Regional Investment Programme · There is a geo-cellular storage unit 0.5km west of Polegate. This...

Registered office Bridge House, 1 Walnut Tree Close, Guildford, GU1 4LZHighways England Company Limited registered in England and Wales number 09346363

Regional Investment ProgrammeA27 East of Lewes Improvements

PCF Stage 3 – Flood Risk AssessmentMARCH 2019

A27 East of Lewes ImprovementsPCF Stage 3 – Flood Risk Assessment

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Notice

This document and its contents have been prepared and are intended solely for Highway England’sinformation and use in relation to the A27 East of Lewes Improvements PCF Stage 3, one of the schemesof the Regional Investment Programme. WSP assumes no responsibility to any other party in respect ofor arising out of or in connection with this document and/or its contents.

Document control

The Project Manager is responsible for production of this document, based on the contributions made byhis/her team existing at each Stage.

Document Title PCF Stage 3: A27 East of Lewes Improvements – Flood Risk Assessment

Author

Owner Tom Beasley, Highways England

Distribution Highways England Consultees, WSP Team

Document Status Final

Revision History

This document is updated at least every stage.

Version Date Description Author

P01 21/02/19 Laura Swettenham /Amanda Nicolaou

P02 08/03/19 Laura Swettenham /Amanda Nicolaou

Reviewer ListName RoleChris Ritchley WSP Project ManagerStuart Craig WSP Project DirectorTom Beasley Highways England, Project ManagerTim Jolley WSP Technical Director

Approvals

The Project SRO is accountable for the content of this document

Name Signature Title Date of Issue Version

Alan Feist Highways EnglandProject SRO

A27 East of Lewes ImprovementsPCF Stage 3 – Flood Risk Assessment

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Project no:HE PIN 552988 East of Lewes Improvements

Prepared forHighways EnglandBridge HouseWalnut Tree CloseGuildfordSurreyGU1 4LZ

WSPThe ForumBarnfield RoadExeterEX1 1QR

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A27 EAST OF LEWESIMPROVEMENTSPCF STAGE 3FLOOD RISK ASSESSMENTHighways England

A27 East of Lewes ImprovementsPCF Stage 3 - Flood Risk Assessment

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Q U A L I T Y M A N A G E M E N TISSUE/REVISION P01 P02

Remarks Draft Final

Date 12/02/19 08/03/19

Prepared by Laura Swettenham /Amanda Nicolaou

Laura Swettenham /Amanda Nicolaou

Signature

Checked by Hilary Hampton / TimJolley

Hilary Hampton / TimJolley

Signature

Authorised by Tim Jolley Tim Jolley

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Project numberPIN: HE552988

WSP ref: 70035418

Report number HE552988-WSP-EWE-SWI-RE-LA-00098

File reference \\uk.wspgroup.com\central data\Projects\700354xx\70035418 - A27EastOfLewes


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TABLE OF CONTENTS1 INTRODUCTION ...........................................................................9

2 EXISTING SCENARIO ...............................................................11

2.1 SCHEME LOCATION ................................................................................... 11

2.2 TOPOGRAPHY ............................................................................................. 13

2.3 SCHEME CATCHMENTS AND DRAINAGE ................................................. 16

3 PROPOSED SCHEME ...............................................................18

3.2 PROPOSED DRAINAGE LAYOUT ............................................................... 19

3.1 INTERNAL DRAINAGE BOARD ................................................................... 19

4 NATIONAL PLANNING POLICY FRAMEWORK .......................21

4.2 FLOOD ZONE DEFINITION .......................................................................... 21

4.3 FLOOD ZONE VULNERABILITY .................................................................. 22

4.4 APPROPRIATE DEVELOPMENT ................................................................. 22

5 PREVIOUS STUDIES AND HISTORIC FLOOD RISK ...............23

5.1 PREVIOUS STUDIES ................................................................................... 23

5.2 HISTORIC FLOODING.................................................................................. 32

6 FLOOD RISK ASSESSMENT ....................................................33

6.1 FLOOD RISK FROM RIVERS AND THE SEA/TIDE...................................... 33

6.2 FLOOD RISK FROM GROUNDWATER ........................................................ 50

6.1 FLOOD RISK FROM ARTIFICIAL SOURCES .............................................. 63

6.2 FLOOD RISK FROM OTHER ARTIFICIAL SOURCES ................................. 65

7 FLOOD RISK MITIGATION ........................................................65

7.2 SURFACE WATER DRAINAGE ................................................................... 66


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8 CONCLUSIONS AND RECOMMENDATIONS ...........................69

8.1 CONCLUSIONS ............................................................................................ 69

8.2 RECOMMENDATIONS ................................................................................. 70


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T A B L E STABLE 2-1 HIGHWAYS ATTENUATION BASINS ......................................................... 17TABLE 2-2 GEO-CELLULAR STORAGE TANKS .......................................................... 17TABLE 3-1 EXISTING AND PROPOSED SCHEME AREAS.......................................... 18TABLE 6-1: ENVIRONMENT AGENCY WATER LEVELS FLUVIAL UNDEFENDED ..... 40TABLE 6-2: ENVIRONMENT AGENCY FLOWS FLUVIAL UNDEFENDED ................... 41

F I G U R E SFIGURE 2-1 SCHEME EXTENTS ................................................................................. 12FIGURE 2-2 LIDAR DATA OF GLYNDE REACH AREA OF THE A27 (1 OF 5) ............. 13FIGURE 2-3 LIDAR DATA OF WESTERN EXTENT OF THE A27 (2 OF 5) ................... 14FIGURE 2-4 LIDAR DATA OF DRUSILLAS ROUNDABOUT (3 OF 5) ........................... 14FIGURE 2-5 LIDAR DATA OF WILMINGTON JUNCTION (4 OF 5) ............................... 14FIGURE 2-6 LIDAR DATA OF POLEGATE JUNCTION (5 OF 5) ................................... 15FIGURE 2-7 WATERCOURSE AND JUNCTION OVERVIEW ....................................... 16FIGURE 3-1 OVERVIEW OF THE MAXIMUM SCHEME EXTENT ................................ 19FIGURE 3-2 RAILWAY LINE ADJACENT TO THE SCHEME (HE552988-WSP-GEN-

SWI-DR-CH-00034.PDF) .............................................................. 19FIGURE 3-3 PEVENSEY AND CUCKMERE WATER LEVEL MANAGEMENT BOARD

(PCWLMB) AREA ........................................................................ 20FIGURE 4-1 NPPF FLOOD RISK VULNERABILITY AND FLOOD ZONE

‘COMPATIBILITY’ ........................................................................ 22FIGURE 5-1 FUTURE FLOOD RISK OF GROUNDWATER FLOODING MAP TAKEN

FROM THE PFRA ........................................................................ 24FIGURE 5-2 OVERVIEW MAP OF THE CUCKMERE AND SUSSEX HAVENS CFMP

AREA ........................................................................................... 25FIGURE 5-3 OVERVIEW MAP OF THE RIVER OUSE CFMP AREA............................. 28FIGURE 5-4 SURFACE WATER HOTSPOTS LOCATED IN POLEGATE (SWMP) ....... 31FIGURE 5-5 HISTORIC SURFACE WATER FLOODING INCIDENTS IN POLEGATE

(SWMP) ....................................................................................... 32FIGURE 6-1 ENVIRONMENT AGENCY FLOOD MAP FOR PLANNING (FLUVIAL AND

TIDAL) ......................................................................................... 34FIGURE 6-2 ENVIRONMENT AGENCY FLOOD MAP FOR PLANNING (GLYNDE

REACH TRIBUTARY EXTENT) (FLUVIAL AND TIDAL) ............... 35FIGURE 6-3 ENVIRONMENT AGENCY FLOOD MAP FOR PLANNING (CUCKMERE

RIVER EXTENT) (FLUVIAL AND TIDAL) ..................................... 36FIGURE 6-4 ENVIRONMENT AGENCY FLOOD MAP FOR PLANNING (LANGNEY

SEWER EXTENT) (FLUVIAL AND TIDAL) ................................... 36FIGURE 6-5 ENVIRONMENT AGENCY FLUVIAL FLOOD WARNING MAP FOR

CUCKMERE RIVER ..................................................................... 37


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FIGURE 6-6 ENVIRONMENT AGENCY FLUVIAL FLOOD WARNING MAP FOR THEGLYNDE REACH TRIBUTARY .................................................... 38

FIGURE 6-7 ENVIRONMENT AGENCY TIDAL FLOOD WARNING MAP ...................... 38FIGURE 6-8 ENVIRONMENT AGENCY RECORDED FLOOD OUTLINES .................... 39FIGURE 6-9 ENVIRONMENT AGENCY LEVEL AND FLOW VALUES OF THE

CUCKMERE RIVER MAP ............................................................ 40FIGURE 6-10 ENVIRONMENT AGENCY FLOOD MAP FOR PLANNING (RIVERS AND

SEA) ............................................................................................ 42FIGURE 6-11 1 IN 100 YEAR + 105% CC FLOOD DEPTH MAP FOR THE CUCKMERE

RIVER .......................................................................................... 43FIGURE 6-12 PROPOSED SCHEME AND RED LINE BOUNDARY AT RIVER

CUCKMERE................................................................................. 44FIGURE 6-13 PROPOSED SCHEME AND RED LINE BOUNDARY AT RIVER

CUCKMERE................................................................................. 45FIGURE 6-14 CHANGE IN WATER LEVEL (MM) FOR 1 IN 100 YEAR + 105% CC

RETURN PERIOD FOR THE CUCKMERE RIVER ....................... 46FIGURE 6-15 1 IN 100 YEAR +105% CC FLOOD DEPTH MAP FOR LANGNEY SEWER47FIGURE 6-16 PROPOSED SCHEME AT POLEGATE BYPASS AT LANGNEY SEWER

CULVERT .................................................................................... 48FIGURE 6-17 CHANGE IN WATER LEVEL (MM) FOR 1 IN 100 YEAR +105% CC

RETURN PERIOD FOR LANGNEY SEWER ................................ 49FIGURE 6-18 BRITISH GEOLOGICAL SURVEY ONLINE BEDROCK MAPPING OF

SCHEME...................................................................................... 50FIGURE 6-19 BRITISH GEOLOGICAL SURVEY ONLINE SUPERFICIAL DEPOSITS

MAPPING OF SCHEME ............................................................... 51FIGURE 6-20 GROUNDWATER VULNERABILITY (DEFRA, 2018) .............................. 52FIGURE 6-21 GROUNDWATER SOURCE PROTECTION ZONES (DEFRA, 2018) ...... 53FIGURE 6-22 ENVIRONMENT AGENCY FLOOD RISK FROM SURFACE WATER ...... 55FIGURE 6-23 ENVIRONMENT AGENCY FLOOD RISK FROM SURFACE WATER-

