Using Satellite imagery to investigate the spatiotemporal development of trough blowouts in coastal foredunes

Abstract

Vegetation growth on coastal foredunes, enhanced by traditional coastal dune management, made the foredunes an artificial wall between the aeolian dynamics of the beach and the hinterland. Naturally, depressions in the foredunes, so-called trough blowouts, may connect the hinterland to the beach, beneficial for ecological diversity. To bring back aeolian dynamics in the hinterland, construction of artificial trough blowouts in foredunes is considered. However, currently there is a scarcity of decadal scale observations of the development of natural foredune blowouts. As the interest in using artificial foredune blowouts as coastal dune management strategy is growing, there is need for better understanding of the long-term development of these trough blowouts. In this thesis, an artificial blowout system (Zuid-Kennemerland) and two natural blowout systems (Padre Island, Haurvig) were monitored using Landsat and Sentinel-2 spectral imagery. Linear Spectral Unmixing was applied on a single image basis. Endmembers were obtained from the image itself by finding representative pixels for sand, water and dune vegetation. The trough blowouts, defined as bare sand areas, were then successfully separated from the surroundings using the unmixed images. Comparison with high resolution drone imagery showed that the satellite images estimated the area of the blowout on average within 2% of their actual size. Initiation of foredune blowouts was observed 1) after two consecutive severe storm surges and, 2) as a gradual multi-annual process probably triggered by local vegetation dying off on the foredune. For the Zuid-Kennemerland and Haurvig study sites, the aerial extent of blowouts increased during winters and decreased during summers. Growth in the winter corresponded to the relatively high energetic wind conditions at Zuid-Kennemerland, with interannual variations related to wind energy. Despite seasonal variations, it was observed that blowouts remained their size given an open connection to the beach is present. Gradual landward expansion of the depositional lobe was found for the blowouts with relatively small entrance widths compared to the full blowout length. The blowouts at Haurvig showed multi-annual growth related to occasional storm surges. Blowout closure was found at Padre Island when the blowouts transgressed further inland losing their open connection to the beach. Future research should include auxiliary data capable of measuring volumes, such LiDAR or Ground Penetrating Radar. This enables to verify whether seasonal and instantaneous increases of the sand area was a continuous expansion of the dune elevation or just a local reorganization of the sand and vegetation coverage.