Bridgwater Tidal Barrier Appraisal Hydraulic Modelling

REPORT

Prepared for Environment Agency

November 2019

Rev: P03

Ref: ENVIMSW002039-CH2-FEV-SW-RP-HY-00105

CH2M HILL Burderop Park Swindon SN4 0QD

Contents

Section Page Acronyms and Abbreviations ...... ix Background ...... 1 1.1 Project need ...... 1 1.2 Purpose of report ...... 2 1.3 Objectives ...... 3 1.4 Scope ...... 3 Approach ...... 5 2.1 Existing models ...... 6 2.2 Model updates ...... 8 2.2.1 Downstream extension ...... 8 2.2.2 Improvement works...... 9 2.2.3 2D Floodplain ...... 10 2.3 Model verification ...... 13 2.4 Model sensitivity ...... 13 Verification of model revisions ...... 14 3.1 Winter 2013-14 tidal event ...... 14 3.1.1 Boundary conditions ...... 14 3.1.2 Results ...... 14 3.1.3 Discussion ...... 20 3.2 Spring tide 2015 ...... 26 3.2.1 Boundary conditions ...... 26 3.2.2 Results ...... 27 3.2.3 Discussion ...... 27 3.3 Summary ...... 30 Design model boundary conditions ...... 32 4.1 Fluvial flow ...... 32 4.1.1 River Parrett upstream of Bridgwater ...... 32 4.1.2 River Parrett downstream of Bridgwater ...... 34 4.1.3 Model fluvial boundaries ...... 34 4.2 Tidal water level ...... 36 4.2.1 Model tidal boundary ...... 36 4.2.2 Model tide curves ...... 37 Model simulations ...... 42 5.1 Baseline flood risk ...... 42 5.1.1 Simulations ...... 42 5.1.2 Results ...... 42 5.1.3 Sensitivity test results ...... 49 5.2 Flood damages ...... 54 5.2.1 Simulations ...... 54 5.2.2 Results ...... 56 5.3 Barrier closure ...... 65 5.3.1 Closure rules ...... 67 5.3.2 Simulations ...... 68 5.3.3 Results ...... 70 5.4 King's Sedgemoor Drain operation ...... 80 5.5 Fluvial flood management ...... 80

iii

5.5.1 Simulations ...... 80 5.5.2 Results ...... 80 5.6 Barrier failure ...... 84 5.6.1 Simulations ...... 84 5.6.2 Results ...... 85 5.7 Adaptation Pathways ...... 87

Appendices Appendix A Barrier Closure Rules

Tables Table 1. Model 2D Domain Roughness Values Table 2. Extreme tide level estimates at km334 in 2008 (Coastal flood boundary conditions 2011) Table 3. Allowances applied in model boundary tide curves for increase in relative sea level between 2008 and future epochs Table 4. Maximum tidal boundary water levels for baseline model simulations Table 5. Modelled water levels in Bridgwater (2024 and 2055) Table 6. Approximate variations in overbank flow in model sensitivity tests Table 7. Approximate variations in overbank flow in model sensitivity tests Table 8. Maximum tidal boundary water levels for flood damage model simulations Table 9. Design simulation scenarios for barrier location No. 5 Table 10. Maximum tidal boundary water levels for barrier design model simulations Table 11. Effect of barrier closure at location No. 5 on discharges from King's Sedgemoor Drain Table 12. Effect of barrier closure on flow in River Parrett at Northmoor Pump Station (locations No. 4 and 5) Table 13. Effect of barrier closure on discharge from Northmoor Pump Station (locations No. 4 and 5)

Figures Figure 1 Location of Bridgwater, River Parrett and the project area Figure 2 Summary of approach to model revisions Figure 3 SLM model schematisation downstream of the M5 motorway Figure 4 Schematisation of downstream model extension Figure 5 Location of Cannington Bends Flood Defence Scheme Figure 6 Location of Northmoor Pump Station to Linden Farm Dredging Figure 7 Extent of 2D model domain Figure 8 Water level gauges in the Parrett estuary Figure 9 Model water levels and gauge data – Winter 2013-14 tidal event (Dunball)

Figure 10 Model water levels and gauge data – Winter 2013-14 tidal event (West Quay) Figure 11 Model water levels and gauge data – Winter 2013-14 tidal event (Northmoor) Figure 12 Model water levels and gauge data – Winter 2013-14 tidal event (Saltmoor) Figure 13 Model water levels and gauge data – 3 January 2014 (West Quay) Figure 14 Model water levels for sensitivity tests (timestep and boundary interpolation) – 3 January 2014 (West Quay) Figure 15 Model high water levels and gauge high water levels at West Quay (1 December 2013 to 31 January 2014) Figure 16 Model water level and gauge data – 18 December 2013 (West Quay) Figure 17 Model water level and gauge data – 18 December 2013 (Dunball) Figure 18 Model water levels and gauge data – 3 January 2014 (West Quay) – effect of fluvial flow Figure 19 Model water levels and gauge data – 3 January 2014 (West Quay) – effect of channel roughness Figure 20 Model water surface profiles downstream of West Quay at low water Figure 21 River Parrett channel downstream of West Quay gauge Figure 22 Model water levels for sensitivity tests (timestep and boundary interpolation) – 3 January 2014 (West Quay) Figure 23 Model water levels and gauge data – Spring tide 2015 (Dunball) Figure 24 Model water levels and gauge data – Spring tide 2015 (West Quay) Figure 25 Model water levels and gauge data – Spring tide 2015 (Northmoor) Figure 26 Model water levels and gauge data – Spring tide 2015 (Saltmoor) Figure 27 Model high water levels and gauge high water levels at West Quay (Winter 2013-14 and Sprig tides 2015) Figure 28 Main features of the lower River Parrett catchment Figure 29 Maximum fluvial flow in the River Parrett upstream of Bridgwater (SLM Design Model) Figure 30 Steady state flow distribution in upper bound fluvial flow model configuration (low water in Bridgwater Bay) Figure 31 Location of tide gauges and Coastal Flood Boundary extreme tide level points Figure 32 Coastal Flood Boundary extreme tide levels in Bridgwater Bay and gauge records December 2013-February 2014 Figure 33 Derivation of 0.5% AEP tide curve in 2008 using Coastal Flood Boundary surge shape Figure 34 Comparison of 0.5% AEP tide curves in 2008 derived using Coastal Flood Boundary and 3 January 2014 surge shapes Figure 35 Comparison of 0.5% AEP tide curves in 2008 and 2125 based on UKCP09 Medium Emission 95th percentile projections Figure 36 Baseline simulation maximum water surface profiles (2024) Figure 37 Baseline simulation maximum water surface profiles (2055)

Figure 38 Baseline simulation maximum water levels in Bridgwater (2024 and 2055) Figure 39 Baseline simulation flood extents in Bridgwater for 50% and 20% AEP (2024 and 2055) Figure 40 Baseline simulation flood extents in Bridgwater for 5% and 2% AEP (2024 and 2055) Figure 41 Baseline simulation flood extents in Bridgwater for 1% and 0.5% AEP (2024 and 2055) Figure 42 Baseline simulation flood extents in Bridgwater at Blake Gardens Figure 43 Baseline simulation flood extents in northern Bridgwater Figure 44 Baseline simulation maximum water surface profiles (20% AEP, 2024) – effect of surge shape Figure 45 Baseline simulation water level at West Quay and tide curves (20% AEP, 2024) – effect of surge shape Figure 46 Baseline simulation flood extent downstream of Bridgwater (20% AEP, 2024) – effect of surge shape Figure 47 Location of sample bank sections for assessing effective weir discharge coefficients in 1D-2D model link line Figure 48 Model results for river water level and flow over banks at sample locations and weir flow equation values Figure 49 Baseline simulation maximum water surface profiles (0.5% AEP, 2015) – effect of variations to overbank flow Figure 50 Baseline simulation maximum flood extent (0.5% AEP, 2015) – effect of variations to overbank flow Figure 51 Location of breaches for flood damage assessment simulations Figure 52 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2024) Figure 53 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2055) Figure 54 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2085) Figure 55 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2125) Figure 56 Flood damage scenario simulations and Baseline simulation maximum water levels in Bridgwater (2024) Figure 57 Flood damage scenario simulations and Baseline simulation maximum water levels in Bridgwater (2055) Figure 58 Scenario 1 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125) Figure 59 Scenario 2 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125) Figure 60 Scenario 3 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125) Figure 61 Flood damages simulation flood extents with breaches for 0.5% AEP tide (2024) : Scenarios 1, 2 and 3 Figure 62 Initial barrier locations Figure 63 Downstream defences with design crest level (excluding settlement allowance)

Figure 64 Location of modelled raised and new flood banks downstream of barrier location No. 5 and Pawlett Hams breach Figure 65 Timing of barrier closure and water level at the barrier for initial closure simulation (0.5% AEP tide, 2015) Figure 66 Maximum water level profile for initial barrier closure simulations (100% AEP tide, 2015) Figure 67 Maximum water level profile for initial barrier closure simulations (0.5% AEP tide, 2015) Figure 68 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 1 (2055) Figure 69 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024) Figure 70 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2055) Figure 71 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 3 (2125) Figure 72 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024) 20% AEP – effect of surge shape Figure 73 Barrier location No. 5 design simulation – effect of barrier closure time on water level at West Quay gauge (0.5% AEP, 2024 tide) Figure 74 Barrier location No. 5 design simulation – effect of barrier closure time on water level at Burrowbridge (0.5% AEP, 2024 tide) Figure 75 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024) 0.5% AEP – effect of barrier closure time on upstream water levels Figure 76 Barrier location No. 5 design simulation – effect of barrier closure on downstream water level and discharge from the King's Sedgemoor Drain (20% AEP, 2024 tide) Figure 77 Barrier location No. 5 design simulation – effect of barrier closure on downstream water level and discharge from the King's Sedgemoor Drain (0.5% AEP, 2024 tide) Figure 78 Scenario 2 barrier design simulation flood extents for 20% AEP (2024 and 2055) Figure 79 Scenario 2 barrier design simulation flood extents for 2% and 0.5% AEP (2024 and 2055) Figure 80 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (6.9mODN offshore tide) Figure 81 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (7.1mODN offshore tide) Figure 82 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (7.4mODN offshore tide) Figure 83 Correlation between simulated offshore tide levels and minimum flow in the River Parrett at barrier locations No. 4 and No. 5 for upper bound fluvial flow conditions Figure 84 Water levels at West Quay and Burrowbridge for barrier failing closed at location No. 5 (upper bound fluvial flow) and locations No. 4 and No. 5 (extreme flow) Figure 85 Barrier failed closed at location No. 5 simulation maximum water surface profiles (upper bound fluvial flow)

vii

Figure 86 Barrier failed closed at locations No. 4 and No. 5 simulation maximum water surface profiles (extreme fluvial flow) Figure 87 “Future banks 1” scenario Figure 88 “Future banks 2” scenario Figure 89 “Future banks 1” scenario flood extent Figure 90 “Future banks 2” scenario flood extent

Acronyms and Abbreviations

1D One Dimensional 2D Two Dimensional AEP Annual Exceedance Probability CFB Coastal flood boundary conditions for UK mainland and islands” (Environment Agency, 2011) DTM Digital Terrain Model HAT Highest Astronomical Tide KSD King’s Sedgemoor Drain MHWN Mean High Water Neaps MHWS Mean High Water Springs TWAO Transport and Works Act Order

Background 1.1 Project need Flooding driven by high tides is a problem faced by communities and infrastructure throughout the United Kingdom. However, the relatively low and flat topography of the Somerset Levels exacerbates this problem, allowing water to widely disperse throughout adjacent areas. Bridgwater, located on the River Parrett, is the only major river in Somerset that does not have a sluice or other structure to exclude high tides. Figure 1 shows the location of the project area.

PROJECT AREA

Figure 1 Location of Bridgwater, River Parrett and the project area (Crown copyright and database rights 2016 Ordnance Survey)

The direct threat posed by tidal flooding close to Bridgwater has been underlined by recent events, including: • the unexpected collapse of a major section of quay and tidal flood wall in Bridgwater in November 2011; and • tidal flooding causing the overtopping of rural flood banks on the River Parrett and near overtopping of flood defences in Bridgwater on 3 January 2014.

The failure or overtopping of flood defences within Bridgwater could lead to the rapid inundation of large areas and could result in severe damage and disruption to both local people and the wider population using transport infrastructure. Approximately 10,000 homes and 600 businesses in Bridgwater and villages in the lower Parrett catchment are protected from tidal flooding by existing flood defences. Further residential and commercial development is proposed in areas at risk of flooding in the coming years, but only on the basis of the delivery of an appropriate strategic flood defence solution that includes the barrier solution. A technical review of the flood defences on the River Parrett (Black & Veatch, 2006) identified the three primary influences of tidal behaviour on flood risk in the River Parrett as: a) High surge tide levels which present a direct flood risk through overtopping or breaching of defences; b) At high tide, tidal waters occupy a volume of the river channel, which may otherwise be available to receive fluvial flood water; and c) Silt is carried upstream and deposited on the tide resulting in a restricted channel area and reducing its fluvial capacity. This also reduces the extent to which the tidal wave will propagate upstream of Bridgwater. The Sedgemoor District Council Level 2 Strategic Flood Risk Assessment (Scott Wilson, 2009) highlighted a preference for a barrier defence accounting for economic and environmental considerations. The Parrett Estuary Flood Risk Management Strategy (Environment Agency, 2010) recommended that the Environment Agency ‘need to carry out major works, including a Bridgwater tidal surge barrier in 2046, to sustain standards of protection’. A further Flood Risk Management Review report (Black & Veatch, 2014), following extensive flooding of the Somerset Levels and Moors in 2012-2014, ‘confirmed the need for a major intervention (in flood risk management) and continued to support the option of a tidal surge barrier’. This review highlighted the fact that, since 2010, several key events had demonstrated that a barrier is required sooner than originally planned. The Somerset Levels and Moors Flood Action Plan (Defra, 2014) was prepared in 2014 to establish a 20-year plan for the sustainable management of the Somerset Levels and Moors. The work identified the need for a tidal barrier or sluice on the River Parrett, and stated a target date for barrier completion by 2024. A Strategic Outline Case (SOC) was approved by LPRG in December 2015 to undertake detailed appraisal and outline design of a tidal surge barrier. The outcome of the appraisal and design process will be the selection of a preferred option for a tidal surge barrier and the submission of an Outline Business Case (OBC) and Full Business Case (FBC) for the scheme. A critical part of the work will be obtaining a Transport and Works Act Order (TWAO) necessary due to the interference with navigation rights on the River Parrett. The completion of the TWAO will include a robust stakeholder and public consultation to ensure that relevant data has been collected and concerns have been addressed. In order to support the detailed appraisal and outline design of the barrier, hydraulic modelling is required to provide design parameters, determine the benefits provided by the barrier and assess the impacts of the barrier on flows, water levels and sediment processes in the Parrett. 1.2 Purpose of report This report documents the use of a hydraulic model to support the appraisal and outline design of the barrier. Results of the model simulations undertaken for the appraisal are presented. The report includes details of the assumptions made and the limitations of the assessment.

1.3 Objectives The objectives of the hydraulic modelling are: 1. Provide estimates of flood levels, extents and depths along the River Parrett in Bridgwater and along the Parrett Estuary for a range of annual exceedance probabilities (AEPs) under both present-day conditions and future conditions, taking into account expected changes in relative sea level, to enable detailed flood damage calculations,to provide data for the selection of the tidal barrier location and the design of the tidal barrier and flood defences associated with the barrier. 2. Determine the impact of the operation of the tidal barrier on flows, water levels and flood extents in the River Parrett, in the floodplain downstream of Bridgwater and in the Levels and Moors upstream of Bridgwater for different barrier locations and for a range of annual exceedance probabilities (AEPs) under both present-day conditions and future conditions and, if required, test the effectiveness of mitigation measures. 3. Provide data for the assessment of the frequency of barrier closures under both present day and future conditions. 4. Provide data for a preliminary broad-scale assessment of the impact of the operation of the barrier on sediment transport in the River Parrett using regime theory. 1.4 Scope 1. Revise the current ‘Somerset Levels and Moors’ (SLM) one dimensional (1D) Flood Modeller model, which includes the Rivers Parrett and Tone, for the purposes of the project: - extend the model at the downstream boundary into Bridgwater Bay in order to improve the representation of tidal flows into and out of the Parrett estuary and to enable the direct application of design sea levels and surge from the National Coastal Flood Boundary data set to the model tidal boundary; - replace the 1D floodplain representation through Bridgwater with a detailed two- dimensional (2D) representation to provide a more accurate simulation of flooding, detailed flood levels for damage calculations and to enable improved visualization of the flood mechanism and impacts of barrier operation for consultation purposes; - review and improve, if necessary, the representation of overbank flow; - verify the original model calibration of simulation of tide propagation in the River Parrett against the January 2014 combined surge tide and high fluvial flow event; - update the model to include relevant physical changes since the development of the SLM model. 2. Derive tidal boundary conditions: - generate design tidal water level boundaries for a range of AEPs under present day and future conditions based on current guidance and methods. 3. Consider joint probability of tidal and fluvial events: - review correlation of tidal and fluvial events and select and test appropriate combinations of joint probabilities for barrier operation simulations, taking account of the long duration of fluvial flows and ‘capping’ of flows through Bridgwater through overflow upstream. 4. Produce a new 1D sediment model of the River Parrett channel - derive a simplified 1D model of the tidal Parrett from the SLM 1D model; - if available, use surveys of channel cross-sections at different dates to derive approximate rates of deposition and erosion and, if possible, ‘reality check’ the 1D sediment model

through a long term simulation (ideally several years) of sediment processes and comparison with results of cross-section survey comparison. 5. Perform simulations to support the appraisal and outline design of the barrier.

