007) ..............................................106

Table 8: Summary of the preferred Natural Flood Management screening processes and the data requirements......................................................................................108

Table 9: Summary of the screening processes which are not recommended without further scientific support or data .............................................................................109

Table 10: Summary of assessment methods for Natural Flood Management measures and the identification of areas where there are currently gaps in assessment capability ............................................................................................112

Table 11: Leaf Area Index values as used in MORECS 2.0 (Hough and Jones, 1997) ...............................................................................................................................117

Table 12: Effect of soil degradation via the use of analogue HOST soil types (Packman et al, 2004) ............................................................................................121

Table 13: The range and quartiles for variables used in the PROPWET descriptor regression analysis ................................................................................................123

Table 14: Summary of flow routing used within existing hydrological models and methods .................................................................................................................127

Table 15: Examples of land cover types and the roughness bands which could be employed ...............................................................................................................130

Table 17: Example of memory requirements for a range of catchment sizes .........136

Table 18: Summary of the data required for the proposed methodology................137

Table 19: Summary of the data which could improve the quality of the tool ............138

Table 20: Summary of how a range of Natural Flood Management measures could be assessed using the proposed methodology .......................................................139

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Appendices

Appendix A - Review of Jacobs Section 20 Review (CREW, 2012)

Appendix B - Summary Table of Natural Flood Management Measures

Appendix C - Other Natural Flood Management considerations

Appendix D - Requirements on SEPA in relation to Natural Flood Management under the FRM Act

Appendix E Runoff generation model examples and initial calibration of flow routing model

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Notation and terms

(The) Act - The Flood Risk Management (Scotland) Act 2009

BFIHOST - Baseflow index (BFI) based on HOST data

iniC - Initial soil moisture capacity (mm)

maxC - Maximum soil moisture capacity (mm)

D - Duration (hours)

DDF - Depth duration frequency model

DPLBAR - Mean drainage path length (km)

DPSBAR - Mean drainage path slope (m/km)

DTM - Digital terrain model

FARL - Flood attenuation from reservoirs and lakes, FEH descriptor

FEH - Flood Estimation Handbook

FSR - Flood Studies Report

G2G - Grid to Grid Hydrological Model

GLU - Grazing livestock unit

HOST - Hydrology of Soil Types (soil classification)

erwsummerLAI int/ - Leaf area index, half the leaf area per unit area of ground (sqm/sqm)

LCM - Land cover map (1990, 2000 or 2007)

Lumped model - A model that treats the whole of a catchment as a single accounting unit and predicts only values of variables averaged over the catchment area

MORECS - Met Office Rainfall and Evaporation Calculation System

PDM - Probability Distributed Model

PE - Potential evapotranspiration (reference land cover) (mm/yr)

PR - Percentage runoff (%)

PROPWET - FEH catchment descriptor, proportion of time when the estimated SMD was less than or equal to 6mm (1961-1990)

PVA - Potentially Vulnerable Area

q - Direct runoff (cumecs)

Q - Flow (cumecs)

QMed - Median annual maximum flood

scr - Vegetation (canopy) resistance (s/m)

ReFH - Revitalised Flood Hydrograph

SAAR - Standard average annual rainfall (mm/yr) (1961-1990)

SAIFF - Scottish Advisory and Implementation Forum for Flooding

Semi-physical model - a hybrid model which represents some hydrological processes in a physical manner and others via mathematical functions that have no direct physical basis.

SEPA - Scottish Environment Protection Agency

SM - Soil moisture depth (mm)

SMD - Soil moisture deficit (mm)

SNH - Scottish Natural Heritage

SPR - Standard percentage runoff (%)

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SuDS - Sustainable (urban) drainage systems

T - Return period (years)

pT - Time to peak (hours)

URBEXT - Extent of urban and suburban land cover (1990 or 2000)

WFD - Water Framework Directive

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1 Introduction

The Flood Risk Management (Scotland) Act 20091 (the 'FRM Act') was enacted in June 2009. The FRM Act places new responsibilities on the Scottish Environment Protection Agency (SEPA) and other Responsible Authorities in relation to the sustainable management of flood risk. Part of the sustainable approach is the consideration of the contribution that Natural Flood Management can make to manage flood risk. This report concerns the delivery of Section 20 of the FRM Act which stipulates that SEPA must undertake an assessment of the role that the restoration, enhancement and alteration of natural features and characteristics can have in managing flood risk in Scotland and subsequent use of this information through the appraisal process and in local flood risk management plans. This restoration, enhancement or alteration of natural features and characteristics is commonly referred to as Natural Flood Management (NFM).

What is Natural Flood Management?

In reflection of the broad-spectrum of measures that have been termed Natural Flood Management there has been some ambiguity over what it entails despite there being apparent consensus on what it was trying to achieve. The Scottish Advisory and Implementation Forum for Flooding (SAIFF, 2011)2 provides the following definition of Natural Flood Management which has been used by this study:

Natural Flood Management can be defined as those techniques that aim to work with natural hydrological and morphological processes, features and characteristics to manage the sources and pathways of flood waters. These techniques include the restoration, enhancement and alteration of natural features and characteristics, but exclude traditional flood defence engineering that works against or disrupts these natural processes.

It should be recognised that alteration has been included within the definition so that techniques associated with changes in land management and partial restoration are not excluded from Natural Flood Management. In some instances it will be difficult to draw a line between what is considered Natural Flood Management and traditional flood defences. Figure 2 provides additional insight to SAIFF interpretation of what is included within Natural Flood Management.