HIGH RISK (TRIBUTARY OF GLYNDE REACH EXTENT) ........... 56FIGURE 6-24 ENVIRONMENT AGENCY FLOOD RISK FROM SURFACE WATER-

MEDIUM RISK (TRIBUTARY OF GLYNDE REACH EXTENT) ..... 57FIGURE 6-25 ENVIRONMENT AGENCY FLOOD RISK FROM SURFACE WATER-

LOW RISK (TRIBUTARY OF GLYNDE REACH EXTENT) ............ 57FIGURE 6-26 ENVIRONMENT AGENCY FLOOD RISK FROM SURFACE WATER-

HIGH RISK (CUCKMERE EXTENT) ............................................. 58FIGURE 6-27 ENVIRONMENT AGENCY SURFACE WATER FLOODING MAP –

MEDIUM RISK (CUCKMERE EXTENT) ....................................... 59FIGURE 6-28 ENVIRONMENT AGENCY SURFACE WATER FLOODING MAP – LOW

RISK (CUCKMERE EXTENT) ...................................................... 59FIGURE 6-29 ENVIRONMENT AGENCY FLOOD RISK SURFACE WATER - HIGH RISK

(LANGNEY SEWER EXTENT) ..................................................... 60FIGURE 6-30 ENVIRONMENT AGENCY FLOOD RISK SURFACE WATER - MEDIUM

RISK (LANGNEY SEWER EXTENT) ............................................ 61FIGURE 6-31 ENVIRONMENT AGENCY FLOOD RISK SURFACE WATER - LOW RISK

(LANGNEY SEWER EXTENT) ..................................................... 61FIGURE 6-32 ENVIRONMENT AGENCY RISK OF SURFACE WATER FLOODING MAP62FIGURE 6-33 SCHEME EXTENT ENVIRONMENT AGENCY FLOODING FROM

RESERVOIRS MAP ..................................................................... 63


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FIGURE 6-34 CUCKMERE RIVER ENVIRONMENT AGENCY FLOODING FROMRESERVOIRS MAP ..................................................................... 64

FIGURE 7-1 CUCKMERE RIVER ENVIRONMENT AGENCY FLOODING FROMRESERVOIRS MAP ..................................................................... 68

A P P E N D I C E SA P P E N D I X A PROPOSED SCHEME RED LINE BOUNDARYA P P E N D I X B IDB AREA AND BYELAWSA P P E N D I X C A27 PROPOSED METHODOLOGY ISSUED TO THE EAA P P E N D I X D ESCC FLOOD RISK AND DRAINAGE REPORTA P P E N D I X E EA CONSULTATION DATAA P P E N D I X F HYDRAULIC MODELLING FOR CUCKMERE RIVERA P P E N D I X G HYDRAULIC MODELLING FOR LANGNEY SEWER AND

CULVERT ASSESMENTSA P P E N D I X H DRAINAGE STRATEGY


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1 INTRODUCTIONSCOPE OF REPORT

WSP have been commissioned by Highways England (HE) to undertake a Flood RiskAssessment (FRA) to support the proposed A27 East of Lewes improvement scheme in EastSussex.

A FRA is required for the proposed scheme as it sits partially within Environment Agency (EA)Flood Zones 2 and 3 and also covers an area greater than 1 hectare (ha), as set out in theNational Planning Policy Framework (NPPF).

This FRA has been carried out with reference to the NPPF, the Planning Practice Guidance(PPG), the Design Manual for Roads and Bridges (DMRB) and the CIRIA Sustainable DrainageSystems (SuDS) Manual. The FRA uses information from the EA, the Pevensey and CuckmereWater Level Management Board (PCWLMB) (the Internal Drainage Board (IDB) for this area) andEast Sussex County Council (ESCC), as Lead Local Flood Authority (LLFA), provided throughconsultation along with existing information available on the respective websites. Local flood riskguidance has also been consulted in conjunction with national guidance.

The FRA sets out flood risk to the proposed scheme and the potential impacts of the proposedscheme on flood risk and outside, either upstream or downstream of the scheme area; as definedby the scheme extent.

Flood level data and flood extent maps for the Cuckmere River have been obtained from the EAfor use in the FRA. The EA state that ‘Whilst the data used is an improvement on the nationalgeneralised modelling from 2004, it is still a catchment scale model. We have used the outlinesto inform the flood map, but the levels and depths are not sufficiently accurate to be used for aflood risk assessment without further site-specific investigations’. As such further hydraulicmodelling has been carried out by WSP in relation to the post scheme scenario as part of thisFRA to assess the flood risk from the Cuckmere River and Langney Sewer (main rivers).Recommendations have been made to ensure the proposed scheme is appropriate andminimises the impact on flood risk in the vicinity of the scheme area.

The FRA consists of the following:

à A description of the existing scenario and the proposed scheme (Sections 2 and 3);

à Planning constraints to be satisfied (Section 4);

à Previous studies and historic flood risk (Section 5);

à Existing flood risk (Section 6);

à Flood risk mitigation and requirements of the drainage design (Section 7); and

à Conclusions and Recommendations (Section 8).

Supporting drawings can be found in the appendices to this report.


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INFORMATION PROVIDED

The following information was provided by the Client, the EA and other sources for use in thisstudy:

à Scheme Extent Drawing (Appendix A);

à Internal Drainage Board Development Control Area and Byelaws (Appendix B);

à A27 Proposed Methodology issued to the Environment Agency. Comments back andresponses (Appendix C);

à East Sussex County Council Flood Risk and Drainage Report (Appendix D);

à Environment Agency Consultation Data (Appendix E).

WSP Highways team have provided us with the latest drawings which are provided in AppendixA.

The following documents have been reviewed to gather information for this FRA:

à East Sussex County Council Preliminary Flood Risk Assessment (PFRA), June 20111;

à Cuckmere and Sussex Havens Catchment Flood Management Plan (CFMP), EA,December 20092;

à River Ouse Catchment Flood Management Plan (CFMP), EA, December 20093;

à South East River Basin District (RBD) Flood Risk Management Plan (FRMP) 2015-20214;

à Lewes District Council Strategic Flood Risk Assessment (SFRA) Levels 1 and 2, October20095;

à East Sussex County Council Stage 1 Surface Water Management Plan (SWMP),September 20176; and

à EA web-based mapping.

The following information is included in the Appendices of this report:

à Hydraulic Modelling details of the Cuckmere River (Appendix F);

à Hydraulic Modelling details of the Langney Sewer (Appendix G);

à Culvert assessment details (Appendix G); and

à Drainage Strategy (Appendix H).

1http://webarchive.nationalarchives.gov.uk/20140328094441/http://www.environment-agency.gov.uk/research/planning/135538.aspx

2https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/293873/Cuckmere_and_Sussex_Havens_Catchment_Flood_Management_Plan.pdf

3https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/293870/Ouse_Sussex_Catchment_Flood_Management_Plan.pdf

4https://www.gov.uk/government/publications/south-east-river-basin-district-flood-risk-management-plan5https://www.lewes-eastbourne.gov.uk/_resources/assets/inline/full/0/257647.pdf6https://www.eastsussex.gov.uk/media/10015/2016s5255-lewes-stage-1-swmp-v3-september-2017.pdf


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2 EXISTING SCENARIO2.1 SCHEME LOCATION

The scheme area consists of stretches of the A27 highway from approximately 5km east of Lewesto the western edge of Polegate, East Sussex (National Grid Reference 547343, 108176) (refer toFigure 2-1). Figure 2-1 represents the scheme extents.

The western extent of the proposed red-line boundary is to the east of the Lacys Hill junction tothe south of Glynde. The eastern extent is approximately 0.2km of the A2270 and the A27immediately before the Polegate / Cop Hall roundabout junction of the A27 and A22. The Drusillasroundabout of the A27, Station Road and Alfriston Road is located 0.8km from the CuckmereRiver.

The surrounding land use is predominantly private agricultural land, with some residentialproperties dispersed along the A27 and associated with the Polegate area of the scheme.


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Figure 2-1 Scheme Extents


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2.2 TOPOGRAPHY

A detailed topographic survey of the existing ground levels outside of the highway boundary wasnot available at the time of this assessment, therefore, general ground levels have been reviewedbased on existing highway topography and LiDAR (Light Detection and Ranging). The LiDAR data(refer to Figure 2-2 to Figure 2-6) covers the east to western extent of the A27 scheme area. Thedata sources show that the road levels are variable along the A27 between Lewes and Polegate.

The LiDAR scale ranges considerably. The highest point of the A27 is at the Wilmington Junctionarea, shown in Figure which is approximately 28mAOD (indicated in red). The lowest point of thescheme is east of Drusillas Roundabout on the western extent of the A27, at approximately3.3mAOD (indicated in light green). Polegate Junction, shown in Figure 2-6 is also an area of hightopography.

Figure 2-2 LiDAR Data of Glynde Reach area of the A27 (1 of 5)


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Figure 2-3 LiDAR Data of western extent of the A27 (2 of 5)

Figure 2-4 LiDAR Data of Drusillas Roundabout (3 of 5)

Figure 2-5 LiDAR Data of Wilmington Junction (4 of 5)


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Figure 2-6 LiDAR Data of Polegate Junction (5 of 5)


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2.3 SCHEME CATCHMENTS AND DRAINAGE

The EA ‘main rivers’ are shown in Figure 2-7. The nearest watercourses to the proposed schemeare three distinct EA ‘main rivers’. The Cuckmere River, which flows across the central section ofthe scheme, Glynde Reach, a tributary of the River Ouse, which runs to the north west of thescheme, and Langney Sewer (also known as Mill Ditch) that crosses the scheme at the easternextent of the A27, south of the Polegate / Cop Hall roundabout. Langney Sewer is culverteddirectly through Polegate, passing underneath the A27 and outfalls immediately west of the A27.

Figure 2-7 Watercourse and Junction Overview

EXISTING ATTENUATION BASINS

As stated in the Drainage Strategy, there is an existing highways attenuation pond locatedadjacent and to the north of the Langney Sewer, which is approximately 0.024km west of the A27Polegate Roundabout section. The details for this attenuation basin are provided in Table 2-1.

At Wilmington junction there are two outfall locations (refer to Table 2-1), including two existingbasins. One of which is located to the north west of Wilmington Junction, and the other is to thenorth east of Wilmington Junction.


