Weston Bay Beach and Dune Management Plan Technical Appendix B Sediment Transport Modelling
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Weston Bay Beach and Dune Management Plan Technical Appendix B Sediment Transport Modelling June 2013 Draft Report 9Y0510 HASKONING UK LTD. RIVERS, DELTAS & COA STS Stratus House Emperor Way Exeter, Devon EX1 3QS United Kingdom +44 1392 447999 Telephone 01392 446148 Fax [email protected] E-mail www.royalhaskoningdhv.com Internet Document title Weston Bay Beach and Dune Management Plan Technical Appendix B Sediment Transport Modelling Document short title Sediment Transport Modelling Status Draft Report Date July 2013 Project name Weston Bay Beach and Dune Management Plan Project number 9Y0510 Client North Somerset Council Reference 9Y0510/R/303395/Exet Drafted by Eddie Crews Checked by Martha Gaches Date/initials check …………………. …………………. Approved by Greg Guthrie Date/initials approval …………………. …………………. A company of Royal HaskoningDHV CONTENTS Page 1 INTRODUCTION 1 1.1 Location 1 2 INPUT DATA FOR SEDIMENT TRANSPORT MODEL 2 2.1 General 2 2.2 Bathymetry 3 2.3 Tide and Still Water Levels 4 2.4 Wave Data 5 2.4.1 General 5 2.5 Tidal Currents 6 2.6 Wind 7 2.7 Sediment Analysis 7 3 LITPACK SEDIMENT TRANSPORT MODEL 8 3.1 Litpack 8 3.2 Analysis of Littoral Drift 8 3.2.1 General 8 3.2.2 Model Set-up 9 3.2.3 Calibration and Validation 9 4 RESULTS 10 4.1 Net Longshore Sediment Transport 10 4.2 Variation in Sediment Transport 10 5 REFERENCES 12 Appendices Appendix A Plots of net, positive and negative annual sediment transport Weston Bay Beach and Dune Management Plan Technical Appendix – Sediment Transport Draft Report June 2013 1 INTRODUCTION This Technical Appendix (B) supports the Weston Bay Beach and Dune Management Plan (Weston Bay B&DMP) commissioned by North Somerset Council (NSC). This document describes the sediment transport modelling conducted for the Weston Bay B&DMP outlining input data and modelling methodology as well as giving an overview of the results. This sediment transport modelling study was designed to complement the results from the two previous sediment transport modelling studies which we are aware of in Weston Bay: • Weston Seafront Strategy Study conducted by Royal Haskoning for North Somerset Council (2004) • Tropicana Sediment Transport Study produced by Royal Haskoning (2007) for HCD Management Ltd Results from all three studies have been used to inform the Weston Bay Beach and Dune Management Plan. 1.1 Location The area covered by the Weston Bay B&DMP) is shown in Figure 1.1, denoted at either end by two black arrows. This area stretches from Brean Cross sluice on the Axe Estuary to the south, to Anchor Head at the north end of Weston Bay. Weston Bay Beach and Dune Management Plan 9Y0510/R/303395/Exet Draft Report - 1 - June 2013 Figure 1.1 Study Area Location Plan 2 INPUT DATA FOR SEDIMENT TRANSPORT MODEL 2.1 General Information for this study has been taken from previous studies including the Weston Seafront Strategy Study conducted by Royal Haskoning for North Somerset Council (2004) and the Bridgwater Bay to Bideford Bay Shoreline Management Plan, Volume 3, Halcrow, 1998. The results from these studies were reviewed and used in the sediment transport model. Modelling was also carried out as part of the proposed redevelopment of the Tropicana building. A baseline was produced by running the model with the existing Tropicana building in place. The model demonstrated that, in general, that the potential long shore net sediment transport in the vicinity of the Tropicana was in a southerly direction, with sand being moved from the northern end of the beach and deposited in the south. During a mean high water spring tide level the potential net sediment transport was found to be approximately 16,170m3/yr across the five beach profiles considered in the study. Weston Bay Beach and Dune Management Plan Technical Appendix –Sediment Transport Draft Report - 2 - June 2013 Further modelling was carried out using Unibest LT in order to revise estimates of overtopping and sediment transport to allow the effect of proposed options to improve the coastal flood defences at Weston – super – Mare. Weston-super-Mare is situated in the Weston Bay process unit of the Severn Estuary Coastal Cell that extends from the rock headland of Birnbeck in the North to Brean Down in the South (as defined in the Severn Estuary Shoreline Management Plan). The limestone headlands protect a bay