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Figure 2: Natural Flood Management techniques (SAIFF, 2011)2

Natural Flood Management under the FRM Act: legislative requirements

Section 20 of the FRM Act requires SEPA, by 22 December 2013, to assess whether alteration (including enhancement) or restoration of natural features and characteristics of any river basin or coastal area in a flood risk management district could contribute to the management of flood risk for that district. Such assessment must be reviewed and updated at least every 6 years or as directed by Ministers.

Section 20 specifies that natural features and characteristics include such features and characteristics that can assist in the retention of flood water, whether on a permanent on temporary basis, (such as floodplains, woodlands and wetlands) or in slowing the flow of such water (such as woodlands or other vegetation), those which contribute to the transporting and depositing of sediments, and the shape of rivers and coastal areas.

The assessment must take into account the flood risk assessment, any flood hazard maps and flood risk maps and any flood risk management plan for the time applicable, and refer to a map showing where natural features and characteristics could contribute to management of flood risk.

The map must be prepared at a scale that would assist SEPA in the identification of measures and assist local authorities in preparing local flood risk management plans. Assessment under Section 20 and maps produced as part of the assessment must be made publically available.

Section 28 further requires SEPA to consider Section 20 assessment when setting objectives and identifying sustainable measures to manage flood risk. Where Section 20 identifies the potential for a natural feature or characteristic to contribute to managing flood risk, and such feature is not selected to be included in a flood risk management plan, a justification must be provided in the relevant part of a flood risk management plan as to the reasons for not selecting such measure. Finally, section 34 (3) requires that a local flood risk management plan (the supplementary part) also includes supplemental information on about how implementing the measures may alter (including enhance) or restore natural features and characteristics.

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The SFM guidance published by the Scottish Government encourages that Section 20 assessment also creates information that can be used in the appraisals of FRM options. This means developing information about the potential benefits and adverse impacts of Natural Flood Management measures that can feed into the appraisal process.

Introduction to this report

Section 2 of this document provides an overview of the review process for a recent report commissioned by SEPA on Natural Flood Management (Jacobs, 2011)3. As part of the quality management process the report was peer reviewed by leading Scottish academics in the field of flood risk management via the Centre of Expertise for Waters (CREW) (Werritty et al., 2012)4. This peer review process identified that the Jacobs literature review was exhaustive, authoritative and up-to-date, leading to only minor suggestions of additional sources with no notable changes to the outcome of the studys findings. A proposed methodology for assessing Natural Flood Management measures was also presented within the report (Jacobs, 2011)3. Unfortunately this methodology is associated with licensing issues, technical difficulties and a catchment based approach which did not lend itself to national identification of opportunities led to this tool not being adopted by SEPA.

The literature base presented by Jacobs has been revisited in Section 3 of this report with efforts made to expand the depth of the review by bringing in additional literature recommended by the peer review process. The range of flood mechanisms was also expanded in an attempt to give due consideration to the role Natural Flood Management can play in all modes of flood risk, including:

Pluvial;

Fluvial;

Groundwater;

Coastal: and

Urban drainage.

The updated literature review has been divided into three sub-sections so that a specific focus can be directed at each area in turn:

Section 3.2 Review of previous literature reviews on Natural Flood Management

Section 3.3 Science relating to a range of Natural Flood Management types

Section 3.4 Review of Natural Flood Management screening and assessment tools

Having updated the literature review a summary table detailing the key information for a broad range of Natural Flood Management measures (29 in total) has been prepared. This table is presented in Appendix B of this report. The table summarises the most relevant scientific literature for each measure, how the flood risk reduction can be quantified and the proposed methodology for inclusion within the methodology for the delivery Section 20 of the Act.

Section 4 details the proposed methodology for the delivery of Section 20 of the Act. This section details how the delivery will be divided into three phases; The identification (or screening) phase using national GIS screening; Appraisal phase to assess NFM measures against and in combination with other measures to manage flood risk, and finally further development of appraised measures through Local Flood Risk Management Plans.

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The Screening, process, will be undertaken at a national level to identify opportunities for Natural Flood Management. The output will be the production of six national maps detailing the potential within the landscape to use Natural Flood Management measures to manage flood risk. Each map will detail the opportunities for a specific type of measure, such that localised opportunities for the source control of runoff or the attenuation of estuarine surge can be rapidly identified following consideration of further catchment information as part of the Appraisal process. The scoring system used within the screening maps will not include non-flood risk constraints or opportunities, nor will the screening process directly specify which measure would be most suited to address flooding at a particular location. It is intended that the output of the screening process will assist in generating ideas at the earliest stages of measures development ahead of more focused assessment during the Appraisal.

The last stage in the process - quantification of the effects of agreed measures - will be taken forward as part of local flood risk management plans. The literature in Section 3 identifies that there are currently gaps in the methodologies that can be used to quantify the effect of Natural Flood Management measures, in particular for fluvial and pluvial driven flooding. Where there are existing tools and methodologies which can be used to for assessment then these should be used, however in the case of fluvial and pluvial the report recommends that a new hydrological methodology is developed. Section 4.4 presents a specification for a proposed semi-physical rainfall-runoff modelling methodology which could be used to assess fluvial and pluvial measures. It should be noted that the method is currently only a proposal, and its successful implementation would require a comprehensive calibration and verification phase prior to it becoming a valuable hydrological assessment tool.

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2 Review of Jacobs Methodology

2.1 Introduction

As part of SEPAs programme to meet the requirements of the Flood Risk Management Act 20091 Jacobs was commissioned to undertake a study to develop a methodology for Section 20 which states:

SEPA must assess whether alteration (including enhancement) or restoration of natural features and characteristics of any river basin or coastal area in a flood risk management district could contribute to the management of flood risk for the district.