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Table 2-1 Highways Attenuation BasinsHIGHWAYS ATTENUATION BASINS

Feature Langney SewerWilmington Junction

Basin to north-western extent Basin to eastern extent

Bottom Level (mAOD) 14.13 23.73 25.49

Assumed CL (mAOD) 15.50 24.70 26.80

30 Year Water Level(mAOD) 15.02 24.46 26.15

100 Year Water Level(mAOD) 15.17 24.68 26.42

Side Slope 1:3 1:3 1:3

To the south west of Polegate Junction there are two underground geo-cellular storage tanks.These are situated to the east and west of Brown Jack Avenue respectively. There is also anoutfall location on the A27, north of Brown Jack Avenue. Refer to Table 2-2

There is a geo-cellular storage unit 0.5km west of Polegate. This is situated within close proximityof the A27, and directly north of Hyperion Avenue.

Drusillas Roundabout consists of two attenuation features (refer to Table 2-2), these being a geo-cellular storage unit and a ditch. The geo-cellular storage unit is located directly at the centre ofthe roundabout, and the ditch runs to the north-eastern extent of the roundabout.

Table 2-2 Geo-cellular Storage TanksGEO-CELLULAR STORAGE TANKS

Feature Langney SewerWilmington Junction

Basin to north-western extent Basin to eastern extentBase Area 14.13 23.73 25.49Depth 15.50 24.70 26.80IL 15.02 24.46 26.15Assumed CL 15.17 24.68 26.42

The flood risk history of these existing drainage features is unknown.


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3 PROPOSED SCHEMEThe proposed scheme for which this FRA covers, involves the below elements:

à Drusillas Roundabout Widening: Enlargement of existing roundabout to include two exit armson the A27 approaches and the provision of improved facilities for NMUs (Non-motorisedusers) including the replacement of the current pedestrian crossing with a Pegasus crossing.

à Gainsborough Lane Junction: Upgrade to a signalised junction with right-turn facilities.

à Polegate: Widening of the A27 Polegate bypass to create a dual carriageway betweenPolegate roundabout and Cop Hall roundabout, plus the upgrade of Polegate junction to asignalised junction including improved crossing facilities. This includes the widening of theexisting railway highway underbridge at Polegate.

à Walking and Cycling Path / Non-Motorised User (NMU) path: Upgrading of the currentfacilities between Glynde and Firle and providing a new pedestrian/cycle path between Firleand Polegate.

The scheme is situated between the south-eastern fringe of Lewes and north west of Polegate, inthe boundary between Lewes District Council and Polegate Town Council. The total scheme areais 19.9ha. The proposed scheme impermeable area is 10.5ha with the remaining 9.4ha beinggreenfield. There is an increase in impermeable area of 5.6ha in comparison to the existing roadwhich made up of the new NMU and alterations to junctions. The proposed scheme alignmentdrawings are provided in Appendix A.

Table 3-1 shows the existing and proposed Scheme areas.

Table 3-1 Existing and Proposed Scheme AreasEXISTING AREA (HA) PROPOSED SCHEME AREA (HA)

Impermeable Surfaces withinRedline Planning Boundary

4.9(24.6%)

10.5(52.8%)

Permeable Surfaces within RedlinePlanning Boundary

15(75.4%)

9.4(47.2%)

Total Area within Redline PlanningBoundary 19.9 19.9

The proposed topography for the planned junction improvements generally follow the existinghighway topography.

Figure 3-1 (refer to Appendix A) shows the maximum scheme extent along the A27, covering atotal distance of 14km. The main line railway between Lewes and Eastbourne is locatedimmediately to the north of the scheme in the east (refer to Figure 3-2).


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Figure 3-1 Overview of the Maximum Scheme Extent

Figure 3-2 Railway line adjacent to the Scheme (HE552988-WSP-GEN-SWI-DR-CH-00034.pdf)

3.2 PROPOSED DRAINAGE LAYOUT

The overall strategy includes attenuation prior to discharge to watercourse at a restricted flowrate. Further information is provided in Appendix H.

3.1 INTERNAL DRAINAGE BOARD

Information has been obtained from Water Management Alliance 7. In which they provided us witha document referred to as ‘Development Control Byelaws’. The proposed scheme is situated inthe PCWLMB (Pevensey and Cuckmere Water Level Management Board). The PCWLMB‘Development Control Byelaws’, along with the charging policy can be found in Appendix B. Theproposed scheme must comply with Byelaw 3 and 10.

After consulting with the Water Management Alliance, they have stated, “If a surface water (ortreated foul water) discharge is proposed to a watercourse within the Internal Drainage District(IDD) (either directly or indirectly), then the proposed scheme will require a land drainage consentin line with the Board’s byelaws (specifically byelaw 3). Any consent granted will likely beconditional, pending the payment a surface water scheme contribution fee, calculated in line withthe Board’s charging policy”.


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Additionally, the Water Management Alliance have stated “If the proposals include works within9m of a Board adopted watercourse (indicated on the mapping), consent is required under byelaw10.” This byelaw would be applicable to the proposed scheme, as the three watercourses are alllocated within 9m of the proposed works.

Figure 3-3 shows the PCWLMB area associated with the Cuckmere River which is situated withinthe Board’s boundary, which is represented by a red outline on the map. There are two furtherIDB watercourses located within the Board’s boundary, these are represented in a blue outline.However, these two watercourses are situated downstream of the Cuckmere River, and to thesouth of the proposed scheme, and are not intercepted by the proposals.

A full version of Figure 3-3 is provided in Appendix B.

Figure 3-3 Pevensey and Cuckmere Water Level Management Board (PCWLMB) Area

7The Water Management Alliance is a grouping of IDBs that include the PCWLMB IDB.


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4 NATIONAL PLANNING POLICY FRAMEWORKThe NPPF (2019) sets out the Government planning policies for England and how these areexpected to be applied. Paragraph 155 of NPPF states that development in areas at risk offlooding should be avoided unless development is necessary. In this instance, the developmentshould not increase flood risk elsewhere.

The document states that a sequential, risk-based approach to the location of development isrequired in order to avoid, where possible, flood risk to people and property and manage anyresidual risk, taking account of the impacts of climate change.

Section 4.2 sets out the various Flood Zones as defined in the NPPF PPG. These Flood Zonesrefer to the probability of the river and sea flooding, ignoring the presence of any formal flooddefences. As set out in Section 6.1, the scheme is partially located in Flood Zones 1, 2 3a and3b. The scheme is not located in an area benefitting from defences.

4.2 FLOOD ZONE DEFINITION

The EA defines four separate Flood Zones, these are listed below.

FLOOD ZONE 1 – LOW PROBABILITY

This zone comprises land assessed as having a less than 1 in 1,000 annual probability of river orsea/tidal flooding (<0.1% Annual Exceedance Probability (AEP)) in any year.

FLOOD ZONE 2 – MEDIUM PROBABILITY

This zone comprises land assessed as having between a 1 in 100 and 1 in 1,000 annualprobability of river flooding (1% – 0.1% AEP), or between a 1 in 200 and 1 in 1,000 annualprobability of sea/tidal flooding (0.5% – 0.1% AEP) in any year.

FLOOD ZONE 3A – HIGH PROBABILITY

This zone comprises land assessed as having a 1 in 100 or greater annual probability of riverflooding (>1% AEP), or a 1 in 200 or greater annual probability of sea/tidal flooding (>0.5% AEP)in any year.

FLOOD ZONE 3B – FUNCTIONAL FLOODPLAIN

This zone comprises land where water has to flow or be stored in times of flood. The identificationof functional floodplain should take account of local circumstances but land which would flood withan annual probability of 1 in 20 (5% AEP) or greater in any year or is designed to flood in an extreme(0.1% AEP) flood, should provide a starting point for consideration.


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AREA BENEFITING FROM DEFENCES

Areas that benefit from the presence of defences in a 1 in 100 (1% AEP) chance of flooding eachyear from rivers; or 1 in 200 (0.5% AEP) chance of sea/tidal flooding each year. If the defenceswere not there, these areas would flood in a 1 in 100 (1% AEP) or 1 in 200 (0.5% AEP) or largerflooding incident.

4.3 FLOOD ZONE VULNERABILITY

Flood risk vulnerability is split into five classifications in NPPF PPG. The NPPF PPG detailstransport infrastructure as ‘Essential infrastructure’.

4.4 APPROPRIATE DEVELOPMENT

The requirements for determining whether a development is appropriate in terms of vulnerabilityand Flood Zone compatibility is set out in Figure 4-1.

Figure 4-1 NPPF Flood Risk Vulnerability and Flood Zone ‘Compatibility’

EXCEPTION TEST

In order to pass the Exception Test set out in the NPPF:

1. 1. It must be demonstrated that the development provides benefits to the community thatoutweigh flood risk, informed by the SFRA referenced in this FRA, and;

2. 2. A site-specific flood risk assessment must demonstrate that the development will be safefor its lifetime taking account of the vulnerability of its users, without increasing flood riskelsewhere, and, where possible, will reduce flood risk overall.


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This FRA only addresses Part 2 of the above. Part 1 is addressed in the Planning Statement forthe scheme.

The proposed scheme is partially in Flood Zone 3b. The scheme is deemed appropriate in theproposed location as long as the Exception Test is passed (highlighted in bold text in Figure 4-1).

5 PREVIOUS STUDIES AND HISTORIC FLOOD RISK

5.1 PREVIOUS STUDIES

The following previous studies have been found that reference flooding in the east Lewes area:

à East Sussex County Council Preliminary Flood Risk Assessment, June 2011;

à Cuckmere and Sussex Havens Catchment Flood Management Plan, EA, December 2009;

à River Ouse Catchment Flood Management Plan, EA, December 2009;

à South East River Basin District Flood Risk Management Plan 2015-2021, EA, March 2016;

à Lewes District Council Strategic Flood Risk Assessment Levels 1 and 2, October 2009;

à East Sussex County Council Stage 1 Surface Water Management Plan, September 2017.

EAST SUSSEX COUNTY COUNCIL PRELIMINARY FLOOD RISKASSESSMENT

East Sussex County Council have produced a PFRA in June 2011. We have used this documentin conjunction with the FRA, as it holds valuable information on previous flood risks in theproposed scheme area.

The PFRA identifies the occurrence of several recent major fluvial events in Lewes, Uckfield andHastings which have been recorded as affecting main river watercourses. Parts of East Sussexsuffered severe flooding in 2000/01. There are a number of main rivers within East Sussex whichpresent a risk of flooding. In particular the Cuckmere River, which runs through Hailsham to itsoutfall east of Seaford. This main river in located within the red line boundary.

Furthermore, East Sussex has suffered from groundwater flooding in 2001 and 2003. As thegeology of the South Downs includes a band of chalk from which, after extended wet periods,springs emerge along the base of the chalk causing localized flooding. Some coastal areas suchas Rye and Camber have experienced groundwater flooding due to the low-lying nature of theland, poor drainage capability and high water levels caused by high tide levels. However, theseparticular locations are not located within the vicinity of the proposed scheme.