Approach

The hydraulic model for the appraisal of the tidal barrier is based on the calibrated ‘Somerset Levels and Moors’ (SLM) 1D Flood Modeller model developed by CH2M HILL in 2016 (Somerset Levels and Moors – Modelling, 2016). This model is an updated and extended version of the Somerset Levels and Moors model produced by Black & Veatch (Somerset Levels Modelling, October 2014). In order to undertake more detailed investigations of the operation of the tidal barrier and provide up to date estimates of flood levels and extents in Bridgwater and the lower Parrett estuary, where tidal flooding is dominant, revisions and further updates to the SLM model have been made, resulting in new ‘barrier model’ versions. Where possible, the changes made to the original model have been verified against gauge data from actual tidal events in the River Parrett estuary. The approach to the model revisions is summarised in Figure 2 below.

Figure 2 Summary of approach to model revisions

The revised Barrier Design Conditions Model has been prepared to provide estimates of flood levels, flows and extents under conditions expected at the time of commissioning of the barrier in 2024. The model includes known improvement works that will have been completed or are proposed to be complete by 2024. The model provides simulations both with and without the barrier and any associated proposals such as bank raising, to assess the impacts of barrier operation on flooding. The without barrier model provides baseline conditions against which the proposals can be assessed.

2.1 Existing models The previous SLM model was developed primarily to produce up to date flood maps and related products for the levels and moors upstream of Bridgwater, where fluvial flooding is dominant, and to test proposals for improvements to flood management in the same area. The updated model for appraisal of the barrier is based on the two principal versions of the SLM 1D model developed by CH2M HILL in 2016: a) SLM 2014 Model This model represents the state of the Parrett and Tone catchment during the period of the 2013-14 winter flood event. The model has been extensively calibrated against gauge data for this event. The flood event includes a spring tide and storm surge on 3 January 2014 during a prolonged period of heavy rainfall and high fluvial flows which resulted in the highest recorded water levels along the Parrett estuary in recent years. The water level recorded at the Hinkley Point tide gauge for the January tidal event corresponds to approximately a 10% AEP. b) SLM Design Model This model represents the state of the Parrett and Tone catchment in 2015, following completion of improvements made to the system as a result of the 2013-14 flood, including dredging of 8Km of channel in the Parrett and Tone rivers. The model has not previously been calibrated. It should be noted that channel roughness values for the dredged reach are the same as in the calibrated 2014 Model since these are considered appropriate mature “design” values. Actual roughness values following the dredging are likely to be lower but would be expected to revert to similar values as in 2014 in the absence of regular maintenance dredging. For the purposes of the “barrier design” model, the calibrated 2014 or “design” roughness values have been retained. In both models, the floodplain areas are represented in 1D using model ‘reservoir’ units which specify the storage volume-depth relationship for discrete compartments of the floodplain. The ‘reservoir’ units are connected to the river channel using model ‘spill’ units which calculate the transfer of flow between the river channel and floodplain using a weir flow equation based on the bank level, river and floodplain water level and a discharge coefficient. Figure 3 shows the outlines of the model floodplain reservoir units in Bridgwater and along the Parrett estuary. While this approach is appropriate for the largely rural areas of the levels and moors upstream of Bridgwater, the representation of the urban area of Bridgwater by a small number of large reservoir units does not allow the flood flow paths from the river banks to the lower lying areas to be accurately simulated and results in a simplistic definition of flood level and extent in this area. The downstream boundary of the SLM models is at Stert Point, where the River Parrett enters Bridgwater Bay. For the SLM model simulations, the tidal water level boundary at this point is based on gauge data (or design water levels) at the Hinkley Point tide gauge to which a simple approximate transformation of the high water level is applied to allow for the difference in location. Figure 3 shows the locations of the model boundary and the tide gauge. This approach to boundary condition is appropriate for simulating flooding upstream of Bridgwater for which the accuracy of the tidal water levels in the Parrett estuary is less important in simulating flooding from large fluvial flows. However, this method does not include the change in shape of the tidal hydrograph between the coastal boundary in Bridgwater Bay and the mouth of the River Parrett or the difference in low water levels in the higher river channel and the deeper coastal water. These factors can both be important in determining water levels in the lower Parrett estuary.

KEY

SLM 1D Model Reservoir Unit

River Parrett Channel

SLM Model Downstream Boundary

Hinkley Point Tide Gauge

M5 Motorway

Figure 3 SLM model schematisation downstream of the M5 motorway

2.2 Model updates Three principal updates have been made to the SLM 1D model for the barrier appraisal study. i) Downstream extension to include a simple 1D representation of Bridgwater Bay. ii) Inclusion of improvement works as at time of barrier commissioning. iii) Floodplain downstream of the M5 motorway represented as a 2D TUFLOW domain. 2.2.1 Downstream extension The model has been extended into Bridgwater Bay using 1D channel cross-sections derived from a Digital Terrain Model assembled from bathymetry data (Steart Managed Realignment project, 2011) and LiDAR data (2013 and 2015). Figure 4 shows the model schematisation.

KEY

DTM

1D Model Cross-section

Estimated Local Flow Direction

Original 1D Model Boundary

Extended 1D Model Boundary

Figure 4 Schematisation of downstream model extension

2.2.2 Improvement works The following improvement works have been included in the ‘design conditions’ model: i) Cannington Bends Flood Defence Scheme The works comprise improvements (level raising) to flood embankments along the left bank of the River Parrett downstream of Bridgwater as detailed in drawing numbers 122507-BVL- FEV-SW-IC-C-00005, 122507-BVL-Z0-XX-ID-C-00009 and 122507-BVL-Z0-XX-ID-C-00010. Figure 5 shows the location of the works.

Cannington Bends scheme in 5 sections

Figure 5 Location of Cannington Bends Flood Defence Scheme ii) Northmoor Pump Station to Linden Farm The works comprise dredging of approximately 700m of the River Parrett channel between Northmoor Pump Station and Linden Farm as detailed in drawing numbers: 190596-BVL-Z0-SW-DR-C-10001.P03-S4; 190596-BVL-Z0-SW-DR-C-10002.P04-S5; 190596-BVL-Z0-SW-DR-C-20001.P03-S4; 190596-BVL-Z0-SW-DR-C-20002.P03-S4; 190596-BVL-Z0-SW-DR-C-20003.P03-S4; 190596-BVL-Z0-SW-DR-C-20004.P03-S4; 190596-BVL-Z0-SW-DR-C-20005.P03-S4; 190596-BVL-Z0-SW-DR-C-20006.P03-S4. Figure 6 shows the location of the works from an extract of drawing number 190596-BVL-Z0- SW-DR-C-10001.P03-S4.

Figure 6 Location of Northmoor Pump Station to Linden Farm Dredging

2.2.3 2D Floodplain In order to improve the accuracy of the simulation of flood flows in Bridgwater and along the Parrett floodplain in the area influenced by the barrier, this part of the floodplain has been represented as a 2D TUFLOW domain linked to the 1D Flood Modeller river channel. The 2D domain cell size is 10m which provides adequate resolution given the nature of the floodplain. The extent of the 2D domain is shown in Figure 7. The domain is bounded to the north by the sea wall along Bridgwater Bay, the Bristol channel and the left bank of the River Brue. To the west the domain is bounded by the 10mODN ground level contour. This is above the highest tide level to be considered in the appraisal and therefore lies above the maximum water levels expected in the model. To the east the domain is bounded by the M5 motorway embankment. The road embankment is typically at least 2m above the adjacent floodplain, generally limiting the landward progression of flooding. Apart from the Parrett, the other principal flow path under the M5 within the model boundary is the King’s Sedgemoor Drain, which is represented in the 1D model domain. To the north of Bridgwater, the Huntspill River and River Brue also allow flow to pass under the M5. These flow

routes are not included in the model. The Huntspill River terminates approximately 4km inland of the M5 at the Gold Corner pumping station and provides limited storage volume above its normal penning level. The River Brue lies outside the model boundary and any overtopping of overland flow into the river channel is ignored in the model.

KEY

LiDAR ground Levels to 10mODN

Boundary of 2D Domain

Figure 7 Extent of 2D model domain

Drainage outfalls into the River Parrett have not been included in the model on the basis that the return flows to the river following any overtopping of defences will be small relative to the overtopping volume and will be subject to tide locking. Ground levels in the 2D domain are based on collated filtered LiDAR data, resampled to a 5m horizontal resolution grid. Roughness values in the 2D domain are assigned according to Mastermap land use classifications as shown in Table 1.

Table 1. Model 2D Domain Roughness Values Mastermap Description Manning’s Feature Roughness Code Coefficient

10021 Buildings Building 1

10053 Land General Surface and entire model default 0.05

10054 Land General Surface 0.05

10056 Land General Surface 0.05

10062 Buildings Glasshouse 0.5

10089 Water Inland Water 0.02

10096 Land Landform 0.05

10111 Land Natural Environment trees , shrubs 0.05

10119 Roads Tracks And Path 0.02 Paths 10123 Roads Tracks And Path 0.02 Paths 10167 Rail Rail 0.03

10172 Roads Tracks And Road Or Track 0.02 Paths 10183 Roads Tracks And Roadside 0.02 Paths 10185 Structures Structure 0.05

10217 Land Unclassified 0.045

The link between the 1D river channel and 2D floodplain domains is specified by an ‘HX’ line along the left and right bank of the river channel. The level of this line represents the river bank level. The banks along the Parrett downstream of the M5 are all raised flood defences – either walls or embankments. The level of the banks on the HX line are defined from the following defence level surveys: i) Burrowbridge to Bridgwater Flood Embankment Survey undertaken in October 2012 by Lewis Brown; ii) Pims Clyse to Combwich Left Bank Flood Embankment Survey undertaken in August 2014 by Longdin & Browning; iii) Bridgwater to Combwich Left Bank Flood Embankment Survey undertaken in April 2010 by 40Severn Ltd; iv) Flood Embankment Survey Undertaken in 2006 by Halcrow. In two locations on the left bank of the river in Bridgwater (between The Clink bridge and the marina and upstream of Broadway Bridge, opposite Salmon Parade), the surveys do not include the level of flood walls along the bank of the river. The surveyed levels have been adjusted to typical levels in the channel cross-section survey. The northern (downstream) boundary of the 2D domain is connected to the river cross-sections in Bridgwater Bay (up to the downstream side of the Highbridge Tidal Exclusion Sluice) in order to include flooding from overtopping of the sea defences. For sections of the sea wall which are not included in the above crest level surveys, the crest levels have been obtained from LiDAR data. LiDAR

data has also been used to modify the bank levels at the breach in the embankment formed for the Steart Managed Realignment project (the defence surveys predate the breach). 2.3 Model verification As indicated in Figure 2, verification checks have been made to confirm the original model calibration has been maintained during the development of the revised barrier models. 1. Check using winter 2013-14 tidal event to confirm the adequacy of the downstream model extension and adequacy of model River Parrett water levels in Bridgwater in the base model. The check has been performed using the SLM 2014 1D model with the inclusion of the downstream model extension and tide data from Hinkley Point gauge for the downstream boundary with no transformation. 2. Check using a spring tide event in 2015 to confirm the adequacy of the downstream model extension and the overall reliability of model River Parrett water levels in Bridgwater for conditions following improvements made after the winter 2013-14 flood. The check has been performed using the SLM Design 1D model with the inclusion of the downstream model extension and tide data from Hinkley Point gauge for the downstream boundary with no transformation. For this second check a period of lower fluvial flow and high (spring) tides has been selected. Hydrological flow inputs for the entire Parrett and Tone catchment have not been derived for this check and only model base flows have been used. Additionally, there has been no attempt to calibrate roughness values in the newly dredged sections of the Parrett and Tone. This version of the model has been carried forward to the “barrier design” version for which the “2014” calibrated mature channel roughness values are considered more appropriate. Although this means that model water levels upstream of Bridgwater are not calibrated, the main purpose of the check is the performance of the model in terms of high water levels in Bridgwater for conditions relevant to the proposed barrier (i.e. for high tides, larger than MHWS). The effect of the fluvial flow and roughness in the dredged reach on the high water level in Bridgwater under these conditions is considered to be small. 2.4 Model sensitivity The sensitivity of the model results to variations in the following parameters has been tested: i) Channel roughness coefficient; ii) Fluvial flow; iii) Downstream tidal boundary level and interpolation; iv) Model timestep; v) Overbank discharge rate.

Verification of model revisions 3.1 Winter 2013-14 tidal event The effect of the downstream extension of the model tidal boundary on the overall model calibration has been verified through simulation of the period 1 December 2013 to 31 January 2014 with the extended SLM 2014 model. The SLM 2014 model was originally calibrated against this event, which includes the largest tidal event recorded in recent years and periods of high fluvial flow. 3.1.1 Boundary conditions As shown in Figure 4, the downstream boundary of the 1D model of the River Parrett is located at the mouth of Bridgwater Bay. The most downstream cross-section in the 1D river model spans Bridgwater Bay from Hinkley Point to Brean Down. For the model verification simulations, the recorded tidal water level series at the Hinkley Point gauge has been applied as the downstream model boundary condition since this gauge provides the most realistic definition of the overall actual tidal water level variations in Bridgwater Bay. In the 1D model the water level across each cross- section is constant. Available gauge data at Hinkley Point and Brean Cross (inland from Brean Down) suggests that the high water level across Bridgwater Bay tends to increase between Hinkley Point and Brean Down. The average high water level across the mouth of the Bay can be around 0.10m to 0.15m higher than that at Hinkley Point. The effect of this level of uncertainty in the downstream boundary condition on water levels in Bridgwater has been assessed through sensitivity tests. Fluvial flow boundaries to the model are the same as for the original SLM 2014 model. 3.1.2 Results Figure 8 shows the location of the water level gauges in the Parrett estuary. Figures 9 to 12 compare the revised model water level results, the original SLM model results and the gauge data for two periods within the simulation: i) 4 to 7 December 2013 : low fluvial flow and high spring tides (6.65mODN at Hinkley Point); ii) 2 to 5 January 2014 : high fluvial flow, high spring tides and storm surge (7.45mODN at Hinkley Point).

Figure 8 Water level gauges in the Parrett estuary

DUNBALL 8

WATER WATER LEVEL (mODN) 3

0 04/12/2013 05/12/2013 06/12/2013 07/12/2013 DATE

Gauge SLM Model Revised Model

DUNBALL 8

WATER WATER LEVEL (mODN) 3

0 02/01/2014 03/01/2014 04/01/2014 05/01/2014 DATE

Gauge SLM Model Revised Model

Figure 9 Model water levels and gauge data – Winter 2013-14 tidal event (Dunball)

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 04/12/2013 05/12/2013 06/12/2013 07/12/2013 DATE

Gauge SLM Model Revised Model

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 02/01/2014 03/01/2014 04/01/2014 05/01/2014 DATE

Gauge SLM Model Revised Model

Figure 10 Model water levels and gauge data – Winter 2013-14 tidal event (West Quay)

NORTHMOOR 9

WATER WATER LEVEL (mODN) 4

1 04/12/2013 05/12/2013 06/12/2013 07/12/2013 DATE

Gauge SLM Model Revised Model

NORTHMOOR 9

WATER WATER LEVEL (mODN) 4

1 02/01/2014 03/01/2014 04/01/2014 05/01/2014 DATE

Gauge SLM Model Revised Model

Figure 11 Model water levels and gauge data – Winter 2013-14 tidal event (Northmoor)

SALTMOOR 9

WATER WATER LEVEL (mODN) 4

1 04/12/2013 05/12/2013 06/12/2013 07/12/2013 DATE

Gauge SLM Model Revised Model

SALTMOOR 9

WATER WATER LEVEL (mODN) 4

1 02/01/2014 03/01/2014 04/01/2014 05/01/2014 DATE

Gauge SLM Model Revised Model

Figure 12 Model water levels and gauge data – Winter 2013-14 tidal event (Saltmoor)

3.1.3 Discussion 1) For lower tides (e.g. 4 to 7 December 2013) the differences between the revised model and the original model results at all gauge locations are negligible and the model calibration is unchanged. 2) For the highest tides in the simulation (3 January 2014) the behavior of the revised model water levels results corresponds more closely to the recorded water levels than the original model water levels and the model calibration is improved. Figure 13 shows the model water levels (at 5 minute intervals) and gauge water levels (at 15 minute intervals) at the West Quay gauge in Bridgwater for the two highest tides (on 3 January 2014). The gauge data shows significant oscillations in the water level at the time of high water. The same effect occurs at the other gauges to a lesser degree. The revised model results show a similar effect at all the gauges which does not occur in the original model. The maximum model water level at the West Quay gauge in Bridgwater is within 0.08m of the recorded level.

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 03/01/2014 04/01/2014 DATE

Gauge SLM Model Revised Model

Figure 13 Model water levels and gauge data – 3 January 2014 (West Quay)

Oscillations in model water levels can sometimes be the result of instabilities resulting from large volumes of overtopping (flow leaving the 1D model) when a large timestep is adopted in the model or due to the method of interpolating water levels at the model boundary. For the model verification simulations an adaptive time stepping solution is adopted, in which the computational timestep can vary between 4.7s and 300s. Water levels at the boundary are specified at 15 minute intervals (from the gauge data) and intermediate water levels are interpolated linearly between the specified values. Sensitivity tests have been performed to assess the effect on water levels of using smaller, fixed, computational timesteps and of using a cubic spline interpolation method for the downstream boundary water levels. Figure 14 shows the results of these sensitivity tests at West Quay for the highest tide of 3 January 2014. The effect of a smaller computational timestep or spline interpolation of the boundary water level time series results in only small differences in the model water level results and does not change the overall behavior.