A literature review of the science relating to Natural Flood Management measures was undertaken as part of this study to inform the configuration of the Jacobs tool. The findings of the study are reported in Development of Methodology for Assessment Required under Section 20 (Jacobs, October 2011)3. On completion the report was reviewed by leading Scottish academics and it was presented to a select group of flood risk management practitioners via a seminar breakout session. As a result two documents provide comment on the Jacobs report.

Werritty A., Arthur S., Ball T. and Wallerstein N., Review of Jacobs Section 20 Review, CREW-The James Hutton Institute, January 20124.

SNIFFER & SEPA Natural Flood Management Implementation Learning from Practice Workshop Report, November 20115

Informal comments have also been provided by Forest Research via email to the author of this document, these have been incorporated within the authors comments in Section 2.4.

2.2 Review of Jacobs Section 20 Review (CREW, 2012)4

A review was undertaken by Scottish academics at CREW (CentRe of Expertise for Waters) base on 11 questions set by SEPA:

Q1) The literature review was limited to those studies that provided quantitative evidence of the effects of Natural Flood Management techniques on catchment scale flooding. In your view, is this the right approach or have any important studies been missed?

Q2) In your view, are the studies identified in the literature review representative of Scottish catchments?

Q3) Do you agree with conclusions and recommendations in sections 2.2.4, 2.3.4 and 2.4.4[of the Jacobs Report]?

Q4) Do you agree with the outlined approach described in section 3.1[of the Jacobs Report]?

Q5) Do you have any observations in relation to the testing of the model described in section 3.3[Of the Jacobs Report]?

Q6) In your opinion, is the preliminary rule set described in section 3.4 [of the Jacobs Report] acceptable and are there sufficient grounds for the rule set? Suggest, if possible, alternative ways to improve the rule set.

Q7) In your opinion is the outline method for coastal floodplains correct?

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Q8) Do you agree with the limitations specified in section 3.7 [of the Jacobs Report] and recommendations in section 8.8[of the Jacobs Report]?

Q9) Do you have any other observations from Section 3 [of the Jacobs Report] that you would like to highlight?

Q10) Overall, would you recommend the use of the proposed GIS tool and the rule for use as the screening tool to provide an indication of the effect of Natural Flood Management techniques?

Q11) Do you have any recommendations for future work that would be beneficial to further develop the rule set and the modelling tool?

To avoid duplication of the CREW review it has been appended to this document in Appendix A.

2.2.1 Comments relating to literature review

Both the CREW reviews are complementary of the literature undertaken by Jacobs with one reviewer describing it as exhaustive, authoritative and up to date. The reviewers pick up on the omission of land management activities such as reducing stock densities, land use change and field drainage as areas where additional information could have been provided. However it is accepted that these omissions could be addressed in later development of the S20 GIS tool as proposed by Jacobs. Both reviewers suggest a handful of additional literature sources to be given consideration in later literature reviews.

The findings relating to reduction in flow introduced by woodland cover are broadly accepted however one reviewer suggests that the reduction in flows are probably over optimistic. The proposed reduction in flows generated by introducing riparian woodland and floodplain woodland are highlighted by the reviewers as being subject to a serious degree of uncertainty due to the small number of studies which can be used to support the proposed reductions.

2.2.2 Comments relating to Section 20 assessment tool

It should be noted that only limited access to the GIS tool was provided to reviewers due to data licensing issues. As a result, reviewers were only able to comment on the principles behind the tool rather than tool functionality. Concerns are raised about the division of the target catchment into thirds (upper, middle and lower by catchment descriptor estimated time to peak) by both reviewers, with one reviewer referring to it as a straight jacket which needs to be loosened.

Both reviewers have issue with how floodplain enhancement is included within the tool, with one reviewer highlighting that it is very dependent on its calibration to the Enrick Water and that it may not be possible to assess floodplain enhancement without site specific hydraulic modelling. The other reviewer considers the underlying science related to floodplain enhancement as too tenuous at present and strongly advises that floodplain enhancement is removed until evidence is available to substantiate the values.

The assumption that the effects of different Natural Flood Management measures within a catchment will be accumulative also raises concern, with one reviewer highlighting it as extremely simplistic, and unlikely to be the case in realitythis issue should be flagged as a serious issue.

The issue of the absence of ground truthing is raised by both reviewers, one reviewer recommends that the intensively gauged Eddleston Water could be used to test the tool against real data.

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2.3 Natural Flood Management Implementation Learning from

Practice Workshop Report (SNIFFER & SEPA, 2011)5

A breakout session was held as part of the above seminar to give delegates the opportunity to comment on the recently released Section 20 methodology proposed by Jacobs. It is reported that in overview the audience considered the tool to be very promising in what is a difficult area. Some delegates did raise concern that the tools should only be used by practitioners who are aware of its limitations. The breakout group delegates made the following recommendations:

The tool should be validated by running it on a range of Scottish catchments

The model should be peer-reviewed

It was suggested that the model incorporate land capability data

Concerns were raised over the validity of the upper/middle/lower division hard coded within the tool. It was suggested that a more subtle subdivision of catchments is needed in catchment headwater areas.

It was suggested that the uncertainty may be so large that decision making will still remain based on local expert judgement.

Many of the encoded Natural Flood Management measures are very generalised and will need to be improved in the future.

It would be helpful to know what factors are important to understand within different types of catchments.

2.4 Additional comments

In undertaking this study the author of this document was able to test the functioning of the proposed Jacobs S20 assessment tool, it should be noted that none of the other reviewers were able to gain access to the tool due to data and software licensing issues. In undertaking the testing of the tool only the prepared data for the Allan Water catchment was used and no other catchments were investigated.

