Exceat Bridge Replacement Flood Risk Assessment

Prepared for East Sussex County Council

Date: 23rd April 2021

East Sussex Highways The Broyle Ringmer East Sussex. BN8 5NP

Contents

Contents ...... 1 Figures, Tables & Appendices ...... 1 Acronyms and Abbreviations ...... 1 1 Executive Summary ...... 2 2 Introduction ...... 3 2.1 Terms of reference ...... 3 2.2 Study Area ...... 3 3 Development location and description ...... 4 3.1 Site location and description ...... 4 3.2 Type of development ...... 4 3.3 Development ...... 5 4 Assessment of Flood Risk in the Area ...... 7 4.1 Historical flooding ...... 7 4.2 Overview ...... 7 4.3 Fluvial and Coastal Flood Risk ...... 7 4.4 Reservoir flood risk ...... 8 4.5 Ground water flood risk ...... 10 4.6 Surface water flood risk ...... 10 4.7 Modelled Impacts ...... 11 4.8 Hazard maps ...... 18 5 Surface Water Drainage Strategy ...... 19 5.1 Design Standards ...... 19 5.2 Topography ...... 19 5.3 Catchments ...... 19 5.4 Surface water collection ...... 20 5.5 Surface water conveyance ...... 20 5.6 Surface water pollution control ...... 21 5.7 Surface water discharge ...... 21 5.8 Surface water exceedance flows ...... 22 6 Conclusion ...... 23 7 Source list ...... 24

Figures, Tables & Appendices

Figures

Figure 3.1 Geographic Location Plan, Google maps 2020 Figure 3.2 Hybrid Design option; August 2020 Figure 4.1 EA National flood map, August 2020 Figure 4.2 Risk of flooding from Reservoirs Figure 4.3 EA National Surface & Groundwater Flood map, August 2020 Figure 4.4 - Extent of flooding from surface water Figure 4.5 Local area difference in maximum flood depth 0.5%AEP(1:200Y)+CC event Figure 4.6. Complete reach difference in maximum flood depth 0.5%AEP(1:200Y)+CC event Figure 4.8;Design scenario hazard map 0.5%AEP(1:200Y)+CC event

Tables

Table 4.1 Peak water levels for pre and post development for a range of scenarios. These are joint probability events, using the worst case fluvial and tidal event Table 5.1

Appendices

Appendix A: Hazard Maps Appendix B: Flood Outlines Appendix C: Modelling Report Appendix D: SUDS technical note

Acronyms and Abbreviations AOD Above Ordinance Datum CC Climate Change CMG Contract Management Group (Client) ESCC East Sussex County Council ESH East Sussex Highways FRA Flood Risk Assessment FSA Flood Storage Area Ha Hectare JV Joint Venture (Costain/ CH2M) NPPF National Planning Policy Framework OS Ordinance Survey PMF Probable Maximum Flood SMB Service Management Board SMT Senior Management Team (JV) SV Social Value SWMP Surface Water Management Plan TWL Top Water Level

1 Executive Summary Jacobs (CH2M) have been appointed by East Sussex Highways (ESH) to produce a Flood Risk Assessment (FRA) in support of a planning application for proposed road bridge upstream of the existing road bridge over the river Cuckmere. This report provides a technical appraisal of flood risk both pre and post development and outlines how the proposal will lead to a reduction in flood risk in the area. This FRA follows the latest Government policy as set out in the National Planning Policy Framework (NPPF), and the latest Highways England design guidance (DMRB). Because the development site lies in Flood Zone 3, a sequential test is required. However, as this is a bridge development, there is no more appropriate location at lower risk for the development to take place. The bridge has been designed according to DMRB guidance to the 0.5%AEP+climate change rainfall and coastal flooding event. Detailed numerical modelling has been undertaken to assess the impact of the development on combined flood risk. SUDS have been incorporated into the drainage strategy. This modelling demonstrates that there is no significant change in flooding or flood hazard due to this development.

2 Introduction 2.1 Terms of reference Jacobs (CH2M) have been appointed by East Sussex Highways (ESH) to produce a Flood Risk Assessment (FRA) in support of a planning application for proposed road bridge upstream of the existing road bridge over the river Cuckmere. This study has been undertaken in accordance with guidelines set out in the National Planning Policy Framework (Department for Communities and Local Government, 2012a) and attendance Technical Guidance (Department for Communities and Local Government, 2012b). This report provides a technical appraisal of flood risk both pre and post development and outlines how the proposal will lead to a reduction in flood risk in the area. 2.1.1 Project and Policy Context This FRA follows the latest Government policy as set out in the National Planning Policy Framework (NPPF). Under the NPPF a site-specific FRA is required for proposals of one hectare or greater in Flood Zone 1, all proposals for new development (including minor development and change of use) in Flood Zones 2 and 3, or in an area within Flood Zone 1 which has critical drainage problems (as notified to the local planning authority by the Environment Agency); and where proposed development or a change of use to a more vulnerable class may be subject to other sources of flooding. The proposed development site is located in Flood Zone 3 and so has to comply with sequential testing from NPPF. The proposed development site comprises an area over one hectare and has the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water run-off. Therefore this FRA should identify and assess the risks of all forms of flooding to and from the development and demonstrate how these flood risks will be managed, taking climate change into account. For major developments in Flood Zone 3, the FRA should identify opportunities to reduce the probability and consequences of flooding through the layout and form of the development. 2.2 Study Area The development relates to the area located upstream of the current Exceat bridge located 2.1miles to the east of Seaford. This new bridge is an improvement to the current A259 (Eastbourne Road). There are three main parts of the development, the east bank the west bank and the bridge itself. The total area of works is 500HA. The development is in a green field site to the north of the current crossing. During construction the existing bridge will be retained, only being removed once the new bridge is operational. The River Cuckmere runs under the bridge, and outfalls at Cuckmere Haven on the English Channel.

3 Development location and description 3.1 Site location and description The development is centred on the Ordnance Survey grid reference TV 51423 99324 (X:551423, Y:099324) located approximately 2.1miles East of Seaford. The proposed development site compromises the banks of the river and the bridge and the returning road adjacent to the Cuckmere Inn. This is predominantly an open space with large fields and salt marshes. This location has the A259 crossing the bridge and bisects the main Cuckmere river floodplain.

Figure 3.1 Geographic Location Plan, Google maps 2020 3.2 Type of development The development is a replacement of the existing bridge with new one that improves traffic flow, minimizes congestion on the A259 and improves pedestrian and cycle facilities across the river Cuckmere. It has been carefully designed to conserve and enhance the special qualities of the South Downs National Park and supports the

4 principle of ‘People Supporting Landscape, Landscape Supporting People’ (through stewardship and ecosystem services)1. 3.3 Development The new bridge is located to the north of the existing bridge as shown in Figure 3.2.

Figure 3.2 Hybrid Design option; August 2020

At Exceat bridge water levels are influenced by both tidal levels and fluvial flows. There are many combinations of these inputs that can lead to an event with a given return period. For example, a 1 in 20 year event could result from a nominal flow and 1 in 20 year tide, or 1 in 2 year flow and 1 in 10 year tide, or a 1 in 4 year flow and 1 in 5 year tide, or a 1 in 5 year flow and 1 in 4 year tide. DMRB states that climate change allowances applied to fluvial flows should be based on the 90th percentile (i.e. Upper End) estimates and for sea levels on the 50th percentile. It is to note that the 50th percentile is not published by the Environment Agency for sea level rises, therefore we followed a precautionary approach, assumed a 100year design life and applied the fluvial Upper End (90th percentile) factor of 105% and sea level Higher central (70th percentile) allowance of 1.2 m rise. Sensitivity tests have been undertaken to the H++ scenario of 120% flow change and 1.9 m rise in sea level. The final designed Bridge Soffit Level, based on a worst case combined tidal and fluvial event is therefore: 1 in 200 Y joint probability level (5.34 mAOD + 0.30 (free board) = 5.64 mAOD.

1 As set out in the South Downs National Park, Partnership Management Plan – Shaping the future of your South Downs National Park, 2014-2019.

We note that the Environment Agency requested that the worst case of a fully blocked outfall caused by a shift in the shingle bank was modelled and used for design. However, this wasn’t the ‘worst case’. The sea level used in our combined tidal and fluvial worst case is higher that the shingle bank, therefore this is actually the worst-case scenario used for design. For further information on the joint probability approach and climate change allowances please see the appended Hydraulic modelling report (Appendix C).

4 Assessment of Flood Risk in the Area 4.1 Historical flooding There have been 28 flood alerts for the river Cuckmere in the last 5 years, with flooding being present for several months in some years. 4.2 Overview The area is currently predominantly salt marsh and swathes (ancient man made ditches), and currently benefits from protection from tidal flooding. The man-made defences south of the A259 have historically been maintained by the Environment Agency, but the maintenance of these defences has now been withdrawn, and over time the defences will degrade and provide reduced benefit. Although the maintenance has been withdrawn, the Environment Agency will continue to remove shale that shifts into river channel during winter storm events. A range of tidal impacts on the river reach have been modelled, including a range of climate change induced sea level rise parameters. The modelling shows that neither the development, nor the proposed construction compound, have any impact on tidal flood risk. Further, the modelling also indicates that the risk of flooding to the development area is unaffected by the development. Whilst the development itself is at risk of tidal flooding, this risk does not change. 4.3 Fluvial and Coastal Flood Risk The Environment Agency national flood map outline for flood zones 2 and 3 indicates the risk of flooding from rivers and the sea close to the site as shown in Figure 3. However, more detailed modelling has been carried out to determine the soffit of the bridge, and to support this Flood Risk Assessment, and this modelling supersedes the National Flood Risk map. The outputs of the modelling are detailed in Section 4.7. Fluvial and coastal flooding is the primary source of flooding in this area.

Figure 4.1 EA National flood map, August 2020 4.4 Reservoir flood risk The Environment Agency national flood outline for inundation from reservoirs indicates that there is a risk from an impounding reservoir in Arlington Reservoir as shown in Figure 4.2 below.

Figure 4.2 Risk of flooding from Reservoirs

4.5 Ground water flood risk There are no recorded incidents of groundwater flooding. A review of the EA surface and groundwater flood maps however suggest a medium - high ground water vulnerability risk probability as shown in Figure 5.

Figure 4.3 EA National Surface & Groundwater Flood map, August 2020 4.6 Surface water flood risk The land use is primarily salt marsh and agricultural land with natural drainage. Any hard standing is a small proportion of to the total area, therefore surface water flood risk in this area is low as shown in Figure 6.

Figure 4.4 - Extent of flooding from surface water 4.7 Modelled Impacts A detailed hydrodynamic model has been used to assess the impacts of climate change on the development design, and to assess the impact of the development on flood risk. The appended modelling and hydrology report (Appendix C) provide further detail on the model software, build and assumptions. Table 1 and Figure 7 detail the difference in maximum flood extent and maximum water level for both pre and post development, and also detail the change in level and extent post development. The scenario used for Figure 7 and Table 1 are the worst-case scenario of 0.5% annual exceedence probability plus climate change. The modelling indicates that there is a difference in level where the new bridge built, and the old bridge is removed as is expected as detailed. However, there are no other significant impacts in the surrounding area. As shown on the Table 4.1Error! Reference source not found. the impact of the new development on the whole reach is not significant across all events.

Table 4.1 Peak water levels for pre and post development for a range of scenarios. These are joint probability events, using the worst case fluvial and tidal event

01.010 01.0 01.0 01.082 01.04 Betwe 22 01.02 20 01.0 S 0 en U/S 1S D/S 01 Worst case joint Climate Simulat Lullingt Seafo new of A259 of Sea Description probability Event change ion on rd bridge new Bridg new Mou Road Lewe and brid e brid th Bridge s sea ge ge mouth Base line 3.71 3.92 3.98 3.99 4.00 4.03 4.04 Increase 1 cm U/S No CC Design 3.71 3.93 3.98 3.99 4.00 4.03 4.04 (01.040) Differen 50% AEP ce 0.00 0.01 0.00 0.00 0.00 0.00 0.00 1 in 2 yr. Base line 4.00 4.24 4.41 4.47 4.48 4.59 4.88 Decrease 1 cm D/S CC Design 4.00 4.24 4.41 4.46 4.48 4.59 4.88 (01.021S) Differen ce 0.00 0.00 0.00 -0.01 0.00 0.00 0.00 Base line 3.78 3.98 4.04 4.05 4.06 4.09 4.11 No CC Design 3.78 3.98 4.04 4.05 4.06 4.09 4.11 No difference Differen ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 20% AEP 1 in 5 yr. Base Increase 1 cm line 4.10 4.24 4.43 4.51 4.52 4.62 4.96 (01.040), 2 cm (01.022) U/S CC Design 4.10 4.25 4.44 4.49 4.51 4.62 4.96 Decrease 1 cm Differen (01.020), 2 cm ce 0.00 0.01 0.02 -0.02 -0.01 0.00 0.00 (01.021S) D/S

01.010 01.0 01.0 01.082 01.04 Betwe 22 01.02 20 01.0 S 0 en U/S 1S D/S 01 Worst case joint Climate Simulat Lullingt Seafo new of A259 of Sea Description probability Event change ion on rd bridge new Bridg new Mou Road Lewe and brid e brid th Bridge s sea ge ge mouth Base line 3.82 4.00 4.08 4.09 4.09 4.13 4.16 No CC Design 3.82 4.00 4.08 4.09 4.09 4.13 4.16 No difference Differen 10% AEP ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1 in 10 yr. Base line 5.00 4.89 4.87 4.87 4.87 4.85 4.90 CC Design 5.00 4.89 4.87 4.87 4.87 4.85 4.90 No difference Differen ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Base line 3.85 4.02 4.10 4.11 4.12 4.16 4.21 No CC Design 3.85 4.02 4.10 4.11 4.12 4.16 4.21 No difference Differen 5% AEP ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1 in 20 yr. Base line 5.20 5.07 5.06 5.05 5.05 5.04 5.04 Decrease 1 cm D/S CC Design 5.20 5.07 5.06 5.05 5.05 5.03 5.04 (01.010) Differen ce 0.00 0.00 0.00 0.00 0.00 -0.01 0.00 Base 2% AEP No CC line 3.88 4.04 4.13 4.14 4.15 4.19 4.27 No difference 1 in 50 yr. Design 3.88 4.04 4.13 4.14 4.15 4.19 4.27

