Impact of the Terminal Groyne on Sediment transport in Minehead Bay

1. Impact of the Terminal Groyne on Sediment transport in Minehead Bay Jane Moon Senior Geomorphologist

2. Minehead Terminal Groyne

3. Background Minehead Terminal Groyne

4. Issues Minehead Terminal Groyne • The source of the shingle in Minehead is derived from erosion of the cliffs to the west of the Bay. • Shingle transport (littoral drift) is from west to east and is being driven by predominant north-westerly waves. Interruption of shingle sediment supply • Harbour groyne • Harbour arm/breakwater • Terminal rock groyne?

5. Aims and Objectives Minehead Terminal Groyne • to assess how the terminal rock groyne, constructed as part of the wider Minehead Flood Defences Scheme in 1997, may be affecting the movement of sediment (sand and shingle), and • to determine if it is having a detrimental impact on the adjacent coastline

6. Key Questions Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? How effective is the groyne system in holding the sand in place, bearing in mind there is unlikely to be any new feed from the west?

7. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process?

8. The shingle deposits form a bar which extends out from the end of the terminal rock groyne. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Fine shingle and coarse gravel is accumulating along the HWM on the western side of the groyne.

9. Minehead Terminal Groyne Methodology: • The wave climate is considered to be the most important factor in controlling the direction and rate of sediment transport along beaches and foreshore, particularly in relation to the movement of coarse sediment such as shingle. • A regional wave model was used to predict the wave climate near the shore for a given offshore wave height, period and direction. • Wave vectors derived can therefore be used to indicate sediment transport processes (long-shore and cross-shore). Where is the shingle, how is it moving and what is the role of the groynes in controlling this process?

10. Minehead Terminal Groyne Inputs: • Time series wave data • Monitoring from 2006 - 2007 • Minehead Directional WaveriderBouy • Dominant north-westerly wave direction • Bathymetry Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Bathymetric grid (mOD)

11. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Shows wave vectors for low water situation where breaking waves are refracted by the shallow gradient of the bathymetry and features such as the offshore shingle bar which extends offshore from the terminal rock groyne.

12. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? • The wave model shows a strong long-shore wave direction which is driving movement of shingle from west to east in this area. • Resulting in the accumulation of shingle against the groyne further east (groyne 4)

13. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? • Shingle is accumulating on both sides of the groyne, but does not ramp up next to the groyne to the same degree as the shingle accumulated next to groyne furthest east (groyne 4). • Shingle in this area is largely being mobilised by cross-shore sediment transport processes, which allows littoral drift to continue to transport material in the direction the waves are breaking (i.e. towards the shore at the end of the terminal rock groyne).

14. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? • In the past, before the groyne was built, littoral drift processes would have been responsible for driving shingle right up to the coast - which would have acted as a supply of shingle along the beach further east. • Wave energy is now being dissipated by the rock armour of the groyne. • As a result shingle is not being driven as far inland as it might have been if the rock groyne were not there. • Lack of available coarse sediment may also be limiting the development of this offshore shingle bar.

15. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Methodology: • LiDAR data - Sept 2001 to Sept 2010 • Analysis of LiDAR data was used to: • Compare changes in ground level around the terminal rock groyne. • Map the pattern of erosion and deposition along the foreshore. • Calculate the change in volume between different surveys

16. The groyne appears to no longer be maintaining beach levels further west in Minehead Bay and the land level is relatively even on both sides of the groyne. analysis of ground levels The influence of the groyne on maintaining the sand beach decreases from section 4 (i.e. from where the rock groyne changes in orientation and elevation). The largest difference in beach elevation occurs here. The groyne is helping to maintain the sand beach in this location. Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Minehead Terminal Groyne The crest elevation of the terminal groyne decreases steeply here, which may be influencing adjacent beach levels.

17. Lidar analysis Significant loss of shingle here due to the 2008 emergency repair works to the shingle ridge. Shingle material was won from the foreshore from this location. Deposition has occurred just below 0mOD, extending beyond the limit of the rock groynes. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? Some erosion and loss of sand has occurred along the beach at MHW

18. Minehead Terminal Groyne Where is the shingle, how is it moving and what is the role of the groynes in controlling this process? • The offshore bar was used to source shingle in 2008 to help repair the storm ridge adjacent to Minehead Golf Course. • Approximately 3,500m3 of shingle was removed during the works from the area at the end of the terminal rock groyne. • In recent years, it appears that this area has recovered both in elevation and volume, which indicates that littoral drift processes are active in this area and are either: • redistributing shingle and or • bringing in a new supply of shingle from further offshore.

19. Minehead Terminal Groyne How effective is the groyne system in holding the sand in place, bearing in mind there is unlikely to be any new feed from the west?

20. Minehead Terminal Groyne • The transport of sand along the beach in Minehead is occurring between all the groynes. • This is primarily caused by wind or wave processes. • The wave model shows that longshore movement of sand is not as significant a process as the cross-shore movement of sand, due to the orientation of waves as they reach the beach in the Bay. • Groynes were built to the length they are to prevent and reduce cross-shore losses. • Wind blown sand is a very significant issue along the beach above MHW. The fine sand is easily mobilised by wind, which has resulted in large volumes of sand being blown over the seawall onto the esplanade. • Most of the net loss of sand on Minehead beach can be attributed to wind processes. • The groyne complex is having limited or no impact on preventing losses of sand associated with wind blown processes.

21. Conclusions Minehead Terminal Groyne • The adjacent coastline to the east is being starved of shingle because of the location of the groyne (and other structures such as the harbour groyne). • This area was stabilised through the recent 2010 works - therefore the risk of breach resulting from lack of sediment supply has been significantly reduced. • There is evidence, however, that some new shingle is being delivered into the Bay. • There may be some benefits in shortening the length of the terminal rock groyne by up to 30m in length. However, there are significant risks associated with this option such as potential increase in loss of sand (resulting from cross-shore processes) from along the recharged beach. • It was therefore concluded that there was unlikely to be any significant benefit gained from altering (shortening) the length of the terminal rock groyne. • Previous physical modelling carried out during the design on the scheme showed that it was the volume of material rather than the beach profile that is key to maintaining the flood risk defence standard provided by the beach. • Due to the amount of sand lost by wind processes it is likely that the removal of sand from the esplanade and redistribution on the beach would need to continue in order to maintain the present level and volume of sand on the beach.

22. Recommendations Minehead Terminal Groyne Repeat bathymetry survey Bathymetry was surveyed in August 2007 by PCO and is due again after 5 years (i.e. August 2012). • This data is extremely valuable and could help us to monitor changes in both the extent and volume of offshore shingle deposits to enable us to better understand the nature of the incoming supply of shingle into this area. • In addition, this information would be helpful in determining if material could be used for any future maintenance works, i.e. to recharge the shingle ridge or beach further east around the Warren.