In addition to the comments raised by the CREW (2012)4 review and at the SNIFFER-SEPA Seminar (2011)5 the author would like to raise the follow points:

The S20 tool is fast and relatively stable (within the normal expectations ESRIs ArcGIS platform). It has an intuitive easy to use interface.

The prescriptive structure of the model does retain a good level of control over the models outputs, this reduces the risk of an inexperienced user applying the tool incorrectly.

The method is reliant on CEH Flow grid coverage for the entire catchment and the tool is hard coded on the assumption that the CEH Flow grid is correct. In essence the tool calculates the change in flows and not the actual flows, therefore the predictions made by the tool are limited by the underlying CEH Flow grid on which it has been built. The tool should not be seen as a hydrological model, rather a method for scaling existing hydrological predictions.

It is unclear what catchment delineation the tool functions with. The sample data provided by Jacobs for the Allan Water shows two catchment delineations for the Allan Water which disagree with each other. It is understood that parts of the tool work with the assumption that the CEH Flow Grid is correct while other elements work with a user supplied catchment delineation. The ambiguity regarding what catchment area is being considered is not helpful. An example of the Forglen

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Burn (a tributary of the Allan Water), which drains the loch at the University of Stirling, is shown in Figure 3.

Figure 3: Example of the differences between the catchment delineations used within the Jacobs S20 tool

The tool allows benefits of riparian planting to be accrued in non-riparian areas. For example using the riparian tool to cover the entire upper two thirds of the Allan Water catchment less all riparian corridors indicates a central estimate of flow reductions for the 1 in 200yr flood event of approximately 0.64% despite no woodland being positioned within the riparian margin. Refer to Figure 4 for more information. The same area covered by forest cover achieves a 1.7% central estimate reduction for the 1 in 200yr event.

Figure 4: Example of a riparian intervention that generated a 0.64% reduction in the 1 in 200yr flood event without any of the proposed riparian woodland being positioned in the riparian zone. (Proposed riparian woodland shown in pink).

The current configuration of the tool does not permit predefined shapefiles to be uploaded into the tool nor can manually drawn Natural Flood Management

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outlines be exported from the tool. It is likely that it would be feasible to include improved import/export functionality in an updated version of the tool.

It is up to the user to classify the function of placed woodland. For example if the woodland tool is used across a broad area which incorporates floodplain and riparian areas the tool will only assess the woodland for its runoff reducing capabilities and not for the riparian or floodplain benefits.

The woodland cover within the LCM2000 dataset is hard coded as the actual woodland cover. While it is accepted that it is a good representation of woodland cover at the time the data was acquired, in many instances the LCM2000 data does not correctly classify the present day local land cover correctly.

The tool does not allow a way for the results to be exported, for example the revised hydrograph cannot be extracted to allow the flows to be applied to a hydraulic model.

Currently the tool only allows the assessment of the hydrological benefits for the 1 in 200yr and 1 in 5yr flood events. It is acknowledged that the report indicates other return periods could be added.

The drain blocking element of the tool does not take account of local soil conditions, drain geometry or drain direction which are likely to play a significant role in the both the magnitude and effect (reducing or increasing) flood risk. (Refer to Section 3.3(D)

The effect of catchment scale on the effectiveness of Natural Flood Management measures is not accounted for within the tool. There is a general consensus within the literature that the effectiveness of runoff reducing measures is inversely related to catchment size.

The adopted hydrological method does not conserve mass. That is, when the flood peak is reduced this is achieved by cropping the top off the hydrograph. This lost flow volume is not reintroduced later in the flood hydrograph. The method of cropping the flood also introduces the curiosity that floodplain restoration might have a notable impact at reducing flows around the peak of the 1 in 5yr event, but for a 1 in 200yr event there is no modification to the hydrograph at that same flow rate, (refer to Figure 5).

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Figure 5: Example of the output from the proposed Jacobs S20 assessment tool showing the hydrological benefits of restoring the floodplain area (purple) to facilitate an increase in flood depths of 300mm with a 50% efficiency factor.

The assessment tool is catchment outlet specific, therefore the tool is not appropriate for national screening of Natural Flood Management opportunities.

In preparing the literature review, Jacobs did not undertake a review of existing hydrological methods which have been used to represent the hydrological benefits for Natural Flood Management measures. Nor does the review take account of the recommendations for future Natural Flood Management assessment tools such as those by OConnell et al. (2004):

The modelling should be distributed and be capable of running continuous simulations. It should also be partially or wholly physically based so that the physical properties of local landscapes, soils and vegetation can be represented, and it should include detailed modelling of surface water flow so that the effects of changes can be tracked downstream.

It is unfortunate that the Jacobs S20 is not aligned with these recommendations, namely it is not a distributed model that is capable of continuous simulation, nor does it have a physical basis or a detailed flow routing engine to permit the effects to be tracked downstream.

The hydrological benefit of floodplain restoration does not include any component to account for the hydrograph volume. The ratio of hydrograph volume to floodplain volume is a key variable (Section 3.3(L)).

The increase in flood depth as a consequence of floodplain planting is weak. It would be beneficial for a stronger physical basis to the selection of increased floodplain depth so that land cover (roughness), discharge intensity (flow/unit width) and hydraulic gradient (or valley slope) are used to estimate the likely increase in floodplain depth.

It is not clear how the Jacobs S20 tool can quantify Natural Flood Management benefits in catchments that have significant lochs or reservoirs, particularly if these waterbodies are artificially managed or are close to the catchment outlet.

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It is unclear how hydrometric data, such as a real data estimation of QMed or the catchments observed time to peak can be incorporated within the tool. It would appear that the tool is hardcoded to remain a no data methodology.