which is comprised predominantly of fine sediments within the intertidal zone, with a sandy beach around the high water mark. The hinterland at Weston is generally low lying across the extent of the bay. In the north, the back shore is highly developed and protected by a stone seawall which was improved as part of the Seafront Enhancements Project. To the south the backshore is occupied by Weston-super-Mare Golf Club which is protected by a robust ridge of sand dunes. Extreme waves approach predominantly from a westerly direction and are considered to be ocean (swell) waves generated in the large fetch of the Atlantic Ocean. However, modelling has shown that the more significant wave conditions influencing Weston Bay are locally generated wind waves approaching from a slightly broader directional sector. There is a large tidal range of 11.2m (mean spring tides) over a shallow sloping beach of sand and mud. There is high suspended sediment content in the water, primarily from the high sediment load of the Severn Estuary, but also from the discharge of the River Axe, south of the Bay. 2.2 Bathymetry In order to assess the sediment transport for the full length of the beach five cross shore beach profiles were established to estimate the potential littoral drift. These five profiles were extracted from the existing wave model created for this project (see technical Appendix A – Wave Modelling). The bathymetry data contains CMAP data (Digital Admiralty Chart soundings), topographic survey data covering an area between Knightstone Island and the Tropicana (Conducted for the Sea Defence Project, 2006) and LiDAR data of the study area. The location of the cross shore profiles can be seen in Figure 2.1. Weston Bay Beach and Dune Management Plan 9Y0510/R/303395/Exet Draft Report - 3 - June 2013 Figure 2.1 Location map of cross shore profiles 2.3 Tide and Still Water Levels Tides at Weston-super-Mare are semi-diurnal i.e. comprising two high tides and two low tides each day. The mean spring tidal range is 11.2m, with a mean neap tidal range of 5.8m. For the purposes of modelling, tide levels were taken from the Admiralty Tide Tables for 2013, which is the standard now used by the Environment Agency for extreme tide predictions in the South West Region. The tide levels used for modelling purposes are shown in Table 2.1. Weston Bay Beach and Dune Management Plan Technical Appendix –Sediment Transport Draft Report - 4 - June 2013 Table 2.1: Spring, neap and extreme tide levels at Weston-super-Mare Levels relative to Levels relative to Tide Level Chart Datum ODNewlyn (m CD) (m) Chart Datum 0.00 -6.00 Mean Low Water Springs (MLWS) 0.80 -5.20 Mean Low Water Neaps (MLWN) 3.00 -3.00 Mean Water Level (MWL) 6.15 0.15 Mean High Water Neaps (MHWN) 8.80 2.80 Mean High Water (MHW) 10.40 4.40 Mean High Water Springs (MHWS) 12.00 6.00 1Yr Return Period 13.62 7.62 200Yr Return Period 14.43 8.43 1000Yr Return Period 14. 76 8. 76 NB. Chart datum at Weston-super-Mare is –6.0m Ordinance Datum 2.4 Wave Data 2.4.1 General All available wave data sources are detailed in the Wave Modelling Technical Appendix (Section 2.4). This sediment transport study only uses annual nearshore Wave Climate created as part of the Weston Seafront Strategy Study (Posfords Haskoning, 2004) using data from the Bridgwater Bay to Bideford Bay Shoreline Management Plan, Volume 3, Halcrow, 1998. The inshore wave heights from this study used for the sediment transport modelling can be seen in Table 2.2 which shows the frequency of wave heights by direction. Table 2.2: Annual wave climate for Weston Bay, wave duration in days Wave Direction (degrees) height o o o o o o o o o o o o Hs(m) 195 -225 225 -255 255 -285 285 -315 315 -345 345 -015 6.0-6.5 0 0 0 0 0 0 5.5-6.0 0 0 0 0 0 0 5.0-5.5 0 0 0 0 0 0 4.5-5.0 0 0 0 0 0 0 , m) , s 4.0-4.5 0 0 0 0 0 0 3.5-4.0 0 0 0 0 0 0 Height(H 3.0-3.5 0 0 0 0 0 0 2.5-3.0 0 0 1 0 0 0 2.0-2.5 0 1 4 0 0 0 1.5-2.0 0 5 10 1 0 0 1.0-1.5 3 10 20 4 1 0 0.5-1.0 8 9 35 5 3 0 0.0-0.5 8 10 34 10 10 172 Weston Bay Beach and Dune Management Plan 9Y0510/R/303395/Exet Draft Report - 5 - June 2013 Further information regarding the derivation of the root mean squared wave height and the zero crossing periods is included in the Wave Modelling Technical Appendix. A wave rose illustrating the wave height and direction can be seen in Figure 2.2. The main directions of wave attack are between 240° and 300° with the largest waves coming directly towards the bay from 270°.