01.010 01.0 01.0 01.082 01.04 Betwe 22 01.02 20 01.0 S 0 en U/S 1S D/S 01 Worst case joint Climate Simulat Lullingt Seafo new of A259 of Sea Description probability Event change ion on rd bridge new Bridg new Mou Road Lewe and brid e brid th Bridge s sea ge ge mouth Differen ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Base line 5.26 5.16 5.14 5.13 5.12 5.11 5.12 Decrease 1 cm U/S CC Design 5.26 5.16 5.13 5.13 5.12 5.11 5.12 (01.022) Differen ce 0.00 0.00 -0.01 0.00 0.00 0.00 0.00 Base line 3.89 4.05 4.15 4.16 4.17 4.22 4.32 No CC Design 3.89 4.05 4.15 4.16 4.17 4.22 4.32 No difference Differen 1% AEP ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1 in 100 yr. Base Decrease 1 cm U/S line 5.50 5.28 5.26 5.24 5.24 5.23 5.20 (01.082S, 01.022) CC Design 5.49 5.28 5.25 5.24 5.24 5.22 5.20 Decrease 1 cm D/S Differen (01.010) ce -0.01 0.00 -0.01 0.00 0.00 -0.01 0.00 Base line 3.89 4.06 4.16 4.18 4.18 4.24 4.37 0.5% AEP Decrease 1 cm D/S No CC Design 3.89 4.06 4.16 4.17 4.18 4.24 4.37 1 in 200 yr. (01.021S) Differen ce 0.00 0.00 0.00 -0.01 0.00 0.00 0.00

01.010 01.0 01.0 01.082 01.04 Betwe 22 01.02 20 01.0 S 0 en U/S 1S D/S 01 Worst case joint Climate Simulat Lullingt Seafo new of A259 of Sea Description probability Event change ion on rd bridge new Bridg new Mou Road Lewe and brid e brid th Bridge s sea ge ge mouth Base line 5.56 5.36 5.33 5.32 5.32 5.30 5.25 CC Design 5.56 5.36 5.33 5.32 5.32 5.30 5.25 No difference Differen ce 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Base Decrease 1 cm U/S line 4.95 4.54 4.38 4.34 4.28 4.08 3.45 (01.022) No CC Design 4.95 4.54 4.37 4.34 4.29 4.08 3.45 Increase 1 cm D/S Differen (01.020) 0.2% AEP ce 0.00 0.00 -0.01 0.00 0.01 0.00 0.00 1 in 500 yr. Base Increase 1 cm U/S line 5.86 5.61 5.56 5.54 5.54 5.50 4.73 (01.040, 01.022) CC Design 5.86 5.62 5.57 5.55 5.55 5.51 4.73 Increase 1 cm D/S Differen (01.021S, 01.020, ce 0.00 0.01 0.01 0.01 0.01 0.01 0.00 01.010) Base Decrease 1 cm U/S line 5.11 4.67 4.63 4.60 4.59 4.55 3.73 (01.022) No CC Design 5.11 4.67 4.62 4.61 4.59 4.55 3.73 Increase 1 cm D/S 0.1% AEP Differen (01.021S) 1 in 1000 yr. ce 0.00 0.00 -0.01 0.01 0.00 0.00 0.00 Base CC line 5.93 5.71 5.66 5.64 5.65 5.61 5.30 No difference Design 5.93 5.71 5.66 5.64 5.65 5.61 5.30

Figure 4.5 Local area difference in maximum flood depth 0.5%AEP(1:200Y)+CC event

Figure 4.6. Complete reach difference in maximum flood depth 0.5%AEP(1:200Y)+CC event

4.8 Hazard maps Hazard maps have been extracted from the modelling to show the Defra hazard rating (which is based on both depth and velocity of flow) for pre and post development scenarios. Figure 9 shows the pre and post development hazard score for the worst-case scenario of 0.5%AEP+CC. There are no areas of increased hazard due to the construction of the new crossing and removal of the old crossing for any of the scenarios modelled.

Figure 4.7;Design scenario hazard map 0.5%AEP(1:200Y)+CC event

5 Surface Water Drainage Strategy 5.1 Design Standards Design Standards The drainage design for this project will be designed and specified using the following:  Interim Code of Practise for Sustainable Drainage Systems (for discharge rates)  Design Manual for Roads and Bridges  Manual of Contract Documents for Highway Works.  SUDS Manual  Preliminary rainfall runoff management for developments  BS EN 752:2008 – Drain and sewer systems outside buildings 5.2 Topography The existing topography of the development area is a bridge crossing a river, with highway works to either side of the bridge. On the west side of the bridge, the highway starts to climb out of the Cuckmere valley. To the east of the bridge the highway remains flat until the extents for the proposed highway works. 5.3 Catchments The overall development can be broken down into two catchments. The catchments will be served by two separate systems with their own pollution control measures and discharge points. The catchments are to either side of the Cuckmere river, named the West bank and the East bank. The table below the area that each of the catchment includes. Table 5.1

Catchment name Catchment area (m²) West Bank 5164 East Bank 1473 Total 6637 The interface between the west bank catchment and the east bank catchment is the high point of the bridge deck at the halfway point of the bridge deck. 5.3.1 West bank The highway works to the West bank are at the bottom of a steep hill. The proposed highway works do not extend up the hill, however, the existing highway does not have any surface water collection facilities, so the surface water runoff from this area will need to be captured within the proposed highway drainage. 5.3.2 East bank The East bank catchment includes the extent of the highway works.

5.4 Surface water collection 5.4.1 Carriageway collection The proposed highway will be kerbed, so it will therefore be necessary to provide a positive surface water collection system. Gullies are the preferred choice as standard highway gullies have sediment traps included. 5.4.2 Bridge deck collection The bridge deck includes a cross fall and a camber. The span of the bridge is approximately 40m. There is insufficient space within the bridge deck structure to include any collection facilities therefore it is proposed that all surface water runoff will be conveyed off the bridge by the kerb system and collected as part of the standard highway drainage on either side of the bridge deck. 5.4.3 Footway and verge collection There are small areas of footways and verges, both adjacent to the highway and on the bridge deck. These areas will be collected by either road gullies, or linear drainage channels. The advantage of linear drainage channel is that they can be more aesthetically pleasing, so can be integrated with surface finishes better. This is of particular importance on the bridge deck area. Any footways to be drained on the bridge deck will be handled on either the East bank or the West bank as detailed above. There are some areas where there is no footway present and there is a verge. Runoff from the verge will be collected as if it were highway runoff. The verge falls to the highway low points, so no additional collection facilities are required. 5.5 Surface water conveyance Surface water runoff from the carriageway will generally be conveyed from the roadside edge to the outfall via a carrier drain network. The hydraulic design of these pipe networks will be undertaken using the Modified Rational method, utilising the latest version of Micro Drainage WinDes software to undertake the hydraulic analysis. The design will seek to achieve the following hydraulic objectives for surface water runoff, in accordance with DMRB HD 33/06: Meet discharge requirements (see section below)  No surcharging during the 1:1 year storm  No carriageway flooding during a 1:30 year storm  Increase in rainfall intensity of 30% to allow for climate change in accordance with the NFH Environmental Statement In addition, sensitivity tests shall be undertaken to determine the extent of flooding from surface water runoff during storm events greater than 1:5 years with the aim of:  Ensuring no carriageway flooding in critical areas (e.g. at sags points) during the 1:10 year storm event  Containing flooding to within the carriageway for storm events up to and including 1:30 years

 Containing flooding within the highway and designated attenuation areas for storm events up to and including 1:100 years. 5.6 Surface water pollution control The development site is located in an area of environmental sensitivity. The proposed works are highway related works, so the surface water runoff will have sediment and hydrocarbons present. In order to provide suitable pollution control measures to protect the ecology in the Cuckmere river and meanders. All gullies and channel drains are to be trapped in order to have sediment traps. Full retention separators are to be provided just upstream of each discharge point. Due to the pollutants that will be collected by a highway scheme, a full retention separator is required as typically the pollutants become mobilised during high intensity short order rainfall events. These events tend to send bypass separators into bypass mode, meaning that not all of the runoff gets treated. Full retention separators treat all the flows which pass through, regardless of the storm intensity. 5.7 Surface water discharge The drainage from the west bank will outfall into the Cuckmere river, just upstream of the new bridge location. The east bank will discharge into the Cuckmere meanders via a designed SUDS system (see SUDS strategy in Appendix D). Both of the outfalls are located approximately 1.5km from the English Channel and therefore, free discharge is achievable. 5.7.1 West bank The west bank discharges directly into the Cuckmere river. This length of the river is heavily influenced by the tide. The Highest Astronomical Tide (HAT) level is 3.94m AOD which is the highest tide every likely to occur. The existing outfall at this location has an invert level of 2.97m AOD, which is to be reused. The impact this outfall level has on the system is that the discharge pipe will be submerged during certain tidal conditions, known as tide lock scenarios. During these scenarios, the outfall pipe will only discharge when the hydraulic head within the piped system is greater than the river level. To mitigate the issues relating with surcharging, the outfall pipe will include a flap valve. The flap valve will prevent river sediments entering the system yet will enable discharge under tide lock, if the hydraulic head is great enough. Further to this, to prevent the entire system from surcharging, the manhole downstream of the full retention separator will have an invert level higher than the HAT. This will ensure that only the final length of pipe (the discharge pipe) will surcharge under tide lock scenarios and protect the full retention separator from backflows. 5.7.2 East bank The east bank discharges into the Cuckmere meanders. The Cuckmere meanders are the historical meanders of the original river, that no longer have significant flows running through them, the area is essentially a wetland. The meanders are protected from tidal influence, therefore the level within the meanders remains more constant and the high water level is not as extreme as in the main Cuckmere river channel. The existing outfall level to the east bank is at a level of 3.44m AOD. The water level in the meanders does not exceed a level of 2.6m AOD, so therefore, the invert of the proposed discharge will be set at this level. A flap valve will be installed on the outfall pipe to protect against wildlife and any surcharging of the system.

5.8 Surface water exceedance flows Due to the topography of the development, any exceedance flows from the west bank will overtop kerbs, down the bank and directly into the Cuckmere river. The east bank will similarly overtop the kerbs and spill over into the Cuckmere meanders. The location of the development and the proximity to a water body, with the ability to discharge to it, there is no requirement for specific exceedance flow measures at this development.

6 Conclusion There is no increase in flood extent or flood risk to property from any source due to the development. There are minor differences in peak water levels at different modelled scenarios. However, none of these impact properties, and the maximum change in depth is <20cm, well within expected model tolerances. The modelling has been used to set the soffit of the bridge such that it will not impede flows for up to the 0.5 annual exceedance probability + cc.

7 Source list References EA flood map- https://flood-map-for-planning.service.gov.uk/confirm- location?easting=551341&northing=99336&placeOrPostcode=BN25%204AB Planning guidance- https://www.gov.uk/guidance/flood-risk-assessment-for-planning- applications#get-information-to-complete-an-assessment Location - https://www.bing.com/maps?q=exceat+bridge&FORM=HDRSC6 LLFA guidance- East Sussex Local, Flood Risk Management, Strategy 2016 – 2026

Appendix A: Hazard Maps

Appendix B: Flood Outlines

Appendix C: Modelling Report

29 Exceat Bridge Replacement Exceat Bridge FRA - Hydraulic Model

Prepared for East Sussex County Council

Date: 23rd April 2021

East Sussex Highways The Broyle Ringmer East Sussex. BN8 5NP

Contents

Contents ...... 1 1 Introduction ...... 4 1.1 Project Background ...... 4 1.2 Study Area ...... 4 1.3 Flood History ...... 5 1.4 Previous Studies ...... 5 1.5 Modelling Objectives ...... 6 1.6 Software ...... 6 2 Data Input Plan ...... 7 2.1 Data ...... 7 2.2 Data Verification ...... 7 3 Hydrology ...... 11 3.1 Catchment Description...... 11 3.2 Data Availability and Review ...... 14 3.3 Rainfall Data ...... 15 3.4 Level Data ...... 15 3.5 Flow Data ...... 15 3.6 River Cuckmere at Cowbeech (41016) ...... 21 3.7 River Cuckmere at Sherman Bridge (41003) ...... 21 3.8 River Bull at Lealands (41029) ...... 22 3.9 Method Statment ...... 23 3.10 Catchment Delineation...... 24 3.11 FEH Statistical Methods ...... 28 3.12 Rainfall-Runoff Approaches ...... 33 3.13 Preferred Hydrology ...... 37 3.14 Tidal Boundaries ...... 37 3.15 Joint Probability ...... 39 3.16 Limitations ...... 39 3.17 Recommendation ...... 39 4 Model Development ...... 40 4.1 Model History ...... 40 4.2 Model Schematisation...... 40 4.3 Model Boundaries ...... 43 4.4 Initial Conditions ...... 44 4.5 Roughness Parameters ...... 44 5 Simulations and Results ...... 46 5.1 Scenarios ...... 46 5.2 Events ...... 46 5.3 Results ...... 47 5.4 Model Performance ...... 48 6 Sensitivity Test ...... 50

Figures, Tables & Appendices

Figures

Figure 3.1 FEH catchments Figure 3.2 Locations of hydrometric gauges Figure 3.3 FEH and LiDAR delineated catchments Figure 3.4 Delineated catchments Figure 3.5 Rainfall-runoff and preferred statistical growth curves for C1 Figure 3.6 Gauged high flow events identified for use in non-dimensional hydrograph analysis Figure 3.7 Non-dimensional hydrographs at Sherman Bridge Figure 3.8 Comparison of hydrograph profiles Figure 3.9 Design tide profile for the 1% AEP event Figure 4.1 Model schematisation Figure 4.2 Comparison of Exceat Bridge alignment Figure 4.3 Bridge nodes locations Figure 4.4 Material layer used to assign manning's n coefficients to 2D domain Figure 5.1 Model performance of baseline and design simulations for 0.5% AEP, no climate change Figure 6.1 Difference in maximum flood depth - Sensitivity test 1 Figure 6.2 Difference in maximum flood depth - Sensitivity test 2 Figure 6.3 Difference in maximum flood depth - Sensitivity test 3 Figure 6.4 Difference in maximum flood depth - Sensitivity test 4 Figure 6.5 Difference in maximum flood depth - Sensitivity test 5 Figure 6.6 Difference in maximum flood depth - Sensitivity test 6

Tables

Table 1.1 Historical flooding reports in Cuckmere and Sussex Havens Table 2.1 Summary of Input Data Table 3.1 Catchment descriptors Table 3.2 Availability of rainfall data Table 3.3 Availability of level data Table 3.4 Availability of flow data Table 3.5 Summary of Cowbeech Gaugings Table 3.6 Summary of Sherman Bridge gaugings Table 3.7 Summary of Lealands gaugings Table 3.8 Peak river flow allowances for the South East (Environment Agency, March 2020) Table 3.9 Sea level allowances for the South East for each epoch in mm per year, with total sea level rise for each epoch in brackets Table 3.10 Catchment delineation Table 3.11 Comparison of Catchment C1 and donor flow gauges Table 3.12 QMED estimates for catchment C1 2