Jacobs recommends that the S20 tool is only used to provide an initial broadscale identification of possible benefits of various Natural Flood Management measures; going on to say that measures should be more fully appraised before adoption. This implies that a different method must be put in place to allow for the detailed assessment and that the tool will not allow Natural Flood Management measures to be directly compared against hard engineered measures. A robust hydrological assessment framework which can be refined as the project develops, and which is consistent with the underlying principles of current British hydrological practice, would be more progressive.

The Jacobs S20 tool only provides a means of assessing the hydrological benefits of Natural Flood Management measures in fluvial systems. Limited discussion of a potential means to configure a saltflats restoration tool for the estimation of reduction in estuarine surge is provided however there are notable gaps in the methodology which is discussed only in broad concept. There is no discussion of using saltflats to dissipate wave energy and the areas of groundwater and pluvial flooding are not considered.

In some cases there has been criticism that the Jacobs S20 tool does not give enough consideration to other benefits. While it would be ideal to have a tool which would consider all benefits and disbenefits of Natural Flood Management, to do so would be immensely challenging. For the time being, the focus of energy should be on the development of a hydrological assessment methodology for Natural Flood Management techniques thus facilitating the fulfilment of Section 20 of the Act.

By direct correspondence with the author, Nadeem Shah at Forest Research provided the following comments:

o Riparian woodland should not be considered a subset of floodplain woodland. He recommended that consideration is given to the definition provided within the UK Forestry Standard Water Guidelines (Forestry Commission, 2011)76

o There is a need for the tool to consider other forest mechanisms for flood alleviation, namely infiltration and water use.

o Would be keen to see more mention of multiple benefits

o The OConnell et al. (2004)83 study identified three fundamental unresolved issues of rainfall-runoff models yet it is unclear that the Jacobs S20 methodology takes appropriate steps to address these issues.

o It would be beneficial for the model output to incorporate confidence bands to assist in decision making.

o The assumptions underlying the simplified and generic models should be commented on in the report or an appropriate paper.

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2.5 Summary

It can be summarised that the literature review undertaken by Jacobs is generally well accepted. Subject to inclusion of a handful of papers, including some recently published, the literature review should be considered authoritative and up to date.

The Jacobs S20 Tool is based on modifications to the CEH Flow Grid. In developing the methodology for modelling the effect of Natural Flood Management measures Jacobs have not drawn on the recommendations of OConnell et al. (2004)83 in regards to how the model should be configured. Instead the method is centred on pro-rata cropping of the hydrograph peak via best estimates made from the literature rather than adopting a more physical based approach to modify parameters contained within the hydrological model.

There are some concerns over the schematisation of the model, with the most notable being related to the absence of a hydraulic based flow routing element to the model (division of catchment into thirds). This formulation prevents accurate accumulation of hydrological benefits moving down through the catchment towards receptors and the assessment of delay/attenuation measures (floodplain enhancement and riparian intervention). There are other areas of concern relating to how the tool accumulates the benefits of multiple types of interventions, the absence of ground truthing and the suggested benefits are questioned for some measures (floodplain enhancement).

The requirement to specify a single location as the catchment outlet will lead to challenges in using the tool to undertake a national screening of Natural Flood Management opportunities. The tool also lacks functionality to identify flood risk management benefits for coastal, groundwater and pluvial flood mechanisms.

It is not recommended that the current formulation of the Jacobs S20 assessment tool is used for the identification or appraisal of Natural Flood Management measures in Scotland.

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3 Literature review

3.1 Introduction

Natural Flood Management is a broad area covering many of the aspects of hydrology, coastal science and their related subjects. This literature review will be divided into the following sections:

A summary of previous Natural Flood Management literature reviews concentrating on reviews relevant to the catchments and coastal conditions typical to Scotland (Section 3.2)

A summary of the most comprehensive science relating to each of the main Natural Flood Management measures within a Scottish context (Section 3.3)

A summary of the existing tools for screening and assessing Natural Flood Management measures (Section 3.4)

3.2 Review of previous literature reviews on Natural Flood

Management

The understanding of Natural Flood Management techniques and their effectiveness in reducing flood risk is a growing area of science. In order to identify key research questions, the major British Natural Flood Management literature reviews were reviewed and outcomes summarised in this part of the report. These reviews included:

(A) FD2114 Review of Impacts of Rural Land Use and Management on Flood Generation (OConnell et al., 2004)83

(B) R&D update review of impact of rural land use and management on flooding (Environment Agency - Atkins, 2007)

(C) FD2120: Analysis of historical data sets to look for impacts of land use and management change on flood generation, R&D Technical Report (Defra / EA, 2008)

(D) Delivery of Making Space for Water, The role of land use and land management in delivering flood risk management (Environment Agency Halcrow, 2008)

(E) Land Use Jigsaw: Prioritising scientific research on land management and FCRM (Environment Agency, 2008)

(F) Evidence Based Review: Does Land Management Attenuate Runoff? (Haycock Associates National Trust, 2008)

(G) The way forward for Natural Flood Management in Scotland - a report to Scottish Environment LINK (MNV, 2008)

(H) Technical review of NFRM techniques, their effectiveness and wider benefits (RSPB, 2009)169

(I) Natural approaches to flood management under the Flood Risk Management (Scotland) Act 2009: Development of Methodology under Section 20 - a report to SEPA (Jacobs, 2011)

(J) Greater working with natural processes in flood and coastal erosion risk management (Environment Agency, 2012)

It is clear that literature reviews vary in their quality and effectiveness. Some of the studies had not been peer reviewed, lessening their accuracy and validity of their conclusions. The most robust technical reviews of Natural Flood Management techniques were commissioned by Defra and the Environment Agency, but these studies

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reflect the issues in England and Wales. There is a tendency for Scottish Natural Flood Management literature reviews to not be as scientifically robust as they have not been subjected to a peer review process, but never the less they do form useful sources.