WEST QUAY

7.5

7 WATER LEVEL (mODN) LEVEL WATER

6.5

6 03/01/2014 06:00 03/01/2014 07:30 03/01/2014 09:00 03/01/2014 10:30 03/01/2014 12:00 DATE

Gauge Revised Model 10s Timestep 5s Timestep Spline Interpolation

Figure 14 Model water levels for sensitivity tests (timestep and boundary interpolation) – 3 January 2014 (West Quay)

3) The revised model high water levels in Bridgwater (at the Dunball and West Quay gauges) generally show a good agreement with the gauge data, particularly for higher high waters, above around 7.0mODN. The gauge at Dunball has a limited recording range and the true values of the highest high waters and most low waters are not recorded, as illustrated in Figure 9. Both the Dunball and West Quay gauge data show occasional anomalies in the water level data, as illustrated in Figures 9 and 10. Figure 15 compares the model and gauge high water levels at the West Quay gauge for all the high waters in the two month simulation period. The mean difference in high waters is +0.16m, indicating a slight bias to higher model levels than recorded, and a root-mean-square difference of 0.20m. These differences are relatively small, amounting to 3-4% of the tidal water level range at the gauge (typically 5 to 6m for higher spring tides). The gauge data for the tide with the largest difference in levels - the high water of the evening of 18 December 2013 highlighted on Figure 14 (7.63mODN) - is likely to be erroneous. As shown in Figure 16, the recorded high water is 1m higher than the water levels recorded 15 minutes earlier or later and there are clear anomalies in the gauge data over the next 2 hours of record. The gauge data at Dunball shows no such anomalies and the model and gauge data agree very well at that location as shown in Figure 17.

GAUGEERROR?

6 MODEL HIGH MODEL HIGH WATER (mODN) 5

3 3 4 5 6 7 8 9 GAUGE HIGH WATER (mODN)

Figure 15 Model high water levels and gauge high water levels at West Quay (1 December 2013 to 31 January 2014)

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 18/12/2013 15:00 18/12/2013 18:00 18/12/2013 21:00 19/12/2013 00:00 DATE

Gauge Revised Model

Figure 16 Model water level and gauge data – 18 December 2013 (West Quay)

DUNBALL 9

WATER WATER LEVEL (mODN) 4

1 18/12/2013 15:00 18/12/2013 18:00 18/12/2013 21:00 19/12/2013 00:00 DATE

Gauge Revised Model

Figure 17 Model water level and gauge data – 18 December 2013 (Dunball)

4) The revised model low water levels in Bridgwater (West Quay gauge) tend to be around 0.5m to 1.0m higher than the gauge low water levels. This could be due to overestimation of the fluvial flow in the model or lower conveyance in the model than in the actual river. Figure 18 shows the model water levels at West Quay with the fluvial flow in the lower estuary artificially reduced in order to improve the agreement with the gauge data. The model low water levels agree more closely with the gauge data but the required reduction in flow (around 60%) is not consistent with the previous satisfactory calibration of the model in the fluvial system further upstream and the revised model results at Northmoor and Saltmoor. As shown in Figure 18, high water levels in Bridgwater for tides of this magnitude are insensitive to the fluvial flow. Figure 19 shows the model water levels at West Quay with the Manning’s roughness coefficient in the lower estuary increased and decreased by 20%. Low water levels are reduced by around 0.2m for a reduction in roughness of 20% but there is still a significant difference between the model and gauge water level. High water levels are not significantly influenced by the variations in roughness value for this tide event. Figure 20 shows a long section of the reach of river downstream of the West Quay gauge and the model water surface profiles for a range of flows at low water in the outer estuary. The results suggest that for a low water level of around 2.2mODN at West Quay, as recorded on 3 January 2014, the fluvial flow would need to be around 15m3/s. This is not consistent with the high fluvial flood flows at the time of the event. The model results indicate a backwater from a section of reduced channel conveyance approximately 3.3km downstream of the gauge at the location indicated in Figure 21. At this point the river channel narrows locally and is in an area of higher morphological activity. These results suggest that low water levels in Bridgwater are relatively sensitive to the shape of the bottom of the channel in this area, which may change in time in response to fluvial and tidal events. The model channel cross-sections were surveyed around 6 months after the January 2014 event, following a period of prolonged high fluvial flows. It is likely that the surveyed cross-sections

therefore differ from the channel cross-section at the time of the event, resulting in differences in the model and gauge low water levels.

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 03/01/2014 04/01/2014 DATE

Gauge Revised Model Revised Model (Reduced Fluvial Flow)

Figure 18 Model water levels and gauge data – 3 January 2014 (West Quay) – effect of fluvial flow

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 03/01/2014 04/01/2014 DATE

Gauge Revised Model Revised Model (Roughnes +20%.) Revised Model (Roughness -20%)

Figure 19 Model water levels and gauge data – 3 January 2014 (West Quay) – effect of channel roughness

40m3/s 35m3/s 30m3/s 25m3/s REDUCED 20m3/s CONVEYANCE 15m3/s 2 10m3/s

5m3/s

1 LEVEL (mODN)LEVEL

THALWEG

-1 0 1000 2000 3000 4000 CHAINAGE DOWNSTREAM OF WEST QUAY GAUGE (m)

Figure 20 Model water surface profiles downstream of West Quay at low water

REDUCED CONVEYANCE

WEST QUAY

Figure 21 River Parrett channel downstream of West Quay gauge

5) The effect of the uncertainty in applying a uniform downstream boundary water level condition based on the Hinkley Point recorded water levels has been tested by raising the entire boundary water level series by 0.10m and 0.15m increments. Figure 22 shows the resulting model water level results at the West Quay gauge. Raising the boundary water level increases the maximum water level at West Quay by 0.05m and 0.06m respectively. This results in slightly better agreement between the model and the recorded maximum water levels. These results indicate that water levels in Bridgwater are best estimated with the model using a water level boundary which is representative of the water level opposite the mouth of the Parrett, approximately one third of the way between Hinkley Point and Brean Down. However, the difference in water level for tides of the magnitude of 3 January 2014 is relatively small.

WEST QUAY

7.5

7 WATER LEVEL (mODN) LEVEL WATER

6.5

6 03/01/2014 06:00 03/01/2014 07:30 03/01/2014 09:00 03/01/2014 10:30 03/01/2014 12:00 DATE

Gauge Revised Model Boundary +0.10m Boundary +0.15m

Figure 22 Model water levels for sensitivity tests (timestep and boundary interpolation) – 3 January 2014 (West Quay)

3.2 Spring tide 2015 The effect of the downstream extension of the model tidal boundary and of the inclusion in the model of the flood risk management improvements implemented following the 2013-14 flood on the overall model calibration, in particular for water levels in Bridgwater, has been verified through simulation of a period of high spring tides in 2015, following completion of the works. The period 31 July to 5 August 2015 has been simulated with the revised “2015 conditions” model. 3.2.1 Boundary conditions The recorded tidal water level series at the Hinkley Point gauge has been used for the downstream model boundary.

Fluvial flows were low during this period and base flow values in the model have been adopted. Sensitivity tests with the Winter 2013-14 event have shown that high water levels in Bridgwater are insensitive to fluvial flow. 3.2.2 Results Figures 23 to 26 compare the 2015 model water level results with the gauge data for the highest tides in the period simulated – 1 to 4 August 2015. 3.2.3 Discussion 1) The model high water levels in Bridgwater (at the West Quay and Dunball gauges) for the highest tide in the simulation (3 August 2014) agree well with the gauged data – the differences between model and gauge high waters at these gauges are +0.09m and +0.14m respectively. Figure 27 compares the model and gauge high water levels at the West Quay gauge for all the high waters in the simulation period, together with the Winter 2013-14 results. The differences between the model and gauge high waters for the two periods is similar, with the model high waters tending to be higher than the gauge data, particularly for lower tides (less than 7mODN). The shape and timing of the model water level hydographs agree well with the gauge records. As for the Winter 2013-14 simulation, the model low water levels tend to be higher than the recorded levels. 2) The model water levels at the gauges upstream of Bridgwater (Northmoor and Saltmoor) agree will with the gauge data in terms of shape and timing. The model levels (high waters and low waters) are generally higher than the recorded levels. Water levels at these gauges are influenced by the effects of the “8km dredge” of the River Parrett and River Tone in 2014. The dredging is represented in the model based on design dredge cross-sections. However, the channel roughness values from the 2014 calibrated model have been retained. In both respects this section of the model will therefore likely differ from the actual channel characteristics during the period simulated, contributing to the differences in water levels.

DUNBALL 8

WATER WATER LEVEL (mODN) 3

0 01/08/2016 02/08/2016 03/08/2016 04/08/2016 DATE

Gauge 2015 Model

Figure 23 Model water levels and gauge data – Spring tide 2015 (Dunball)

WEST QUAY 9

WATER WATER LEVEL (mODN) 4

1 01/08/2016 02/08/2016 03/08/2016 04/08/2016 DATE

Gauge 2015 Model

Figure 24 Model water levels and gauge data – Spring tide 2015 (West Quay)

NORTHMOOR 9

WATER WATER LEVEL (mODN) 4

1 01/08/2016 02/08/2016 03/08/2016 04/08/2016 DATE

Gauge 2015 Model

Figure 25 Model water levels and gauge data – Spring tide 2015 (Northmoor)

SALTMOOR 9

WATER WATER LEVEL (mODN) 4

1 01/08/2016 02/08/2016 03/08/2016 04/08/2016 DATE

Gauge 2015 Model

Figure 26 Model water levels and gauge data – Spring tide 2015 (Saltmoor)

2013-14 WINTER FLOOD LIKELY BARRIER CLOSURE 2015 SPRING TIDE TRIGGER RANGE

GAUGE ERROR? 7

RISK OF FLOODING (APPROX.)

6 MODEL HIGH MODEL HIGH WATER (mODN) 5

BARRIER MODEL PRINCIPAL SIMULATION RANGE

3 3 4 5 6 7 8 9 GAUGE HIGH WATER (mODN) Figure 27 Model high water levels and gauge high water levels at West Quay (Winter 2013-14 and Sprig tides 2015) 3.3 Summary The results for the Winter 2013-14 verification simulation confirm that the original model calibration has not deteriorated as a result of the revisions to the model and that within Bridgwater and the lower Parrett estuary the model performance has been improved. Model high water levels and the shape and timing of the model water level hydrographs agree well with the gauge records, particularly for the highest tides. In general, as shown in Figure 27, the model tends to overestimate high waters in in the lower Parrett estuary and the difference is greater at lower tides, below around 7.0mODN. For the purposes of assessment of the barrier, model performance at the highest tides in the calibration events is most relevant since these tides are closest to the primary range of events to be simulated with the model. Barrier closure trigger levels are likely to be in the range of 7.3mODN to 7.7mODN in Bridgwater and the typical defence level or onset of flooding is Bridgwater is around 8.3mODN, as shown in Figure 27. As illustrated in Figure 13, for tides higher than around 7mODN in Bridgwater, complex transient effects occur as the tidal wave travels along the river. This results in significant (e.g. 0.5m in Bridgwater) and rapid (in 15 minutes or less) fluctuations in the high water level in the river. Although the model results show similar variations, it is difficult to accurately simulate such behavior, which differs on every tide. The water level gauge records also include anomalous data, as shown in Figure 16, which can affect high water records. These factors need to be considered when comparing the model water levels and gauge data. Model low water levels are generally higher than the recorded low water levels. Low water levels are sensitive to fluvial flow, channel roughness and the bathymetry of the bottom of the channel, since the flow at low water is generally very small relative to the size of the channel downstream of Bridgwater. However, sensitivity tests indicate that model high water levels for higher tides (above 7.0mODN at Bridgwater) are not sensitive to low water levels and therefore the model performance at low water is not considered critical for the purposes of the project. The “2015 conditions” model, with the addition of the further improvement works implemented between 2015 and the planned barrier commissioning in 2024 (River Parrett “700m dredge”,

“Cannington Bends”) forms the “barrier design model” model (as shown in the flow chart in Figure 2) for simulation of scenarios both with and without the barrier in the baseline year for the scheme (2024) and future epochs (up to 2125). Although the model includes the changes to channel cross- sections resulting from dredging of the River Parrett and River Tone upstream of Bridgwater following the 2013-14 flood, the channel roughness values from the calibrated model prior to dredging have been retained. These “mature” channel conditions are considered more appropriate for the barrier baseline configuration. The results of the verification simulations confirm that the model is suitable for the purposes of the project given the availability and quality of record data. The available calibration data does not cover the full range of tidal events that need to be considered for the project. In particular, there are no recent records for events which result in overtopping of the defences within Bridgwater or downstream of Bridgwater (an important mechanism in determining water levels in Bridgwater under more extreme tidal events). Further assessment of the model performance and the sensitivity of model results to uncertainties in the overtopping flow is presented in Section 6 of this report. The uncertainties in the model results and the complexity of the behaviour of high water levels need to be considered in the design of the barrier and the assessment of impacts.

Design model boundary conditions 4.1 Fluvial flow Figure 28 shows the main features of the lower River Parrett catchment which together determine the fluvial flow in the river at Bridgwater.

DUNBALL SLUICE

DRAIANGE RIVER PUMP STATIONS SOWY (PRC) BEAZLEYS SPILLWAY ALLERMOOR SPILLWAY / MONKSLEAZE CLYSE HOOKBRIDGE NTL SPILLWAY NTL

Figure 28 Main features of the lower River Parrett catchment 4.1.1 River Parrett upstream of Bridgwater Fluvial flow in the River Parrett upstream of Bridgwater is regulated by the overtopping of spillways on the right bank of the River Parrett (Allermoor and Beazleys spillways) and on the left bank of the River Tone (Hookbridge spillway). Overflow occurs at relatively low flows – i.e. for the 50% AEP (1 in 2 years) and lower flows. The flow over the River Tone spillway is temporarily stored in the low-lying moors between the River Tone and the Parrett. Water is returned to the rivers by pump stations on the Tone (Currymoor) and Parrett (Saltmoor and Northmoor) when river levels permit. Additional temporary pumps can be used to drain the flood water more rapidly if the water level in the moors exceeds a critical trigger level.

The flow over the River Parrett spillways passes down the River Sowy (Parrett Relief Channel or PRC) and into the King’s Sedgemoor Drain which discharges into the River Parrett at Dunball, downstream of Bridgwater. The tidal sluice at Dunball prevents discharge when the tidal water level in the River Parrett is higher than the water level in the King’s Sedgemoor Drain. Excess flow is stored in the moors along the River Sowy and the King’s Sedgemoor Drain. In extreme events, temporary pumps can be used to allow discharge at Dunball to continue during the period of tide lock and drain the moors more rapidly. Flood water in the Penzoy part of the King’s Sedgemoor catchment can also be drained by pumping into the Parrett at Westonzoyland pump station when the river level permits. The system of controlled overflow from the rivers acts to limit the fluvial flow in the Parrett downstream of the Parrett and Tone confluence. Provided sufficient flood water has entered the moors, the flow in the Parrett downstream of the Tone confluence may be augmented by discharges from the drainage pump stations, principally at Saltmoor and Northmoor which can contribute a combined flow of up to 20m3/s provided the water level in the river does not exceed a high threshold level. For higher tides, pumped discharges may only occur during the low water period. In this way although the fluvial flow in the Parrett at the upstream end of Bridgwater depends to a certain extent on the magnitude and duration of rainfall events in the catchment, the extent of flooding of the moors and the magnitude of the coincident tides, the variation in flow for different AEPs is relatively small due to the relatively large capacity of the spillways and storage volumes in the moors and the relatively smaller maximum capacities of the pump stations. Figure 29 shows the maximum fluvial flow in the River Parrett from the results of the SLM Design Model simulations for fluvially dominated flood events of AEP from 50% (1 in 2 year) to 0.1% (1 in 1000 year) and a low order tidal water level. There is very little variation in the flow immediately downstream of the River Tone confluence for this range of AEPs. The flow at the M5 crossing just upstream of Bridgwater gradually increases with smaller AEPs due to increasing contributions from the pump stations. For smaller AEPs the volume of water entering the moors and the hence the pumped discharges are both greater. However, for AEPs less than 2% (greater than 1 in 50 year) there is little further increase in the flow upstream of Bridgwater since the maximum pumping capacity is reached and the maximum model fluvial flow at Bridgwater is around 66m3/s to 70m3/s.