Table 3.13 Comparison of East tributary catchment and donor flow gauges Table 3.14 QMED estimates for East Tributary Table 3.15 Z values for Growth Curve distributions for Catchment C1. Table 3.16 Z values for growth curve distribution on East Tributary Table 3.17 Preferred statistically derived flood frequency curves Table 3.18 ReFH and ReFH2 model parameters Table 3.19 FEH model parameters Table 3.20 Events used to create non-dimensional hydrograph Table 3.21 Preferred design peak flow estimates Table 3.22 Extreme sea levels and confidence intervals at Exceat (chainage 4,520 km) Table 4.1 Modelled Structures Table 4.2 Model changes Table 4.3 Combination of fluvial and tidal events Table 4.4 2D Manning's n Value Table 5.1 Combinations of fluvial and tidal events Table 5.2 Equivalence [AEP - Return period] for the simulated events Table 5.3 Relationship between locations and model nodes Table 5.4 1D maximum water level results Table 6.1 Sensitivity Tests Table 6.2 Comparison of maximum water levels of sensitivity tests

Appendices

Appendix A: Appendix Title Appendix B: Pooling Groups Appendix C: Flood Estimation Calculation proforma Appendix D: Joint Probability Combinations

1 Introduction

1.1 Project Background 1.2 Study Area The study area covers the Cuckmere River located in the east Sussex area. It has an area of approximately 16 km² to its confluence with the Strait of Dover to the east of Brighton. The Cuckmere River is located at east of Seaford and continues up to the north to the town of Hailsham. The river pass through an area of mostly natural zones although it goes nearby some small villages (Alfriston, Litlington). The Cuckmere Valley contains natural marsh areas, forest and farms. The Cuckmere Valley is surrounded by high banks that have the function of working as a flood defence system and as walking paths across the valley. At the upper part (approx. 8 km from mouth) there is a barrier installed to avoid higher tides propagating upstream, but to our knowledge, the barrier is inoperative and kept opened. Two bridges cross the valley, the A259 bridge and the Lullington Road Bridge. There are also 3 additional footbridges, one in the located in the Footpath Alfriston 23a, another located in Cow lane, and the last one located next to Alfriston village. Exceat Bridge carries the A259, Eastbourne Road, over the River Cuckmere between Seaford and Friston (National Grid ref: 551420 99323). The bridge is narrow with priority working for traffic travelling eastbound creating a bottleneck which frequently causes traffic to queue on both sides, but predominately on the Eastbourne side particularly in peak periods. On the western approach to the bridge (from Seaford) there is poor horizontal and vertical alignment and the T junction access to the Cuckmere Inn is located on the corner of the western approach to the bridge. The bridge deck has an overall length of 19.53m and a span between bearing centres of 18.0m. the deck is 5m wide with a skew of approximately 8 degrees. The original Exceat bridge was constructed circa 1870 but the superstructure was replaced with a composite corten steel/concrete deck in 1976. Due to financial constraints at the time the original brick masonary abutments, wrought iron edge girders and poor alignment were retained. The bridge is located within the South Downs National Park (SDNP) and a Site of Special Scientific Interest (SSSI). As a result of the special environmental issues the new bridge and associated infrastructure is likely to require planning permission and acquisition of land on which to build it. A Bridge Options report undertaken in September 2015 recommended that a full bridge replacement would address all the deficiencies, but the cost would likely exceed available funds and therefore promoted a substantial refurbishment and upgrade. In October 2018, another bid to the South East Local Enterprise Partnerships was made for additional funding to improve Exceat Bridge, and improve the footway facilities on the eastern approach to accommodate pedestrian and cyclists.

1.3 Flood History Research of the flood history for Cuckmere and Sussex Havens was conducted from data provided by the Environment Agency and the Strategic Flood Risk Assessment (2008). Table 1.1 provides an account of historical flooding reports. None of the historical flooding events listed in Table 1.1 resulted in significant property damages within the study area, this could however be attributed to the lack of significant urbanisation in the catchment and may not be indicative to the severity of these flooding events. Email communication with the Environment Agency received 19th June 2020 supports this. Therefore, it is highly likely that Table 1.1 does not list all of the historical flooding events that have occurred in the study area. The River Cuckmere at Cowbeech (NRFA Gauge No: 41016) is located towards the north-eastern corner of the upstream catchment and drains a catchment area of 21.8 km2 and may provide insight into the dates of high flow events within the catchment. The five largest flow records for the River Cuckmere at Cowbeech (41016) were in December 2000, November 1974, May 2000, December 1993, December 1979. Of these records, only November 1974 is mentioned within the historical flooding reports. The largest flow records for the River Cuckmere at Sherman Bridge (NRFA Gauge No: 41003) were not compared to historical flooding reports as the NRFA report that flows at this site above 10 m3/s are unreliable and become truncated above approximately 27 m3/s. Table 1.1 Historical flooding reports in Cuckmere and Sussex Havens

Event Date Location Details November 1973 Alfriston Roads, property and garden flooding noted in Alfriston. November 1974 Alfriston, Berwick Many garden and low-lying land flooded and Chalvington throughout the catchment. Flooding of several properties and roads. March 1995 Polegate and Surface water flooding observed which Wannock resulted in 56 properties being inundated as well as flooding of a number of roads causing disruption. October 2000 Hellingly Hellingly experienced its worst event on record with fluvial flooding caused by backing up at the confluence and possible blockages in the watercourses. October 2019 Near Exceat Sussex Air Imaging (2019) observed heavy Bridge rainfall in Cuckmere Haven caused flooding near Exceat Bridge. River mouth was de- shingled after the rainfall. 1.4 Previous Studies There is no knowledge of previous studies at this location.

1.5 Modelling Objectives The objective of this modelling is to calculate the effect of new Exceat Bridge on flood extent and maximum flood depth. New 1D-2D model has been built from scratch to simulate the existing situation and the post development situation. The model was run with different hydrological event for pre- and post- development scenario. 1.6 Software The software used is a combination of Flood Modeller and TUFLOW. Flood Modeller-TUFLOW combines two software packages for managing overland flow and rapid inundation modelling. It provides a flexible and comprehensive range of tools for designing cost effective engineering schemes, flood forecasting, flood risk mapping and developing catchment management strategies. The project used the following version of Flood Modeller and TUFLOW:  Flood Modeller Version 4.6 (double precision)  TUFLOW Version 2020-01-AA-iDP-w64

2 Data Input Plan

2.1 Data A range of data has been provided by our client, whilst other data has been gathered from online resources, in order to develop the model. The data is summarised in Table 2.1 below. Table 2.1 Summary of Input Data

ID Data Type Data Format Comment Obtained from 1 Topographical DWG A Cuckmere River Delivered by Survey Channel Topographical client Survey has been done by Maltby Land Survey. We received Cross Sections, Long Sections, Flood Modeller Bed Data, XYZ data, EACSD data, Photos, a detailed Topographic survey of the A239 bridge area and the survey report. 2 Digital Terrain ASC A 2 metres grid Digital Downloaded from Model Terrain Model has been data.gov.uk/ obtained from the government website data.gov.uk. The downloaded tiles are the TQ50SW and TV59NW 3 Land use SHP Information about terrain Downloaded from type and land uses has osmaxx.hsr.ch been obtained from Open Street Maps layers. 4 Meteorological web A statistical analysis Seen in: show that the most willyweather.co.uk common wind direction is from South-West. With an average of 3-4 m/s. 2.2 Data Verification 2.2.1 Topography versus DTM: Cross Section Analysis A random set of cross sections has been taken in order to compare the available topography [1] with the Digital Terrain Model [2]. In this comparison it can be appreciated that both topography and DTM have very similar shapes, being the only difference that the DTM fails to capture information below the 2 mAOD. This could be explained because the LiDAR has captured the dense vegetation in the area, or because the tidal level was rather high than low, or, more likely, a combination of both. In general, it seems to be a good concordance between the elevations of both datasets. 7

Mind that cross sections are named from Downstream to Upstream in the survey, and the same name convention has been carried on in the model, therefore cross section 01.006 is closer to the downstream part (Sea) than 01.026.

DTM / Topography Comparison [XS 01.006] 4.5 4 3.5 3 2.5 2 Topography 1.5

Elevation [m] Elevation DTM 1 0.5 0 0 10 20 30 40 50 60 70 Distance [m]

DTM / Topography Comparison [XS 01.026] 5

2 Topography

Elevation [m] Elevation DTM 1

0 0 10 20 30 40 50 Distance [m]

DTM / Topography Comparison [XS 01.046] 5

2 Topography

Elevation [m] Elevation DTM 1

0 0 10 20 30 40 50 Distance [m]

DTM / Topography Comparison [XS 01.066] 5

2 Topography

Elevation [m] Elevation DTM 1

0 0 10 20 30 40 50 Distance [m]

DTM / Topography Comparison [XS 01.086] 6 5 4 3 Topography 2

Elevation [m] Elevation DTM 1 0 -5 5 15 25 35 45 55 Distance [m]

DTM / Topography Comparison [XS 01.095] 6

3 Topography 2

Elevation [m] Elevation DTM 1

0 0 10 20 30 40 50 60 70 Distance [m]

2.2.2 Topography versus DTM: Long Section In this test we have compared the Topography and the 2 metres width cell DTM during a long section in both embankments. It can be observed that the curve sampled with the DTM has a sample rate much higher than the topography, therefore making it sharper. However, both curves share a similar trend.

DTM / Topography Comparison: East bank 6 5.5 5 4.5 4 3.5

Elevation [mAOD] Elevation 3 2.5 2 0 1000 2000 3000 4000 5000 6000 7000 8000 Distance from 01.095 [m]

DTM Topography

DTM / Topography Comparison: West bank 6 Max at 6.745 5.5 5 4.5 4 3.5

Elevation [mAOD] Elevation 3 2.5 2 0 1000 2000 3000 4000 5000 6000 7000 8000 Distance from 01.095 [m]

DTM Topography

3 Hydrology

A hydrodynamic model is required to support a Flood Risk Assessment (FRA), assess scour on the new structure and identify the required bridge deck height. In addition, Jacobs understand that there is ongoing discussion about the management of the area downstream of Exceat Bridge. Whilst the hydrodynamic model will be developed such that changes related to land management can be readily incorporated, this report focuses solely on the hydrological analysis undertaken to produce design inflows for the River Cuckmere for the purposes of producing a model for Flood Risk Assessment and Scour Assessment. 3.1 Catchment Description 3.1.1 Catchment Characteristics Exceat Bridge (the focus of this study) passes over the River Cuckmere as part of the A259 between Westdean and Seaford in East Sussex. Located at NGR: TV 514 993 it is about 1.8 km upriver of the coast. The River Cuckmere drains a catchment between Seaford and Eastbourne in East Sussex. The catchment is largely rural, laid predominantly to grassland with some arable land and areas of woodland. It contains a number of villages and in the west, the urban area of Hailsham. Arlington pumped storage reservoir lies about 11 km upriver of Exceat Bridge. Abstraction for this influences flow but is unlikely to have a significant impact on high flows. As an offline reservoir, it will have limited flow attenuation impact. Downriver of Exceat Bridge the channel is canalised with a historic meander to the east of the channel forming Cuckmere Haven. The limit of FEH catchment delineation of the river in this area is a catchment of 141.90 km2, where the Lullington Road crosses the River Cuckmere. This is about 6 km up river of Exceat Bridge to the north of Alfriston (NGR: TQ 524 036). The catchment downstream of this is to the mouth of the River Cuckmere is 39 km2 The catchment upstream of Lullington Road is underlain by mixed geology including Wealden Group – Mudstone, Siltstone and Sandstone, Lower Greensand Group, Gault Formation and Upper Greensand (British Geological Survey, 2019). Downstream of this the catchment is underlain by Chalk. This change in geology likely results in differing catchment responses, with low to moderate permeability and a more responsive catchment upstream of Lullington Road and a more permeable and baseflow influenced, slower responding catchment downstream. The principal FEH catchment descriptors for both sub catchments are shown in Table 3.1. Both catchments are shown in Figure 3.1.

Table 3.1 Catchment descriptors

Catchment descriptor North Catchment East catchment (Upstream of Lullington Road) Downstream grid ref TQ 524 036 TV 512 996 Area (km2) 141.9 12.145 BFIHOST 0.428 0.975 DPLBAR (km) 16.63 6.45 FARL 0.98 1 SAAR (mm) 814 819 SPRHOST (%) 41.92 4.29 URBEXT1990 0.206 0.0079 URBEXT2000 0.0246 0.0081

Figure 3.1 FEH catchments

3.2 Data Availability and Review A range of hydrometric data was supplied by the Environment Agency. The locations are illustrated in Figure 3.2. Analysis of data availability and quality on the gauging stations is provided below.

Figure 3.2 Locations of hydrometric gauges

3.3 Rainfall Data The availability of rainfall data is listed in Table 3.2. A review of the data shows that all of the measured rainfall gauges have an overall good data quality. There are five rainfall gauges located within the catchment upstream of Lullington Road: Arlington RS, Chalvington, Cowbeech TBR, Hailsham 2 and Vines Cross TBR. Of the five gauges Cowbeech TBR is the only 15-minute rainfall gauge located within the catchment and as a single gauged station may struggle to represent the rainfall response of the entire catchment. The majority of the rainfall gauges lie to the east/south-east of the catchment. Of the gauges to the east/south-east of the catchment, the Ringwood gauge is the only gauge that has large periods of missing data. 3.4 Level Data Stage data was provided for the seven gauges detailed in Table 3.3. Three gauges are located within the catchment of the River Cuckmere; River Cuckmere at Cowbeech (41016), River Bull at Lealands (41029) and River Cuckmere at Sherman Bridge (41003). All three gauges are located upstream of Exceat Bridge. Of the three gauges located within the upstream catchment, Cowbeech is the only gauge that has multiple instances of days with missing data. However, none of the gauges have significant periods of missing or suspect data. 3.5 Flow Data Three flow gauges lie within the bounds of the upstream catchment; River Cuckmere at Sherman Bridge (41003), River Bull at Lealands (41029) and River Cuckmere at Cowbeech (41016). The Sherman Bridge gauge is located approximately 1.3 km north-east of Milton Lock, the upstream limit of the hydraulic model. Lealands and Cowbeech gauging stations are both located approximately 10 km and 14 km north- east of Milton Lock respectively. Table 3.4 presents the data available at the three gauging stations including the spot gaugings. Comparison of the spot gauged values with rated flows has been used to assess the rating performance and identify any systematic errors present within the rating.