(A) FD2114 Review of Impacts of Rural Land Use and Management on Flood Generation (OConnell et al., 2004)83

This was the first major review of the impacts of land use and management on flood generation undertaken by Defra and the Environment Agency. The purpose of the study was to review the factors contributing to runoff and flooding in the rural (managed, not natural) environment, and to scope the research needed to improve the identification of the management policies and interventions to reduce the impact of flooding. The study focused on inland flooding mechanisms such as pluvial and fluvial flooding.

The study was undertaken by a team drawn from the disciplines of agriculture, soil science, hydrology, hydrogeology and socio-economic science and provides much of our current understanding of the science with respect to land use management and its impact on flood generation.

It is worth noting that this review focused on land use and land management techniques, therefore focusing on runoff generation and thus excluding other Natural Flood Management techniques such as floodplain restoration, river restoration, washlands creation and coastal realignment.

The main conclusions of FD2114 are as follows (taken from the Summary of the document):

Significant changes in land use and management practices in the last fifty years have resulted in a general intensification of agricultural land use;

There is substantial evidence that changes in land use and management practices affect surface runoff generation at the local scale, but the effects are complex;

There is only limited evidence that local changes in runoff are transferred to the surface water network and propagate downstream;

Analyses of peak runoff records provide very little firm evidence of catchment scale impacts of land use management;

There are many measures that can be taken to mitigate local flooding by delaying runoff;

There is considerable uncertainty about how effectively land managers will respond to promotions or policies related to particular flood prevention or mitigation measures;

Rainfall-runoff modelling to predict the effects of changes in rural land use and management on flood generation is in its infancy: there is no generally accepted theoretical basis for the design of a model suitable to predict impacts; and

The uncertainty in the response of land managers noted above needs to be accounted for when modelling the overall outcomes when flood prevention and mitigation practices are promoted.

FD2114 also proposed a number of research priorities centring on collating existing knowledge, data gathering and the development of assessment tools.

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(B) R&D update review of impact of rural land use and management on flooding (Environment Agency - Atkins, 2007)6

Subsequently in 2007, Defra and the EA commissioned Atkins to undertake a further review of the evidence base, updating the findings of the 2004 Defra Research project FD2114 with the latest research. This project was commissioned as part of delivery of Making Space for Water (Defra activities HA6 and HA7) and included a review of current research relevant to the role of land use in flood risk management in conjunction with the identification of future research and development needs. This research project was mainly concerned with the processes governing runoff response of catchments and the findings of this review are of relevance to Natural Flood Management in Scotland. This report is one of the most valuable and authoritative sources of information on Natural Flood Management measures.

The objectives of the review were to:

Update evidence from Defra Research project FD2114 on the impacts of rural land use changes and management on flood generation in rural areas, at both the local and catchment scale within the UK; and

To review and prioritise future research needs in this area.

The review was developed from published literature, ongoing studies and additional information obtained through consultation with a number of key individuals between January and March 2007. The review reflected the most up-to-date evidence and provided an indication of where current research is being targeted.

The R&D update focused upon the most important land use management practices, as identified within FD2114 and subsequent consultation with the Environment Agency. Unlike FD2114, the update was limited to UK sources and evidence. The findings were presented according to five main land use or management practices:

Afforestation and deforestation;

Agricultural drainage;

Peat drainage and moorland gripping;

Pasture; and

Cultivation techniques.

The key findings of the study were:

There is continued uncertainty in how local scale effects propagate downstream.

For large catchments, existing modelling studies suggest that a large extent of land-use or land management change is required to produce a relatively modest reduction or delay in downstream flood peaks.

The location of local changes or interventions within a catchment is critically important. Changes will have different effects depending on the relative phasing within the catchment. Catchment scale models that couple hillslope responses to downstream channel networks are vital to enable prediction of these effects.

For particular soil types and geologies (and therefore locations within the UK), groundwater becomes important for flooding. The effects of land use on groundwater flooding remains an under-researched area.

Multiple interventions have and continue to hamper our ability to predict the impacts from specific land-use changes.

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Climate variability is the dominant factor in influencing the frequency and magnitude of flood events within any given catchment.

Afforestation and deforestation:

There is strong evidence that upland conifer forests have a negligible effect on the reduction of flood flows. (This point is subsequently contradicted implying that it relates to out of date forestry practices.)

There is very limited potential for afforestation to reduce flood risk at catchment scale. While upland plantations may reduce peak flows for smaller events, they are thought to have a negligible impact on extreme events. Equally there is scant evidence that deforestation increases flood risk although there is evidence that poor logging practices may have temporary negative impacts.

Land management practices associated with conifer afforestation such as plough drainage and access road construction can exacerbate flooding. However these effects are limited by current best practice.

There is evidence that at local scale there is some potential for strategic tree planting to increase infiltration in compacted soils.

There is limited knowledge on the hydrology of natural unmanaged woodland in the UK. It is not thought that these woodlands would be any more likely to reduce extreme floods than conifer plantations, except perhaps for short duration summer and autumn events. In some circumstances the increased water use may lower antecedent water thus reducing flood risk.

There is a growing body of evidence that suggests that riparian and floodplain woodland can attenuate flood propagation.

Agricultural practices:

Agricultural policy has been a major driver of land management practices. As yet there is little evidence of the impact of new agri-environment schemes.

The evidence shows that there is a need to consider the land management history of a catchment down to the field scale when assessing its flows.

