The long-term bio-geomorphic development of foredune blowouts quantified from PlanetScope imagery and Airborne LiDAR data

Abstract

Nowadays the Dutch coastal dune management is focussing more on ‘dynamic preservation’ of the foredunes whereby it is the objective to enhance dune biodiversity and elevate the hinterland to keep up with the sea level rise. A recent addition to this strategy is stimulating natural blowouts or digging man-made blowouts to increase the aeolian transport from the beach to the back dunes. However much is unknown about the long-term (bio)-geomorphic development of these foredune blowouts and their effectiveness on different timescales, annual and seasonal. Here, the differences and similarities in morphological development between six different Dutch blowout sites near Noordvoort, Egmond-Bergen, Bloemendaal, Schouwen and two on Terschelling, are shown. A positive trend in sand area gain at the edges of the blowout with depositional lobe expansions from the end of 2016 to the beginning of 2021 is seen for Schouwen (40.9%, ~20169 m2 /year), Egmond-Bergen (36.2%, ~3473m2 /year), Noordvoort (27.6%,~962m2 /year), Terschelling 2 (15.3%,~2729 m2 /year), Bloemendaal (15.0%,~4486 m2 /year) and Terschelling 1 (27.5% from 2019- 2021 ~8984m2 /year). Clear seasonal variations in sand area occurred around the edges of all six blowouts with peaks of the highest and lowest sand areas at the beginning of the growing season and at the end of the growing season which corresponds with the greenness cycle of vegetation. EgmondBergen (0.38x104 m3 /year), Bloemendaal (1x104 m3 /year) and Terschelling 2 (0.67x104 m3 /year) showed an increase in cumulative sand volume and Noordvoort (~1.04x103 and ~0.77x103 m3 /year), Schouwen (~2.2x104 and ~2.3x104 m3 /year) and Terschelling 1 (~1.75x104 and ~0.485x104 m3 /year) showed an increase followed by a decrease in cumulative sand volume. Sand area changes were determined since 2016 using 3x3m PlanetScope multispectral satellite imagery and volumetric changes area were determined since 2012 using LiDAR airborne elevation data. The morphological blowout development which consisted of a deepened and widened deflation basin and a landwards expanded and migrated depositional lobe, assumed that all blowout sites were still in the geomorphological stage following the conceptual model of Schwarz et. al. (2018). Overall, no strong relation in size, age, man-made/natural or beach connection which might drive the sand area or volume increment difference, was found by looking at these results. However this study may be the starting point for a more defined comparison between the factors that determine the obtained differences in the long-term (bio)-geomorphic blowout development. For example, distribution of the height changes for several landcover changes may determine a deposition threshold for a blowout stage transition. Furthermore, research on a considerable amount of different blowouts with detailed sand budget computation involving an increased satellite resolution and decreased time span, will be relevant.