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The PFRA also states that the duration of a flood event from groundwater event is often muchlonger than a flood event from fluvial flooding and can last from weeks or months.

Sewer flooding is often caused by excess surface water entering the drainage network. Therehave been previous incidents of sewer flooding in East Sussex, which are provided by SouthernWater. As stated in the PFRA, there was a significant sewer flooding event in Billingshurst on 1stJune 1981, whereby a blockage of a culvert resulted in surface water flooding occurred in HighStreet. Consequently the “Drainage system not able to cope with rainfall intensity of 20mm perhour”

According to the PFRA there has not been any past floods with significant harmful consequencesto report in the East Sussex area.

Figure 5-1 shows the future risk of groundwater flooding map. This has identified areas inPolegate and Lewes which are susceptible to groundwater flooding in the future. The proposedscheme is shown to be susceptible to groundwater flooding in the future at both the westernextent and eastern extent of the proposed scheme. The PFRA has no present-day groundwaterflood mapping.

Figure 5-1 Future flood risk of groundwater flooding map taken from the PFRA

CATCHMENT FLOOD MANAGEMENT PLANS (CFMP)

The CFMPs were completed in 2009 by the EA. They provide an overview of catchment wideflood risk and set out the EA’s (and working partners) plan for sustainable flood risk managementover the next 50 – 100 years. The CFMPs describes how future changes in flood risk to thecatchment will be driven by climate change and, to a lesser extent, by urbanisation.

The EA takes the lead responsibility for planning flood and coastal erosion risk managementfunctions in relation to flooding from the sea and main rivers. The EA’s flood map outline indicatesareas at risk of river flooding for the 1 in 100 and 1 in 1,000 return period events.

The proposed scheme lies within two separate CFMPs. The eastern extent of the scheme beingapplicable to the Cuckmere and Sussex Havens CFMP and the western extent of the scheme lieswithin the River Ouse CFMP.


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CUCKMERE AND SUSSEX HAVENS CATCHMENT FLOOD MANAGEMENTPLAN

An overview of the area covered by the Cuckmere and Sussex Havens CFMP is provided inFigure 5-2.

The proposed scheme lies within three different sub-areas of the Cuckmere and Sussex HavensCFMP, these are sub-areas 5, 6 and 9. Each sub-area has a different policy attributed to it.

Figure 5-2 Overview Map of the Cuckmere and Sussex Havens CFMP Area

Sub-area 5 recommends Policy Option 6. This sub-area covers The High and Low Weald andThe Levels. The vision of this sub-area is that risk to property is very low and the main focus ofthe area is the SSSI / Ramsar site which the EA manage with a view to enhancing. The chosenpolicy (Policy 6) whereby there are areas of low to moderate flood risk. This policy aims to takeaction with others to store water or manage run-off in locations that provide overall flood riskreduction or environmental benefits.

The key messages from the CFMP for sub-area 5: High and Low Weald and The Levels, forwhich the A27 scheme lies within are:


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à The current risk to human life and property is low. Flood risk throughout the Cuckmere andSussex Havens CFMP area could be reduced by increasing flooding in low risk upstreamareas, thereby potentially reducing the rate at which floodwaters move downstream tolarger urban areas;

à Increased flooding in this sub-area could have a positive effect on biodiversity, wouldincrease the sustainability of flood risk management and would provide opportunities toenhance landscapes and improve recreation and amenity value;

à Emphasis would be placed on making the public better understand the risk of flooding andclimate change; encouraging close partnership with local communities and ruraldevelopment authorities; and building policy objectives into planning documents;

Existing flood risk management (use of pumps and drainage channels) also limits the amount ofnatural wetland habitats. Increased flooding, whether freshwater, tidal or a combination of bothcould offer localised environmental benefits and would act as floodwater storage areas in thesame way that the artificial drains do now.

The proposed EA (and working partners) aspirational non-statutory actions to implement thepreferred approach of this policy are listed below:

à Complete a System Asset Management Plan (SAMP) to determine how existing flooddefences will be managed;

à Work with Natural England and other partners to develop a strategy plan to look at riverrestoration and naturalisation, with the objectives of reducing run-off and contributing to widercatchment benefits (biodiversity, soil conservation and water quality improvements);

à Encourage sustainable land use practices to reduce run-off rates from agricultural land,working with landowners, such as entry level stewardship programmes. This shouldencourage increasing forest cover and changes in farming practices;

à Work with Wealden District Council and Rother District Council to influence spatialdevelopment with the aims of ensuring no net increase in run-off from new developments;

à Work with Wealden District Council and the water companies to develop a Surface WaterManagement Plan (SWMP) for Heathfield to review capacity and vulnerability of groundwater,soil percolation, watercourses, foul and surface water sewers and consider the effects of newdevelopment;

à Develop fully integrated Water Level Management Plan with linked Eastbourne ParkManagement Plan and the Eastbourne Park Water Level Management Plan which covers thearea of this sub-area south of the A27;

à Prepare a fluvial study linked to the Water Level Management Plan looking at the options for,and feasibility of, river restoration and naturalised drainage through the Levels including waterlevel management and appropriate agricultural land management.

à Support England Catchment Sensitive Farming Initiative;

à Continue Pevensey outfalls study to assess the effectiveness of the sea outfalls in the area.


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Sub-area 6 recommends Policy Option 4. This sub-area covers the Brooks and Lower Ouse. Thevision of this sub-area is that risk to property is very low and the main focus of the area is theSSSI/Ramsar site which the EA manage with a view to enhancing. The preferred policy (Policy 6)whereby there are areas of low to moderate flood risk that will take action with others to storewater or manage run-off in locations that provide overall flood risk reduction or environmentalbenefits.

The key message from the CFMP for Sub-area 6: The Brooks and Lower Ouse, for which the A27scheme lies within is “The chosen policy can deliver benefits for people and the environmentlocally or in other sub areas. By increasing flooding locally in this sub-area, flood risk in Lewescan be reduced. Increasing frequency of inundation is likely to improve wetland biodiversity, asflooding is an essential part off floodplain ecosystems. There is potential to increase the area ofexisting Lewes Brooks Site of Special Scientific Interest (SSSI) wetland habitat; identified withinambition five of The South Downs Management Plan (Draft June 2006)”.

The proposed EA (and working partners) aspirational non-statutory actions to implement thepreferred approach of this policy are listed below:

à Develop the Lewes Brooks Water Level Management Plan to identify water levelmanagement that meets the need of flood risk management and the enhancement of thewetland habitat.

à A pre-feasibility study should be undertaken to identify realignment options to increaseflood storage and inundation of areas to reduce flood risk and benefit the environment.

à Implement schemes to increase floodplain inundation downstream of Lewes and upstreamof Newhaven.

à Develop a System Asset Management Plan (SAMP) to review maintenance regimes.

Sub-area 9 recommends Policy Option 1. This sub-area covers the South Downs (east and west).There are few issues in this sub-area as there are no rivers in this sparsely populated chalk downland sub-area, so no properties are at risk from river flooding. There is a chance of surface waterrun-off across the South Downs, however no residential property is affected in these sparselypopulated sub-areas. Agricultural land is at risk of soil erosion.

The key message for policy Option 1 is that “there are no existing flood risk managementactivities here”. The proposed actions to implement the preferred approach are to continue tomonitor and advise.

There is currently one property at current and future risk of a 1% annual probability flood event.

RIVER OUSE CATCHMENT FLOOD MANAGEMENT PLAN

An overview of the area covered by the River Ouse CFMP is provided in Figure 5-3.

The western extent of the proposed scheme lies within the River Ouse CFMP and is locatedwithin two different sub-areas, these are sub-areas 4 and 8. Each sub-area has a different policyattributed to it.


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Figure 5-3 Overview Map of the River Ouse CFMP Area

The River Ouse CFMP Policy Option 2 for sub-area 4: Ouse Low Weald (East and west) is areasof low to moderate flood risk where the EA can generally reduce existing flood risk managementactions. ‘This policy was selected as areas of moderate to high flood risk where we [the EA] cangenerally take further action to reduce flood risk. The current flood risk is considered to be lowwith small areas of localised fluvial flooding associated with the tributaries of the River Ouse, andsurface water flooding in Wivelsfield Green, Plumpton Green and Ringmer. Flood risk is notexpected to increase in the future due to climate change.

The key messages from the CFMP for the sub-area 4: Ouse Low Weald (east and west) for whichthe scheme lies within are:

à This policy applies where the current level of flood risk is low and flood risk is not expected tosignificantly increase in the future. It will allow a controlled reduction in flood risk managementmaintenance costs to a more appropriate level for the level of flood risk in the sub-area, whilefocusing flood risk management actions within the communities at higher risk of flooding fromlocal fluvial and surface water sources. It is recognised that flood risk will change in the future,and management actions may change in time to gain efficiencies or improve effectiveness.

The proposed EA (and working partners) aspirational non-statutory actions to implement thepreferred approach are:


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à Understanding of flood risk in this sub-area should be enhanced through a pre-feasibilitystudy. This should concentrate on the present flood risk at Plumpton Green and Ringmer.

à Maintain existing level of maintenance within the communities of Ringmer, Plumpton Greenand Wivelsfield Green, looking for efficiencies and improvements to ensure the existing floodrisk to these communities does not get worse in the future.

à Undertake a System Asset Management Plan (SAMP) to investigate options to better targetthe available funds for flood risk management.

à Encourage the take up of flood resistance and resilience measures by people at risk ofsurface water flooding.

The River Ouse CFMP Policy Option 1 for sub-area 8: South Downs (east and west)/ Saltdeanand Peacehaven is areas of little or no flood risk where we [the EA] will continue to monitor andadvise. The South Downs is highly rural with only a very small risk of fluvial flooding from theGlynde Reach. There are raised manmade flood defences, with a design standard for the 3.3%AEP flood event along parts of the Glynde Reach.

The key messages from the CFMP for the sub-area 8: South Downs (east and west)/ Saltdeanand Peacehaven for which the scheme lies within are:

à The chosen policy promotes active monitoring and providing advice but does not supportany flood risk management actions within the sub-area. For a very small area of this sub-area, maintenance associated with the existing raised man-made defences along theGlynde Reach will continue.

à This policy also encourages a shift towards best practice in land management. By workingwith landowners and government this could reduce disruption from surface water floodingto locally important transport links, such as the railway. The southern part of this sub-areais coastal. Any actions in this coastal area will have to be undertaken in accordance withthe Beachy Head to Selsey Bill Shoreline Management Plan (SMP).

The proposed EA (and working partners) aspirational non-statutory actions to implement thepreferred approach is to continue to monitor and advise.


