Surface-area development of foredune trough blowouts and associated parabolic dunes quantified from time series of satellite imagery

Foredune trough blowouts are elongated wind-eroded depressions in the most seaward dune and their adjoining depositional lobes. Despite their importance to the sand budget and floral diversity of coastal dunes, the spatiotemporal evolution of trough blowouts is not well understood. We designed an automated workflow in the Google Earth Engine platform to produce time series of blowout surface area from medium-resolution satellite imagery available since the mid-1980s and applied it to a blowout system in the Netherlands, Denmark and the USA. Blowout surface areas were found to vary on multi-annual, seasonal and episodic time scales. Multi-annual change reflects successive development through stages of growth, stabilization and decay. The transition from growth to stabilization appears to be related to a change in blowout shape (width-to-length ratio). The decay phase starts with vegetation obstructing the blowout connection to the beach; the lobe can still migrate inland and develop into a parabolic dune before also becoming fully vegetated. The seasonal variations in blowout area increase with latitude; the observed larger areas in winter at the Dutch and Danish site presumably reflect seasonal plant development and the effect of stronger winds in winter. Episodic increases in blowout area, observed during winter at the Danish site only, are associated with pronounced foredune erosion. None of the episodic events changed blowouts into a different stage or persistently affected seasonal dynamics. Future work should focus on the combined analysis of changes in blowout area and sand volume to improve our understanding of sand-vegetation interactions driving blowout dynamics.