30 FLOW (m3/s) FLOW

0 100 10 1 0.1 0.01 ANNUAL EXCEEDANCE PROBABILITY % DOWNSTREAM TONE CONFLUENCE UPSTREAM OF BRIDGWATER (M5 CROSSING)

Figure 29 Maximum fluvial flow in the River Parrett upstream of Bridgwater (SLM Design Model)

4.1.2 River Parrett downstream of Bridgwater Fluvial flow in the River Parrett downstream of Bridgwater is determined primarily by the flow through Bridgwater and the contribution from the King’s Sedgemoor Drain which discharges into the Parrett through the tidal sluice at Dunball. Other smaller drainage outfalls and watercourses contribute flow to the Parrett in Bridgwater and downstream of the town but the contributions are small in comparison to the flow in the Parrett and the King’s Sedgemoor Drain. In a fluvial event, the discharge from the King’s Sedgemoor Drain is initially limited by the hydraulic capacities of the channel and outfall and by tide locking due to high tidal water levels in the River Parrett. The duration of tide locking in each tide cycle depends on both the tide height in the River Parrett and the water level or flow in the King's Sedgemoor Drain. Flow in excess of the discharge capacity overtops the elevated banks of the channel and is stored in the low-lying moors along the channel. The SLM Design Model simulations indicate an initial maximum mean daily discharge of around 30m3/s to 35m3/s, although instantaneous flows are variable due to the storage of flow during the tide lock period and the initial rapid drawdown of the channel at the end of the tide lock period. When the flow into the King's Sedgemoor Drain exceeds the discharge capacity for prolonged periods, the moors fill up to the channel bank level and the subsequent increase in channel water level results in increased discharge. In the SLM 2014 Model simulation of the 2013-2014 winter flood event, the maximum mean daily discharge is around 65m3/s and the maximum instantaneous discharge is around 70m3/s. In this way the discharge from the King's Sedgemoor Drain and hence the fluvial flow in the River Parrett downstream of Bridgwater in any given tide depends on the magnitude and duration of fluvial events in the River Parrett and the magnitude of the tide. 4.1.3 Model fluvial boundaries The fluvial flow boundaries of the Barrier Design Model (derived from the SLM Design Model) comprise hydrological rainfall-runoff flow boundaries and direct rainfall boundaries into the lower lying moors. The model includes a detailed representation of the overbank flows from the river channels into the moors upstream of Bridgwater and the drainage of the moors back into the river channels via drainage pumps - both permanent pump stations and temporary additional pumps which are mobilized by the Environment Agency once flood levels reach the critical “trigger” levels. As described in Section 5.1.2, the fluvial flow in the River Parrett upstream of Bridgwater does not increase continuously with reducing AEP. Design flood event simulations with the SLM Design Model indicate an upper bound to the fluvial flow of approximately 66m3/s to 70m3/s. The “upper bound” is approached at relatively high probability flows – i.e. 54m3/s at 50% AEP, 56m3/s at 10% AEP and 68m3/s at 2% AEP. Given the low AEP at which fluvial flows approach the upper bound flow and the generally long duration of sustained fluvial flows in the river (several days or weeks is typical), there is a relatively high likelihood of extreme tides occurring during periods of high fluvial flow (close to the upper bound). Model sensitivity tests (Section 4.1.2 of this report) indicate that, without the barrier, high water levels in Bridgwater are relatively insensitive to fluvial flow magnitude for tides over around 7.0mODN, the primary range of events relevant to the project. However, for barrier closure simulations the water level upstream of the barrier during closure will depend on the fluvial flow. For the Barrier Design Model simulations, an “upper bound” fluvial flow condition is adopted for most simulations since this results in the highest ponded water level upstream of the barrier. Simulations with “lower bound” fluvial flows, corresponding to base flows in the rivers have also been performed for some scenarios to assess the sensitivity of model results to the fluvial flow.

4.1.3.1 Boundary configuration for upper bound fluvial flow in Bridgwater For simulations with upper bound fluvial flow the model boundaries are configured as follows: - Upstream end of River Parrett truncated at Great Bow Bridge, Langport, and a high constant flow applied to the river boundary (70m3/s); - High constant flow (70m3/s) applied to River Tone upstream boundary at Ham weir (original SLM Design model boundary); - High water level boundaries applied to Currymoor (7.3mODN) and Northmoor (4.0mODN) model floodplain units to activate maximum discharge capacities at Currymoor, Saltmoor and Northmoor drainage pump stations; The model steady state flows in this configuration with a low water level in Bridgwater Bay are summarised diagrammatically in Figure 30.

KEY To Dunball ~40 51 Flow (m3/s) M5

15 51

5 24 22 Beazleys Spillway

Currymoor Allermoor Spillway 34 New Bridge West Currymoor Parrett 43 Great Bow Bridge 70 West Moor, 70 Haymoor

Figure 30 Steady state flow distribution in upper bound fluvial flow model configuration (low water in Bridgwater Bay)

The steady state fluvial flow upstream of Bridgwater at low water is 66m3/s, similar to the upper bound values of flow derived from the SLM Design Model design flood hydrograph simulations. Discharge from Currymoor Pump Station does not occur in the simulation due to the high water level in the River Tone. For higher water levels in Bridgwater Bay the tide tends to reduce the fluvial flow upstream of Bridgwater and flow reversal can occur. Similarly, during a barrier closure, the rising ponded water level in the reach upstream of the barrier tends to reduce the fluvial flow entering the ponded reach. In the upper bound fluvial flow model configuration, the model upstream fixed flow boundaries are located sufficiently far upstream that the effect of tides and water levels on flow in the lower estuary is included in simulations. For some initial model investigations, the upstream end of the River Parrett was truncated at the confluence of the River Tone and a constant upper bound estimate of fluvial was applied to the river

35 boundary. These simulations result in a more conservative estimate of ponding level since the effect of the rising water level in reducing flow in the river is not included. The steady state flow in the King’s Sedgemoor Drain in this model configuration, with free discharge at the Dunball tidal sluice, is approximately 40m3/s. As described in Section 5.1.2, higher mean discharges are possible once the moors alongside the King's Sedgemoor Drain have filled, raising the water level in the channel. For model investigations of the effect of the barrier with higher discharges from the King's Sedgemoor Drain (up to 70m3/s), the King's Sedgemoor Drain has been truncated at Parchey Bridge, 5.5km upstream of Dunball, and a high water level boundary applied to the channel (4.4mODN, the approximate maximum water level that occurred in the winter 2013-14 flood event). The fluvial boundary conditions for each type of simulation are summarised in the description of the model simulations in Section 6. 4.1.3.2 Changes to fluvial flows in future epochs The Barrier Design Model is a representation of the river channels and other flood risk management infrastructure such as spillways and drainage pump stations in the present day (2017-2018), both in terms of physical arrangement and operating philosophy. Simulation of “upper bound” fluvial flows with the model results in the flows described in Section 5.1.3.1. In future epochs, fluvial flows at Bridgwater may change because of changes in rainfall, land use and relative sea level or because of changes in flood risk management in the Parrett and Tone catchments such as channel enlargement, defence raising or increased flood storage in the moors. Given the current infrastructure, any increases in flow in the Parrett or Tone rivers at the upstream boundaries of the model will not significantly increase the “upper bound” fluvial flow at Bridgwater since this is currently limited by overflow to the moors and the capacity of the existing permanent and temporary land drainage pumps, as illustrated by the design flood results in Figure 29. Future enlargement of the River Parrett or Tone channels would tend to increase the flow at a given water level. However, since this will reduce the volume of water entering the moors the pumped flows to the river may also reduce and hence the net increase in “upper bound” fluvial flow at Bridgwater may not be larger. In addition, any future increases in channel capacity may not be sustained continuously and it is therefore not clear what the long-term impact of such changes on fluvial flows may be. For these reasons, the present-day model representation of the system (allowing for physical changes currently in progress) and current “upper bound” fluvial flow estimate has been retained for simulation of tide events in future epochs. 4.2 Tidal water level High water levels in Bridgwater are currently determined principally by the tidally varying water level at the mouth of Bridgwater Bay. The continuous contraction in channel cross-section along the river estuary from Bridgwater Bay to Bridgwater means that high water levels are generally higher in Bridgwater than in the Bay as the tidal wave passes up the river. 4.2.1 Model tidal boundary The model downstream boundary crosses the mouth of Bridgwater Bay from Hinkley Point to Brean Down as shown in Figure 31. The closest tide gauge to the model downstream boundary is at Hinkley Point, at the southern end of Bridgwater Bay. The gauge on the downstream side of the tidal sluice on the River Axe at Brean Cross provides an indication of tidal levels at Brean Down at the northern end of the model boundary. However, since this gauge is located approximately 3.5km inland along the River Axe, the water level at the gauge is likely to be higher than at the coast. Figure 31 also shows the locations along the coastline (defined by chainage along the coast in kilometres) at which the current best estimates of extreme peak sea levels are available, as derived in the study “Coastal flood boundary conditions for UK mainland and islands” (Environment Agency, 2011).

Figure 31 Location of tide gauges and Coastal Flood Boundary extreme tide level points 4.2.2 Model tide curves 4.2.2.1 Tide levels Extreme tide levels for the model boundary have been obtained from the Coastal Flood Boundary dataset at chainage km334 and are listed in Table 2, together with the corresponding confidence intervals. This data point corresponds approximately to the centroid of Bridgwater Bay. In the model, the water level is constant along the model boundary. The Coastal Flood Boundary data indicates a variation in extreme tide levels across the Bay. Figure 32 shows the tide levels at point km334 together with the levels at points km326 (Hinkley Point) and km346 (Brean Down) at the south and north ends of the model boundary. The water levels at either end of the model boundary differ by between 0.4m and 0.5m, depending on AEP. Intermediate water levels vary linearly with chainage between the end points. The high water levels recorded at the Hinkley Point and Brean Cross gauges for the winter of December 2013 to February 2014 are also shown on Figure 32. For plotting purposes, the AEP of the recorded high waters at Hinkley Point is estimated from the Coastal Flood Boundary data at km326, ignoring relative sea level rise between 2008 and 2014. The gauge data suggests that the difference in levels between the two ends of the model boundary (0.3m on average) is less than that indicated

37 by the Coastal Flood Boundary data. For the tide of 3 January 2014 the difference in level at Hinkley Point (km326) and the point adopted for the model boundary (km334) is 2014 is around 0.1m.

Table 2. Extreme tide level estimates at km334 in 2008 (Coastal flood boundary conditions 2011) AEP (%) Annual chance (1 IN X) of tide levels Tide level (mODN) Confidence interval (m)

100 1 7.26 0.2

50 2 7.35 0.2

20 5 7.47 0.2

10 10 7.57 0.2

5 20 7.66 0.2

4 25 7.70 0.2

2 50 7.80 0.2

1.33 75 7.87 0.2

1 100 7.91 0.3

0.67 150 7.98 0.3

0.5 200 8.02 0.3

0.4 250 8.06 0.3

0.33 300 8.09 0.3

0.2 500 8.18 0.4

0.1 1000 8.30 0.5

EWL km334 EWL km326 EWL km346 HINKLEY POINT 2013-14 BREAN CROSS 2013-14 9

Tidal Event 8

3/1/14 LEVEL (mODN)LEVEL

7 100 10 1 0.1 AEP (%)

Figure 32 Coastal Flood Boundary extreme tide levels in Bridgwater Bay and gauge records December 2013- February 2014

4.2.2.2 Derivation of model tide curves The tidal water level time series, or tide curve, for the model downstream boundary has been derived by the method recommended in the guidance document to the Coastal Flood Boundary database. The astronomical tide curve at Hinkley Point for the five day period from 31 December 2013 to 5 January 2014 has been used as the base astronomical curve. The highest astronomical level in this period (6.71mODN at 0800 on 3 January 2014) is between Mean High Water Springs (5.93mODN) and Highest Astronomical Tide (7.12mODN) for the station. The period extends for three days before the highest astronomical level and for two days after the highest level. Design tide curves have been produced by adding a scaled surge shape to the astronomical tide curve to achieve the required maximum design tide level, obtained from the Coastal Flood Boundary data in Table 2, for the year 2008. The Coastal Flood Boundary design surge shape for Hinkley Point has been used for most of the design model simulations. For initial model simulations, the actual surge shape for the 3 January 2014 event was used and these results provide an indication of the sensitivity of the model results to the surge shape. The Coastal Flood Boundary surge shape is applied coincidentally with the peak astronomical tide level whereas the 3 January 2014 surge has been applied at the same time as the actual surge, relative to the astronomical tide. Figure 33 shows an example of the derivation of the 0.5% AEP tide curve in year 2008 using the Coastal Flood Boundary surge shape for Hinkley Point. Figure 34 shows a comparison of tide curves for the same tide as Figure 33, derived using both the Coastal Flood Boundary surge shape and the 3 January 2014 surge shape, normalized at time of highest astronomical tide. Although the peak tide levels are the same in each case, the tide curve derived using the 3 January 2014 surge shape rises more rapidly to the highest tide level than the curve derived using the Coastal Flood Boundary surge shape. The tide curves adopted for each type of simulation are summarised in the description of the model simulations in Section 6. The sensitivity of model results to the uncertainty in the extreme tide level estimates has been tested for selected scenarios by adding the confidence intervals in Table 2 to the tide level prior to deriving the tide curve for the model boundary.

ASTRONOMICAL TIDE CURVE CFB SURGE (NORMALISED) 0.5% AEP TIDE CURVE 9

LEVEL (mODN)LEVEL 0

-1

-2

-3

-4

-5

-6 -3 -2 -1 0 1 2 DAYS AFTER HIGHEST ASTRONOMICAL TIDE

Figure 33 Derivation of 0.5% AEP tide curve in 2008 using Coastal Flood Boundary surge shape

CFB SURGE (NORM.) 3/1/2014 SURGE (NORM.) 0.5% AEP TIDE (CFB SURGE) 0.5% AEP TIDE (2014 SURGE) 9

LEVEL (mODN)LEVEL 0

-1

-2

-3

-4

-5

-6 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 DAYS AFTER HIGHEST ASTRONOMICAL TIDE

Figure 34 Comparison of 0.5% AEP tide curves in 2008 derived using Coastal Flood Boundary and 3 January 2014 surge shapes

4.2.2.3 Changes to tide curves in future epochs Relative sea level rise is expected to change the tide curves in Bridgwater Bay in future epochs and an allowance has been included in the tide curves produced for Barrier Design Model simulations based on the following guidance. 1. “Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities”, Environment Agency, 13 April 2016. The recommended allowances for relative sea level rise are applied to model simulations for assessing the impact of barrier closure on water levels upstream and downstream of the barrier, design levels and to determine flood damages for benefit-cost analysis. In most cases the UKCP09 relative sea level rise medium emission upper confidence band (95th percentile) or “change factor” projections for the barrier location are applied. For assessing design water levels at the barrier itself, the Upper End Estimate projections are applied due to the strategic importance of the structure and the difficulty in making modifications in the future.

2. “Flood risk assessments: climate change allowances”, Environment Agency, 19 February 2016 The recommended allowances for relative sea level rise are applied to model simulations for the assessment of residual flood risk with the barrier in operation. The relevant allowances for relative sea level rise for the principal epochs considered in the model simulations are presented in Table 3 (some allowances are only relevant to specific epochs, according to the purpose of the simulation). The relative sea level rises are applied to the entire tide curve, as illustrated in Figure 35 for the 0.5% AEP tide in 2008 and in 2125 based on the UKCP09 Medium Emission 95th percentile projections.

Table 3. Allowances applied in model boundary tide curves for increase in relative sea level between 2008 and future epochs Guidance Allowance Increase in relative sea level from 2008 (m)

2024 2055 2085 2125 UKCP09 Medium Emission th Adapting to Climate Change 95 percentile (“Change 0.086 0.285 0.517 0.869 (April 2016) factor”) Upper End Estimate 1.248 Flood risk assessments: climate change allowances South West Region 0.056 1.225 (February 2016)

ASTRONOMICAL TIDE CURVE CFB SURGE (NORMALISED) 0.5% AEP TIDE CURVE (2008) 0.5% AEP TIDE CURVE (2125) 9

LEVEL (mODN)LEVEL 0

-1

-2

-3

-4

-5

-6 -3 -2 -1 0 1 2 DAYS AFTER HIGHEST ASTRONOMICAL TIDE

Figure 35 Comparison of 0.5% AEP tide curves in 2008 and 2125 based on UKCP09 Medium Emission 95th percentile projections

Model simulations 5.1 Baseline flood risk Simulations of the present-day configuration of the River Parrett, without the barrier, have been performed to understand the baseline flood risk at the time of barrier commissioning (2024) and in the future (2055). 5.1.1 Simulations The Barrier Design Model (without barrier) has been configured for the upper bound fluvial flow condition and with downstream boundary tide curves for 50%, 20%, 5%, 2%, 1% and 0.5% AEP tide levels and relative sea level rise at two epochs (2024 and 2055) based on the “change factor” allowances. Table 4 summarises the maximum tidal boundary levels for each of the simulations. The tide curves use the Coastal Flood Boundary design surge shape. Simulations have also been made to assess the sensitivity of the model results to the surge shape and overbank flow rates downstream of Bridgwater.

Table 4. Maximum tidal boundary water levels for baseline model simulations AEP (%) Annual chance (1 IN X) of tide levels 2024 Tide level 2055 Tide level (mODN) (mODN)

50 2 7.436 7.635

20 5 7.556 7.755

5 20 7.746 7.945

2 50 7.886 8.085

1 100 7.996 8.195

0.5 200 8.106 8.305

5.1.2 Results Figures 36 and 37 show, for the 2024 and 2055 epochs respectively, the baseline maximum water level profiles in the River Parrett between Somerset Bridge and Stert Point for the 50%, 20%, 5%, 2%, 1% and 0.5% AEP tides and upper bound fluvial flow. Left and right bank levels from the defence surveys used to define the banks in the model (undertaken between 2006 and 2014 depending on location, as detailed in Section 3.2.3) are also shown. Figure 38 shows compares the maximum water levels in Bridgwater (at the West Quay gauge) to the offshore tide level for the two epochs. The modelled water levels at West Quay are tabulated in Table 5. Figures 39, 40 and 41 show the model flood extents for all AEPs and both epochs.