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SOUTH EAST RIVER BASIN DISTRICT (RBD) FLOOD RISK MANAGEMENTPLAN (FRMP) 2015-2021

The FRMP was produced by the EA in March 2016, providing an overview of the sub-areas in theSouth East river basin district. The scheme lies partially in the Cuckmere and Pevensey Levelscatchment and the Adur and Ouse catchment.

In the Cuckmere and Pevensey Levels catchment it was identified that infrastructure within theriver basin district can be significantly affected by flooding. As in recent years key roads such asthe A27, A24, A259 and M23 have all flooded. Additionally, there are no significant flood defencesalong the non-tidal Cuckmere River network and so out of bank flows happen fairly often, causinginundation of the natural floodplain.

In the Adur and Ouse catchment it was highlighted that flooding does still occur most winterswithin rural floodplain areas. The wide, low lying valleys of the Rivers Adur and Ouse can oftenremain under water for prolonged periods in the winter and can have an impact on rural roadnetworks. Surface water flooding can occur when sewer systems receive more rainwater thenthey are designed to handle which can cause surcharging. Surface water flooding is an issue inparts of Burgess Hill, Uckfield, Lancing, Lewes, Hassocks, Haywards Heath and Worthing.

LEWES DISTRICT COUNCIL STRATEGIC FLOOD RISK ASSESSMENT(SFRA)

The Lewes District Council Strategic Flood Risk Assessment (SFRA) Levels 1 and 2 covers themajority of the River Ouse catchment as well as relatively small sections of the River Adur and theCuckmere catchments. The vast majority of flooding incidents in the District can be attributed tothe River Ouse or the sea. However, the Cuckmere River runs south through a relatively narrowand well defined floodplain, cutting through the eastern end of the South Downs, before passingthrough the embanked lower reaches and estuary to the sea.

The SFRA identified that the only flooding records found during the study were those relating tothe 1960 and 2000 floods. Which further states that the River Ouse Catchment FloodManagement Plan (CFMP) identified known and potential flood prone areas. These particularflood prone areas are as follows:

à Fluvial flooding in Uckfield and Lewes.

à Surface water flooding in Lewes, Barcombe and Seaford.

à Combined fluvial and tidal flooding in Newhaven, Lewes and Barcombe Mills.

à Periodic groundwater flooding from the South Downs in various locations across thecatchment, including Kingston south of Lewes.

Lewes is not directly located within the proposed scheme, as it is located 4.1km north west fromthe proposed scheme.


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EAST SUSSEX COUNTY COUNCIL EASTBOURNE AREA SURFACE WATERMANAGEMENT PLAN (SWMP)

The East Sussex County Council Eastbourne Area SWMP was completed in 2012 and aims toinvestigate the local flood risks of surface water, groundwater and ordinary watercourses acrossthe urban centre of Lewes.

Figure 5-4 represents surface water hotspots which identifies Langney Sewer (Mill Ditch) as anintermediate hotspot and therefore a high priority flood location. An intermediate hotspot isdefined as a discrete area of flooding that affects houses, businesses and / or local infrastructure.The boundary is defined by the readily available information on surface water flood risk (mainlythe EA’s Flood Map for Surface Water dataset) and confirmed by partners.

Figure 5-4 Surface Water Hotspots Located in Polegate (SWMP)


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Figure 5-5 Historic Surface Water Flooding Incidents in Polegate (SWMP)

As shown in Error! Reference source not found., much of Polegate Junction is located in aHighways Agency flood record boundary. A Southern Water sewer is situated directly on the A27,west of Polegate. There are two pink stars located on the A27 which represent Highway Agencyflooding. Identified as a green node, there are two additional recorded incidents of HighwaysAuthority historic flood events, within the Highways Agency flood record boundary. Furtherinformation on these particular historic flood incidents can be accessed in the SWMP.

5.2 HISTORIC FLOODING

The South East River Basin District FRMP stated that Lewes experienced significant flooding inthe autumn of 2000 and also in the winter of 1960, leading to hundreds of properties flooding.

The floods in October 2000 were due to torrential rain causing the River Ouse to burst its banks,which consequently led to 600 homes in Lewes being evacuated. In October 2000 the townsuffered major flooding during an intense period of severe weather throughout the UnitedKingdom. In a government report into the nationwide flooding, Lewes was officially noted the mostseverely affected location. As a result of the impact of this flood event the Lewes Flood ActionGroup formed, to press for better flood protection measures.

Whilst both flood events in Lewes are not directly associated with the proposed scheme, thescheme lies partially in the wider catchment of the River Ouse.


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6 FLOOD RISK ASSESSMENT6.1 FLOOD RISK FROM RIVERS AND THE SEA/TIDE

ENVIRONMENT AGENCY WEB BASED DATA

There are three EA Main Rivers within close proximity of the proposed scheme boundary. Thesebeing Glynde Reach, Cuckmere River and Langney Sewer (also known as Mill Ditch).

To the western extent of the proposed scheme lies Glynde Reach, which is a tributary of the RiverOuse. An ordinary watercourse tributary of the Glynde Reach flows from south to north, awayfrom the A27 and proposed scheme as shown on the EA Flood Map for Planning (Figure 6-1). TheCuckmere River is located further to the east of the Glynde Reach tributary, in the central extentof the proposed scheme, and flows in a north to south direction, crossing the proposed scheme.To the eastern extent of the proposed scheme lies Langney Sewer. This watercourse flows fromwest to east, and is culverted underneath the A27 and Polegate, with its outfall to the east ofPolegate.


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Figure 6-1 Environment Agency Flood Map for Planning (fluvial and tidal)


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The scheme lies largely within Flood Zone 1 but is in close proximity to Flood Zones 2 and 3,associated with the tributary of the Glynde Reach (refer to Figure 6-2) and is located within FloodZones 2, 3a and 3b associated with both the Cuckmere River (refer to Figure 6-3) and LangneySewer (refer to Figure 6-4) More detailed information has been provided by the EA for this schemeand is provided in the Environment Agency Consultation Data section of this FRA.

Figure 6-2 Environment Agency Flood Map for Planning (Glynde Reach Tributary Extent) (fluvial andtidal)


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Figure 6-3 Environment Agency Flood Map for Planning (Cuckmere River Extent) (fluvial and tidal)

Figure 6-4 Environment Agency Flood Map for Planning (Langney Sewer Extent) (fluvial and tidal)


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The scheme is situated approximately 7.5km inland and approximately 1.1km upstream of thetidal limit of the Cuckmere River at Milton Lock, south west of Milton Street.

The majority of the proposed scheme alignment has a low risk of flooding from rivers, however,the sections of the proposed scheme which cross the Cuckmere River and Langney Sewer areconsidered as being at a high risk of fluvial flooding. Based on the information available, thescheme overall has a medium risk of flooding from rivers and a low risk of flooding from thesea/tide.

ENVIRONMENT AGENCY FLOOD WARNING AREAS

The proposed scheme is within EA fluvial flood warning areas associated with the CuckmereRiver and the Glynde Reach tributary as shown in Figure 6-5 and Figure 6-6.

Figure 6-5 Environment Agency Fluvial Flood Warning Map for Cuckmere River


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Figure 6-6 Environment Agency Fluvial Flood Warning Map for the Glynde Reach Tributary

The flood warning map for the tidal flooding of the Cuckmere River is shown in Figure 6-6 thisshows that the A27 is located immediately upstream of the flood warning area.

Figure 6-7 Environment Agency Tidal Flood Warning Map


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The EA Fluvial flood warning map in Figure 6-5 shows that the Cuckmere extent of the A27 iscurrently at risk from fluvial flooding. However, as shown in Figure 6-7, the scheme is not at riskfrom tidal flooding. There are currently no flood warning maps for Langney Sewer area of the A27.

Based on the information available, there is a high risk of fluvial flooding where the watercoursesintercept the A27, and there is a low risk of fluvial flooding on the remainder of the highway.Therefore, it can be concluded that the overall fluvial flood risk is medium.

Based on the information available, there is a high risk of fluvial flooding where the watercoursesintercept the A27, and there is a low risk of fluvial flooding on the remainder of the highway.Therefore, it can be concluded that the overall fluvial flood risk is medium.

ENVIRONMENT AGENCY CONSULTATION DATA

Flood level data for the Cuckmere River adjacent to the proposed scheme was provided by theEA (refer to Appendix E). The information provided by the EA in Appendix E is based on thenational level modelling and does not consider structures or specific channel survey. As such it isa very high model of the Cuckmere River.

In relation to flood history and historic flood records along the vicinity of the scheme the EAstated, “The area floods most years, but due to its rural nature we generally do not get the extentssurveyed” and that “The floodplain is very defined so the extent does not change much. However,the depths may vary significantly.” Figure 6-8 shows mapping of all EA recorded flood extentsfrom the Cuckmere River.

Figure 6-8 Environment Agency Recorded Flood Outlines


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The EA has provided water levels and flow values which are referenced as nodes C0646 andC0647. Figure 6-9 shows the location of these nodes within the broad scale catchment model datafor the Cuckmere River. Table 6-1 and Table 6-2 reference to the nodes C0646 and C0647 whichare displayed in Figure 6-9. Node C0646 is situated underneath the point at which the A27intercepts the Cuckmere River. Node C0647 is situated slightly further downstream of the A27crossing of the Cuckmere River.

Table 6-1 provides the water levels recorded in metres, for the fluvial undefended scenario for the1% AEP event (1 in 100 year flood event) and the 0.1% AEP event (1 in 1000 year flood event).Table 6-2 provides the flow values recorded in cubic meters per second (cumecs / m3/s), for thefluvial undefended 1% AEP (1 in 100) event and 0.1% AEP (1 in 1000) event.

Figure 6-9 Environment Agency Level and Flow Values of the Cuckmere River Map

Table 6-1: Environment Agency Water Levels Fluvial Undefended

NODE REFERENCE

WATER LEVEL (M)UNDEFENDED ANNUAL EXCEEDANCE

PROBABILITY1% 0.1%

CO646 6.36 6.80

CO647 6.09 6.70


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Table 6-2: Environment Agency Flows Fluvial Undefended

NODE REFERENCE

FLOW (M3/S)UNDEFENDED ANNUAL EXCEEDANCE

PROBABILITY1% 0.1%

CO646 100.08 178.36

CO647 100.08 178.36


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The EA have provided the Flood Map for Planning (Rivers and Sea) for the proposed scheme(refer to Figure 6-10). A relatively small amount of the proposed scheme is located within a FloodZone 3. Flood Zone 3 is described as an area that could be affected by flooding from the sea witha 1 in 200 or greater chance of happening in one year, or from a river with a 1 in 100 or greaterchance of happening each year.

