Statistics of High-Water Events Controlling the After-Storm Recovery of Coastal Dunes

Coastal dunes protect the coast from storm-driven flooding and erosion, and contribute to the development of unique habitats. Although mature dune erosion and overwash caused by large storms have been extensively studied, there is a knowledge gap in the effect of lower-intensity, but higher frequency, high water events (HWE). These events are especially relevant in the after-storm dune recovery, when the elevation of proto-dunes is small and vegetation is colonizing low-lying washover fans (Duran and Moore, Nature Clim. Change, 2015). Here, we analyze the stochastic properties of HWE from locations in the East, West and Gulf coasts of the US (Virginia Eastern shore, Oregon and Texas). We calculate high-water events by comparing the 2% exceedance total water elevation (driven mostly by wave run-up) to a threshold elevation, typically the average elevation of the beach berm. Our results show that, even though there exists regional variability of total water elevations among the different coastlines, relevant high-water events in all study areas are random with both inter-arrival times and event magnitude exponentially-distributed. We find that the average arrival frequency of beach elevation overtopping events is around one month, well below typical dune recovery times. Their average magnitude is of the order of tens of centimeters, enough to disrupt vegetation and erode low proto-dunes. Our findings support the conclusion that dune recovery is controlled by low-intensity high-frequency events rather than extreme local storms.