MAXIMUM WATER LEVELS : BASELINE (2024) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE

DRAIN

A39/WESTGAUGEQUAY KING'SSEDGEMOOR

6.5 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 21,000 CHAINAGE FROM SOMERSET BRIDGE (m) LEFT BANK (EXIST) RIGHT BANK (EXIST) 50% AEP 20% AEP 5% AEP 2% AEP 1% AEP 0.5% AEP

Figure 36 Baseline simulation maximum water surface profiles (2024)

MAXIMUM WATER LEVELS : BASELINE (2055) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE

DRAIN

A39/WESTGAUGEQUAY KING'SSEDGEMOOR

Figure 37 Baseline simulation maximum water surface profiles (2055)

8.4

TYPICAL DEFENCE LEVEL IN BRIDGWATER ~8.3mODN 8.3 0.5% 2% 1%

8.2 5% 0.5% 20% 1%

8.1 2% 50% 5%

20% MAXIMUM WATER LEVEL IN BRIDGWATER (mODN) BRIDGWATER IN LEVEL MAXIMUM WATER 7.9 2024

50% 2055

7.8 7.4 7.5 7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 MAXIMUM TIDE LEVEL OFFSHORE (mODN)

Figure 38 Baseline simulation maximum water levels in Bridgwater (2024 and 2055)

Table 5. Modelled water levels in Bridgwater (2024 and 2055)

AEP (%) Annual chance (1 IN X) of tide levels Modelled water level at West Modelled water level at West Quay 2024 (mODN) Quay 2055 (mODN) 50 2 7.9 8.0 20 5 8.0 8.1 5 20 8.1 8.2 2 50 8.1 8.2 1 100 8.2 8.2 0.5 200 8.2 8.3

50% AEP : 2024 50% AEP : 2055

20% AEP : 2024 20% AEP : 2055

Figure 39 Baseline simulation flood extents in Bridgwater for 50% and 20% AEP (2024 and 2055)

5% AEP : 2024 5% AEP : 2055

2% AEP : 2024 2% AEP : 2055

Figure 40 Baseline simulation flood extents in Bridgwater for 5% and 2% AEP (2024 and 2055)

1% AEP : 2024 1% AEP : 2055

0.5% AEP : 2024 0.5% AEP : 2055

Figure 41 Baseline simulation flood extents in Bridgwater for 1% and 0.5% AEP (2024 and 2055)

As shown in Figure 38, the increase in maximum water level in Bridgwater with increasing tide level is limited by overtopping of long lengths of river bank downstream of Bridgwater, particularly over the left bank which is typically 0.5m lower than the right bank between Bridgwater and Combwich. Overtopping occurs at a relatively high AEP – in the 2024 epoch, minor overtopping occurs at the 50% AEP – but the short duration and small depth of overtopping means the extent of flooding is very small for high AEP tides. As shown in Figures 39, 40 and 41, at lower AEPs extensive flooding downstream of Bridgwater is predicted. Figure 38 also gives an indication of the sensitivity of water levels in Bridgwater to the uncertainty in the estimates of extreme tide levels in Bridgwater Bay. The confidence interval for AEPs greater than 1% is +/-0.2m and for the 1% and 0.5% AEP the confidence interval is +/-0.3m. Figure 38 shows that variations in tide levels by these amounts results in a variation in maximum water level in Bridgwater of a little less than +/-0.1m. For the range of tides simulated, the maximum water level in Bridgwater is slightly lower (50- 100mm) than the current general defence level (typically around 8.3mODN) in both epochs. However, bank levels vary through the town and some flooding in Bridgwater is predicted. At Blake Gardens, flooding is limited to a small area around Blake Gardens – the bank level is locally low (less than 8mODN) and overtops at a low AEP (50%). However, in the 2024 epoch flooding is restricted mainly to the park, reaching a small number of properties between the 2% and 1% AEP and the A39 Broadway road between the 1% and 0.5% AEP. For the 2055 epoch, the predicted flooding is more extensive, crossing the A39 road and reaching properties in Taunton Road and Old Taunton Road in the 0.5% AEP. Figure 42 shows the model flood extents in this area.

50% AEP (2024)

0.5% AEP (2024)

0.5% AEP (2055)

Figure 42 Baseline simulation flood extents in Bridgwater at Blake Gardens

Flooding is also predicted in the northern part of Bridgwater, on the left bank of the River Parrett. In the 2024 epoch, flooding is predicted between the 1% and 0.5% AEP due to water from the left bank floodplain downstream of Bridgwater flowing south through Chilton Trinity and into the town. Local overtopping of the left bank at Linham Road is also predicted. For the 2055 epoch flooding of this area is predicted between the 5% and 2% AEP. Figure 41 shows the model flood extents in this area.

2% AEP (2055)

0.5% AEP (2055)

0.5% AEP (2024)

Figure 43 Baseline simulation flood extents in northern Bridgwater 5.1.3 Sensitivity test results 5.1.3.1 Surge shape Figure 44 shows the maximum water level profiles in the River Parrett for the 2024 20% AEP tide with tide curves based on the Coastal Flood Boundary design surge shape and the 3 January 2014 surge shape. For the simulation of the tide curve based on the 2014 surge shape the maximum water level in Bridgwater is 0.14m higher than the simulation based on the design surge shape. Figure 45 shows the water level at West Quay and the tide curves for the two simulations. For the 2014 surge shape the transient oscillations in water level are greater (similar to those recorded for the actual January 2014 event) and this is most likely due to the more rapid rise in tide level in Bridgwater Bay. Although the absolute maximum water level in Bridgwater is slightly higher with the 2014 surge shape, the time for which the water level exceeds a given threshold level above 7.5mODN is shorter due to the oscillation in water level. Overall this results in a smaller volume of flooding downstream of Bridgwater with the 2014 surge shape compared to the design surge shape, as illustrated by the model flood extents in Figure 46.

MAXIMUM WATER LEVELS : BASELINE (2024) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE

DRAIN

A39/WESTGAUGEQUAY KING'SSEDGEMOOR

Figure 44 Baseline simulation maximum water surface profiles (20% AEP, 2024) – effect of surge shape

TIDE CURVE (DESIGN SURGE)

8 TIDE CURVE (2014 SURGE)

WEST QUAY (DESIGN SURGE)

WEST QUAY (2014 SURGE) 7

6 LEVEL (mODN) LEVEL 5

3 0 1 2 3 4 5 6 TIME (hrs)

Figure 45 Baseline simulation water level at West Quay and tide curves (20% AEP, 2024) – effect of surge shape

2014 SURGE

DESIGN SURGE

Figure 46 Baseline simulation flood extent downstream of Bridgwater (20% AEP, 2024) – effect of surge shape

5.1.3.2 Overbank flow The baseline model results indicate the importance of flow over the lower level river banks downstream of Bridgwater in limiting the maximum water levels in Bridgwater for low AEP tides. The model simulation of flow over the banks cannot be calibrated or validated against observations due to a lack of overtopping events over the available period of water level records. The bank levels in the model are based on recent topographic surveys of the defence level and form the boundary between the 1D model domain of the river channel and the 2D model domain of the floodplain. Flow over the 1D-2D model boundary TUFLOW “HX line” (i.e. the flow from the river into the floodplain) is calculated according to the water levels in the river and the floodplain and the defence level. Default calculation parameters are adopted in the model which resulting in a frictional flow calculation across the boundary. By adjusting the calculation parameters, it is possible to force a weir flow calculation on the boundary line and to adjust the effective discharge coefficient in the weir flow equation. Due to the importance of downstream flooding on the water level in Bridgwater and the lack of observations for model calibration, the sensitivity of model water levels to the effective discharge coefficient of the banks has been assessed through simulations with three alternative TUFLOW HX line parameters as follows: a) Baseline model – default parameters; b) Increased overflow – weir flow forced on HX line with default parameters; c) Reduced overflow – weir flow forced on HX line with “B” coefficient adjusted to reduce effective discharge coefficient. The simulations have all been performed with a tide curve for the 0.5% AEP tide in 2015 using the 2014 surge shape. Since the effective weir discharge coefficient is not specified directly in the model, the model results for flow over two sample sections of bank and the corresponding river water level have been compared to calculated weir flow curves for the same sections of bank, calculated from the original 1D SLM Design Model “spill” units, to allow the effective discharge coefficients to be estimated and compared. Figure 47 shows the location of the two sample sections of bank. Figure 48 shows the

51 model results for flow over the bank and river water level in each scenario together with calculated weir flow curves which best fit the model results. The results are summarised in Table 6.

MODEL CROSS- SECTIONS

Figure 47 Location of sample bank sections for assessing effective weir discharge coefficients in 1D-2D model link line

8.2

8.1

LEFT BANK MODEL : BASELINE 8.0 LEFT BANK MODEL : FORCE WEIR

LEFT BANK MODEL : FORCE WEIR, B=3

LEFT BANK WEIR FLOW : Cd=0.92 7.9 LEFT BANK WEIR FLOW : Cd=1.17

LEFT BANK WEIR FLOW : Cd=0.76

7.8 RIGHT BANK MODEL : BASELINE WATER LEVEL IN RIVER (mODN)RIVER IN LEVEL WATER RIGHT BANK MODEL : FORCE WEIR

RIGHT BANK MODEL : FORCE WEIR, B=3

7.7 RIGHT BANK WEIR FLOW : Cd=0.80 RIGHT BANK WEIR FLOW : Cd=1.15

RIGHT BANK WEIR FLOW : Cd=0.67

7.6 0 10 20 30 40 50 60 70 80 90 100 FLOW OVER BANK (m3/s)

Figure 48 Model results for river water level and flow over banks at sample locations and weir flow equation values

Table 6. Approximate variations in overbank flow in model sensitivity tests Bank HX line parameters Approximate effective Change in overbank flow discharge coefficient compared to baseline

Default 0.92

LEFT Force weir 1.17 +27%

Force weir – B=3 0.76 -17%

Default 0.80

RIGHT Force weir 1.15 +44%

Force weir – B=3 0.67 -16%

The baseline effective discharge coefficients (0.8 to 0.9 approximately) are considered appropriate for the raised flood banks along the river. The effect of increasing the effective discharge coefficient and overbank flow by around 30-40% and reducing the effective discharge coefficient and overbank flow by around 15-20% on maximum water levels in the river is shown in Figure 49. The corresponding changes in maximum water levels are within the range of +/-60mm. The effect on the model flood extent is shown in Figure 50.

MAXIMUM WATER LEVELS : BASELINE (2015) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE

DRAIN

A39/WESTGAUGEQUAY KING'SSEDGEMOOR

Figure 49 Baseline simulation maximum water surface profiles (0.5% AEP, 2015) – effect of variations to overbank flow

Maximum Flood Extents

C) Bank Flow Reduced OVERBANK~17% FLOW REDUCED

A)BASELINE Baseline

OVERBANKB) Bank Flow FLOW INCREASED Increased ~27%

Figure 50 Baseline simulation maximum flood extent (0.5% AEP, 2015) – effect of variations to overbank flow

5.2 Flood damages Simulations of the present-day configuration of the River Parrett, without the barrier, have been performed for various future epochs to produce flood depth grids to allow calculation of the damages which will be avoided through operation of the barrier. The sensitivity of the damages to uncertainties and assumptions in the model and in relation to the future management of flood risk downstream of Bridgwater has been assessed through alternative model scenarios. 5.2.1 Simulations Simulations have been performed for four future epochs (2024, 2055, 2085, 2125) and nine AEPs (50%, 20%, 10%, 5%, 2%, 1.33%, 1%, 0.5%, 0.1%). Three scenarios, representing “lower”, “middle” and “realistic upper bound” estimates of flood damages in Bridgwater have been simulated for all epochs and AEPs. The scenarios consider increasing allowances for the uncertainty in offshore tide level estimates and increasing allowances for defence level raising downstream of Bridgwater (based on the approach envisaged in the Parrett Estuary Flood Risk Management Strategy 2010), both of which tend to increase water levels and hence flood damages in Bridgwater. In all cases the Barrier Design Model (without barrier) has been configured for the upper bound fluvial flow condition and with downstream boundary tide curves based on the Coastal Flood Boundary design surge shape and relative sea level rise based on the “change factor” allowances. A reduced effective discharge coefficient for overbank flow (as defined in Section 6.1.3.2) downstream of Bridgwater is included in all scenarios. The scenario definitions are summarised in Table 7 and the maximum tidal boundary water levels are presented in Table 8. Additional simulations have also been performed for the 2024 epoch

scenarios with breaches separately in the left and right banks at the locations shown in Figure 51. The breach locations have been selected as they are the highest risk locations based on topography of the floodplain.

Table 7. Approximate variations in overbank flow in model sensitivity tests Scenario Defence raising downstream of Bridgwater Allowance for uncertainty in tide level

2024 None Scenario 1 2055 All banks raised to 2% AEP 2085 water level* Nil “Lower” 2085 and 2125 All banks raised to 2% AEP 2125 water level* 50% of left bank raised to 2% AEP 2055 water 2024 level# Scenario 2 50% of CFB confidence 2055 All banks raised to 2% AEP 2085 water level# “Middle” interval 2085 and 2125 All banks raised to 2% AEP 2125 water level# All of left bank raised to 2% AEP 2055 water 2024 Scenario 3 level# “Realistic 100% of CFB confidence 2055 All banks raised to 2% AEP 2085 water level# Upper interval Bound” 2085 and 2125 All banks raised to 2% AEP 2125 water level#

* water level determined for Scenario 1 tide level # water level determined for Scenario 3 tide level

Table 8. Maximum tidal boundary water levels for flood damage model simulations AEP Annual Scenario 1 Scenario 2 Scenario 3 (%) chance (1 IN X) 2024 2055 2085 2125 2024 2055 2085 2125 2024 2055 2085 2125 of tide levels

50 2 7.436 7.635 7.867 8.219 7.536 7.735 7.967 8.319 7.636 7.835 8.067 8.419

20 5 7.556 7.755 7.987 8.339 7.656 7.855 8.087 8.439 7.756 7.955 8.187 8.539

10 10 7.656 7.855 8.087 8.439 7.756 7.955 8.187 8.539 7.856 8.055 8.287 8.639

5 20 7.746 7.945 8.177 8.529 7.846 8.045 8.277 8.629 7.946 8.145 8.377 8.729

2 50 7.886 8.085 8.317 8.669 7.986 8.185 8.417 8.769 8.086 8.285 8.517 8.869

1.33 75 7.956 8.155 8.387 8.739 8.056 8.255 8.487 8.839 8.156 8.355 8.587 8.939

1 100 7.996 8.195 8.427 8.779 8.146 8.345 8.577 8.929 8.296 8.495 8.727 9.079

0.5 200 8.106 8.305 8.537 8.889 8.256 8.455 8.687 9.039 8.406 8.605 8.837 9.189

0.1 1000 8.386 8.585 8.817 9.169 8.636 8.835 9.067 9.419 8.886 9.085 9.317 9.669

BREACHES

Figure 51 Location of breaches for flood damage assessment simulations 5.2.2 Results Figures 52, 53, 54 and 55 show, for the 2024, 2055, 2085 and 2125 epochs respectively, the maximum water level profiles in the River Parrett between Somerset Bridge and Stert Point for the 0.5% AEP tides for Scenarios 1, 2 and 3. The raised bank levels adopted in each scenario downstream of Bridgwater are also indicated.

MAXIMUM WATER LEVELS : FLOOD DAMAGES (2024) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE DRAIN A39/WESTGAUGEQUAY SCENARIO 3 RAISED BANK

SCENARIO 2 RAISED BANK KING'SSEDGEMOOR

Figure 52 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2024)

MAXIMUM WATER LEVELS : FLOOD DAMAGES (2055) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE DRAIN A39/WESTGAUGEQUAY SCENARIO 3 RAISED BANK

SCENARIO 2 RAISED BANK KING'SSEDGEMOOR SCENARIO 1 RAISED BANK 6.5 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 21,000 CHAINAGE FROM SOMERSET BRIDGE (m) LEFT BANK (EXIST) RIGHT BANK (EXIST) 0.5% AEP SCENARIO 1 0.5% AEP SCENARIO 2 0.5% AEP SCENARIO 3

Figure 53 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2055)

MAXIMUM WATER LEVELS : FLOOD DAMAGES (2085) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE DRAIN A39/WESTGAUGEQUAY SCENARIO 3 RAISED BANK

Figure 54 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2085)

MAXIMUM WATER LEVELS : FLOOD DAMAGES (2125) 9.5

9.0 COMBWICH STEARTBREACH

8.5

8.0 LEVEL LEVEL (mODN)

7.5 TOWNBRIDGE

7.0 SOMERSETBRIDGE DRAIN A39/WESTGAUGEQUAY SCENARIO 3 RAISED BANK

Figure 55 Flood damages simulation maximum water surface profiles for 0.5% AEP tide (2125)

Figures 56 and 57 compare the maximum water levels in Bridgwater (at the West Quay gauge) for the three scenarios in 2024 and 2055 respectively, together with the maximum water levels from the baseline model simulations presented in Figure 38. (It should be noted that, as shown in Figures 52 to 55, the maximum water level varies more significantly through Bridgwater in the three flood damages scenarios due to the overtopping of defences in the town). In Figure 56 (2024), the difference in water level at Bridgwater between the baseline simulation and Scenario 1 is due to the reduced bank discharge coefficient for the downstream banks in Scenario 1, as discussed in Section 6.1.3.2. The differences in water level at Bridgwater between Scenarios 1, 2 and 3 (for a given offshore tide level) are due to the different extent of bank raising downstream of Bridgwater in each scenario. Figure 57 (2055) shows that, in this epoch, the water level at Bridgwater for a given offshore tide level is similar in Scenarios 1, 2 and 3, due to the same extent of bank raising in this epoch (downstream water levels are generally in-bank, as shown in Figure 53).