To the west of the scheme, south east of Glynde, there is a small area of Flood Zone 3 on theA27 highway. There is a large area of Flood Zone 3 where the Cuckmere River intercepts theproposed scheme.

There is a significantly small area of Flood Zone 2 in Polegate, to the east of the proposedscheme. Flood Zone 2 shows the extent of an extreme flood from rivers or the sea with up to a 1in 1000 chance of occurring each year.

The EA has stated that floodplain compensation may be required. Further information in relationto the provision of floodplain compensation is provided in Section 7.

There are currently no flood defences in place within the scheme area.

Figure 6-10 Environment Agency Flood Map for Planning (Rivers and Sea)

The scheme area is not at direct flood risk from the sea and is not presently located in a tidalreach. The tidal limit of the Cuckmere River is at is at Milton Lock, north east of Winton. As suchthe proposed scheme is not susceptible to tidally influenced flooding.


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HYDRAULIC MODELLING OF THE CUCKMERE RIVER

A summary of the hydraulic modelling is provided here and the Hydraulic Modelling Report of theCuckmere River can be found in Appendix F. Details of the derivation of hydrology is provided inAppendix F: Flood Estimation Calculation Record. Correspondence with the EA on methodologyand results is also provided in Appendix F.

A 1D – 2D linked ICM hydraulic model of Cuckmere River was built in Infoworks ICM forassessment of changes to the A27. The model includes the main A27 Sherman Bridge crossingof the Cuckmere River, as well as the flood relief culverts connecting the Cuckmere floodplainsupstream and downstream of the A27. The upstream boundary is 1.8km north of Sherman Bridgeand the downstream boundary is 1.5km south.

The model was run for a range of return periods including 1 in 100 year + 105% climate change(CC). The climate change allowance projection is the “Upper End” category for South East riverbasin district, anticipated change for 2070-2115, within Flood Zone 3 and “EssentialInfrastructure” flood risk vulnerability category.

The sensitivity tests carried out on the model show that the model is not very sensitive toblockage, changes in roughness or changes to downstream boundary conditions. It is nominallysensitivity to changes in flow. The 100yr +105% CC flood depth map for the Cuckmere River isshown in Figure 6-11 with the red line boundary for the A27 indicated.

Figure 6-11 1 in 100 year + 105% CC flood depth map for the Cuckmere River


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PROPOSED SCHEME AT CUCKMERE RIVER

There are no changes proposed to the existing A27 road embankment or existing ShermanBridge crossing, but there is a new Non-Motorised User (MNU) route proposed that runs adjacentto the A27 highway. The options appraisal process for the NMU included two primary optionswhich were tested with variations in terms of opening size and skew. These were tested in thehydraulic model and are presented in Appendix F.

On 21st September 2018, the Environment team confirmed that the design team had fixed theNMU route to be Option 1 (parallel to the existing A27 road). In January 2019 the design teamsent through revisions to the NMU in the vicinity of the River Cuckmere floodplain. A proposedground model and drawings of the NMU on full embankment across the whole of the RiverCuckmere floodplain were received. A proposed drawing of the NMU at Cuckmere River isprovided in Figure 6-12 and full drawings are provided in Appendix A.

Figure 6-12 Proposed scheme and red line boundary at River Cuckmere

The following design is proposed for NMU Option 1C on Embankment (as shown schematically inFigure 6-13):

à 4m wide NMU route parallel to existing carriageway with approx. 1:3 embankments

à NMU route sits at a minimum of 6.0mAOD across the whole Cuckmere floodplain and isapprox. 315m long.

à NMU bridge crossing of the main River Cuckmere channel has soffit at 6.0mAOD, deck levelat ~6.4mAOD and bridge opening is 22m wide.

à The channel on the floodplain that takes flow from the 2 existing connectivity culverts has aboardwalk proposed above the 100 year + 105% CC flood level. It will have a 10m spanbetween the NMU embankments with deck level 6.8mAOD.

à A maintenance access embankment is proposed between the existing A27 road and the NMUembankment. Twin 900mm pipes are proposed to allow flow to pass along this narrowfloodplain.


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Figure 6-13 Proposed scheme and red line boundary at River Cuckmere

The average increase in water level on the floodplain upstream of the A27 is 20mm and 39mmrespectively for the 1 in 100 year and 1 in 100 year + CC (105%) return period design events. Theincrease in water level on the floodplain between the option model and the baseline (the detrimentin mm) for the 100 year + CC (105%) return period is shown as a thematic map in Figure 6-14.


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Figure 6-14 Change in water level (mm) for 1 in 100 year + 105% CC return period for the CuckmereRiver

The average change in water level on the upstream floodplain is within the “Minor” (between10mm and 50mm for the 1% probability) category for Magnitude of Impact on an Attribute. TheRiver Cuckmere fluvial floodplain does not affect any residential or industrial properties thereforethe Importance of Water Environment Attributes is estimated to be “Low”. The overall Significanceof Potential Effects is categorised as “Neutral”.

Localised increases in water level in the main river Cuckmere channel for the 1 in 100 year returnperiod are up 71mm. In between the existing A27 road embankment and the new NMUembankment increases in water level are up to 120mm for the 1 in 100 year return period, but thisconstricted only to this narrow area within the red line boundary. In these localised areas theincrease in water level is considered to be within the “Significant” (greater than 100mm for the 1%probability) category for Magnitude of Impact on an Attribute. These localised areas are far fromany properties and hence the overall Significant of Potential Effects would be categorised as“Slight/Moderate”.

Discussion on mitigation is provided in Section 7.


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HYDRAULIC MODELLING OF THE LANGNEY SEWER

A summary of the hydraulic modelling is provided below and the Hydraulic Modelling Report ofthe Langney Sewer can be found in Appendix G. Details of the derivation of hydrology is alsoprovided in Appendix G: Flood Estimation Calculation Record. Correspondence with the EA onmethodology and results is provided in Appendix F.

A 1D – 2D linked ICM hydraulic model of Langney Sewer was built in Infoworks ICM forassessment of changes to the Polegate Bypass (part of the A27 East of Lewes upgrade). Themodel included a short reach of open watercourse and the culvert known as Langney Sewer.

The model was run for a range of return periods including 1 in 100 year + 105% climate change.The climate change allowance projection is the “Upper End” category for South East river basindistrict, anticipated change for 2070-2115, within Flood Zone 3 and “Essential Infrastructure” floodrisk vulnerability category.

The sensitivity results were used to check for possible instabilities within the model, these showedno issues indicating the model to be stable. The sensitivity tests carried out on the model showthat the model is not very sensitive to blockage or sensitive to changes in flow. The 100yr +105%CC flood depth map for Langney Sewer is shown in Figure 6-15.

Figure 6-15 1 in 100 year +105% CC flood depth map for Langney Sewer


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PROPOSED SCHEME AT LANGNEY SEWER

The proposed changes to the Polegate Bypass (as part of the A27 EoL project) include slightwidening of the existing road embankments either side of Langney Sewer culvert inlet. There areno proposed changes to the Langney Sewer culvert itself. The proposed scheme drawing fromJanuary 2019 is shown in Figure 6-16.

Figure 6-16 Proposed Scheme at Polegate Bypass at Langney Sewer Culvert


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The changes to road embankments at Polegate Bypass cause a small displacement of thefloodplain to the west by approximately 5m (rather than an increase in water level as such). Thevolume of 1 in 100 year +105% climate change floodplain that is being displaced westwards dueto the widened embankments of the Polegate Bypass is approximately 230m3.

The model results for detriment (increase in water level between option and baseline) upstream isindicated as up to 136mm and 187mm for the 1 in 100 year and 1 in 100 year + climate change(105%) return periods respectively, shown by the dark orange of the thematic map in Figure 6-17.

Figure 6-17 Change in water level (mm) for 1 in 100 year +105% CC return period for Langney Sewer


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The detriment is within the Major Adverse (greater than 100mm for the 1% probability) categoryfor Magnitude of Impact on an Attribute. As the Langney Sewer fluvial floodplain does not affectany residential or industrial properties, the Importance of Water Environment Attributes isestimated to be “Low”. The overall Significance of Potential Effects is categorised as“Slight/Moderate”.

Discussion on mitigation is provided in Section 7.

6.2 FLOOD RISK FROM GROUNDWATER

GEOLOGY AND GROUNDWATER

Based on the British Geological Survey (BGS) online mapping, the bedrock geology of thescheme comprises mainly of Mudstone within the Gault formation; sedimentary bedrock formed inthe Cretaceous Period (refer to Figure 6-18). To the east of the scheme, a minor extent isconsidered Sandstone, Siltstone and Mudstone within the lower Greensand group. A short extentof the scheme is a mudstone of Weald Clay Formation. To the west of the scheme, a short extentis Chalk within the West Melbury Marly Chalk Formation.

Figure 6-18 British Geological Survey Online Bedrock Mapping of Scheme


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The majority of superficial geology for this stretch of the A27 comprises of Clay and Silt Tidal FlatDeposits formed in the Quaternary Period. Noticeably, along the Cuckmere River Valley, thesuperficial deposits consist of sand and gravel from River Terrace deposits. There are alsoalluvium deposits consisting of clay, silt, sand and peat along the Cuckmere River. Thesesuperficial deposits are shown in Figure 6-19.

Figure 6-19 British Geological Survey Online Superficial Deposits Mapping of Scheme


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The scheme is situated within the BGS aquifer classification ‘principle aquifer’ for bedrockgeology. Principle aquifers are defined as ‘layers of rock or drift deposits that have highintergranular and/or fracture permeability - meaning they usually provide a high level of waterstorage. They may support water supply and/or river base flow on a strategic scale. In mostcases, ‘principal aquifers’ are aquifers previously designated as ‘major aquifer’.

The scheme is situated within the BGS aquifer classification Secondary (undifferentiated) forsuperficial geology. Secondary (undifferentiated) classification is defined as ‘in cases where it hasnot been possible to attribute either category A or B to a rock type. In most cases, this means thatthe layer in question has previously been designated as both minor and non-aquifer in differentlocations due to the variable characteristics of the rock type’.

There is a significantly small area of superficial geology along the Cuckmere River Valley which isclassified as a Secondary A Aquifer. A Secondary A classification is defined as ‘permeable layerscapable of supporting water supplies at a local rather than strategic scale, and in some casesforming an important source of base flow to rivers. These are generally aquifers formerlyclassified as minor aquifers’.

The scheme is partially located within the EA Major Aquifer High, Major Aquifer Intermediate,Major Aquifer Low, Minor Aquifer High and Minor Aquifer Intermediate Groundwater VulnerabilityZones (refer to Figure 6-20). A high vulnerability groundwater zone is an area that has highleaching soils and an absence of low permeability drift deposits. However, much of the route isnot located in any vulnerability classification.