Table 3.2 Availability of rainfall data

Gauge NGR Start Date End Date Data Interval Comments Amberstone TQ 598 10/03/1949 09:00 30/04/1999 09:00 24 hours Missing data: 112  02/09/1998 09:00 to 31/12/1998 09:00 Arlington RS TQ 538 01/03/2008 09:00 28/02/2015 09:00 24 hours Missing data: (Current) 068  01/02/2009 09:00 – 31/01/2009 09:00 due to snow  01/06/2009 09:00 – 30/06/2009 09:00 Arlington RS TQ 538 01/01/1974 09:00 29/02/2008 09:00 24 hours Missing data: (Old) 068  01/10/2000 09:00 – 31/10/2000 09:00  01/06/2004 09:00 – 30/06/20004 09:00  20/03/2007 09:00  21/03/2007 09:00 Chalvington TQ 518 02/02/1967 09:00 31/01/1994 09:00 24 hours No data gaps identified 095 Cowbeech TBR TQ 610 01/10/1999 00:00 18/06/2020 09:00 15 minutes Missing data: 149  01/10/1999 00:00 – 17/02/2000 16:45  27/05/2009 16:00 – 07/06/2009 01:00  17/01/2010 05:00 – 21/02/2010 04:45  07/12/2010 00:15 – 07/01/2011 12:15 Cranedown TQ 567 01/04/1968 09:00 01/06/2000 09:00 24 hours No data gaps identified 032 Eastbourne R TQ 598 02/01/1979 09:00 01/06/1993 09:00 24 hours No data gaps identified 005

Gauge NGR Start Date End Date Data Interval Comments Eastbourne Wilmington TV 611 01/03/2008 09:00 31/03/2011 09:00 24 hours No data gaps identified (Current) 983 Eastbourne Wilmington TV 611 31/01/1886 09:00 29/02/2008 09:00 24 hours No data gaps identified (Old) 983 Hailsham 2 TQ 587 01/03/2008 09:00 30/09/2014 09:00 24 hours No data gaps identified 095 Hailsham TQ 596 02/03/1962 09:00 01/11/1998 09:00 24 hours Missing data: 087  01/07/1998 09:00 – 31/07/1998 09:00 Hellingly HP TQ 597 10/03/1949 09:00 01/11/2007 09:00 24 hrs hours No data gaps identified 124 Horseye TBR TQ 627 06/12/1995 13:15 07/03/2011 22:00 15 minutes Missing data: 083  27/05/2009 16:15:00 – 07/06/2009 01:00:00  11/02/2001 – 26/02/2001 Langney Friday Street TQ 621 01/03/1999 09:00 29/02/2008 09:00 24 hours No data gaps identified (Old) 039 Langney Friday Street TQ 621 01/03/2008 09:00 30/04/2020 09:00 24 hours Missing data: (Present) 039  07/01/2009 09:00  01/12/2019 09:00 – 31/12/2019 09:00 Polegate CD TQ 584 01/01/1980 09:00 31/12/2006 09:00 24 hours Missing data: 047  01/10/2003 09:00 – 31/10/2003 09:00 Ringwood TV 02/02/1966 09:00 01/03/2000 09:00 24 hours Missing data: 578989  02/02/1966 09:00 – 31/12/1989 09:00  02/05/1996 09:00 – 31/12/1996 09:00  02/01/1998 09:00 – 31/12/1998 09:00  02/11/1999 09:00 – 31/12/1999 09:00 17

Gauge NGR Start Date End Date Data Interval Comments Vines Cross TBR TQ 594 01/10/1999 09:00 18/06/2020 09:00 24 hours Missing data: 169  06/06/2002 09:00 – 02/07/2002 09:00  05/05/2004 09:00 – 08/06/2004 09:00  Data from 20/03/2011 09:00 – 22/03/2011 09:00 marked as suspect. Willingdon TQ 601 02/12/1952 09:00 01/01/1955 09:00 24 hours Missing Data: 022  01/01/1955 09:00 Willingdon101 TQ 585 01/03/2008 09:00 30/04/2020 09:00 24 hours No data gaps identified (Current) 034 Willingdon101 TQ 585 02/01/2007 09:00 01/03/2008 09:00 24 hours Missing data: (Old) 034  01/03/2008 09:00

Table 3.3 Availability of level data

Gauge NGR Start Date End Date Data Interval Comments Cowbeech GS TQ 611 01/10/1999 00:00 18/06/2020 09:00 15 Minutes Data missing: 150  11/10/2002 12:45 – 11/10/2002 14:45  15/10/2002 10:30 – 15/10/2002 11:45  22/11/2008 14:00 – 31/12/2008 23:45  30/07/2009 10:00 – 30/07/2009 17:00  24/02/2010 23:00 – 24/02/2010 23:30  18/03/2010 05:00 – 19/03/2010 09:00 – Infilled from back-up logger data Eastbourne New TQ 641 26/04/2018 12:45 17/06/2020 04:30 15 Minutes Data marked as uncertain at 17/05/2018 11:45 Tide Gauge 015 Data marked as uncertain from 27/04/2020 – 17/06/2020 Eastbourne TQ 641 01/01/2011 12:45 17/06/2020 04:30 15 Minutes Data marked as uncertain or suspect from Tidal 016 01/01/2011 – 11/09/2014 (Old Gauge) Data marked as suspect from 30/11/2016 11:45 – 14/08/2018 13:45 Data marked as uncertain or missing from 14/08/2018-17/06/2020 Langney RL TQ 631 26/08/2005 00:00 17/06/2020 04:30 15 Minutes Data marked as uncertain from 26/08/2005 019 00:00 – 30/04/2006 23:45 Data marked as uncertain from 15/01/2008 00:00 – 26/10/2009 Data marked as uncertain from 27/04/2020 – 17/06/2020 Lealands GS TQ 576 01/10/1999 00:00 18/06/2020 09:00 15 Minutes Data missing: 130  07/05/2010 13:15– 18/06/2010 10:00 Sherman Bridge TQ 532 01/10/1999 00:00 18/06/2020 09:00 15 Minutes No data gaps identified GS 051 Data marked as uncertain 17/06/2020 04:45 – 18/06/2020 04:30 Wannock Mill TQ 586 01/07/2007 00:00 12/06/2020 04:30 15 Minutes Data marked as uncertain or suspect from 046 01/07/2007 00:00 – 02/01/2011 07:00 Data marked as uncertain from 17/02/2020 00:15 – 12/06/2020

Table 3.4 Availability of flow data Gauge NGR Start Date End Date Data Interval Comments Cowbeech GS TQ 611 150 01/10/1999 00:00 18/06/2020 09:00 15 minutes No data gaps identified. Data marked as uncertain from 03/06/2020 04:30 – 18/06/2020 04:30 TQ 611 150 05/01/2000 15:00 06/03/2020 15:33 Spot flow Flow data for 65 spot gaugings provided. gauge data Ten spot gaugings marked as uncertain Lealands GS TQ 576 130 01/10/1999 00:00 18/06/2020 09:00 15 minutes Missing data:  07/05/2010 13:15 – 18/06/2010 10:00  09/05/2017 11:45 – 17/05/2017 16:30  Data marked as uncertain from 27/04/2020 04:45 – 18/06/2020 04:30 TQ 576 130 23/11/2000 19:31 17/02/2020 13:47 Spot flow Flow data for 26 spot gaugings provided. gauge data One spot gauging marked as uncertain Sherman TQ 532 051 01/10/1999 00:00 18/06/2020 09:00 15 minutes No data gaps identified. Bridge GS Data marked as uncertain from 17/06/2020 04:45 – 18/06/2020 04:30 TQ 532 051 25/08/2009 13:20 17/02/2020 15:04 Spot flow Flow data for 26 spot gaugings provided. gauge data Two spot gaugings marked as uncertain.

3.6 River Cuckmere at Cowbeech (41016) The River Cuckmere at Cowbeech (41016) lies about 20 km north-east (upstream) of Exceat Bridge and records flows from an 18.7 km2 rural catchment. The Cowbeech gauging station has recorded flows since 1939 using a crump profile weir, the crest of which was replaced in 2002. The gauge is included in the National River Flow Archive (NRFA) and is a high flows station suitable for QMED. The station has been gauged to near QMED however is not suitable for pooling as the modelled rating has not been confirmed above QMED. Additionally, the NRFA notes that a bridge located downstream causes choking at the station, affecting readings. A velocity-area station is located downstream of the Cowbeech station that the NRFA states provides “good check gauging”. A total of 65 spot flow gaugings are available at Cowbeech between 2000 and 2020, eleven of these gaugings are prior to the crest replacement in 2002. It is of note that of the 65 spot flow gaugings conducted, only five gaugings were during flow events of 5 m3/s or greater and the results may not be representative of the station performance during high flow events. A comparison of the rated flow gaugings to the spot flow gaugings is presented in Table 3.5. A negative change is seen in 43 of the gaugings (66% of the spot gauged flows), suggesting that the rating tends to underestimate flows. This underestimation is seen both before and after the new crest was installed (Table 3.4). Throughout the period of record the spot flow has a tendency to be higher than the rated flow. The majority of the percentage differences lie within the ±0.2% range. It is of note that there are two significantly high negative deviations in January 2008, however appear to be erroneous when compared to other deviations. Table 3.5 Summary of Cowbeech Gaugings

Full record Pre-crest Post-crest replacement replacement Period of record 2000-2020 2000-2002 2003-2020 Number of gaugings 65 12 53 Number negative 43 9 34 gaugings (i.e. flows are underestimated) Percentage negative 66% 75% 64% gaugings 3.7 River Cuckmere at Sherman Bridge (41003) Sherman Bridge is located just north of the A27. It is approximately 7 km north-east of Exceat Bridge and drains a catchment of 126.3 km2. The Sherman Bridge station uses a flat-V weir and gauges high flows at the velocity-area section upstream of Arlington. The gauge was included in the NRFA as a high flows station, however was removed from the peak flow dataset in May 2016. The NRFA states that flows at Sherman Bridge become truncated at approximately 27 m3/s and some bypassing may occur at high flows. Additionally, NRFA note that the gauge becomes unreliable above 10 m3/s.

A total of 26 spot gaugings are available at Sherman Bridge between 2009 and 2020. All are at relatively low flows, only six were observed to be flow events above 10 m3/s (which is about 20 m3/s less than our estimate of QMED in this part of the catchment). The rating underestimates flows in 13 of the gaugings and overestimate flows in 12 of the gaugings (Table 3.6). The largest deviations were seen for flows above 10 m3/s with an average percentage deviation of 44% (i.e. flows are overestimated) compared to flows below 10 m3/s that had an average percentage deviation of -13% (i.e. flows are underestimated). This supports the information in NRFA. Table 3.6 Summary of Sherman Bridge gaugings

Sherman Bridge Full record Period of record 2009-2020 Number of gaugings 26 Number negative gaugings (i.e. flows are 13 underestimated) Percentage negative gaugings 50% 3.8 River Bull at Lealands (41029) The River Bull at Lealands (41029) is located just north of Hellingly. It is approximately 15 km north-east of Exceat Bridge and drains a catchment of 40.8 km2. The catchment is underlain by mixed geology draining the High Weald. The River Bull at Lealands uses a flat-V weir and is theoretically rated. All flows are contained within the structure and the NRFA states that the station has “very good high flow performance”. However, the NRFA also advise that very few gaugings have been carried out due to difficult site access at high flows and recommend caution if using this station to interpret high flow behaviour. The station is not included within the NRFA high flows database. A total of 26 spot gaugings are available at Lealands between 2000 and 2020 (Table 3.7). Of the 26 spot flow gaugings, only four were gauged to be above 5m3/s. The rating underestimates flows in 13 of the gaugings and overestimate flows in the other 13 of the gaugings. On average the percentage difference between recorded and spot gauged flow is –2% indicating that the station has a tendency to underestimate flow. Table 3.7 Summary of Lealands gaugings

Lealands Full record Period of record 2000-2020 Number of gaugings 26 Number negative gaugings (i.e. flows are 14 underestimated) Percentage negative gaugings 54%

3.9 Method Statment The upstream boundary of the model is at Milton Lock (NGR: TQ 525 040), which is represented by Catchment C1 (NGR: TQ 524 036) as shown in Figure 3.1. At this point the channel drains a largely rural and impermeable catchment of 141.9 km2. Downstream of this the river receives flows over a more permeable catchment. Given the relative size of the catchments flow from the upstream catchment will have a greater influence on levels than the much smaller permeable catchment. Water levels will also be tidally influenced, up to the tidal limit at Milton Lock. Fluvial and tidal boundaries are therefore required, and a joint probability approach followed to develop design events. The primary purpose of the modelling is to support design of the replacement bridge at Exceat and therefore peak flows rather than volumes are of primary interest. 3.9.1 Fluvial Inflows As discussed above, FEH does not delineate the catchment to Exceat Bridge, as it is located within a tidally influenced reach. FEH identifies the tidal limit of the River Cuckmere at Alfriston, upstream of Exceat Bridge. The catchment here is largely impermeable (SPRHOST = 41.92), essentially rural (URBEXT2000 = 0.0246) and has very limited flow attenuation from upstream reservoirs or lakes (FARL = 0.98). Application of standard FEH procedures is therefore appropriate for this catchment. The change in bedrock downstream of Alfriston results in a much more permeable catchment. Rainfall-runoff methods are less representative of catchment response. Statistical analysis can however be applied with appropriate adjustment for permeability. The catchment area downstream of Alfriston is much smaller and will therefore have a limited contribution to peak flows and volumes. Steady state inflows are applied for this area, estimated using statistical analysis based on a representative tributary and then scaled by catchment area. A permeable adjustment is applied to the growth curve. 3.9.2 Tidal Boundary The downstream boundary (sea levels and tide profile) are established following the methods described in Coastal flood boundary conditions for the UK: Update 2018 (Environment Agency, 2019). This is detailed in later section. 3.9.3 Joint Probability The water levels at the bridge are influenced by both tidal levels and fluvial flows. Fluvial and tidal flows were combined using the method described in Technical Report FD2308 ”Use of Joint Probability Methods in Flood Management, A guide to Best Practice” (Hawkes, 2005). The gauges within the Cuckmere catchment are not included within the dependency analysis undertaken as part of the FD2308 study. A representative neighbouring gauge was therefore used as a donor for the dependence measure. 3.9.4 Climate Change Climate change allowances will be applied to both fluvial and tidal inflows using latest Environment Agency guidance and the Design Manual for Roads and Bridges (DMRB) CD 356 (Highways England, 2020). Exceat Bridge lies within the South East river basin district. The peak river flow allowances for the South East are shown in Table 3.8 and the peak sea level allowances are Table 3.9.

