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The effect of fetch limitation on coastal dune erosion and recovery

Jasper Donker, Pam Hage, Gerben Ruessink
Utrecht University, Utrecht, Utrecht, The Netherlands

Coastal sand dunes protect a large portion of the world's shorelines from marine flooding during storm surges. The impact of storm surges on dune erosion has been studied intensively and the processes are well understood, leading to reasonably accurate predictive erosion models. To make predictions of long-term (> years) dune development, more knowledge is required on the aeolian processes that govern dune recovery and growth. It is expected that the long-term aeolian input depends on beach width because of the fetch effect. To study this, topographic data of the beach and foredune are required at both high spatial and temporal resolution. Here, the applicability of mobile laser scanning (MLS) to monitor dune growth and erosion is studied and the relation between dune activity and beach width is investigated. A MLS system attached to an internal navigation system with RTK-GPS was used to carry out 10 surveys over a 3 km beach stretch with foredune (height ~25 m) between Castricum and Egmond aan Zee, the Netherlands over a 2.5-year period. The first survey was conducted in December 2013; 5 days after a large storm surge hit the Dutch coast. Survey frequency increased from bi-annually in 2014 to bimonthly in 2015 to link erosion and growth to specific wave and wind events. The height observations, typically 300-1000 per m2, were averaged into a 1x1 m DEM. Foredune and beach activity were determined by calculating the standard deviation of the height per grid cell over all surveys. Comparing the point cloud with manual RTK-GPS measurements confirmed the high accuracy of the MLS system. Foredune activity was largest where beach width was smallest (~35 m). Here, most activity was caused by erosion during storm surges. Where the beach was wider (~60 m), the area in front of the foredune was the most active because of the formation and erosion of embryo dunes. Apparently, the embryo dunes protected the foredune against erosion for the storm surges in our data set. The formation of embryo dunes and the positive correlation between beach width and dune volume change confirm that the fetch is a critical parameter for dune erosion and recovery.