Scenario 1 Scenario 2 Scenario 3 BASELINE

8.8

8.7

1000yr 8.6

200yr 1000yr 8.5 100yr 75yr 50yr 200yr 8.4 100yr 1000yr 20yr 75yr 50yr TYPICAL DEFENCE LEVEL IN 8.3 10yr BRIDGWATER ~8.3mODN 200yr 20yr 100yr 75yr 8.2 5yr 50yr 10yr 20yr 8.1 2yr

5yr HIGH HIGH WATER LEVEL BRIDGWATER IN (mODN)

10yr 8.0 2yr 5yr

7.9

2yr

7.8 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9.0 9.1 9.2 HIGH WATER LEVEL OFFSHORE (mODN)

Figure 56 Flood damage scenario simulations and Baseline simulation maximum water levels in Bridgwater (2024)

Scenario 1 Scenario 2 Scenario 3 BASELINE

8.8

1000yr 8.7 1000yr 200yr 100yr 200yr 8.6 100yr 50yr 1000yr 75yr 75yr 50yr 8.5 20yr 200yr 10yr 100yr 20yr 75yr 8.4 50yr 10yr 5yr TYPICAL DEFENCE LEVEL IN 8.3 20yr BRIDGWATER ~8.3mODN 5yr 2yr 10yr 8.2 2yr

5yr

8.1 HIGH HIGH WATER LEVEL BRIDGWATER IN (mODN) 2yr 8.0

7.9

7.8 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9.0 9.1 9.2 HIGH WATER LEVEL OFFSHORE (mODN)

Figure 57 Flood damage scenario simulations and Baseline simulation maximum water levels in Bridgwater (2055)

Figures 58, 59 and 60 show the model 0.5% AEP flood extents for Scenarios 1, 2 and 3 respectively for all four epochs.

Figure 61 shows the model flood extents in Bridgwater with left and right bank breaches for the 0.5% AEP tide in 2024 for each of the three scenarios.

0.5% AEP : 2024 0.5% AEP : 2055

0.5% AEP : 2085 0.5% AEP : 2125

Figure 58 Scenario 1 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125)

0.5% AEP : 2024 0.5% AEP : 2055

0.5% AEP : 2085 0.5% AEP : 2125

Figure 59 Scenario 2 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125)

0.5% AEP : 2024 0.5% AEP : 2055

0.5% AEP : 2085 0.5% AEP : 2125

Figure 60 Scenario 3 flood damages simulation flood extents for 0.5% AEP (2024, 2055, 2085, 2125)

LEFT : SCENARIO 1 RIGHT : SCENARIO 1

LEFT : SCENARIO 2 RIGHT : SCENARIO 2

LEFT : SCENARIO 3 RIGHT : SCENARIO 3

Figure 61 Flood damages simulation flood extents with breaches for 0.5% AEP tide (2024) : Scenarios 1, 2 and 3

5.3 Barrier closure Simulations of the present-day configuration of the River Parrett, including barrier closure, have been performed to assess the impact on water levels upstream and downstream of the barrier and inform design levels for the barrier and associated flood defence works. The barrier design has been developed based on an assessment of hydraulic conditions, potential navigation requirements through the barrier, operation and maintenance requirements, and cost. The arrangement of two 15m wide gates was selected to provide flexibility in operation and maintenance and to allow vessels to pass safely whilst having a minimal impact on water levels and flow velocities. The gate crest level has been determined from the modelled 1 in 200-year (0.5% AEP) tidal flood level in 2125 (based on the Upper End Estimate climate change allowance1) with an additional Residual Uncertainty Allowance (RUA). This is to build resilience in to the barrier design to allow for a future adaptive approach to flood defence in the Parrett estuary, as planned under the Parrett Estuary Flood Risk Management Strategy2. The gate crest level is 10.3mOAD and the sill level for the gates, based on the existing bed level, is 0.3mAOD. The raised platforms on either side of the barrier and the operational building will be at a level of 10.1mAOD. Further information on the future adaptive approach proposed in the Parrett can be found in Section 5.7.Initially simulations were performed for the seven potential locations identified for the barrier shown in Figure 62 to support assessment of the preferred location. Further simulations were performed for the preferred locations (No. 5 in Figure 62). Simulations include the consideration of alternative scenarios for flood protection downstream of Bridgwater (raising of the primary on-line river bank defences and construction of additional secondary floodplain embankments) in order to improve the current standard of protection. The primary and secondary defence crest levels have been selected based on the 1 in 200-year (0.5% AEP) tidal flood level in 2055 (based on the Change Factor climate change allowance3) with an additional Residual Uncertainty Allowance (RUA). The Residual Uncertainty Allowance (RUA) assessment has been carried out to determine the required freeboard for the defences. The water levels in the floodplain are very sensitive to on-line bank level, and the tidal boundary. The RUA for the primary defences is approximately 300mm and for the secondary defences is approximately 500mm. An allowance of 300mm has also been added to allow for potential settlement of the banks. The crest levels for the defences are shown in Figure 63. The primary defences on the left bank immediately downstream of the barrier will tie in to the barrier structure to prevent bypassing.

1 Refer to Table 5 of Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities (Environment Agency, 2016)

2 Environment Agency, 2010. Parrett Estuary Flood Risk Management Strategy. IMSW000584. WXS003E/001A/001A. FSOD F/1011/0135.

3 Refer to Table 5 of Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities (Environment Agency, 2016)

Figure 62 Initial barrier locations

Figure 63 Downstream defences with design crest level (excluding settlement allowance)

The barrier is represented in the model as a single vertical sluice gate with the gate raised fully above the highest water levels except for the period of the highest tide. As both gates will be operated simultaneously this representation is considered appropriate. The gate width has been set to 30m in order to represent the two 15m wide openings. In the simulations, the sluice gate is closed instantaneously just before the water level starts to rise from the low water level at the barrier location. Depending on the time it takes to fully close the actual barrier, in practice closure will need to commence some time earlier in order that the barrier is fully closed by the time the tidally driven flow reaches the barrier and the volume of tidally driven flow entering the river upstream of the barrier is minimised. In the simulations, the barrier is opened instantaneously as soon as the water level downstream of the barrier falls below the ponded water level upstream of the barrier to drain the river reach and minimise the ponded water level. In practice drainage may be slower depending on the speed of barrier operation. Simulations have been made to assess the sensitivity of the model results to the timing of barrier closure as well as the uncertainty in tide level estimates and surge shape. 5.3.1 Closure rules The tidal barrier would be closed on the low tide ahead of a forecast of a surge tide exceeding a trigger level in Bridgwater. The forecast tide level in Bridgwater would be based on the forecast total water level (tide and surge) at Hinkley Point applied to a model to generate the water levels in the Parrett Estuary.

The minimum and maximum barrier trigger levels are discussed in a technical note, Bridgwater Tidal Barrier Closure Rules, CH2M, 2017 in Appendix A. The trigger levels account for factors such as uncertainty in the extreme tide forecast, existing flood defence levels in Bridgwater, freeboard and upstream fluvial storage requirement. The minimum trigger level would be an estimated water level in Bridgwater of 7.3mODN and this is likely to be the trigger level used when the barrier first becomes operational. With experience of operating the barrier and improved accuracy of forecasting, this may be increased up to 7.7mODN with time. 5.3.2 Simulations 5.3.2.1 Initial upstream ponding level for 7 barrier locations Initial simulations were performed to assess the maximum water level upstream of the barrier as a result of closure at each of the seven barrier locations. Simulations were performed for the 100% AEP and 0.5% AEP tides (2015 epoch) based on the January 2014 surge shape, an upper bound fluvial flow of 65m3/s in the River Parrett and an upper bound discharge of 70m3/s from the King's Sedgemoor Drain. For these initial simulations, the upstream boundary of the model is located at the confluence of the River Parrett and River Tone and a constant flow boundary is applied. This results in a higher water level in the ponded reach upstream of the boundary since the effect of the rising water level in this reach in reducing the flow entering the reach is not included in the simulation. 5.3.2.2 Design simulations for barrier location No. 5 For the selected barrier location, simulations have been performed for the three principal scenarios summarised in Table 9. In all cases the CFB design surge shape and “upper bound” fluvial flow scenario as described in Section 5.1.3.1 are applied. The maximum tidal boundary levels are presented in Table 10. The model scenarios include local raising of the online flood banks along the River Parrett downstream of the barrier together with new “secondary” flood banks to provide additional protection to communities downstream of Bridgwater. The raised and new banks have generally been included as “glass walls” in the model (i.e. set infinitely high to enable the design height required to contain the water to be determined), except for a section of raised online bank to the south of Combwich. For the 2125 epoch simulations, a breach in the right bank at Pawlett Hams is included in the model to represent the managed realignment of the defence at this location, as envisaged in the Parrett Estuary Flood Risk Management Strategy 2010. The locations of the sections of raised online banks, new secondary banks and the Pawlett Hams breach are shown in Figure 64.

Table 9. Design simulation scenarios for barrier location No. 5 Scenario and Purpose Epochs Flood bank arrangement Tide levels Relative sea level rise allowance

Establish the design water 0.5% AEP level for the crest of the “Change 1 2055 0.5% AEP + secondary banks for and test All secondary banks and sections confidence Factor” uncertainty in tide level of raised online banks “glass interval walls”, except online bank south of Combwich - set to 2055 0.5% Establish the impact of 50%, 20%, 5%, 2024 & AEP water level. “Change 2 barrier closures on water 2%, 1%, 0.5% 2055 Factor” levels downstream AEP

As Scenarios 1 and 2 but Establish design water levels including 1%, 0.5%, 0.1% “Upper End 3 2125 at the barrier Pawlett Hams managed AEP Estimate” realignment breach

Table 10. Maximum tidal boundary water levels for barrier design model simulations AEP (%) Annual Scenario 1 Scenario 2 Scenario 3 chance (1 IN X) 2055 Tide level 2055 Tide level 2024 Tide level 2055 Tide level 2125 Tide level of tide (mODN) + confidence (mODN) (mODN) (mODN) levels interval (mODN)

50 2 7.436 7.635

20 5 7.556 7.755

5 20 7.746 7.945

2 50 7.886 8.085 1 100 7.996 8.195 9.158 0.5 200 8.305 8.605 8.106 8.305 9.268 0.1 1000 9.548

The primary defence to the south of Combwich was raised to the 2055 0.5% AEP water level in order to control the volume of water overtopping the defences into the floodplain. Scenario 1 includes a test of the sensitivity of model water levels to the uncertainty in tide level estimates through simulation of the 0.5% AEP tide increased by the CFB confidence interval (0.3m). For Scenario 2, the effect of different surge shapes (CFB design shape and January 2014 actual shape) has been tested for the 20% AEP 2024 tide. The 20% AEP event was selected as this was similar to the 2014 flood event. For Scenario 2, the sensitivity of upstream ponded water level to the timing of barrier closure has been tested for the 0.5% AEP 2024 tide by closing the barrier 1 hour and 2 hours earlier than the baseline simulation (closure just before rise in low water level). The 0.5% AEP event was used for this test as this is the design standard for the scheme.

Figure 64 Location of modelled raised and new flood banks downstream of barrier location No. 5 and Pawlett Hams breach 5.3.3 Results 5.3.3.1 Initial upstream ponding level for 7 barrier locations An example illustration of the water levels at the barrier compared to the offshore tide level is given Figure 65 for the simulation of barrier closure at location no. 5 for the 0.5% AEP tide. In this simulation the barrier is fully closed around 3 hours after low water offshore (e.g. at the Hinkley Point tide gauge) and re-opened around 1 hour after high water offshore.

2 BARRIER CLOSURE PERIOD

1 LEVEL (mODN) LEVEL 0 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 -1

-2

-3

-4

-5 TIME AFTER HIGH WATER OFFSHORE (hours)

OFFSHORE TIDE WATER LEVEL DOWNSTREAM BARRIER WATER LEVEL UPSTREAM BARRIER

Figure 65 Timing of barrier closure and water level at the barrier for initial closure simulation (0.5% AEP tide, 2015)

Figures 66 and 67 show the maximum water level profile in the simulations for barrier closure at all seven barrier locations for the 100% and 0.5% AEP tides respectively.

1 LEVEL (mODN)

0 Broadway

-1 Burrowbridge M5

-2

Combwich StertPoint TownBridge

-3 SteartBreach

Dunball

SaltmoorPS NorthmoorPS

-4 RailwayBridge

A39 A39 RoadBridge WestonzoylandPS

-5 TheClink Bridge -6 0 5000 10000 15000 20000 25000 30000 CHAINAGE FROM TONE CONFLUENCE (m)

BED LEVEL NO BARRIER L1 L2 L3 L4 L5 L6 L7

Figure 66 Maximum water level profile for initial barrier closure simulations (100% AEP tide, 2015)

1 LEVEL (mODN)

0 Broadway

-1 Burrowbridge M5

-2

Combwich StertPoint TownBridge

-3 SteartBreach

Dunball

SaltmoorPS NorthmoorPS

-4 RailwayBridge

A39 A39 RoadBridge WestonzoylandPS

-5 TheClink Bridge -6 0 5000 10000 15000 20000 25000 30000 CHAINAGE FROM TONE CONFLUENCE (m)

BED LEVEL NO BARRIER L1 L2 L3 L4 L5 L6 L7

Figure 67 Maximum water level profile for initial barrier closure simulations (0.5% AEP tide, 2015)

5.3.3.2 Design simulations for barrier location No. 5 Figure 68 shows the maximum water level profile for the Scenario 1 design simulations for barrier closure at location No. 5 for the 0.5% AEP tide in 2055 and for the same tide increased by the CFB confidence interval (0.3m). Figures 69 and 70 show, for the 2024 and 2055 epochs respectively, the maximum water level profiles for the 20%, 2% and 0.5% AEP tides for the Scenario 2 design simulations. The maximum water level profiles for the baseline simulations of the same tides are shown for comparison. Figure 71 shows the maximum water level profiles for the Scenario 3 design simulations. Figure 72 shows the maximum water level profiles for the Scenario 2 design simulations for the 20% AEP tide in 2024 with CFB design surge shape and the January 2014 actual surge shape. Figures 73 and 74 show the effect of barrier closure time on water level in Bridgwater (at the West Quay gauge) and at Burrowbridge, respectively, for the Scenario 2 2024 0.5% AEP tide simulation. In the standard scenario simulation, full barrier closure is triggered by rising water level downstream of the barrier, 2 hours and 10 minutes before high water offshore (e.g. at the Hinkley Point gauge). Simulations with the barrier full closure advanced to 3.25 hours and 4.25 hours before high water offshore have been performed to assess the impact on water level in Bridgwater. With the upper bound fluvial flow condition the maximum water level at the West Quay gauge is increased by 0.23m and 0.44m through closing the barrier one hour and two hours earlier respectively. The maximum water level for the two hours earlier closure (7.57mODN) is around 0.7m below the typical defence level in Bridgwater and 0.63m lower than the maximum water level without barrier closure. The differences in maximum water level at Burrowbridge are much smaller – 0.07m and 0.14m respectively – and the maximum water level for the two hours earlier closure (7.63mODN) is 0.40m lower than the maximum water level without closure. Figure 75 shows the maximum water level profiles for the different barrier closure times simulated.