It is possible to predict with a significant degree of confidence the effect of agricultural drainage on runoff and river flow from a given site by an examination and understanding of the individual site properties. Areas could be mapped at local scale and high risk areas identified.

Peat and moorland drainage

Tools have been and are being developed that help to predict the effect of artificial drainage or drain blocking on the timing and magnitude of runoff. These should help target interventions. Catchment scale models also need to be used to take account of the location of the land-use change within the catchment and its influence on the timing and magnitude at receptors.

There is uncertainty to whether long-term changes to peat can be reversed.

Pasture

There is evidence that land management in grassland environments influences runoff at a local scale and small catchment scale. There is also evidence for year-to-year variability in soil structure, soil vulnerability to degradation, and in runoff processes.

The Pontbren study will be important at bridging the gaps between hillslopes, small catchment and downstream impacts on flood frequency.

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Existing modelling studies suggest that a large extent of grassland land-use or land management change is required to produce a relatively modest modification to downstream floods.

(C) FD2120: Analysis of historical data sets to look for impacts of land use and management change on flood generation, R&D Technical Report (Defra / EA, 2008)7

This study was commissioned as part of joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme. Its purpose was to provide an analysis of historical data on land use and land management to ascertain whether any impacts of land use and management change on flood generation could be identified. The change identification methodologies used were Dynamic Harmonic Regression to examine seasonal scale trends, and Data Based Mechanistic models were used to examine changes in storm responses.

The study found that, in general the variability between years and inconsistencies in the rainfall and flow data appear to dominate any tendency of other changes. Only seasonal flows could be shown to exhibit any significant temporal trend and then only for two of the nine catchments studied. The assessment of hydrograph responses showed no clear changes over time. Where tendencies in hydrograph characteristics with time were evident they were obscured by year to year variability.

The project concluded with a set of policy recommendations:

Both climate variability (particularly rainfall variability) and land use affect flood runoff. Changes in discharge should not be analysed without consideration of changes in catchment rainfall inputs.

The preliminary study of catchment responses within different event classifications (antecedent conditions) was the most promising form of analysis developed during this project. Different classification schemes should be investigated to check the nature of changes, including a more complete uncertainty analysis. Careful quality control of existing datasets is necessary in carrying out such analyses.

Adequate information about past land management changes and soil conditions is not readily available but will need to be collected and made available in future for different land use categories if improved understanding of the links between runoff and land management is to be gained and used at catchment scales.

The results of this project show that there will be a real difficulty of estimating the benefits of land use change measures in respect of any reduction of flood risk. Further monitoring of studies aimed at reducing runoff should be carried out to evaluate the effectiveness of different types of measure at the local level in the context of agri-environment schemes.

The difficulty in identifying consistent changes given the limitations of the available data means that land management measures cannot be relied on as alternatives to more proven flood risk management options.

The difficulty in identifying consistent change given the limitations of the available data does not mean that change is not happening and should not be taken to imply a policy of doing nothing. Contextually relevant management practice guidelines (linked to land use, soil type, antecedent condition) should be developed and monitored to deliver multiple benefits including reduced runoff generation and local flood risk.

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(D) Delivery of Making Space for Water, The role of land use and land management in delivering flood risk management (Environment Agency Halcrow, 2008)8

This study was commissioned by the Environment Agency with the objectives to:

Summarise the strength of underlying science and knowledge gaps with respect to land-use/management impacts on flood risk;

Illustrate how land-use/management changes relevant to flood risk are being delivered through a review of case studies; and

Identify policy mechanisms for delivering changes in land-use management relevant to flood risk.

The study provides a summary of recent catchment scale modelling studies:

The lumped sub-catchment hydrological model with 1-D flow routing model approach used in the Ripon study potentially provides a simple framework for testing future land use scenarios which could be applied generically to different catchments. However, this approach is as yet untested on other catchments:

o Scale issues -Land use change modelling is undertaken at a sub-catchment scale and does not take account of localised changes;

o Underlying science issues Land use changes are simulated by making changes to parameters within the Flood Estimation Handbook (FEH) rainfall-runoff method, which provides only crude assessments of surface runoff impacts; and

o Limitations in model input data quality and setup.

The physically based distributed hydrological model used in the Parrett catchment is considered to be more robust because surface, subsurface and groundwater are better represented throughout the catchment using a grid approach. This approach can potentially be more reliably applied to catchments with very different characteristics and as the Natural Flood Management interventions are modelled using physically based modifications to the model there is better scope to accurately represent the accumulative impact of Natural Flood Management measures at a catchment scale. The study identified:

o The potential to identify sub-catchments where land management changes would offer the greatest benefit;

o Potential to appraise land management measures such as buffer strips and contour planting;

o Models are data hungry and computationally expensive; and

o Models can be costly to setup and require an experienced user.

Simple modelling tools (such as the lumped models applied to the Ripon) should be used wherever only basic level information is available. It should be recognised that simpler tools will provide simple and more approximate predictions.

Research undertaken in Upper Wharfedale has shown that rapid aggradation is a principle driver of flooding and that the sedimentation risk could be controlled via Natural Flood Management measures.

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(E) Land Use Jigsaw: Prioritising scientific research on land management and FCRM (Environment Agency, 2008)9

The Land Use Jigsaw is an ongoing (as of time of writing) research prioritisation tool maintained by the Environment Agency that seeks to guide the development of policy relevant research for Flood and Coastal Risk Management (FCRM) in England and Wales. The tool, which takes the form of a community maintained register, seeks to promote current research and determine gaps and overlaps in evidence. It is worth noting that the Land Use Jigsaw has been focused on the evidence base for individual measures and has not considered the combined effects of measures on reducing flood risk. In addition to this it should be noted that it is an Environment Agency tool and therefore focused on issues faced by the Environment Agency in England and Wales. While there are marked similarities between hydrological processes across the British Isles care should be taken in applying the findings in Scotland.

