Modelling storm-induced beach/dune evolution: Sefton coast, Liverpool Bay, UK

Storm-induced dune evolution on a sandy coastal system is investigated using a nested modelling approach applied to the Sefton coast, Liverpool Bay, UK. Real-time offshore water levels and waves were used as model boundary forcings. A Delft3D coarse grid setup is used to simulate time and space varying sea surface elevations on which offshore waves are transformed (by applying the SWAN model) to establish the wave boundary for the high resolution morphological model (XBeach). Statistical comparisons between model predicted and measured post-storm profiles at a number of locations along the coast suggest that XBeach successfully captures storm-induced beach change along the Sefton coast. Predicted bed evolution of the beach/dune system shows alternate erosion and sedimentation areas in the nearshore. Strong bed level changes are found at the northern part of the Sefton coast when north-westerly (NW) extreme waves and winds coincide with spring-high tide. Morphological changes in the southern part are significantly lower than that in the north as a result of NW wave dissipation on the shoals located to the north of the Crosby channel, which creates low wave actions in that area. In addition, erosion of the dune foot is observed at some locations along the beach. Temporal simulation of beach/dune evolution as a result of variable forcing conditions during storms provides useful insight into the morphodynamic processes of beach/dune systems during storms (using Sefton as an example), which is very useful for developing coastal management strategies over the existing conceptual tools.