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2055) 9.5

LEFT RIGHT LEFT

BANK BANK BANK COMBWICH RAISED RAISED RAISED

9.0 STEART STEART BREACH

8.5

8.0 LEVEL (mODN) LEVEL

7.5 TOWN TOWN BRIDGE

7.0 SOMERSET BRIDGE

DRAIN

A39/WEST GAUGE A39/WEST QUAY KING'S SEDGEMOOR KING'S

Figure 68 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 1 (2055)

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2024) 9.5

LEFT RIGHT LEFT BANK BANK BANK RAISED RAISED COMBWICH RAISED

9.0 STEART STEART BREACH

8.5

8.0 LEVEL (mODN) LEVEL

7.5 TOWN TOWN BRIDGE

7.0 SOMERSET BRIDGE

DRAIN

A39/WEST GAUGE A39/WEST QUAY KING'S SEDGEMOOR KING'S

Figure 69 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024)

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2055) 9.5

LEFT RIGHT LEFT BANK BANK

BANK COMBWICH RAISED RAISED RAISED

9.0 STEART STEART BREACH

8.5

8.0 LEVEL (mODN) LEVEL

7.5 TOWN TOWN BRIDGE

7.0 SOMERSET BRIDGE

DRAIN

A39/WEST GAUGE A39/WEST QUAY KING'S SEDGEMOOR KING'S

Figure 70 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2055)

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2125) LEFT BANK RIGHT BANK LEFT BANK RAISED RAISED RAISED

9.5 COMBWICH

9.0 STEART BREACH STEART

8.5

8.0 LEVEL (mODN) LEVEL

7.5

7.0 TOWN BRIDGE

SOMERSET BRIDGE SOMERSET DRAIN A39/WEST GAUGE A39/WEST QUAY PAWLETT HAMS

KING'S SEDGEMOOR KING'S BREACH

Figure 71 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 3 (2125)

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2024) 9.5

LEFT RIGHT LEFT BANK BANK BANK RAISED RAISED COMBWICH RAISED

9.0 STEART STEART BREACH

8.5

8.0 LEVEL (mODN) LEVEL

7.5 TOWN TOWN BRIDGE

7.0 BRIDGE SOMERSET

DRAIN

A39/WEST GAUGE QUAY A39/WEST KING'S SEDGEMOOR KING'S

Figure 72 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024) 20% AEP – effect of surge shape

8 BARRIER CLOSURE TIMES 7

5 LEVEL (mODN) LEVEL

2 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 TIME AFTER HIGH WATER OFFSHORE (hours)

OFFSHORE TIDE NO BARRIER CLOSURE -2h10m CLOSURE -3h15m CLOSURE -4h15m

Figure 73 Barrier location No. 5 design simulation – effect of barrier closure time on water level at West Quay gauge (0.5% AEP, 2024 tide)

BARRIER 8 CLOSURE TIMES

5 LEVEL (mODN) LEVEL

2 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 TIME AFTER HIGH WATER OFFSHORE (hours)

OFFSHORE TIDE NO BARRIER CLOSURE -2h10m CLOSURE -3h15m CLOSURE -4h15m

Figure 74 Barrier location No. 5 design simulation – effect of barrier closure time on water level at Burrowbridge (0.5% AEP, 2024 tide)

MAXIMUM WATER LEVELS : BARRIER CLOSURE (2024) 9.5

LEFT RIGHT LEFT BANK BANK BANK RAISED RAISED COMBWICH RAISED

9.0 STEART STEART BREACH

8.5

8.0 LEVEL (mODN) LEVEL

7.5 TOWN TOWN BRIDGE

7.0 SOMERSET BRIDGE

DRAIN

A39/WEST GAUGE A39/WEST QUAY KING'S SEDGEMOOR KING'S

Figure 75 Barrier location No. 5 design simulation maximum water surface profiles for Scenario 2 (2024) 0.5% AEP – effect of barrier closure time on upstream water levels

The effect of changes in downstream water level through barrier closure at location No. 5 on the discharge from the King's Sedgemoor Drain is illustrated in Figures 76 and 77 for the 2024 20% and 0.5% AEP tide simulations respectively. The results show that the changes in downstream water level result in a small reduction (less than 0.2%) in the volume discharged from the King's Sedgemoor Drain due to a slightly longer tide lock period and slightly higher water levels in the River Parrett over part of the tide cycle and hence slightly lower gravity discharge. Table 11 summarises the differences in discharge with barrier closure. The upper bound fluvial flow condition includes the use of temporary pumps during the tide lock period. Figures 78 and 79 show the simulated extent of flooding in the Scenario 2 barrier design simulations for the 20% AEP, 2% AEP and 0.5% AEP tides in 2024 and 2055. The baseline flood extents are also shown for comparison.

Table 11. Effect of barrier closure at location No. 5 on discharges from King's Sedgemoor Drain Total volume discharged over 12hrs (m3) Tide Difference (m3) Difference (%) No closure With closure 20% AEP 1,504,705 1,504,368 -337 -0.02

0.5% AEP 1,469,702 1,467,179 -2523 -0.17

WATER LEVEL IN PARRETT (NO BARRIER) WATER LEVEL IN PARRET (BARRIER) KSD FLOW (NO BARRIER) KSD FLOW (BARRIER) 10 100

8 90

6 80

4 70

2 60

0 50

-2 40 (m3/s) FLOW WATER LEVEL (mODN)LEVEL WATER

-4 30

-6 20

-8 10

-10 0 -6 -4 -2 0 2 4 6 TIME AFTER HIGH WATER OFFSHORE (hrs)

Figure 76 Barrier location No. 5 design simulation – effect of barrier closure on downstream water level and discharge from the King's Sedgemoor Drain (20% AEP, 2024 tide)

WATER LEVEL IN PARRETT (NO BARRIER) WATER LEVEL IN PARRET (BARRIER) KSD FLOW (NO BARRIER) KSD FLOW (BARRIER) 10 100

8 90

6 80

4 70

2 60

0 50

-2 40 (m3/s) FLOW WATER LEVEL (mODN)LEVEL WATER

-4 30

-6 20

-8 10

-10 0 -6 -4 -2 0 2 4 6 TIME AFTER HIGH WATER OFFSHORE (hrs)

Figure 77 Barrier location No. 5 design simulation – effect of barrier closure on downstream water level and discharge from the King's Sedgemoor Drain (0.5% AEP, 2024 tide)

20% AEP : 2024 20% AEP : 2055

Figure 78 Scenario 2 barrier design simulation flood extents for 20% AEP (2024 and 2055)

2% AEP : 2024 2% AEP : 2055

0.5% AEP : 2024 0.5% AEP : 2055

Figure 79 Scenario 2 barrier design simulation flood extents for 2% and 0.5% AEP (2024 and 2055)

5.4 King's Sedgemoor Drain operation See Technical Note “BTB Discharge from KSD Technical Note v1.pdf”. 5.5 Fluvial flood management Simulations have been performed to assess the potential for using barrier closures to reduce fluvial flood risk in the moors upstream of Bridgwater during lower tidal events. By preventing tidally driven flow from entering the River Parrett at Bridgwater, discharge of fluvial flood water can be increased, potentially reducing the fluvial flood risk. For tides below a certain level, positive discharge can be maintained at the barrier location over the whole tide cycle during high fluvial flow. Barrier closure for tides below this level will tend to increase water levels in the river by restricting discharge. A simulation has been performed to estimate the minimum tide levels at which barrier closure would be beneficial for fluvial water levels. 5.5.1 Simulations The Barrier Design Model has been configured for the upper bound fluvial flow condition described in Section 5.1.3.1 and with downstream boundary tide curves derived for a range of maximum offshore tide levels (6.9, 7.0, 7.1, 7.2, 7.3 and 7.4mODN). Simulations have been performed both with and without barrier closures, for both barrier location No. 4 and barrier location No. 5 (locations shown in Figure 62). In order to investigate the minimum tide level for which barrier closure would be beneficial, a simulation of the period of actual tides from November 2014 to September 2015 (using data from the Hinkley Point tide gauge) and a constant flow of 65m3/s at Burrowbridge has been performed. For all the simulations, the 2D floodplain domain has been decoupled since it is inactive for the range of tides simulated. 5.5.2 Results Figures 80, 81 and 82 show, for the 6.9mODN, 7.1mODN and 7.4mODN tides respectively, the water level and flow in the River Parrett at Northmoor Pump Station (location shown on Figure 8) without barrier closure and with barrier close at location No. 4 and location No. 5. Discharge from the pumps (which drain flood water from Northmoor) is halted when the water level in the river exceeds 7.5mODN. At the lowest tide simulated (6.9mODN) the water level at Northmoor just reaches the pump stop level with no barrier closure. For lower tides, the pumps will operate and discharge into the River Parrett continuously over the whole tide cycle and pump discharges will not be affected by barrier operation. However, the total flow in the River Parrett still falls at high tide due to the backwater from the tide in the lower estuary. For higher tides, the reduction in flow is greater and reverse flow occurs. For both barrier locations, the water level and flow in the River Parrett at Northmoor Pump Station with barrier closure do not vary greatly for the range of tides simulated. For the same fluvial flow condition, the water level in the river upstream of the barrier depends principally on the duration of closure. Although the closure period increases slightly with increasing tide level, the effect on water levels further upstream is small. As well as allowing the pumps to continue operating at higher tides, barrier closure helps to maintain the fluvial flow for all the tides simulated. Tables 12 and 13 summarise the benefits of barrier closure for the range of tides simulated in terms of the additional volume of water evacuated down the Parrett and out of Northmoor via the pump station. The additional volume that could be pumped of out Northmoor is around 2%-11%. However, the larger additional volumes correspond to the upper end of the range of tides simulated. Tables 12 and 13 also indicate the maximum water levels at West Quay in Bridgwater, assuming no barrier

closure. Allowing for uncertainties in forecast tide level and depending on choice of closure trigger level, the barrier would likely be closed in any case for tidal flood protection for offshore tide levels of 7.2mODN or less. In terms of total fluvial discharge, barrier closure results in an increased volume for the range of tides simulated, representing a reduction in flood volume in the moors for the upper bound fluvial flow scenario (in which the spillways discharge continuously to the moors). For most of the tide range simulated the barrier would likely be closed in any case for tidal flood protection. For the lowest tide simulated (6.9mODN) the increase in discharge volume over one tide cycle is 3.8% for barrier location No. 4 and 7.1% for barrier location No. 5. The increase in discharge volume will be less for lower tides.

Table 12. Effect of barrier closure on flow in River Parrett at Northmoor Pump Station (locations No. 4 and 5) Additional Volume Additional Volume Average additional Maximum water Volume in one tide cycle in one tide cycle in one tide cycle flow over one tide level (mODN) (Mm3) (Mm3) (%) cycle (m3/s)

Offshore West No Closure Closure Closure Closure Closure Closure Closure Closure Tide Quay* barrier L4 L5 L4 L5 L4 L5 L4 L5 6.9 7.37 2.69 2.79 2.88 0.10 0.19 3.8 7.1 2.3 4.3 7.0 7.48 2.66 2.78 2.88 0.13 0.23 4.8 8.5 2.9 5.1 7.1 7.59 2.61 2.78 2.88 0.17 0.27 6.6 10.5 3.9 6.2 7.2 7.70 2.56 2.77 2.88 0.21 0.32 8.4 12.7 4.9 7.4 7.3 7.81 2.52 2.76 2.88 0.24 0.36 9.7 14.3 5.5 8.2 7.4 7.93 2.46 2.76 2.88 0.30 0.42 12.1 16.9 6.8 9.5 * without barrier closure

Table 13. Effect of barrier closure on discharge from Northmoor Pump Station (locations No. 4 and 5) Additional Volume Additional Volume Average additional Maximum water Volume in one tide cycle in one tide cycle in one tide cycle flow over one tide level (mODN) (Mm3) (Mm3) (%) cycle (m3/s)

Offshore West No Closure Closure Closure Closure Closure Closure Closure Closure Tide Quay* barrier L4 L5 L4 L5 L4 L5 L4 L5 6.9 7.37 0.65 0.66 0.66 0.01 0.01 2.0 2.0 0.3 0.3 7.0 7.48 0.63 0.66 0.66 0.03 0.03 4.2 4.2 0.6 0.6 7.1 7.59 0.62 0.66 0.66 0.04 0.04 6.4 6.4 0.9 0.9 7.2 7.70 0.60 0.66 0.66 0.06 0.06 9.6 9.6 1.3 1.3 7.3 7.81 0.59 0.66 0.66 0.07 0.07 11.1 11.1 1.5 1.5 7.4 7.93 0.59 0.66 0.66 0.07 0.07 11.1 11.1 1.5 1.5 * without barrier closure

8.0 120

NORTHMOOR PUMP STOP LEVEL = 7.5mODN 7.5 100

7.0 80

6.5 60

6.0 40

FLOW (m3/s) FLOW LEVEL (mODN)LEVEL 5.5 20

5.0 0

4.5 -20

4.0 -40 75 80 85 90 95 100 105 110 TIME (HOURS)

NO CLOSURE CLOSURE (L5) CLOSURE (L4) NO CLOSURE CLOSURE (L5) CLOSURE (L4)

Figure 80 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (6.9mODN offshore tide)

8.0 120

NORTHMOOR PUMP STOP LEVEL = 7.5mODN 7.5 100

7.0 80

6.5 60

6.0 40

FLOW (m3/s) FLOW LEVEL (mODN)LEVEL 5.5 20

5.0 0

4.5 -20

4.0 -40 75 80 85 90 95 100 105 110 TIME (HOURS)

NO CLOSURE CLOSURE (L5) CLOSURE (L4) NO CLOSURE CLOSURE (L5) CLOSURE (L4)

Figure 81 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (7.1mODN offshore tide)

8.0 120

NORTHMOOR PUMP STOP LEVEL = 7.5mODN 7.5 100

7.0 80

6.5 60

6.0 40

FLOW (m3/s) FLOW LEVEL (mODN)LEVEL 5.5 20

5.0 0

4.5 -20

4.0 -40 75 80 85 90 95 100 105 110 TIME (HOURS)

NO CLOSURE CLOSURE (L5) CLOSURE (L4) NO CLOSURE CLOSURE (L5) CLOSURE (L4)

Figure 82 Effect of barrier closure at locations No. 4 and No. 5 on upstream fluvial water level at Northmoor pump station (7.4mODN offshore tide)

Figure 83 shows the results of the simulation of the period of actual tides from November 2014 to September 2015, without barrier operation, and a constant flow of 65m3/s at Burrowbridge. Figure 83 shows, for each tide simulated, the offshore high water level and the corresponding minimum flow in the tide cycle at barrier locations No. 4 and No. 5. Positive flow values indicate flow downstream, towards the sea, negative flow values indicate flow upstream, inland. Barrier closure is likely to be beneficial only for tides in which the minimum flow between tides is negative such that barrier closure prevents tidally driven flow entering the reach upstream of the barrier. For tides where the minimum flow between tides without a barrier is positive then some drainage to sea over the whole tide period is maintained. Closing the barrier on such tides will tend to impede drainage. The results in Figure 83 suggest that the lowest tide for which barrier closure is still beneficial for the upper bound fluvial flow estimate would be around 4.5mODN for location No. 5 and around 3.6mODN for location No. 4. The simulation does not contribution from the King's Sedgemoor Drain, which would tend to increase the minimum tide level for zero flow at the barrier due to the greater volume of freshwater in the estuary below the barrier. For lower fluvial flows in the River Parrett, the minimum flow in a tide cycle will tend to reduce, resulting in a lower tide level at which closure is beneficial.

8 FLOW

7 65m3/s FLUVIAL 65m3/s

6 MHWS = 5.93mODN

4 OFFSHORE HIGH WATER LEVEL (mODN)LEVEL WATER HIGH OFFSHORE

MHWN = 3.01mODN 3

BARRIER LOCATION No. 4 BARRIER LOCATION No. 5 2 -350 -300 -250 -200 -150 -100 -50 0 50 100 MINIMUM FLOW AT BARRIER SITE (m3/s)

Figure 83 Correlation between simulated offshore tide levels and minimum flow in the River Parrett at barrier locations No. 4 and No. 5 for upper bound fluvial flow conditions

5.6 Barrier failure Simulations have been performed to assess the consequences of the barrier failing to re-open after a closure. By preventing discharge of the fluvial flow in the River Parrett during the low water part of the tide cycle, flooding may occur in Bridgwater upstream of the barrier. 5.6.1 Simulations In the event of the barrier failing in the closed position, the water level in Bridgwater upstream of the barrier will ultimately be determined by the total fluvial flow in the river and the discharge characteristics (crest level and discharge coefficient) of the banks upstream of the barrier. The lowest bank levels upstream of the barrier are at the Beazleys spillway (7.48mODN) and Allermoor Spillway (7.8mODN) on the River Parrett and the Hookbridge spillway (7.45mODN) on the River Tone. Free discharge is always achieved over Beazleys and Allermoor spillways due to the very large and low lying area of floodplain along the River Sowy and King's Sedgemoor Drain corridor. The Hookbridge spillway discharges into Currymoor, a much smaller area of floodplain bounded by higher ground and river banks. The water level in Currymoor is controlled by the overflows to the Northmoor floodplain at Lyng Cutting (7.08mODN) and Athelney Spillway (7.06mODN). Once Currymoor has filled and is overflowing to Northmoor, flow over Hookbridge spillway is limited by the water level in Currymoor and the flow in the River Tone downstream increases. Simulations have been performed for two fluvial flow conditions: i) “upper bound” fluvial flow condition as described in Section 5.1.3.1 (free discharge over all spillways) – for barrier failed closed at location No. 5; ii) “extreme” flow condition, boundaries for the upper bound fluvial flow condition increased to force the filling of Currymoor – for barrier failed closed at locations No. 4 and No. 5.

5.6.2 Results Figure 84 shows the simulated water levels in the River Parrett at Bridgwater (West Quay) and at Burrowbridge for the barrier failed closed. Water levels initially rise more rapidly for the upper bound condition since this includes pumping from Northmoor and Saltmoor pump stations such that the initial fluvial flow downstream of Northmoor pump station is higher than for the “extreme” scenario. However, the final water level for the extreme scenario (7.83mODN) is higher than for the upper bound simulation (7.65mODN) due to the higher flow over the banks of the Parrett upstream of the Tone confluence. The maximum water level is independent of barrier location and is 0.47m lower than the typical defence level in Bridgwater for the extreme scenario and 0.65m lower for the upper bound simulation. The maximum water level is reached more rapidly for barrier location No. 5 due to the smaller upstream storage volume. Figures 85 and 86 show the maximum water level profiles along the River Parrett and the indicative bank levels, together with the water level profile at low water for the two scenarios simulated.

LEVEL (mODN)LEVEL 5

3 -6 0 6 12 18 24 30 TIME AFTER BARRIER CLOSURE (HOURS) B'BRIDGE (L5) - UPR. BND. B'BRIDGE (L4) B'BRIDGE (L5) W. QUAY (L5) - UPR. BND. W. QUAY (L4) W. QUAY (L5)

Figure 84 Water levels at West Quay and Burrowbridge for barrier failing closed at location No. 5 (upper bound fluvial flow) and locations No. 4 and No. 5 (extreme flow)

7 ALLERMOOR BEAZLEYS 6 SPILLWAY SPILLWAY

4 LEVEL (mODN)LEVEL

2 TONE CONFL.TONE 1

B'BRIDGE

WESTQUAY SALTMOORPS -1 NORTHMOORPS 0 5000 10000 15000 20000 25000 CHAINAGE FROM LANGPORT GREAT BOW BRIDGE (m)

BED LEFT BANK (IND.) RIGHT BANK (IND.) BARRIER OPEN AT LOW WATER FAILED CLOSED L5

Figure 85 Barrier failed closed at location No. 5 simulation maximum water surface profiles (upper bound fluvial flow)

7 ALLERMOOR SPILLWAY BEAZLEYS 6 SPILLWAY

4 LEVEL (mODN)LEVEL

2 TONE CONFL.TONE 1

B'BRIDGE

WESTQUAY SALTMOORPS -1 NORTHMOORPS 0 5000 10000 15000 20000 25000 CHAINAGE FROM LANGPORT GREAT BOW BRIDGE (m)

BARRIER OPEN AT LOW WATER FAILED CLOSED L5 FAILED CLOSED L4 BED LEFT BANK (IND.) RIGHT BANK (IND.)

