Design requirement for mixed sand and gravel beach defences under scenarios of sea level rise

Along many coastlines of the world, beaches provide the primary defence against flooding or erosion with their fate under rising sea levels still only poorly researched. This is particularly the case for the coastline of Southeast England, where similar to 190 km of shingle barriers protect low-lying hinterland or a coastal plain that is at or barely above the present reach of waves. During the Holocene transgression, these beaches moved into their present position through longshore extension and cross-shore roll back. This process stopped more than a century ago with port developments affecting longshore transport and the construction of groynes to hold the beach in place. This was followed by beach recharge and recycling towards the end of the 20(th) century to build up and maintain beaches as coastal defences in their mid 19(th) century position. This paper explores the design requirements for these beaches under future sea level rise scenarios of 1 to 5 m using recently developed tools. It shows that the presently still semi-natural beaches have to increase in size with crest height elevations having to rise by at least up to 1.26 times the rate of sea level rise and that due to higher longshore wave power, especially during storm conditions in the future, higher and stronger groynes are needed to hold these larger beaches in place. Future design requirements for beaches are sensitive to foreshore levels and orientation of the beach to the dominant waves with those presently characterised by shallow foreshores and oblique wave approach requiring the biggest adjustments. Required size increases will be difficult to implement due to the built-up nature of the hinterland. The engineering alternative would be to replace beaches with hard structures, a process that has already started where maintaining a beach is no longer economically viable or the residual risk associated with overwashing and an eventual tidal breach has become unacceptable.