DMRB states that climate change allowances applied to fluvial flows should be based on the 90th percentile (i.e. Upper End) estimates and for sea levels on the 50th ercentile. We note that the 50th percentile is not published by the Environment Agency for sea level rises, therefore we will follow a precautionary approach, assume a 100 year design life and apply the fluvial Upper End (90th percentile) factor of 105% and sea level Higher central (70th percentile) allowance of 1.2 m rise. We will also sensitivity test to the H++ scenario of 120% flow change and 1.9m rise in sea level. Table 3.8 Peak river flow allowances for the South East (Environment Agency, March 2020)

Allowance Total potential Total potential Total potential category change change change anticipated for anticipated for anticipated for the 2020s (2015 the 2050s (2040 the 2080s (2070 to 2039) to 2069) to 2115) H++ 30% 60% 120% Upper end (90th 25% 50% 105% percentile) Higher central 15% 30% 45% (70th percentile) Central (50th 10% 20% 35% percentile) Table 3.9 Sea level allowances for the South East for each epoch in mm per year, with total sea level rise for each epoch in brackets

Allowance 2000 to 2036 to 2066 to 2096 to Cumulative 2035 (mm) 2065 (mm) 2095 (mm) 2125 (mm) rise 2000 to 2125 (metres) H++ 1.9 m Upper end 6.9 (242) 11.3 (339) 15.8 (474) 18.2 (546) 1.60 (95th percentile) Higher 5.7 (200) 8.7 (261) 11.6 (348) 13.1 (393) 1.20 central (70th percentile) 3.10 Catchment Delineation The catchment delineation is based on the location of interest (Exceat Bridge), hydrological features, including tributaries and changes in permeability, consideration of location of flow gauges and the model extent. The most downstream point of River Cuckmere catchment delineated by the FEH webservice (https://fehweb.ceh.ac.uk/) is about 6 km up river of Exceat Bridge to the north of Alfriston (NGR: TQ 524 036). At this point the catchment has an area of 141.90 km2. The FEH webservice also identifies eight tributaries between this upstream catchment at Alfriston and Exceat Bridge. The catchment boundary between the upstream FEH boundary and the mouth of the River Cuckmere was identified through LiDAR analysis. It was combined with the FEH boundary to identify a lumped catchment of 180.91 km2, for the River Cuckmere to the downstream extent. The catchment boundary was reviewed against Ordnance Survey (OS) mapping and the Environment Agency River Centre Lines. No adjustments were required. 24

Figure3.3 identifies the catchment to the upstream model extent (in red), the LIDAR identified boundary (in black) and two of the eight tributary boundaries (in purple and green). These latter two boundaries are shown to highlight correlation between these FEH boundaries and the LiDAR derived boundary. The tributary to the east (FEH Catchment 551200 99600, referred to in this report as East Tributary) is selected as representative of the permeable area and used in the estimation of peak flows for the permeable part of the catchment. Two model inflows were developed; the first for flows from Catchment C1 to the limit of the FEH catchment delineation of the River Cuckmere (Catchment C1), the second to account for the more permeable catchment between Catchment C1 and the river mouth (Catchment C2). This second inflow will be distributed through the model using lateral inflows based on areal weighting. Details of the inflow locations are described in Table 3.10 and shown in Figure 3.4. Table 3.10 Catchment delineation

Name C1 C2 Description Lumped inflow to the limit Flows from the more of the FEH catchment permeable part of the delineation of the River catchment downstream of Cuckmere. Represents TQ 524036. Represented flow from the largely by the East Tributary impermeable catchment. (FEH Catchment 551200 A lumped inflow at the 99600), with flows scaled upstream model extent at by area Milton Lock Downstream extent of sub Lullington Road Mouth of the River catchment NGR: TQ 524 036 Cuckmere, NGR: TV 515 978. Model inflow location A lumped inflow at the Distributed along channel upstream model extent at reach downstream of Milton Lock NGR TQ 525 NGR: TQ 524 036. 040 Sub catchment area 141.9 km2 39 km2 % of total catchment 78.4% 21.6%

Figure 3.3 FEH and LiDAR delineated catchments

Figure 3.4 Delineated catchments

3.11 FEH Statistical Methods Statistical analysis was carried out to produce flood frequency estimates for the two sub catchments. The first for the lumped catchment to NGR: TQ 524 036 (Catchment C1), the second for the East Tributary catchment which acts a donor for the downstream permeable reach of the channel (Catchment C2). 3.11.1 QMED Estimation QMED estimates were obtained from catchment descriptors by applying the current QMED equation (Environment Agency Science Report SC050050, 2008). Possible donors for each site were initially selected in a semi-automated manner from a short- list of NRFA stations classified as suitable for QMED or pooling. Only stations located less than 75 km from the subject catchments were included. The search also considered similarity of AREA, SAAR, PROPWET, BFIHOST, FARL and DPSBAR catchment descriptors. Stations with short records (< 14 years) were excluded, as were urbanised sites (URBEXT2000 > 0.03). Based on these criteria the most similar sites were selected for more detailed consideration. The River Cuckmere at Cowbeech (41016) lies upstream of Exceat Bridge. It was selected as a donor for catchment C2 based on the criteria outlined above, however it is considerably smaller than catchment C1 and therefore did not fulfil the catchment descriptor criteria used to identify suitable donors for this catchment. Despite this, Cuckmere at Cowbeech was included for consideration as a donor for catchment C1 because of its location. 3.11.1.1 Catchment C1 Table 3.11 compares the key features of the donor catchments with those of the upstream catchment on the Cuckmere (C1). The resulting estimates of QMED are shown in Table 3.12. The confidence intervals of catchment descriptor derived estimates were evaluated using Environment Agency Science Report SC050050, 2008. The confidence intervals for donor adjusted catchment descriptor based estimates were derived assuming Factorial Standard Error (FSE) from donor sites where AMAX record length exceed 15 years, as per Flood Estimation Handbook Volume 3, table 13.10 (CEH, 1999). Urban adjustment has been applied as described in Kjeldsen 2010. Table 3.11 Comparison of Catchment C1 and donor flow gauges

Site Area SAAR BFIHOST FARL Hiflows Distance (km2) suitability between centroids (km) Catchment C1 141.9 814 0.428 0.98 N/A N/A Bevern Stream 34.6 886 0.355 0.993 Suitable 19.54 at Clappers for QMED Bridge (41020) Rother at Udiam 206 857 0.388 0.975 Suitable 15.31 (40004) for QMED Cuckmere at 18.7 855 0.471 0.966 Suitable 7.18 Cowbeech for QMED (41016)

Table 3.12 QMED estimates for catchment C1

QMED method QMED (m3/s) QMED (m3/s) QMED (m3/s) Central estimate 68% confidence 95% confidence interval interval Catchment 29.93 20.9 – 42.8 14.6 – 61.3 descriptors Catchment 30.72 21.5 – 44.0 15.0 – 62.9 descriptors with urban adjustment Donor adjustment 30.68 27.7 – 34.1 24.9 – 37.7 using Bevern Stream at Clappers Bridge Donor adjustment 27.99 25.2 – 31.1 22.8 – 34.4 using Rother at Udiam Donor adjustment 38.50 34.7 – 42.7 31.3 – 47.4 using Cuckmere at Cowbeech Donor adjustment 32.10 29.0 – 35.6 26.1 – 39.5 using all three donors As shown in Table 3.12 there is notable variation in estimates of QMED depending on the donor selected for adjustment. The Cuckmere at Cowbeech increases the estimate of QMED by about 8 m3/s whilst the Rother at Udiam reduces QMED by about 2 m3/s. When a weighted average of the adjustments using all three gauges is applied the estimate of QMED increases by about 1.5 m3/s compared to the catchment descriptor estimate. The most conservative approach to selecting QMED would be based on use of Cuckmere at Cowbeech. This gauge was not selected as the single donor for QMED because despite being located upstream of the subject location it is only about an eighth of the size of the subject catchment and is not the most representative donor of those reviewed. The weighted average approach is selected as the preferred estimate for QMED. 3.11.1.2 East Tributary Table 3.13 compares the key features of the donor catchments with those of the East Tributary. The resulting estimates of QMED are shown in Table 3.14. The Law Brook at Albury was selected as the preferred donor. Despite being the furthest donor from the subject catchment, Law Brook at Albury is the most representative donor being very similar in terms of size, rainfall, urbanisation and permeability. The other two donors, whilst geographically closer, measure flows from largely impermeable catchments and are likely to be less representative of the subject catchment. QMED for the East Tributary is therefore 0.383 m3/s.

Table 3.13 Comparison of East tributary catchment and donor flow gauges

Site Area SAAR BFIHOST FARL Hiflows Distance (km2) suitability between centroids (km) East Tributary 12.15 819 0.975 1 N/A 0 Law Brook at 16 819 0.888 0.96 Suitable 68.81 Albury (39036) for QMED Cuckmere at 18.7 855 0.471 0.966 Suitable 19.28 Cowbeech for QMED (41016) Bevern Stream 34.6 886 0.355 0.993 Suitable 24.05 at Clappers for QMED Bridge (41020) Table 3.14 QMED estimates for East Tributary

QMED method QMED QMED QMED Central estimate 68% confidence 95% confidence interval interval Catchment descriptors 0.378 0.27 – 0.55 0.19 – 0.78 Catchment descriptors 0.404 0.28 – 0.58 0.20 – 0.83 with urban adjustment Donor adjustment 0.383 0.35 – 0.43 0.31 – 0.47 using Law Brook at Albury Donor adjustment 0.479 0.43 – 0.53 0.39 – 0.59 using Cuckmere at Cowbeech Donor adjustment 0.404 0.36 – 0.45 0.33 – 0.50 using Bevern Stream at Clappers Bridge 3.11.2 Growth Curve Development Growth curves for the two catchments were developed using WinFAP software version 4 and the NRFA Peak Flow Dataset version 8 (downloaded in April 2020). Two separate pooling groups were developed because of the differences in catchment descriptors. In both cases the default pooling group was reviewed with a focus on data quality and sites removed where quality was of concern. Sites were then added to the pooling group to achieve a target pooling group of 500 station years. This group is referred to as the Reviewed pooling group. Three statistical distributions were examined to identify the most suitable distribution at each site; Generalised Logistic (GL), Generalised Extreme Value (GEV) and Pearson Type III (PIII). In order to account for the influence of urbanisation on flood peak estimates, flood frequency curve development was carried out updating URBEXT2000 to 2020 through the National Urban Expansion Factor (UEF).

Full details of pooling groups are described in Appendix A. Details of growth curve development specific to each of the two catchments are described below. 3.11.2.1 Catchment C1 The default pooling group for Catchment C1 contained 505 years of data. Following a review this reduced to 492 years. The pooling group was then tailored to sensitivity test the impact of including only impermeable sites (SPRHOST>20) and contained 354 years of data. Details of all pooling groups are included in Appendix A. The full list of growth factors is provided within the FEH proforma in Appendix B alongside plots of the growth factors and flood frequency curves. Growth factors resulting from the Default and Tailored pooling groups are very similar, whilst review of the pooling group reduces growth factors, most notably for events with an annual exceedance probability of 1% or less. As illustrated in Table 3.15, both the GEV and PIII distributions provided acceptable fits to the data for all pooling groups, whereas the GL distribution did not provide an acceptable fit for any pooling group. The growth curve from the reviewed pooling group with GEV distribution was selected as the preferred growth curve for this study. It is considered to contain sites with better quality data and greater similarity to the subject site than the default pooling group. Table 3.15 Z values for Growth Curve distributions for Catchment C1.

Pooling Group Generalised Generalised Pearson Type III Logistic Extreme Value Default 1.79 0.13* -1.59* Reviewed 2.84 0.87* -0.67* Tailored 3.11 -1.52* -0.04* Note: * indicates an acceptable fit 3.11.2.2 East Tributary The default pooling group for the East Tributary contained 501 years of data. Following a review this changed to 504 years. Permeability is not automatically considered within pooling analysis and therefore a further pooling group was developed to sensitivity test the impact of only including permeable sites (SPRHOST<20) within the pooling group (the Tailored group), this contained 403 years of data. The reviewed pooling group was changed significantly to form the Tailored pooling group. While the additional sites in the Tailored group were similar in terms of permeability, they differed greatly in other aspects such as area or rainfall. Thus, it is unlikely that the Tailored pooling group would produce growth curves that are close to the true growth curve of the study site (Environment Agency, 2008). The Tailored growth curve was therefore not used but provided a useful sensitivity test to help understand uncertainty in flow estimates. Permeable adjustment was carried out on both the Reviewed and Tailored pooling groups to remove the influence of non-flood years.