Figure 6-20 Groundwater Vulnerability (Defra, 2018)


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The scheme is not located within any EA Groundwater Source Protection Zones, as displayed inFigure 6-21.

Figure 6-21 Groundwater Source Protection Zones (Defra, 2018)

The PFRA states that East Sussex has suffered from groundwater flooding in 2001 and 2003.This is because the geology of the South Downs includes a band of chalk from which, afterextended wet periods, springs emerge along the base of the chalk thus causing localised flooding.These factors do not necessarily apply to the proposed scheme area. Furthermore, East SussexCounty Council state in the PRFA “This makes it difficult to draw any conclusions about thehistoric groundwater incidents or to tie them in with any of the other historic flooding incidentrecords.”

Therefore, based on the information available, the proposed scheme is identified as being at amedium risk from groundwater flooding.

FLOOD RISK FROM SEWERS

The SFRA mentions that surface water sewer flooding in the area could be expected to occur asa result of intense, short duration rainstorms, such as summer thunderstorms. The rapid runofffrom impermeable areas overwhelms the capacity of the urban drainage system and the sewersbecome surcharged. Water escapes from the sewer at manholes and flows over the groundsurface, generally along the line of the sewer.

The Lewes Integrated Urban Drainage Pilot Scheme is being implemented by the EnvironmentAgency, Southern Water, East Sussex County Council and Lewes District Council with a view tointegrate the management of all facets of the drainage system and provide the best possible levelof service to the town of Lewes.

Based on the current information available, the proposed scheme is considered to be at low floodrisk from sewers.


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FLOOD RISK FROM LAND

The EA’s flood risk from surface water map (shown in Figure 6-22) identifies surface waterflooding within the proposed scheme.

As a result of surface water runoff from land surfaces, there is a considerable amount of high riskof surface water flooding in the scheme. ‘High’ risk is defined as an area that each year this areahas a chance of flooding of greater than 3.3%. This takes into account the effect of any flooddefences in the area. These defences reduce but do not completely stop the chance of floodingas they can be overtopped, or fail.’

Some areas in Figure 6-22 are shown to be medium risk of surface water flooding. ‘Medium’ riskis defined as an area that each year has a chance of surface water flooding of between 1% and3.3%. This takes into account the effect of any flood defences in the area. These defences reducebut do not completely stop the chance of flooding as they can be overtopped, or fail.’

Several areas adjacent to the proposed scheme are identified as being at low risk of surfacewater flooding. ‘Low’ risk is defined as an area that each year has a chance of surface waterflooding of between 1 in 1000 (0.1% AEP) and 1 in 100 (1% AEP).

Figure 6-22 shows that the scheme has a significantly low coverage of high risk flooding,however, surface water flooding is still a high risk to the proposed scheme in these discreet areas.


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Figure 6-22 Environment Agency Flood Risk from Surface Water


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Figure 6-23, Figure 6-24 and Figure 6-25 are areas associated with a key tributary of GlyndeReach, which is culverted underneath the A27. The affected area, is at a considerable risk ofsurface water flooding. As such, the low risk scenario is a higher threat to the A27 and proposedworks. Each scenario has a potential to flood over 900mm on the highway.

Figure 6-23 Environment Agency Flood Risk from Surface Water- High Risk (Tributary of GlyndeReach extent)


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Figure 6-24 Environment Agency Flood Risk from Surface Water- Medium Risk (Tributary of GlyndeReach extent)

Figure 6-25 Environment Agency Flood Risk from Surface Water- Low Risk (Tributary of GlyndeReach extent)


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Figure 6-26, Figure 6-27 and Figure 6-28 are areas associated with the Cuckmere River extent.The affected area, is at a considerable risk of surface water flooding. As such, each scenario hasa potential to flood over the highway, especially shown in Figure 6-28, 900mm on the highway.The low risk scenario was far more impacting than the high-risk scenario.

Figure 6-26 Environment Agency Flood Risk from Surface Water- High Risk (Cuckmere extent)


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Figure 6-27 Environment Agency Surface Water Flooding Map – Medium Risk (Cuckmere extent)

Figure 6-28 Environment Agency Surface Water Flooding Map – Low Risk (Cuckmere extent)


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Figure 6-29, Figure 6-31 and Figure 6-31 are areas associated with the Langney Sewer extent.The affected area, is at a considerable high risk of surface water flooding. However, there is asmall area of flooding shown in Figure 6-31, low risk scenario map.

Figure 6-29 Environment Agency Flood Risk Surface Water - High Risk (Langney Sewer extent)


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Figure 6-30 Environment Agency Flood Risk Surface Water - Medium Risk (Langney Sewer extent)

Figure 6-31 Environment Agency Flood Risk Surface Water - Low Risk (Langney Sewer extent)


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Figure 6-32 is taken from the EA Consultation Data in Appendix E. It shows the risk of surfacewater flooding along the Cuckmere River. A high proportion of the surface water flooding lieswithin the 1 in 30 Flood Extent. However, along the A27 there is mainly a flood extent of 1 in 1000which encroaches the highway.

Figure 6-32 Environment Agency Risk of Surface Water Flooding Map


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Therefore, based on the information available the proposed scheme is identified as beingsusceptible to flooding from surface water. The overall risk of flooding from land is considered tobe high.

CULVERT ASSESSMENT

A new hydraulic model was constructed for assessing the performance of culverts at the locationof 15 ordinary watercourse crossings along the A27. The 1D-2D hydraulic model was developedusing Infoworks ICM. A topographic survey was undertaken in June 2018, which included riverchannel and structure cross sections at the location of each culvert.

Hydrological catchments were delineated for each culvert using an EA LiDAR 1m DTM. Inflowswere applied to a small section of river reach upstream of each culvert used for flow routing aswell as allowing flood depth analysis at each headwall. 1 in 5 year, 1 in 10 year, 1 in 25 year, 1 in50 year, 1 in 75 year, 1 in 100 year, 1 in 100 year plus climate change (105%) fluvial flood eventsfor the critical storm duration of 4.5 hours were analysed. Details of the derivation of hydrology isprovided in Appendix G: Flood Estimation Calculation Record.

Peak water levels have been assessed in the hydraulic model at the inlet and outlets for eachculvert structure for the range of return periods. Tables of results with culvert grid references areincluded in Appendix G and were issued to the drainage design team to help with highwaysdrainage outfall design.

6.1 FLOOD RISK FROM ARTIFICIAL SOURCES

FLOOD RISK FROM RESERVOIRS

The nearest reservoir associated with the A27 is Arlington Reservoir, which is locatedapproximately 1.9km north east of the proposed scheme.

The EA web-based mapping identifies the proposed scheme as being inside the maximum extentof flooding from reservoirs (refer to Figure 6-33). The scheme area is identified as being inside ofthe maximum extent of reservoir flooding, particularly along the Cuckmere River Valley area ofthe A27 (refer to Figure 6-34).

Figure 6-33 Scheme Extent Environment Agency Flooding from Reservoirs Map


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Figure 6-34 Cuckmere River Environment Agency Flooding from Reservoirs Map


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The South East River Basin District FRMP states that there are two notable reservoirs used forwater supply within the catchment; Arlington which is adjacent to the Cuckmere River, andBarcombe which is adjacent to the River Ouse. Barcombe Reservoir is a 5.6km distance from theA27, so is not considered a potential risk of flooding to the scheme area. In the FRMP there is nomention of flooding from these two reservoirs.

The proposed scheme is identified as being susceptible to flooding from reservoirs. Therefore,based on the information available, the overall flood risk from reservoirs is considered to bemedium.

6.2 FLOOD RISK FROM OTHER ARTIFICIAL SOURCES

There are no canals or pumped catchments in the vicinity of the scheme. However, the scheme islocated in an Internal Drainage Board area.

Based on the information available, the proposed scheme is identified as not being susceptible toflooding from other artificial sources. Therefore, the overall flood risk from other artificial sourcesis considered to be low

7 FLOOD RISK MITIGATIONFLUVIAL FLOOD RISK MITIGATION

CUCKMERE RIVER

As the increase in water level upstream of the scheme is greater than 15mm for 1 in 100 year +CC (105%) return period for the proposed “Option 1C on Embankment” scenario (latest designfrom January 2019), flood mitigation is likely to be required.

If the EA require “like-for-like” volume of compensatory storage for the 1 in 100 year + CC (105%)return period, the volume of floodplain removed by the new NMU embankment is 2600m3. Thisvolume could be provided in either of the 2 areas identified for compensatory storage which havebeen included within the red line boundary of the scheme and shown previously in Figure 6-12.These areas have been chosen for the following reasons:

à sufficient space for “like-for-like” compensation volume for the 1 in 100year + 105% climatechange return period,

à “level for-level” so could be provided between ~3.8mAOD and 5.8mAOD,

à located as close to and upstream of the proposed works as possible i.e. on the north side ofthe A27 road,

à not located in an existing floodplain,

à would need to be sympathetic to the natural contours of the land and are provided at the edgeof an existing floodplain.


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Further consultation with the EA is to be undertaken over the coming months. The hydraulicmodel will be packaged up and sent across and a formal response will be sought on mitigationrequirements. The red line boundary allows for some contingency in the areas identified forcompensatory storage should design changes in later stages of the project need to beaccommodated.

If the EA require further reductions in water levels then a number of other mitigations options havebeen shown to be effective by the hydraulic modelling and these could be incorporated in latterstages of the design:

à Lowering the right bank level between the weir and Sherman Bridge to allowing spilling fromthe floodplain back in to the main channel (the red line boundary therefore includes this area).

à Lowering the bed (dredging) by 0.5m for a 400m reach downstream of the weir.

LANGNEY SEWER

The proposed changes to the Polegate Bypass and culvert inlet cause a small displacement ofthe floodplain to the west by approximately 5m (rather than a direct increase in water level).

As the detriment is greater than 15mm for the 1 in 100 year + 105% climate change scenario, it ispossible the Environment Agency will require compensatory storage to be provided. If the EArequire “like-for-like” volume of compensatory storage for the 1 in 100 year + CC (105%) returnperiod, the volume of floodplain removed by the widened Polegate Bypass embankments is230m3. In terms of location it is proposed to be placed within the red line boundary shownpreviously in Figure 6-16 and as close to the watercourse and proposed changes as possible.

7.2 SURFACE WATER DRAINAGE

DESIGN MANUAL FOR ROADS AND BRIDGES (DMRB)

Section 2.2 of the DMRB9 (Volume 4 Section 2 Part 3 HD33/16) provides a hierarchical approachto the disposal of rainwater with the preferred option being discharge to the ground (via asoakaway or other infiltration system). If this is not possible, the next favoured option isattenuated discharge to a surface watercourse. If neither are possible the scheme shoulddischarge to a surface water sewer.

