The impact of storms, wind and vegetation recovery on washover development on Rottumeroog, studied using high-resolution satellite imagery

Abstract

Washover openings are naturally occurring, low-lying features on barrier islands, which occasionally experience overwash. Allowing washovers on barrier islands is currently being considered in order to increase the sediment supply and to allow the islands to accrete with sea-level rise. However, washover evolution has not been researched extensively, and studies so far have used a coarse temporal resolution. This study has investigated the evolution of a washover on Rottumeroog from 2009 to 2019, on both a long-term and on a seasonal timescale, using high-resolution satellite imagery with a spatial resolution of 3 to 5 metres and weekly to monthly coverage. The satellite imagery was classified using a pixel-based classification and compared to supporting data. It was found that the washover formed in 2012. At first, the washover was sheltered by dunes to the northeast, which likely reduced the frequency of overwash events. When these dunes were eroded in 2014, the washover rapidly expanded to the south and west, until it reached higher areas beyond which the washover could not expand. The expansion of the washover was coupled with a southward migration of the island as the beach was eroded as sediment from the beach was deposited in the washover. The washover showed strong seasonal patterns in vegetation growth as a result of washover frequency: All inundation events occurred from October to April. After the overwash season in winter and early spring, vegetation starts to grow in the washover as conditions become more favourable, demonstrating that vegetation can already develop in a washover shortly after overwash. As washovers start to occur in late autumn, vegetated areas change to sandy areas and conditions become unfavourable for vegetation growth. When the washover was sheltered by the dunes, conditions were more favourable for vegetation than after these dunes were eroded in 2014. Vegetation regrowth was high again (~70% ±10% of the washover area) from 2016 onwards, which is likely because of an increase in elevation as a result of overwash. Aeolian changes were common in spring but not in other months, as vegetation reduces the fetch in summer and the sand is too wet in winter because of the frequent overwash. Overwash was dominant in determining the long-term evolution of the washover, while aeolian transport did not lead to any permanent changes in land cover.