The full list of growth factors is provided within Appendix B alongside plots of the growth curves and flood frequency curves. Review and tailoring of the pooling groups provide increasingly reduced growth factors, particularly for events with an annual exceedance probability (AEP) of 2% or lower. Permeable adjustment also serves to reduce growth factors slightly. As illustrated in Table 3.16, both the GEV and PIII distributions provided acceptable fits to the data for the Reviewed and Tailored pooling groups. Whilst the GL distribution provides an acceptable fit to data for the Default and Tailored pooling groups. Due to the need for permeable adjustment the GL distribution was selected. We note that this differs to that for catchment C1 which could cause inconsistencies, however the impact of this is deemed to be limited because of the relatively small proportion of total flow contributed by catchment C2. The permeable adjusted GL distribution growth curve based on the Reviewed pooling group was therefore selected as the preferred growth curve for the study. It is considered to contain sites with better quality data and greater similarity to the subject site than the Default pooling group. Table 3.16 Z values for growth curve distribution on East Tributary

Pooling Group Generalised Generalised Pearson Type III Logistic Extreme Value Default 0.95* -0.48* -2.25 Reviewed 2.03 0.41* -1.44* Tailored 1.31* -0.22* -1.49* Note: * indicates an acceptable fit 3.11.3 Selected Statistical Flood Frequency Curves Flood frequency curves were developed by combining the preferred QMED estimate and growth factors for each sub catchment. The flood frequency curve for the East Tributary was scaled to the area of Catchment C2 using an area weighting factor. The flow estimates from the selected flood frequency curves are listed in Table 3.17. Tables of all flood frequency curves are included in Appendix B. Further adjustments to the flood frequency curves are made in later sections and the final flood frequency curve is shown in section 3.13, Table 3.17 Preferred statistically derived flood frequency curves

Return period Annual Exceedance Catchment C1 Catchment C2 (Years) Probability (%) Flow (m3/s) Flow (m3/s) 2 50 32.10 1.23 5 20 46.03 1.77 10 10 55.81 2.19 20 5 65.60 2.65 25 4 68.81 2.81 30 3.33 71.44 2.95 40 2.5 75.68 3.18 50 2 78.99 3.37 75 1.33 85.09 3.73 100 1 89.51 4.02 200 0.5 100.49 4.78 500 0.2 115.70 6.01 1000 0.1 127.77 7.14

3.12 Rainfall-Runoff Approaches As described above, rainfall-runoff approaches are highly uncertain on permeable catchments and the Environment Agency (2015) recommends that design events methods such as FEH and ReFH should be avoided for highly permeable catchments. Rainfall-runoff approaches are however appropriate for the upstream catchment (C1) and the development of flood hydrographs for this catchment is described here. 3.12.1 Flood Frequency Curve Development The flood hydrograph profiles were created using the FEH, ReFH and ReFH2 methods for catchment C1. As the catchment is essentially rural, ReFH urban was not applied. Rainfall parameters, including critical storm duration were applied for the lumped catchment. ReFH model parameters are shown in Table 3.18 and FEH parameters in Table 3.19. Growth curves created by the rainfall-runoff models are shown in Figure 3.5, plotted against the preferred statistical growth curve1. Values are included within Appendix B. Table 3.18 ReFH and ReFH2 model parameters

Parameter ReFH ReFH2 Tp (hours) 8.42 9.30 Cmax (mm) 345.20 338.79 BL (hours) 51.39 56.04 BR 1.02 1.03 Season of event Winter Winter Storm duration (hours) 15.25 17 Timestep (hours) 0.25 1 Table 3.19 FEH model parameters

Parameter Value Tp (hours) 10.25 Base Flow 3.12 Season of event Winter Storm duration 18.75 (hours) Timestep (hours) 0.25

1 Preferred statistical growth curve is derived from reviewed pooling group using GEV distribution 33

Figure 3.5 Rainfall-runoff and preferred statistical growth curves for C1 3.12.2 Non-Dimensional Hydrograph The gauge at Sherman Bridge is not appropriate for use to determine peak flows however it can provide useful information on hydrograph profile. A non-dimensional hydrograph was created following the method outlined by Archer et al (2000). An average hydrograph profile for the catchment to Sherman Bridge was derived by identifying a series of observed flow events and averaging them to create a design hydrograph. This design hydrograph can then be scaled to the appropriate peak value. As part of this hydrological review, several high flow events for the Sherman Bridge station were identified and used to generate a design hydrograph. This gauging station was selected as it is closest to the upstream extent of the hydraulic model. It is important to note that the gauge truncates the highest flows and is noted to be uncertain about flows of 10 m3/s, and the resulting hydrograph profiles will therefore be uncertain. Any flow events above 15 m3/s within the last five years of data were identified and analysed to determine whether they were suitable to be used in the non-dimensional hydrograph. Events were considered suitable for use if they met the following criteria:  Data contained no anomalies  The flow event was dominated by a single peak  The peak flow event was below 27 m3/s (as flows above 27 m3/s are truncated) The five highest flow events that met the above criteria were used to generate the non-dimensional hydrograph. Details of the events chosen are provided below.

Table 3.20 Events used to create non-dimensional hydrograph

Name Date Start Date End Peak Flow (m3/s) Hydrograph 1 27/01/2020 29/01/2020 21 Hydrograph 2 03/03/2019 06/03/2019 23.2 Hydrograph 3 30/03/2018 01/04/2018 20.6 Hydrograph 4 10/12/2015 13/12/2015 23.2 Hydrograph 5 26/12/2017 29/12/2017 20.5

Figure 3.6 Gauged high flow events identified for use in non-dimensional hydrograph analysis

Figure 3.7 Non-dimensional hydrographs at Sherman Bridge

Figure 3.8 Comparison of hydrograph profiles

The hydrograph profiles created from FEH, ReFH and ReFH2 were compared with the average non-dimensional hydrograph to identify whether the rainfall-runoff hydrograph profiles were representative of high flow events. The plots in Figure 3.8 illustrate the differences between the rainfall-runoff hydrographs and the average non-dimensional hydrograph. The non-dimensional hydrograph for Sherman Bridge produces a larger volume of runoff during a peak flow event than any of the rainfall-runoff hydrographs. As discussed in above section, there is significant uncertainty about the accuracy of the flow gauge and these hydrographs may be missing the peak flow data therefore appearing ‘flatter’ with a more slowly reacting rising and falling limb than in reality, however careful selection of events has attempted to reduce this risk. 36

Of the rainfall-runoff hydrograph profiles considered, the ReFH appears to provide the most suitable representation yet all hydrographs have a weak fit to the observed hydrograph profiles. As there are no gauges within the model extent it will not be possible to calibrate a rainfall-runoff produced hydrograph within the model. Therefore, the non-dimensional hydrograph will be used for calibration purposes. 3.13 Preferred Hydrology The fluvial design flows for use in the hydraulic modelling are shown in Table 3.21. These were produced from the preferred statistical growth curves shown in above section, with on Catchment C1, the ReFH ratio method applied for return periods greater than 1 in 100 years. The ReFH ratio method was applied as the Statistical Method alone is unsuitable for deriving low probability events due to the length of record available. This method was not suitable to apply for Catchment C2 as rainfall- runoff methods perform poorly for very permeable catchments. The non-dimensional hydrograph for Sherman Bridge was scaled to fit the fluvial design flows for catchment C1 and used as the model inflow for the upstream model extent. Due to the poor performance of rainfall-runoff methods on permeable catchments, the lack of gauged data on catchment C2 and the relatively small contribution of this area to overall flow, the fluvial inflows for catchment C2 were entered as steady state values. Table 3.21 Preferred design peak flow estimates

Return period AEP (%) Catchment C1 Catchment C2 (Years) (m3/s) (m3/s) 2 50 32.10 1.23 5 20 46.03 1.77 10 10 55.81 2.19 20 5 65.60 2.65 25 4 68.81 2.81 30 3.33 71.44 2.95 50 2 78.99 3.37 75 1.33 85.09 3.73 100 1 89.51 4.02 200 0.5 106.00 4.78 500 0.2 134.48 6.01 1000 0.1 162.96 7.14 3.14 Tidal Boundaries The downstream boundary (sea levels and tide profile) was established following the methods described in Coastal flood boundary conditions for the UK: Update 2018 (Environment Agency, 2019). The most appropriate chainage point from which to take data on extreme sea levels is chainage 4,520 km. Extremes data for this point are listed in Table 3.22. The Highest Astronomical Tide (HAT) is 4.82 mOD and Mean High Water Springs (MHWS) is 3.92 mOD. The surge shape to apply at this location is that for Dover.

Newhaven was identified, from Admiralty Tide Tables, as the standard port for the study area. A time series of tide data was extracted from the tide tables and the peak tide level was identified. The peak tide level was aligned with the peak of the Dover surge shape and a superposition of the two curves results in the design tidal inflow. An example of this for the 1% AEP event is shown in Figure 3.9.

Figure 3.9 Design tide profile for the 1% AEP event

Table 3.22 Extreme sea levels and confidence intervals at Exceat (chainage 4,520 km)

Return period Extreme sea 2.5% confidence 97.5% confidence (years) level (mOD) interval levels (mOD) interval levels (mOD) 1 4.01 3.99 4.03 2 4.08 4.06 4.11 5 4.18 4.16 4.23 10 4.26 4.22 4.32 20 4.34 4.29 4.43 25 4.35 4.31 4.46 50 4.44 4.38 4.58 75 4.49 4.41 4.66 100 4.52 4.43 4.72 150 4.56 4.46 4.8 200 4.6 4.49 4.87 250 4.62 4.51 4.92 300 4.64 4.52 4.97 500 4.7 4.56 5.11 1,000 4.78 4.61 5.32 10,000 5.06 4.77 6.12

3.15 Joint Probability Water levels at Exceat Bridge are affected by both fluvial and tidal levels. The FD2308 desk study approach (Hawkes, 2005) was used to calculate the marginal return periods/AEP combinations required to develop the joint exceedance return periods. FD2308 provides dependency measures for a series of flow gauges and tidal sites around the UK. The closest gauge to the River Cuckmere is the Ouse at Barcombe Mills (41004). The dependency between this and the closest surge station (Newhaven) is (Chi =)-0.01. This indicates that the flows and tidal surge are essentially independent. The resulting fluvial and tidal combinations for the design even runs are described in Appendix C. 3.16 Limitations Whilst there are three flow gauges within the catchment of the River Cuckmere, all lie upstream of the modelled area and therefore it was not possible to directly calibrate the hydrology and model. To mitigate this flow data at from the gauge at Sherman Bridge was used to create a hydrograph profile which was then scaled to fit statistically derived design flows. Use of the flow data at Sherman Bridge, however, is another limitation/uncertainty as the gauge performs poorly for high flow measurement and records truncated flows above 27 m3/s. Given the permeable nature of the catchment downstream of Lullington Road it is possible that groundwater will influence water levels in this area. Groundwater has not been explicitly considered as part of this study as we consider that fluvial flows from the much larger upper catchment of sea levels will have a greater influence on water level at the bridge location. 3.17 Recommendation Confidence in estimation of flow in the River Cuckmere could be improved if data quality at Sherman Bridge were improved. It is recommended that alterations to the gauge structure and subsequent rating review be considered to provide a better record of high flows within the catchment. In addition, all gauges in the catchment would benefit from increased spot gaugings at high flows to help validate the ratings. The hydrological analysis present in this report was created solely to support hydraulic modelling as part of the development of a new bridge at Exceat. It should not be used for any other purpose without further review.

4 Model Development

4.1 Model History There is no previous model of the area. Unfortunately, model calibration is not possible due to lack of calibration data. Therefore, the model will be tested against different sensitivity factors in order to check that its behaviour is reasonable. 4.2 Model Schematisation A new 1D-2D model was developed using the datasets listed in Table 2.1. Based on topography of the floodplain, the model was schematised as 1D for the river and the floodplain has been modelled on 2D domain as detailed in Figure 4.1.

Figure 4.1 Model schematisation

Baseline model The baseline model represents the current situation of the Cuckmere River (and Exceat Bridge). There are defence banks surrounding the river bed. These embankments have been included in the model as Z lines using the DTM values for elevation sampled with a 3 metres frequency. The banks are being included using two different layers: 2d_zln_EXC_ExternalBanks_003 and 2d_zln_EXC_banks_003. Design model The difference between the baseline and design model is the location and dimension of the Exceat Bridge. The longitudinal axis of the bridge moved ~20m in upstream direction, the width of the bridge opening increased from 17.5m to 20.0m, the soffit level increased from 4.60mAOD to 5.64mAOD. Another difference is the ground level around the new bridge. The ground surface around the bridge has been modified according to Figure 4.2, using the 2d_zsh_Road_R.shp and 2d_zsh_Road_P.shp layers. 4.2.1 1D Schematisation The model was schematised using embanked 1D cross section. There are few exceptions in some areas where the embankment distances itself from the river bed, and therefore it was not picked up by the topography survey. These areas are:  From cross-section 01.029 to 01.026, the west (high) bank distances away from riverbed.  From cross-section 01.019 to 01.017, the west (high) bank distances away from riverbed.  From cross-section 01.014 to 01.009, the west (high) bank distances away from riverbed.  From cross-section 01.011 to 01.001 the east (high) bank distances away from riverbed. Structures There is a total of 6 structures represented in the model, which are summarised in Table 4.1. They are represented using a combination of two units, being one of them a spill to account for water overtopping the structures. The 2D domain has been deactivated on top of the structures to avoid double counting the water spilled. There are pictures and extract from the survey at Appendix A. Table 4.1 Modelled Structures Structure Cross section Location Modelled using chainage Gate 01.094 552598,104032 Vertical Sluice unit + Spill unit Bridge 01.082 Long Bridge Road Bridge unit + Spill unit Footbridge 01.075 Near Alfriston War Bridge unit + Spill unit Memorial Hall Footbridge 01.057 Cow Lane Bridge unit + Spill unit Footbridge 01.049 Footpath Alfriston Bridge unit + Spill unit 23A Bridge 01.021 A259 Bridge Bridge unit + Spill unit The A259 Bridge was modified for the design simulations compared to baseline simulations (see Figure 4.2). The modifications include changes in bridge location, width and soffit level. Differences between baseline and design scenarios are summarised in Table 4.2. Figure 4.3 displays the difference in location between the 2 scenarios. 41

Figure 4.2 Comparison of Exceat Bridge alignment

Table 4.2 Model changes

Baseline file Design file Description EXC061.DAT EXC062.DAT Change the width and soffit level for the bridge. Change the level of the spill unit. Change the distance to next node for 2 nodes to schematise the new location of the bridge 1d_nd_EXC_002_P.shp 1d_nd_EXC_004_P.shp Modify the location of the bridge nodes 1d_nwk_EXC_002_L.shp 1d_nwk_EXC_003_L.shp Modified to meet the change in location of the bridge 1d_WLL_EXC_002_L.shp 1d_WLL_EXC_003_L.shp Modified to meet the change in location of the bridge 2d_bc_hx_EXC_002_L.shp 2d_bc_hx_EXC_003_L.shp Modified to meet the change in location of the bridge 2d_zsh_Road_R.shp New files to modify the 2d_zsh_Road_P.shp topography at the edges of the bridge to represent the road

Figure 4.3 Bridge nodes locations 4.2.2 2D Schematisation The cell size of the 2D model domain has been set at 6 metres, which is appropriate for the size of Cuckmere River and able to represent the flow paths in natural areas. Elevations for the 2D model were taken from the 2m digital terrain model. In the downstream 1D-2D connection, the elevations of the 2D domain have been smoothed to make a smooth connection between the two domains. 4.3 Model Boundaries 4.3.1 Inflows The model includes 2 inflow boundaries. One that applies as point inflow to the top node of the model (01.095) and one that distributes as lateral inflow (Catchment_C2) to 4 nodes (01.073, 01.065, 01.055 and 01.043). The joint probability tables (see Appendix D) identify 52 combination of fluvial and tidal events. 156 model runs were accomplished (including climate change and H++) to choose the worst case for each return period. The worst-case scenarios for present day and climate change scenarios listed in Table 4.3. Table 4.3 Combination of fluvial and tidal events

Hydrological Present Day Climate Change Return Period Fluvial Tidal Fluvial Tidal 50% AEP Nominal 50% AEP Nominal 50% AEP (1 in 2 year) (1 in 2 year) (1 in 2 year) 20% AEP Nominal 20% AEP Nominal 20% AEP (1 in 5 year) (1 in 5 year) (1 in 5 year) 10% AEP Nominal 10% AEP 20% AEP 50% AEP (1 in 10 year) (1 in 10 year) (1 in 5 year) (1 in 2 year) 5% AEP Nominal 5% AEP 10% AEP 50% AEP (1 in 20 year) (1 in 20 year) (1 in 10 year) (1 in 2 year) 2% AEP Nominal 2% AEP 10% AEP 20% AEP (1 in 50 year) (1 in 50 year) (1 in 10 year) (1 in 5 year)

Hydrological Present Day Climate Change Return Period Fluvial Tidal Fluvial Tidal 1% AEP Nominal 1% AEP 2% AEP 50% AEP (1 in 100 year) (1 in 100 year) (1 in 50 year) (1 in 2 year) 0.5% AEP Nominal 0.5% AEP 1% AEP 50% AEP (1 in 200 year) (1 in 200 year) (1 in 100 year) (1 in 2 year) 0.2% AEP 0.2% AEP Nominal 0.2% AEP Nominal (1 in 500 year) (1 in 500 year) (1 in 500 year) 0.1% AEP 0.2% AEP 50% AEP 0.2% AEP 50% AEP (1 in 1000 year) (1 in 500 year) (1 in 2 year) (1 in 500 year) (1 in 2 year) Details for the calculations of the hydrology can be found in the hydrology part of the report. 4.3.2 Downstream boundary The downstream (tidal) boundary represented as HT boundary within the 1D and 2D domain. A short section of the sea has been included in the 2D domain. This boundary represents the water level according to the tide forces. 4.4 Initial Conditions Model has been prepared to start with a tidal water level of 1.6 metres above the datum and a baseflow of 0.1 m³/s. Shall the user to start a model with a different tidal value, it is recommended that the user will start in the previous sinusoidal part to be modelled at the time this one reach a value of 1.6 metres. The value of 1.6 metres has been chosen because this value would keep the whole 1D domain wet without overtopping (in the defenced model) to the 2D domain. 4.5 Roughness Parameters Roughness parameters for the 1D cross sections were specified using Manning’s n friction parameters. The roughness values are a means of representing the channel and floodplain conveyance based on the vegetation, composition and sinuosity. The whole 1D domain uses a manning value of 0.040 푠/푚. This value is to represent the muddy terrain with vegetation that conforms the Cuckmere River.