HD33/16 also states a requirement for no surcharge of the drainage system in a 1 year storm andno flooding in a 5 year storm. Compliance with these requirements maintains manageableattenuation requirements, and a level of resilience in-line with the Strategic Road Network. For a100 year storm these requirements have been adhered to.

Where highway runoff outfalls to a fluvial surface of body, the following design return periodsapply:

à 1 in 1 year – No surcharge of the drainage system;

à 1 in 5 years – No flooding from the drainage system;

à 1 in 100 years – Flood risk assessment for receiving surface water.


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Section 3.2 of the DMRB states that the consideration of drainage is a basic requirement in theestablishment of road alignments and cross-sections. This is to ensure that outfall levels areachievable and subgrade drainage can discharge above the design flood level of any outfallwatercourses.

Section 6.2 of the DMRB states that all runoff from the existing land drainage system must bekept entirely separate from the highway drainage systems and surface water from the externalcatchment must not be discharged into the highway drainage system. Furthermore, new surfacewater connections from sites and/or proposed developments adjacent to the road must not beaccepted.

CLIMATE CHANGE

The effects of climate change have been taken into account in the scheme drainage design. Asper the current climate change allowances stated in the NPPF PPG, and also the likelyrecommendations in the forthcoming update to the DMRB, a 40% increase in rainfall intensitieshas been applied to all calculations to account for climate change.


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SURFACE WATER RUNOFF FLOODING MITIGATION

The proposed scheme will increase the impermeable areas and in turn the surface water runofffrom the site. In order to mitigate this SuDS are incorporated into the drainage network for theproposed scheme. SuDS aim to reduce the risk of flooding on the site by imitating naturaldrainage and managing surface water runoff in a more sustainable way. In addition to reducingthe risk of flooding, SuDS can also produce water quality improvements and environmentalenhancements on site.

Figure 7-1 sets out the range of SuDS components outlined in the SuDS Manual (CIRIA C753,2015).

Figure 7-1 Cuckmere River Environment Agency Flooding from Reservoirs Map


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PROPOSED DRAINAGE STRATEGY

The proposed drainage strategy includes utilisation of the existing outfalls and provision ofattenuation basins to attenuate the additional surface water runoff from the proposed scheme.Further information on the proposed drainage strategy has been provided in the DrainageStrategy in Appendix H.

8 CONCLUSIONS AND RECOMMENDATIONS

8.1 CONCLUSIONS

The flood risk to the proposed A27 East of Lewes Improvement Scheme has been assessed inaccordance with the National Planning Policy Framework (NPPF), the Planning PracticeGuidance (PPG), with reference to the Design Manual for Roads and Bridges (DMRB), the CIRIASustainable Drainage Systems (SuDS), the Internal Drainage Board (IDB) guidance and throughconsultation with the Environment Agency (EA). The following published documents have alsobeen reviewed to gather information for this study; East Sussex County Council Preliminary FloodRisk Assessment (PFRA), Cuckmere and Sussex Havens Catchment Flood Management Plan(CFMP), River Ouse Catchment Flood Management Plan (CFMP), South East River Basin District(RBD) Flood Risk Management Plan (FRMP), Lewes District Council Strategic Flood RiskAssessment (SFRA), and East Sussex County Council Stage 1 Surface Water Management Plan(SWMP).

The proposed scheme, which is classified as ‘Essential infrastructure’ under NPPF PPG, isappropriate in this area subject to passing the Exception Test. This FRA addresses Part 2 of theException Test only. Part 1 is addressed in the Planning Statement for the scheme.

The scheme area is not at direct flood risk from the sea, and is not presently located in a tidalreach. The tidal limit of the Cuckmere River is at is at Milton Lock, north east of Winton. As suchthe proposed scheme is not susceptible to tidally influenced flooding.

The majority of the scheme is located within Flood Zone 1; however, parts of the scheme arewithin Flood Zone 2 and Flood Zone 3 (incorporating both 3a and 3b). The main rivers locatedwithin the proposed scheme area are situated in Flood Zone 3. Based on the informationavailable it can be said that the areas where watercourses cross the A27 are at a higher risk tofluvial flood risk.

The Environment Agency Flood Warning Areas maps identify that the Cuckmere River is at a highrisk of fluvial flooding and a low risk of tidal flooding. Glynde Reach is also identified as an areawith a high fluvial flood risk, however, there are currently no tidal flood warning maps available forGlynde Reach. There are currently no fluvial or tidal flood warning maps for Langney Sewer areaof the A27.

Environment Agency Consultation data focuses directly on the Cuckmere River, based on theinformation available the proposed scheme is identified as being susceptible to flooding fromsurface water runoff from land. The overall risk of flooding from land is considered to be high;subject to the implementation of a suitable drainage strategy for the scheme.


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Hydraulic modelling of the Cuckmere River identified the change in water level for the majority ofthe floodplain is within the “Minor Adverse” (between 10mm and 50mm for the 1% probability)category for Magnitude of Impact on an Attribute. The Cuckmere River fluvial floodplain does notaffect any residential or industrial properties therefore the Importance of Water EnvironmentAttributes is estimated to be “Low”. The overall Significance of Potential Effects is categorised as“Neutral”. There are localised increases in water level in the main river Cuckmere channel andfloodplain that are considered to be within the “Significant” (greater than 100mm for the 1%probability) category for Magnitude of Impact on an Attribute. These localised areas are far fromany properties and hence the overall Significant of Potential Effects would be categorised as“Slight/Moderate.

The scheme changes at Langney Sewer cause a small displacement of the floodplain byapproximately 5m to the west of the widened bypass. Hydraulic modelling of the Langney Seweridentified the change in water level is within the Significant Adverse (greater than 100mm for the1% probability) category for Magnitude of Impact on an Attribute. The Langney Sewer fluvialfloodplain does not affect any residential or industrial properties therefore the Importance of WaterEnvironment Attributes is estimated to be “Low”. The overall Significance of Potential Effects iscategorised as “Slight/Moderate”.

The majority of the proposed scheme alignment has a low risk of flooding from rivers, however,the sections of the proposed scheme which cross the Cuckmere River and Langney Sewer areconsidered as being at a high risk of flooding from rivers. Based on the information available, thescheme overall has a medium risk of flooding from rivers.

There is no evidence of historic flooding along the proposed section of the A27. However,groundwater flooding is prevalent in the South Downs, and therefore based on the informationavailable the overall flood risk from groundwater flooding is considered to be medium.

There is no historic evidence of sewage flooding in the surrounding area. Therefore, based oninformation available the proposed scheme is considered to be at low risk from sewer flooding.

From a review of the EA’s surface water flooding map the scheme area is identified as beingsusceptible to flooding from surface water runoff from land. There are areas of low to high surfacewater flood risk in the scheme area indicating an annual probability of flooding of between 1 in1000 (0.1%) and 1 in 30 (3.3%). However, based on the information available, the overall floodrisk from surface water runoff from land is considered to be high.

The EA’s flooding from reservoirs map identifies a small area of the A27 Cuckmere River extentwhich is classified as an area with “maximum extent of flooding from reservoirs.” The majority ofthe proposed scheme is identified as not being susceptible to flooding from reservoirs or otherartificial sources. Therefore, based on the available information, the overall flood risk fromreservoirs and other artificial sources is considered to be medium.

The effects of climate change have been taken into account in the scheme drainage design. Asper the current climate change allowances stated in the NPPF PPG, and also the likelyrecommendations in the forthcoming update to the DMRB, a 40% increase in rainfall intensitieshas been applied to all calculations to account for climate change.

8.2 RECOMMENDATIONS

The following mitigation measures are recommended to manage flood risk to and from thescheme. However, it is important to note, that given the close proximity of the Cuckmere River,Glynde Reach and Langney Sewer to the proposed scheme, there will always be a degree ofresidual flood risk even with the mitigation measures outlined below.


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à Pollution control elements may be required to reduce the risk of oil and hydrocarbonsentering the drainage system or polluting groundwater and to provide appropriate waterquality improvements and environmental enhancements. We recommend that an oil /petrol interceptor should be installed before the discharge point to provide a minimumlevel of treatment;

à The effect on surface water quality should considered for the proposed drainage;

à Implementation of the proposed Drainage Strategy for the scheme and consideration of a40% increase in rainfall intensities in accordance with the NPPF;

à The mitigation measure proposed for the NMU crossing over the Cuckmere River in itsproposed alignment (Option 1C: parallel to existing A27 carriageway and on fullembankment) is to ensure the opening width is 22m and perpendicular to the flow.

à It is recommended to continue liaison with the EA to determine whether compensatorystorage is also required for the Cuckmere River or whether further reduction in waterlevel is required. It is possible the EA would require compensatory storage to be provided(on a like-for-like volume basis of approx. 2600m3) or that detriment be further reduced.Formal comment has not yet been received from the EA for the Cuckmere River at thetime of writing this FRA.

à The proposed changes at Polegate Bypass simply displace the existing floodplainapproximately 5m westwards. It is recommended to continue liaison with the EA todetermine whether compensatory storage is also required for the Langney Sewer. It ispossible the EA would require compensatory storage to be provided (on a like-for-likevolume basis of approx. 230m3). Formal comment has not yet been received from the EAfor the Langney Sewer at the time of writing this FRA.

à Peak water levels for a range of return periods have been provided to the drainagedesign team at the 15No. existing culvert crossings of the A27. It is recommended thatthese water levels are used to inform the design of highway drainage outfalls.

WSP UK LIMITED MAKES NO WARRANTIES OR GUARANTEES, ACTUAL OR IMPLIED, INRELATION TO THIS REPORT, OR THE ULTIMATE COMMERCIAL, TECHNICAL, ECONOMIC,OR FINANCIAL EFFECT ON THE PROJECT TO WHICH IT RELATES, AND BEARS NORESPONSIBILITY OR LIABILITY RELATED TO ITS USE OTHER THAN AS SET OUT IN THECONTRACT UNDER WHICH IT WAS SUPPLIED


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1

Appendix APROPOSED SCHEME RED LINE BOUNDARY

2

Appendix BIDB AREA AND BYELAWS

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Appendix CA27 PROPOSED METHODOLOGY ISSUED TO THE EA

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Appendix DESCC FLOOD RISK AND DRAINAGE REPORT

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Appendix EEA CONSULTATION DATA

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Appendix FHYDRAULIC MODELLING FOR CUCKMERE RIVER

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Appendix GHYDRAULIC MODELLING FOR LANGNEY SEWER AND CULVERT

ASSESMENTS

8

Appendix HDRAINAGE STRATEGY

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