The starting point for the Land Use Jigsaw was the current understanding as presented in previous studies as discussed above. In summary:

Effects of land use management on flood risk at a large catchment scale are difficult to determine as they require aggregating many local-scale effects and are dependent on catchment characteristics;

The lack of understanding does not necessarily mean that there is no catchment-scale effect, but rather that the nature of the effect differs between different catchments and is difficult to detect;

There is strong evidence that small scale catchment management approaches deliver flood risk management benefits. Farmland can store floodwater to reduce downstream flood risk. Large washland (storage) or managed realignment areas can be designed to store excess water and slow down flood peaks;

Although there can be benefits for local flood mitigation, the evidence to date has not demonstrated the reduction of extreme flood events at the catchment scale when using most land management interventions. The project will continue to support research into large catchment scale impacts of land management changes on flood risk, water quality and resource protection and will actively seek partnership projects; and

Policy needs to be based on sound scientific evidence. In order to deliver the evidence base there needs to be a coordinated effort between all those undertaking research in this area to focus their research efforts in the correct areas.

The key priority research questions identified in the 2008 summary report are summarised in Table 1.

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No. Evidence Need Category Question

Q020 Evidence gap

Needed now

Under sowing

2a Are there quantifiable flood risk benefits at the local scale?

Q031 Evidence gap

Needed now

Reduce soil compaction

3a Where in the catchment will this be most effective?

Q038 Evidence gap

Needed now

Crop choice 2a Are there quantifiable flood risk benefits at the local scale?

Q039 Evidence gap

Needed now

Crop choice 2b What are the costs for implementation and maintenance?

Q070 Evidence gap

Needed now

Wetland creation

4a Is there a quantifiable flood risk benefit at the catchment scale?

Q088 Evidence gap

Needed now

On farm storage

4a Is there a quantifiable flood risk benefit at the catchment scale?

Table 1: Priority research questions

The most significant gaps in scientific knowledge identified in the 2008 summary report are summarised in Table 2.

No. Evidence Need Category Question

Q112 Ongoing work

Needed now

Grip blocking

3a Where in the catchment will this be most effective?

Q113 Ongoing work

Needed now

Grip blocking

3b How long does the intervention last?

Q121 Ongoing work

Needed now

Peat restoration

3a Where in the catchment will this be most effective?

Q122 Ongoing work

Needed now

Peat restoration

3b How long does the intervention last?

Q139 Ongoing work

Needed now

Floodplain woodland

3a Where in the catchment will this be most effective?

Table 2: Questions requiring further research

(F) Evidence Based Review: Does Land Management Attenuate Runoff? (Haycock Associates National Trust, 2008)10

This report was commissioned by the National Trust to provide an overview of the current literature in order that future operational policy is relevant and informed. It focuses on changing land use impacts upon runoff and the impact of land management upon catchment level studies (>100sqkm).

The study used a systematic approach to identify published and unpublished research relating to land use/management and runoff with a strict quality control procedure. In total 29,772 articles (including duplicates) were indentified by the literature search. Of these articles, 108 were identified as sufficiently relevant for full text viewing and

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subsequently 31 were found to be sufficiently relevant and robust. These 31 articles covered a total of 34 study sites.

The review found that there is significant evidence that land management is an effective method for impacting runoff at an experimental scale, however there is not significant evidence at representative catchment scales. Despite this there is increasing evidence of local scale impacts propagating downstream.

There is consistent evidence to support that at the representative catchment and experience scale afforestation will lead to an overall reduction in runoff. Similar strength trends are observed in relation to agricultural intensification and deforestation which suggest increases in discharge, peak flows, baseflow and surface flow are frequent to observe in studies at the representative catchment and experimental level.

Studies into the impact of moorland drainage on runoff follow similar trends as found in deforested and intensified areas, although surface flow is actually found to be lower in these areas. In the small number of studies that looked into the impact of soil degradation on runoff, it was demonstrated that discharge and peak flows were found to be higher in degraded soils.

There were no trends to be observed in the impact of agricultural drainage upon overall runoff. However, the lack of a trend may be due to the fact that the exact impact that agricultural drainage has upon the hydrology of a site is largely dependent upon the soil type as well as the type, size and extent of the installed drainage. Further complications associated with the life expectancy of a drainage system need to be understood. It has been identified that the impacts that a newly installed drainage system has upon runoff, cannot be assumed to be applicable indefinitely. For example mole drainage channels (traditionally used in clayey soil types) are typically effective for 2-10 years, again dependent upon the soil type. Pipe drainage can last anywhere between 10-50 years, dependent upon the gradients used and the proportion of fine silts in the soil.

The following observations were made within the studys findings:

In an experimental scale there is substantial quantitative evidence than land management attenuates runoff;

The number of representative catchment studies is relatively low;

There is not substantial quantitative evidence at a representative catchment scale that land management attenuates runoff, however there is an increasing number of computational studies and a large range of qualitative evidence. There is emphasis that a lack of evidence does not necessarily equate to a lack of effect;

Need to adopt the precautionary principle when assessing the influence of land management upon runoff;

At a large catchment scale there is some evidence that land use changes impact runoff;

Need to appraise current methodologies for investigating impact of land management, in response to the lack of statistics and unit hydrograph analysis in representative catchment studies;

From the papers reviewed it is not clear what impact land management has on different return periods; and

It is important to note that following permanent changes in vegetation it takes more than 5 years for a catchment to reach