Figure 4.4 Material layer used to assign manning's n coefficients to 2D domain On the 2D domain, the Open Street Maps layer have been used to determine the land use or type of terrain as seen in Figure 4.4 above. The default value for the whole 2D domain is a manning value of 0.057 푠/푚. This value has been overridden with the values of the Table 4.4, if they fall under one of the following Open Street Maps attributes. Table 4.4 2D Manning's n Value Manning’s n value Open Street Map Attribute 0.300 Building: residential, industrial, commercial, covered reservoir 0.050 Manmade natural surface: farm, park 0.060 Non-manmade natural surface: natural, wetland, scrub, heath, meadow orchad 0.200 Greenhouse 0.045 Inland Water: water, waterway, reservoir 0.120 Forest, wood 0.035 Road, Track, Path, Railway. (Lines buffered to 6.5 meters) 0.055 Tidal Water: sand, beach, sea 0.070 Bare Rock 0.057 Default unassigned value

5 Simulations and Results

5.1 Scenarios 5.1.1 Baseline This scenario represents the current situation of the Exceat River. In this scenario the existing banks are represented as Z-lines using the existing DTM and ensuring continuity throughout them. In the downstream connection to the sea, no blockage is represented and the river (FM-1D) is connected to the sea (TUFLOW-2D) using a head-time unit. 5.1.2 Design This scenario represents the new location of the bridge with the new width and soffit level. The rest parameters like defences and downstream boundary are the same as the previous scenario. 5.2 Events Nine events have been simulated for each scenario, and each of these events has been simulated for the current situation and for a future climate change situation. Each event is a combination of fluvial and tidal inflows. These combinations were selected for each return period. Table 5.1 displays the combinations of fluvial and tidal inflows for each return period. Table 5.1 Combinations of fluvial and tidal events

Event Climate change Fluvial Tidal 50% AEP No Nominal 2 Yes Nominal 2 20% AEP No Nominal 5 Yes Nominal 5 10% AEP No Nominal 10 Yes 5 2 5% AEP No Nominal 20 Yes 10 2 2% AEP No Nominal 50 Yes 10 5 1% AEP No Nominal 100 Yes 50 2 0.5% AEP No Nominal 200 Yes 100 2 0.2% AEP No 500 Nominal Yes 500 Nominal 0.1% AEP No 1000 Nominal Yes 500 2

The equivalence between AEP and return period can be found on Table 5.2. Table 5.2 Equivalence [AEP - Return period] for the simulated events

Annual 50% 20% 10% 5% 2% 1% 0.5% 0.2% 0.1% Exceedance Probability Return 1 in 2 1 in 5 1 in 10 1 in 20 1 in 50 1 in 100 1 in 200 1 in 500 1 in Period years years years years years years years years 1000 years 5.3 Results Results have been extracted at 7 different locations on the model and presented in Table 5.4. Difference is calculated by subtracting baseline result from design result. Correspondence between location and model nodes is found on Table 5.3. When the location is a footbridge or bridge, the reference node is the immediate downstream river cross section. Table 5.3 Relationship between locations and model nodes

Location Lullington Seaford U/S of A259 D/S of Location Sea Road Lewes bridge bridge bridge between Mouth bridge bridge and sea mouth Model 01.082S 01.040 01.022 01.021S 01.020 01.010 01.001 Node Table 5.4 1D maximum water level results

Event Climate Simulation

change

Lullington Road Road Lullington bridge Lewes Seaford ofbridge U/S bridge A259 ofbridge D/S Location bridge between mouth sea and Mouth Sea 50% No Baseline 3.71 3.92 3.98 3.99 4.00 4.03 4.04 AEP Design 3.71 3.93 3.98 3.99 4.00 4.03 4.04 Difference (m) 0.00 0.01 0.00 0.00 0.00 0.00 0.00 Yes Baseline 4.00 4.24 4.41 4.47 4.48 4.59 4.88 Design 4.00 4.24 4.42 4.46 4.47 4.59 4.88 Difference (m) 0.00 0.00 0.01 -0.01 -0.01 0.00 0.00 20% No Baseline 3.78 3.98 4.04 4.05 4.06 4.09 4.11 AEP Design 3.78 3.98 4.04 4.05 4.06 4.09 4.11 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yes Baseline 4.10 4.24 4.43 4.51 4.52 4.62 4.96 Design 4.10 4.25 4.45 4.49 4.51 4.62 4.96 Difference (m) 0.00 0.01 0.02 -0.02 -0.01 0.00 0.00 10% No Baseline 3.82 4.00 4.08 4.09 4.09 4.13 4.16 AEP Design 3.82 4.01 4.08 4.09 4.09 4.13 4.16 Difference (m) 0.00 0.01 0.00 0.00 0.00 0.00 0.00 Yes Baseline 5.00 4.89 4.87 4.87 4.87 4.85 4.90 Design 5.00 4.89 4.87 4.87 4.87 4.85 4.90 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Event Climate Simulation

change

Lullington Road Road Lullington bridge Lewes Seaford ofbridge U/S bridge A259 ofbridge D/S Location bridge between mouth sea and Mouth Sea 5% No Baseline 3.85 4.02 4.10 4.11 4.12 4.16 4.21 AEP Design 3.85 4.02 4.10 4.11 4.12 4.16 4.21 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yes Baseline 5.20 5.07 5.06 5.05 5.05 5.04 5.04 Design 5.20 5.07 5.05 5.05 5.05 5.03 5.04 Difference (m) 0.00 0.00 -0.01 0.00 0.00 -0.01 0.00 2% No Baseline 3.88 4.04 4.13 4.14 4.15 4.19 4.27 AEP Design 3.88 4.04 4.13 4.14 4.15 4.19 4.27 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yes Baseline 5.26 5.16 5.14 5.13 5.12 5.11 5.12 Design 5.26 5.16 5.13 5.13 5.12 5.11 5.12 Difference (m) 0.00 0.00 -0.01 0.00 0.00 0.00 0.00 1% No Baseline 3.89 4.05 4.15 4.16 4.17 4.22 4.32 AEP Design 3.89 4.05 4.15 4.16 4.17 4.22 4.32 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Yes Baseline 5.50 5.28 5.26 5.24 5.24 5.23 5.20 Design 5.49 5.28 5.25 5.24 5.24 5.22 5.20 Difference (m) -0.01 0.00 -0.01 0.00 0.00 -0.01 0.00 0.5% No Baseline 3.89 4.06 4.16 4.18 4.18 4.24 4.37 AEP Design 3.89 4.06 4.16 4.17 4.18 4.24 4.37 Difference (m) 0.00 0.00 0.00 -0.01 0.00 0.00 0.00 Yes Baseline 5.56 5.36 5.33 5.32 5.32 5.30 5.25 Design 5.56 5.36 5.33 5.32 5.32 5.30 5.25 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.2% No Baseline 4.95 4.54 4.38 4.34 4.28 4.08 3.45 AEP Design 4.95 4.54 4.37 4.34 4.29 4.08 3.45 Difference (m) 0.00 0.00 -0.01 0.00 0.01 0.00 0.00 Yes Baseline 5.86 5.61 5.56 5.54 5.54 5.50 4.73 Design 5.86 5.62 5.57 5.55 5.55 5.51 4.73 Difference (m) 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.1% No Baseline 5.11 4.67 4.63 4.60 4.59 4.55 3.73 AEP Design 5.11 4.67 4.62 4.61 4.59 4.55 3.73 Difference (m) 0.00 0.00 -0.01 0.01 0.00 0.00 0.00 Yes Baseline 5.93 5.71 5.66 5.64 5.65 5.61 5.30 Design 5.93 5.71 5.66 5.64 5.65 5.61 5.30 Difference (m) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 The analysis of model result shows the new Exceat Bridge has no effect on maximum water level and flood extent. 5.4 Model Performance The model simulations were run in an unsteady state with a 1.5 second 1D timestep and a 3 second 2D timestep (2D domain has 6m grid cell size). The model shows good performance, as shown in Figure 5.1, with no convergence issues.

Baseline 0.5% AEP Design 0.5% AEP

Figure 5.1 Model performance of baseline and design simulations for 0.5% AEP, no climate change

6 Sensitivity Test Sensitivity tests were undertaken to explore a wider and credible range of alternative parameter values. Sensitivity tests are listed in Table 6.1. The sensitivity tests were based on the 1% AEP hydrological event (fluvial: nominal, tidal: 1% AEP) with design scenario. Table 6.1 Sensitivity Tests

Test Description Sensitivity test 1 +20% inflow Sensitivity test 2 -20% inflow Sensitivity test 3 Downstream boundary increased by 0.2m Sensitivity test 4 Downstream boundary decreased by 0.2m Sensitivity test 5 +20% Manning’s n Sensitivity test 6 -20% Manning’s n Based on all model nodes, the effect of increasing the roughness by 20% results in an average increase in peak flood level of 0.025m (maximum 0.030m). Reducing the roughness by 20% shows an average reduction of 0.055m (maximum reduction - 0.07m). Increasing the downstream boundary by 0.20m impacts water levels up to Lullington. ±20% change in inflow has no effect on maximum water level. The nominal flow was applied during the test and it is low flow and 20% of this flow is so small that has no effect on the water level. The result of sensitivity test are presented in Table 6.2, and Figure 6.1 to Figure 6.6. Table 6.2 Comparison of maximum water levels of sensitivity tests

Figure 6.1 Difference in maximum flood depth - Sensitivity test 1

Figure 6.2 Difference in maximum flood depth - Sensitivity test 2

Figure 6.3 Difference in maximum flood depth - Sensitivity test 3

Figure 6.4 Difference in maximum flood depth - Sensitivity test 4

Figure 6.5 Difference in maximum flood depth - Sensitivity test 5

Figure 6.6 Difference in maximum flood depth - Sensitivity test 6

Appendix A: Appendix Title

A.1 Bridges

A.1.1 Bridge A259

A.1.2 Footbridge at Foothpath Alfriston 23A

A.1.3 Footbridge at Cow Lane

A.1.4 Footbridge near Alfriston War Memorial Hall

A.1.5 Bridge at Lullington Road

A.2 Gates

A.2.1 Flood Gate

Appendix B: Pooling Groups

Table B.1 Catchment C1 default pooling group

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 52010 0.224 54 36.21 0.278 0.338 0.491 137.82 866 0.082 0.997 0.011 41.92 (Brue @ Lovington) 39025 0.23 51 17 0.198 0.158 1.13 142.01 789 0.075 0.978 0.02 32.78 (Enborne @ Brimpton) 21027 0.341 32 40.298 0.321 0.268 1.174 155.34 774 0.07 0.997 0.004 36.77 (Blackadder Water @ Mouth Bridge) 76010 0.382 44 33.117 0.225 0.274 0.357 157.58 940 0.077 0.993 0.007 30.52 (Petteril @ Harraby Green) 43018 0.418 44 7.026 0.241 0.126 1.567 170.88 860 0.067 0.979 0.005 8.17 (Allen @ Walford Mill) 15008 0.472 53 26.832 0.132 0.059 1.536 176.61 840 0.127 0.973 0.015 37.21 (Dean Water @ Cookston) 13001 0.485 27 35.577 0.212 0.141 0.656 124.47 890 0.059 0.998 0.002 40.16 (Bervie @ Inverbervie) 54040 0.494 45 4.736 0.246 0.29 1.35 159.87 700 0.112 0.931 0.007 30.18 (Meese @ Tibberton)

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 43004 0.513 45 2.157 0.328 0.31 0.928 165.21 768 0.056 1 0.024 5.94 (Bourne @ Laverstock) 25005 0.514 48 43.54 0.241 0.269 0.607 194.15 726 0.107 0.994 0.014 40.45 (Leven @ Leven Bridge) 42003 0.528 23 27.4 0.276 0.36 0.909 99.87 854 0.107 0.997 0.013 39.18 (Lymington @ Brockenhurst) 205011 0.529 39 40.622 0.144 0.17 1.296 186.21 968 0.104 0.96 0.011 44.65 (Annacloy @ Kilmore Bridge)

Table B.2 Sites removed from Catchment C1 Default Pooling Group

Removed Station Reason for removal 15008 Dean Water is located in Scotland and has a much larger FPEXT compared to Exceat (Dean Water @ Cookston) catchment and has no high flow gaugings since 1988. It overlies a different geology (more permeable catchment) and has the shallowest growth curve, potentially skewing the pooled growth curve to underestimate flows. 54040 There was no record of a reservoir in the catchment and FEH guidance advises that (Meese @ Tibberton) stations with a FARL <0.95 should not be included in pooling groups 204011 Annacloy is located on the Eastern cost of Northern Ireland, not likely to be representative (Annacloy @ Kilmore Bridge) of the target catchment. Annacloy was identified as having potentially spurious L-moments and growth curves, which could lead to peak flows being underestimated.

