Understanding bi-modal seas, shingle beach response and flood risk at Hayling island, UK

Abstract

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 which have caused beach erosion and localised flooding. Using long-term nearshore wave datasets and state-of-the-art statistical methods, new multivariate extreme wave conditions were derived. The conditions were applied to simulate beach profile response, estimated bimodal overtopping rates and resulting flood inundation, then validated against recent storm events. The 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 roll back erosion, wave overwash and therefore flood inundation under certain bimodal wave conditions, causing uncertainty around the future standard of protection.It remains unclear if the unexpected beach response at Eastoke resulted from increased storminess due to climate change, additional loading by mixed sea/swell events, increasing bimodal events, or the assumptions within the original bivariate analysis, leading to incorrect AEP attribution.Communicating these issues 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.