Understanding bimodal seas, shingle beach response and flood risk at Hayling Island, UK

Beach management through nourishment and annual recycling has been applied for over 37 years to manage flood and coastal erosion risk to 1700 properties at Eastoke, Hayling Island, UK. This form of natural flood risk management has proved successful in avoiding the annual flooding that blighted this area previously. However, there have been unexpected storm events that have caused beach erosion and localised flooding. Using long-term nearshore wave datasets and state-of-the-art statistical methods, new multi-variate extreme wave conditions were derived and applied to assess beach performance. Eastoke beach was found to meet its original design criteria of a 0.5% AEP standard of protection for unimodal wave conditions. However, it experiences greater rollback erosion, wave overwash and therefore flood inundation under certain (but not all) bimodal wave conditions, causing uncertainty around the future standard of protection. Communicating these inconsistencies to practitioners and a non-specialist audience is challenging as we tend to oversimplify, despite every beach and storm being unique. Better understanding of mixed beaches, both in situ and through parametric and numerical models, would reduce uncertainty to ensure communities are resilient to climate change.