Table B.3 Catchment C1 Reviewed Pooling Group

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 52010 0.224 54 36.21 0.278 0.338 0.601 137.82 866 0.082 0.997 0.011 41.92 (Brue @ Lovington) 39025 0.23 51 17 0.198 0.158 1.165 142.01 789 0.075 0.978 0.02 32.78 (Enborne @ Brimpton) 21027 0.341 32 40.298 0.321 0.268 1.46 155.34 774 0.07 0.997 0.004 36.77 (Blackadder Water @ Mouth Bridge) 76010 0.382 44 33.117 0.225 0.274 0.782 157.58 940 0.077 0.993 0.007 30.52 (Petteril @ Harraby Green) 43018 0.418 44 7.026 0.241 0.126 1.737 170.88 860 0.067 0.979 0.005 8.17 (Allen @ Walford Mill) 13001 (Bervie @ 0.485 27 35.577 0.212 0.141 0.293 124.47 890 0.059 0.998 0.002 40.16 Inverbervie) 43004 0.513 45 2.157 0.328 0.31 1.276 165.21 768 0.056 1 0.024 5.94 (Bourne @ Laverstock) 25005 0.514 48 43.54 0.241 0.269 1.429 194.15 726 0.107 0.994 0.014 40.45 (Leven @ Leven Bridge) 42003 0.528 23 27.4 0.276 0.36 1.018 99.87 854 0.107 0.997 0.013 39.18 (Lymington @ Brockenhurst) 43012 0.562 49 4.637 0.18 0.049 0.834 114.01 925 0.059 0.975 0.026 11.19 (Wylye @ Norton Bavant)

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 21025 0.576 33 51.665 0.214 0.097 0.829 173.79 926 0.06 0.948 0 46.62 (Ale Water @ Ancrum) 54041 0.588 42 12.444 0.187 0.109 0.578 193.49 719 0.119 0.954 0.015 27.5 (Tern @ Eaton Upon Tern)

Table B.4 Sites removed from Cuckmere Reviewed Pooling Group

Removed Stations Reason for removal 43012 Bypassing occurs at very extreme events and upper limit of rating is 0.41m, extrapolated (Wylye @ Norton Bavant) beyond that limit. Flow is augmented and groundwater abstractions affect flow. Only a few high flows gauged. The station has a significantly different SPRHOST to the target catchment and was removed for the tailored pooling group. 43004 The station has a significantly different SPRHOST to the target catchment and was removed (Bourne @ Laverstock) for the tailored pooling group. 43018 The station has a significantly different SPRHOST to the target catchment and was removed (Allen @ Walford Mill) for the tailored pooling group.

Table B.5 Catchment C1 Tailored Pooing Group

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 52010 0.224 54 36.21 0.278 0.338 0.502 137.82 866 0.082 0.997 0.011 41.92 (Brue @ Lovington) 39025 0.23 51 17 0.198 0.158 1.123 142.01 789 0.075 0.978 0.02 32.78 (Enborne @ Brimpton) 21027 0.341 32 40.298 0.321 0.268 2.113 155.34 774 0.07 0.997 0.004 36.77 (Blackadder Water @ Mouth Bridge) 76010 0.382 44 33.117 0.225 0.274 0.57 157.58 940 0.077 0.993 0.007 30.52 (Petteril @ Harraby Green) 13001 (Bervie @ 0.485 27 35.577 0.212 0.141 0.281 124.47 890 0.059 0.998 0.002 40.16 Inverbervie) 25005 0.514 48 43.54 0.241 0.269 1.534 194.15 726 0.107 0.994 0.014 40.45 (Leven @ Leven Bridge) 42003 0.528 23 27.4 0.276 0.36 1.033 99.87 854 0.107 0.997 0.013 39.18 (Lymington @ Brockenhurst) 21025 0.576 33 51.665 0.214 0.097 0.811 173.79 926 0.06 0.948 0 46.62 (Ale Water @ Ancrum) 54041 0.588 42 12.444 0.187 0.109 1.034 193.49 719 0.119 0.954 0.015 27.5 (Tern @ Eaton Upon Tern)

Table B.6 East Tributary Default Pooling Group

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of AM SKEW 2000 data 25019 0.31 40 5.384 0.343 0.378 0.755 15.09 830 0.019 1 0.004 38.58 (Leven @ Easby) 26802 0.406 19 0.109 0.309 0.183 0.243 15.85 757 0.03 1 0 5.67 (Gypsey Race @ Kirby Grindalythe) 27051 0.575 46 4.539 0.219 0.148 0.407 8.17 855 0.013 1 0.006 40.77 (Crimple @ Burn Bridge) 49005 0.611 8 6.511 0.262 0.049 2.656 16.08 1044 0.023 0.991 0.006 31.92 (Bolingey Stream @ Bolingey Cocks Bridge) 27010 0.729 41 9.42 0.224 0.293 0.386 18.82 987 0.009 1 0.001 50.58 (Hodge Beck @ Bransdale Weir) 44008 0.824 39 0.448 0.411 0.328 1.648 20.18 1012 0.015 1 0.004 19.53 (South Winterbourne @ Winterbourne Steepleton 47022 1.101 25 6.18 0.273 0.149 0.312 13.43 1403 0.023 0.942 0.014 44.18 (Tory Brook @ Newnham Park) 45816 1.11 25 3.456 0.306 0.399 0.638 6.81 1210 0.011 1 0.005 31.27 (Haddeo @ Upton) 25011 1.12 32 15.533 0.235 0.334 1.215 12.79 1463 0.012 1 0.001 58.21 (Langdon Beck @ Langdon)

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of AM SKEW 2000 data 28033 1.139 43 4.205 0.231 0.369 0.815 7.92 1346 0.007 1 0 42.5 (Dove @ Hollinsclough) 36010 1.328 51 7.5 0.372 0.184 1.5 27.58 588 0.045 0.999 0.007 44.57 (Bumpstead Brook @ Broad Green) 27032 1.364 52 3.923 0.207 0.244 0.67 22.25 1433 0.021 0.997 0 57.36 (Hebden Beck @ Hebden) 26014 1.397 20 0.431 0.297 0.127 0.458 32.42 721 0.016 1 0.007 6.51 (Water Forlornes @ Driffield) 206006 1.426 48 15.33 0.189 0.052 2.609 14.44 1704 0.023 0.981 0 51.72 (Annalong @ Recorder) 48009 1.475 12 8.469 0.245 0.373 0.689 22.97 1511 0.023 0.982 0.002 39.93 (st Neot @ Craigshill Wood)

Table B.7 Sites removed from East Tributary Default Pooling Group

Removed Sites Reason for removal 206006 Annalong is located in Northern Ireland, experiencing a different climate to Exceat (Annalong @ Recorder) Catchment. Annalong experiences more rainfall (SAAR of 1704) compared to the East Catchment (SAAR of 819). The station was decommissioned in 1943 and no new data recorded since then. Station has the flattest growth curve, possibly skewing pooled growth curve. 25011 Large period of missing data between 1981-2001. Any flows above 1.5m stage are out of (Langdon Beck @ Langdon) bank and will be estimated. Recent gaugings suggest rating may underestimate peak flows and rating review required. River ice may affect winter rating. 28033 Left bank experiences bank accretion and heave undergrowth. Difficult to gauge higher (Dove @ Hollingsclough) flows suggesting rating underestimates flow, rating revised in 2016.

Table B.8 East Tributary Reviewed Pooling Group

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 25019 0.31 40 5.384 0.343 0.378 0.912 15.09 830 0.019 1 0.004 38.58 (Leven @ Easby) 26802 0.406 19 0.109 0.309 0.183 0.314 15.85 757 0.03 1 0 5.67 (Gypsey Race @ Kirby Grindalythe) 27051 0.575 46 4.539 0.219 0.148 0.478 8.17 855 0.013 1 0.006 40.77 (Crimple @ Burn Bridge) 49005 0.611 8 6.511 0.262 0.049 2.99 16.08 1044 0.023 0.991 0.006 31.92 (Bolingey Stream @ Bolingey Cocks Bridge) 27010 0.729 41 9.42 0.224 0.293 0.462 18.82 987 0.009 1 0.001 50.58 (Hodge Beck @ Bransdale Weir)

Station Distance Years QMED L-CV L- Discordancy AREA SAAR FPEXT FARL URBEXT SPRHOST of data AM SKEW 2000 44008 0.824 39 0.448 0.411 0.328 1.72 20.18 1012 0.015 1 0.004 19.53 (South Winterbourne @ Winterbourne Steepleton 47022 1.101 25 6.18 0.273 0.149 0.279 13.43 1403 0.023 0.942 0.014 44.18 (Tory Brook @ Newnham Park) 45816 1.11 25 3.456 0.306 0.399 1.15 6.81 1210 0.011 1 0.005 31.27 (Haddeo @ Upton) 36010 1.328 51 7.5 0.372 0.184 1.953 27.58 588 0.045 0.999 0.007 44.57 (Bumpstead Brook @ Broad Green) 27032 1.364 52 3.923 0.207 0.244 1.219 22.25 1433 0.021 0.997 0 57.36 (Hebden Beck @ Hebden) 26014 1.397 20 0.431 0.297 0.127 0.497 32.42 721 0.016 1 0.007 6.51 (Water Forlornes @ Driffield) 48009 1.475 12 8.469 0.245 0.373 1.11 22.97 1511 0.023 0.982 0.002 39.93 (st Neot @ Craigshill Wood) 73015 1.539 27 12.33 0.205 0.281 0.612 30.04 1158 0.074 0.976 0.003 35.79 (Keer @ High Keer Weir) 72014 1.552 50 16.465 0.233 0.162 0.446 28.99 1183 0.082 0.975 0.006 35.96 (Conder @ Galgate) 25012 1.594 49 33.265 0.19 0.225 0.859 24.58 1577 0.021 1 0 53.45 (Harwood Beck @ Harwood)

Table B.9 Sites removed from East Tributary Reviewed Pooling Group

Removed Stations Reason for removal 2012 Station was only gauged at low flows until 2013 and current rating underestimates high (Harwood Beck @ Harwood) flows compared to check gaugings. Out of bank flows occur beyond 1m stage. A very impermeable small catchment with peat and Boulder Clay cover. 27010 Catchment in very steep and impermeable, likely to have a different runoff response to the (Hodge Beck @ Bransdale Weir) target catchment. Station affected by sand and gravel accumulation, was also closed in 1979 and no new data collected since then. 27032 An impermeable and steep catchment, catchment also partially karstic so true drainage area (Hebden Beck @ Hebden) unknown. Significant upstream accretion occurs and erosion observed on weir. 25019 Station is largely impermeable, significantly higher SPRHOST than target catchment (Leven @ Easby) 27051 Station is largely impermeable, significantly higher SPRHOST than target catchment (Crimple @ Burn Bridge) 49005 Station is largely impermeable, significantly higher SPRHOST than target catchment (Bolingey Stream @ Bolingey Cocks Bridge) 27010 Station is largely impermeable, significantly higher SPRHOST than target catchment (Hodge Beck @ Bransdale Weir) 47022 Station is largely impermeable, significantly higher SPRHOST than target catchment (Tory Brook @ Newnham Park) 45816 Station is largely impermeable, significantly higher SPRHOST than target catchment (Haddeo @ Upton) 36010 Station is largely impermeable, significantly higher SPRHOST than target catchment (Bumpstead Brook @ Broad Green) 27032 Station is largely impermeable, significantly higher SPRHOST than target catchment (Hebden Beck @ Hebden) 48009 Station is largely impermeable, significantly higher SPRHOST than target catchment (st Neot @ Craigshill Wood) 73015 Station is largely impermeable, significantly higher SPRHOST than target catchment (Keer @ High Keer Weir) 72014 Station is largely impermeable, significantly higher SPRHOST than target catchment (Conder @ Galgate) 25012 Station is largely impermeable, significantly higher SPRHOST than target catchment (Harwood Beck @ Harwood)

Table B.10 East Tributary Tailored Pooling Group

Station Distan Years of QMED L- L- Discorda AR SA FPE FA URBEXT SPRHO ce data AM CV SKE ncy EA AR XT RL 2000 ST W 26802 0.406 19 0.109 0.3 0.183 0.243 15.8 757 0.03 1 0 5.67 (Gypsey Race @ Kirby Grindalythe) 09 5 44008 0.824 39 0.448 0.4 0.328 1.648 20.1 101 0.01 1 0.004 19.53 (South Winterbourne @ 11 8 2 5 Winterbourne Steepleton 26014 1.397 20 0.431 0.2 0.127 0.458 32.4 721 0.01 1 0.007 6.51 (Water Forlornes @ Driffield) 97 2 6 26013 2.23 8 2.78 0.2 0.218 6.054 53.3 690 0.09 0.99 0.006 17.61 (Driffield Trout Stream @ Driffield) 9 3 3 7 33054 2.255 42 1.132 0.2 0.08 0.261 48.5 686 0.11 0.94 0.005 9.74 (Babingley @ Castle Rising) 01 3 8 4 27095 2.371 18 8.73 0.3 0.305 0.601 66.2 837 0.02 1 0.003 20.02 (Pickering Beck @ Pickering) 01 6 33032 2.396 50 0.442 0.3 0.124 0.478 56.1 688 0.11 0.98 0.006 6.01 (Heacham @ Heacham) 04 6 6 3 26003 2.423 57 1.76 0.2 -0.009 0.714 59.5 698 0.10 0.98 0.004 10.43 (Foston Beck @ Foston Mill) 48 9 6 7 27073 2.455 37 0.82 0.2 0.047 0.538 8.06 721 0.23 1 0.008 17.77 (Brompton Beck @ Snainton Ings) 7 42008 2.543 47 1.36 0.2 0.409 1.361 74.3 885 0.04 0.99 0.009 6.89 (Cheriton Stream @ Sewards 51 4 5 Bridge) 34012 2.85 52 1.038 0.2 -0.148 2.052 83.8 668 0.09 0.99 0.005 6.29 (Burn @ Burnham Overy) 15 7 8 7 46013 2.916 14 24.8 0.2 0.245 0.583 86.9 135 0.02 0.99 0.004 13.8 (Bovey @ Bovey Parke) 19 5 6 2 7

Appendix C: Flood Estimation Calculation proforma

Appendix D: Joint Probability Combinations

1 in 2 (50% AEP) 1 in 5 (20% AEP)