Occurrences of Predicted Wave Overtopping Assessed Using In Situ Winter Observations at an Emergent Beach-Fronted Sea Wall

A growing reliance on (ageing) sea walls to protect people, property and infrastructure is a global coastal management issue. Wave overtopping hazard is increasing in likelihood throughout the year due to sea-level rise and is not only associated with winter storms. Adaptive management requires accurate predictive tools to explore future options and forecast hazard. Predictive tools, based on empirical rules derived mainly from scaled experiments, can be highly uncertain in real-world application due to limited multi-process representation. We analyse a unique 4-month field dataset of coastal wave overtopping at two locations, ~345 m apart, along a sea wall fronted by an emergent beach, in SW England. Nearly one order of magnitude difference in the number of waves overtopping occurs between the two sites, most likely due to the variability in the beach influencing the coastal dynamics. Higher beach levels reduce but do not always eliminate overtopping, even when the beach is emergent above the high tide level. The number of waves overtopping increases with higher water elevations. The likelihood of overtopping will potentially increase with sea-level rise while the emergent beach is sustained. During winter 2021/2022, overtopping was incorrectly predicted for 49% of the tides using an empirical approach that disregards wind influence and the temporal variability in and emergence of the beach levels in front of the sea wall. Predictive tools and approaches for beach-fronted vertical structures must account for non-static and emergent beaches, wind influence and broken waves.