Figure 86 Barrier failed closed at locations No. 4 and No. 5 simulation maximum water surface profiles (extreme fluvial flow)

5.7 Adaptation Pathways Based on current climate change projections, the tidal flood risk in the area is expected to increase over time due to rising sea levels. Without intervention, this will result in an increase in flood risk to properties and infrastructure. The proposed Bridgwater Tidal Barrier Scheme has been designed to provide a 0.5% AEP Standard of Protection up to 2055 (under the “Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities” guidance). After 2055 further works would be required to maintain a 0.5% AEP Standard of Protection to properties and infrastructure. These are set out in the Parrett Estuary Flood Risk Management Strategy. The following adaptation pathway is proposed:

• 2022-2025: Bridgwater Tidal Barrier Scheme constructed, providing a 0.5% AEP Standard of Protection. Based on current climate change guidance the Scheme will maintain this Standard of Protection up to 2055.

• Up to 2055: Primary defences on the left bank of the River Parrett, between Chilton Trinity and Combwich, raised in a series of interventions – this has already commenced with bank raising at Cannington Bends.

• Beyond 2055: Secondary defences raised as required to maintain the Standard of Protection with sea level rise.

• Managed realignment at Pawlett Hams will also be implemented through breach of a section of the primary defences. A set-back defence may be required to maintain protection to properties and infrastructure. These works will allow intertidal habitat to be created, compensating for coastal squeeze elsewhere. Whilst climate change guidance can be used to identify the peak level for events in a given year, it should be noted that there is some uncertainty in the underlying projections. This means that a sea level with a given probability which is predicted in 2055 could actually occur much earlier or much later. The adaptation pathway described above should therefore be considered the current best estimate for how flood risk and flood defence infrastructure may evolve over time but may be updated in future. Hydraulic modelling has been undertaken to identify how flood risk may change over time and with implementation of the adaptation pathway. The following scenarios have been modelled:

• “Future banks 1”: See Figure 87. Defences proposed as part of the Bridgwater Tidal Barrier Scheme in place. Raising of the primary defences on the left bank of the Parrett between Chilton Trinity and Combwich to a level of 8.1mOAD. This is considered to be a conservative representation of the position in 2055 and has been used to determine the design level of the secondary defences.

• “Future banks 2”: See Figure 88. Raising of the primary defences on the left bank of the Parrett between Chilton Trinity and Combwich to a level of 8.3mAOD. “Glass-walling” of the primary and secondary defences which form part of the Bridgwater Tidal Barrier Scheme to identify the required level for bank raising to maintain a 0.5% AEP Standard of Protection. Defences extended where required to prevent bypassing. Breach of the existing right bank primary defence at Pawlett Hams and inclusion of a new set-back defence “glass-walled” to identify the required bank level. “Glass-walled” primary defences along the right bank of the Parrett and on the left bank at Combwich to identify the required defence level. Run for the 2125 climate change scenario.

Figure 87 “Future banks 1” scenario

Figure 88 “Future banks 2” scenario

The model results are shown in Figure 89 and 90.

Figure 89 “Future banks 1” scenario flood extent

Figure 90 “Future banks 2” scenario flood extent

Appendix A Barrier Closure Rules

Bridgwater Tidal Barrier: Closure Rules Page 1

Bridgwater Tidal Barrier Closure Rules

PREPARED FOR: Environment Agency

COPY TO:

PREPARED BY: Jack Mason, Ed Hill

DATE: 23 February 2017

PROJECT NUMBER: 672450 REF: ENVIMSW002039-CH2-FBS-00-TN-C-00077

REVISION NO.: Draft v6

APPROVED BY: Russell Corney Bridgwater Tidal Barrier: Closure Rules Page 2

Bridgwater Barrier – Closure Rules

Objective

1. When and how frequently the Bridgwater Tidal Barrier (BTB) is closed has a significant bearing on a number of factors including:

a) Residual flood risk to Bridgwater

b) Costs associated with operating the barrier and long term operating and maintenance planning

c) The function of the existing tidal defences in Bridgwater and their future management

d) Impacts on geomorphology and associated influence on habitats, species and navigation

2. This technical note aims to summarise the closure logic taking into account the various drivers. The main focus in the tidal flood risk operation but fluvial management operation is also discussed.

3. This technical note generally focusses on the tidal closure rules associated with a barrier at Site 5. However the implications of the barrier at Site 4 are discussed at the end of the note.

Tidal Closure Analysis

4. Figure 1 (see end of memo) summarises the assessment of the proposed barrier closure logic which is discussed in detail below. Steps in the figure are labelled (e.g. A. B. etc).

5. The costs of using the barrier will increase with the frequency of operation due to manning and operational costs and mechanical wear leading to increased maintenance or replacement investment. A high frequency of operation also has the potential to have a more significant environmental impact.

6. The existing flood defences in Bridgwater protect the town to a level of approximately 8.3m AOD (A), without any allowance for freeboard. The existing ground levels in the centre of Bridgwater are about 7.5m AOD, whilst outside of the centre they are lower at about 6.5m AOD or in some places lower.

7. Tide level forecast information would inform the closure of the barrier. Following a forecast of a tide exceeding the operation trigger level the barrier would be closed on the preceding low tide. The barrier would be reopened as soon as water levels were lower downstream of the barrier than upstream.

8. Modelling has shown for Site 5 under high fluvial flows (approximately 65m3/s in the River Parrett) the ponded level behind the barrier would reach approximately 7.24m (B). In this modelled scenario the barrier was shut against a 1 in 200 year tide in 2015. The analysis includes the recent dredging works which does increase flows on the River Parrett, and the expected pumping volumes from both permanent and temporary pumps on the moors. Bridgwater Tidal Barrier: Closure Rules Page 3

9. In the future higher fluvial flows are possible either due to more pumping capacity from the moors, dredging and climate change. Whilst higher fluvial flows associated with climate change may increase flooding on the moors upstream of the barrier, the impact on flows in the River Parrett will be limited as excess flow is taken by the moors. However it is prudent to accommodate future changes. Modelling has been undertaken to increase flows by 25% downstream of Northmoor pumping station as a further sensitivity and the assessment shows a further increase of about 250mm. Therefore an allowance of 300mm is considered suitable (C). Therefore the defences in Bridgwater must retain water up to of at least 7.6m (rounded up to 100mm) (D) for Site 5.

10. The current defences at 8.3m (A) are above that needed to store fluvial flows when the barrier is closed. However, even if forecast tide levels were accurate (see discussion below) additional freeboard should be provided to:

a) Provide flexibility in the future management of the Bridgwater defences taking into account the age of the defences and future fragility.

b) Acknowledge that the defences have failed in the past (November 2011 – albeit under an unusual load case)

c) Accommodate any future settlement without needing major defence rebuilding (earth embankments in particular)

d) Minor allowances for any wave action associated with the barrier operation.

11. It is difficult to provide a fully justified assessment of these factors. However if a freeboard allowance of 300mm (E) is allowed for these aspects the maximum design still water level should be taken as 8.3m – 0.3m = 8.0m AOD (F). The maximum water level at West Quay on 3rd January 2014 was 8.117m AOD. The defences performed well during this event and this provides evidence that the defences are effective up to this level.

12. The barrier would be closed at low tide based on a forecast of the following high tide. There is uncertainty in the forecast which means that the barrier will have to be closed more frequently to ensure that the tide is not under-forecast (i.e. this will mean that in retrospect the barrier would have been closed unnecessarily in some cases). If an allowance of 300mm is allowed [this is the best information we have at the moment based on a suggestion from Roger Quinn for Hinkley] for the uncertainty then the forecast flood level could be 7.7m AOD to ensure that a level of 8.0m is not exceeded.

13. So in theory the closure trigger level can be 7.7m AOD or less (H). For Site 5 it is necessary to retain the fluvial flood water up to at least 7.6m for Site 5. So the lowest trigger level could be 7.3m (including the forecast allowance). A lower trigger level is not worthwhile as the defence level is determined by the fluvial storage. The range of potential trigger levels is therefore 7.3m to 7.7m AOD.

14. The frequency of tidal operation is summarised below for the potential range of trigger levels. The table does not include for maintenance testing, potential fluvial operation or potential sediment management. The closure frequency is very sensitive to future sea level rise. The assessment of frequency is based on analysis of 20 years of recorded water levels at West Quay, Bridgwater and at Hinkley Point. Bridgwater Tidal Barrier: Closure Rules Page 4

Site 5 Trigger Maximum Fluvial Average Average Average Level mAOD Water Level Closures/Year Closures/Year Closures/Year Upstream of 2015 2055* 2125* Barrier mAOD

7.3m 7.6m 5 12 55

7.7m 8.0m 1 2 20

* based on the medium emissions 95%ile projection

15. For maintenance purposes the barrier may be operated up to twice a month, although it is anticipated that this would generally be a short closure, less than 1 hour (unless there was the need to check seals). If the barrier was operated to manage flood risk this would negate the need for a further maintenance test if the activation occurred within the appropriate period.

16. This assessment should be reviewed during the later design phases to take into account changes (for example agreed dredging regime, changing operation of the Sowy River, better tide forecasting, pump changes etc.).

17. The fluvial storage provided by a barrier at Site 4 is significantly higher than at Site 5 due to the shape of the estuary. The fluvial ponded water level is approximately 6.48m compared with 7.24m for Site 5. Allowing for future changes and rounding up the minimum storage level for Site 4 would be 6.8m. AOD. In theory a trigger level of as low as 6.5m could be adopted for Site 4.

18. A trigger level as low as 6.5m would mean that the barrier would be operated more frequently as shown below.

Site 4 Trigger Maximum Fluvial Average Average Average Level mAOD Water Level Closures/Year Closures/Year Closures/Year Upstream of 2015 2055* 2125* Barrier mAOD

6.5m 6.8m 50 90 200 (minimum)

6.8m 7.1m 25 50 115 (illustrating 300mm higher than minimum)

* based on medium emissions 95%ile projection

19. A lower maximum water level upstream of the barrier provides the opportunity to reduce the length of flood defences upstream of the barrier which have to be maintained. The impact is summarised below for a maximum water level upstream of the barrier of 7.6m AOD for Site 5 and 6.8m AOD for Site 4. Bridgwater Tidal Barrier: Closure Rules Page 5

Site Maximum Length of Existing Length of Existing Ground Total Length Fluvial Ground Levels Behind Levels Behind Defences m AOD (both Water level Defences Below Water Above Fluvial Water left and right Upstream Level (so requiring Level (so not requiring bank) mAOD defences to be retained) defences to be retained) m m

5 7.6m 6638 (93%) 529 (7%) 7167

4 6.8m 4044 (56%) 3123 (44%) 7167

Note: In many cases the tidal defences are also retaining walls or footpaths, so maintenance may be required for these functions even if flood defence is not required.

20. High frequencies of operation are unlikely to be acceptable due to the potential impact on estuary geomorphology, protected sites, navigation and the additional cost involved. Therefore it is considered that the operational trigger levels for tidal surge are likely to be in the range 7.3 – 7.7mAOD. This assessment is being tested from an environmental impact assessment.

Fluvial Closure Analysis

21. An assessment was carried out to identify if the barrier could be used to reduce flooding on the Somerset Levels and Moors by excluding the tide during periods of high fluvial flow. The potential benefit is related to the ability to continuously operate Northmoor pumping station (PS), where pumping currently stops when the water level in the River Parrett reaches 7.5mAOD.

22. The model was run with a high fluvial flow at Northmoor PS of 66m3/s against various tides. A 66m3/s flow in the Parrett represents an estimate of the maximum flow that can physically be achieved in the channel downstream of Northmoor PS for the current channel size during lower magnitude tides. The current modelled channel size includes the design cross-sections for the “8Km” Parrett and Tone dredge in 2014 (assuming the channel sections have been maintained) and the recently completed “750m” dredge of the Parrett downstream of Northmoor. The estimate allows for:

§ overflow from the Parrett into the Sowy via Allermmoor and Beazleys spillways;

§ overflow from the Tone into Currymoor via Hookbridge spillway; and

§ maximum pumped flow into the Parrett at Saltmoor (5m3/s) and Northmoor PS (15m3/s) based on current operational procedures including temporary pumps (N.B. Currymoor PS is prevented from discharging to the Tone under such conditions due to high water level in the Tone).

The flow value has been obtained from model simulation of the above conditions using the latest Parrett and Tone model as updated for the Bridgwater Tidal Barrier project.

23. The modelling showed that for a fluvial flow of 66m3/s the water level at Northmoor PS will only exceed 7.5mAOD when the tide level in Bridgwater exceeds 7.3mAOD. For tide levels in Bridgwater of 7.5mAOD and 7.8mAOD, the water level at Northmoor PS would currently Bridgwater Tidal Barrier: Closure Rules Page 6

exceed 7.5mAOD for approximately 1 and 2 hours at the peak of the tidal cycle respectively. Closure of the barrier during these events would maintain the water level at Northmoor PS below 7.5mAOD and allow pumping to continue. Appendix A Figure 1 shows the effect on water levels at Northmoor PS with and without the barrier for a peak tide level at Hinkley of 7.4mAOD. A tide level of 7.4mOD at Hinkley approximately corresponds to a peak water level in Bridgwater of 7.8mOD and a 1 in 2 year return period event during which it is anticipated that the barrier would be closed for tidal exclusion.

24. The ongoing dredging of the River Parrett has changed how the water level at Northmoor PS responds to high tide levels. The increase in channel area has apparently dampened the river level response at Northmoor PS to extreme tides. For this reason, direct validation of modelling results using historic gauged river level data for an extreme event was not possible. However, we have extracted gauge data for Hinkley, West Quay and Northmoor PS on 17 October 2016, when there was a 7.0m tide at West Quay and a moderate flow in the River Parrett, refer to Appendix A. This shows that with a large tide, approaching when the barrier might be closed for tidal purposes, the response at Northmoor is not great enough to require the pumps to be switched off. Also on 14 January 2016 there was a high flow in the Parrett and a mean spring tide and again this did not generate sufficient response to require the Northmoor pumps to be switched off.

25. Closure of the barrier to exclude extreme tides during periods of high fluvial flow will allow a greater total discharge from the River Parrett and from the Northmoor pumps over a tidal cycle. Appendix A Table 1 summarises the benefits of closing the barrier for a range of tides in terms of additional volume evacuated down the Parrett and out of Northmoor. The tides included in Table 1 are in the range when the barrier would be closed against the tide. The trend is for reducing fluvial benefit with reducing tide level, therefore it is expected that at tides lower than those presented the fluvial benefit would be smaller still.

26. In summary, the barrier can be used to reduce fluvial flood levels and volumes in the River Parrett however, the tide level threshold from which the barrier would be effective for this purpose is likely to be greater than 7.3mAOD and this is at the lower end of the expected range for tidal closures. Bridgwater Tidal Barrier: Closure Rules Page 7

Figure 1: Barrier Closure Trigger Logic

BRIDGWATER DEFENCE LEVEL AND BARRIER TRIGGER LEVEL SITE 5 8.5

8.4

8.3 A. EXISTING DEFENCE LEVEL ~ 8.3mODN

8.2 E. 300mm 8.1 FREEBOARD

8.0 F. MAXIMUM DESIGN STILL WATER LEVEL 8.0m ODN

7.9 G.UNCERTAINTY IN FORECAST 7.8 TIDE LEVEL

7.7 D. MIMIMUM REQUIRED FLUVIAL WATER RETAIN LEVEL = 7.6m

LEVEL (mODN) 7.6 ODN 7.5 H RANGE OF BARRIER UNCERTAINTY IN CLOSURE TRIGGER LEVELS 7.4 C. MINIMUM 300mmFLUVIAL WATER 7.3m to 7.7m AOD FREEBOARD LEVEL 7.3 B.FLUVIAL PONDED LEVEL = 7.24mODN 7.2 (BARRIER LOCATION 5, 65m3/s FLUVIAL FLOW)

7.1

7.0 Bridgwater Tidal Barrier: Closure Rules Page 8

Appendix A Figure 1

TIDE AT HINKLEY POINT = 7.4mODN PARRETT AT NORTHMOOR PS 8.0 120

7.5 100

7.0 80

6.5 60

6.0 40 FLOW (m3/s) LEVEL LEVEL (mODN)

5.5 20

5.0 0

4.5 -20

4.0 -40 75 80 85 90 95 100 105 110 TIME (HOURS)

NO CLOSURE CLOSURE (L5) CLOSURE (L4) NO CLOSURE CLOSURE (L5) CLOSURE (L4)

L4 = Barrier at Site 4 L5 = Barrier at Site 5 Bridgwater Tidal Barrier: Closure Rules Page 9 Water level gauge records for Hinkley, West Quay and Northmoor

NB – Hinkley gauge time base appears to be incorrect for 14 Jan 2016 although level compares well to tide tables Bridgwater Tidal Barrier: Closure Rules Page 10

Appendix A Table 1

L4 = Barrier at Site 4 L5 = Barrier at Site