Detecting coastal change around the North Sea from open-source satellite images

Wave-dominated sandy coastlines worldwide are susceptible to change under the expected climate-change induced developments in sea level rise, mean wave conditions and storm events. For coastal management it remains important to observe and quantify these coastal changes, especially in low-lying developed coastal areas susceptible to flooding. The beaches surrounding an ocean basin have a variety of orientations, tidal ranges and management strategies, to name a few, which will lead to a range of morphological responses to future changes in hydrodynamic conditions within the basin. In addition, the conditions under which these varied responses mainly occur (e.g., under regular conditions or only during storm conditions) is not clear. Here, we used satellite imagery to compare the morphological response of a selection of beaches surrounding the North Sea.

The position of the shoreline is generally considered as a key variable to monitor the morphological evolution of sandy coasts. This research used the open-source software toolkit CoastSat (Vos et al., 2019) to automatically map shorelines from publicly available satellite imagery from 1984 to present, which are retrieved via Google Earth Engine (Gorelick et al., 2017). We selected five long, sandy beaches around the North Sea with varying tidal ranges, orientations and wave exposure for our analysis: (1) Skallingen in Denmark, (2) Egmond aan Zee and (3) the barrier island of Schiermonnikoog, both in the Netherlands, (4) Groenendijk in Belgium, and (5), Theddlethorpe in the UK. Approximately 2000 images per site were used for the shoreline extraction. Offshore wave buoy measurements and numerical model output provided the tidal water levels and wave conditions for the different sites. To account for tidal correction of the shoreline to a reference elevation, we used the dataset of Athanasiou et al. (2019) to estimate characteristic beach face slopes. At the conference we will present our analysis of the shoreline responses around the North Sea over the last few decades.

Athanasiou, P., Van Dongeren, A., Giardino, A., Vousdoukas, M., Gaytan-Aguilar, S., & Ranasinghe, R. (2019). Global distribution of nearshore slopes with implications for coastal retreat. Earth system science data, 11(4).

Gorelick, N., Hancher, M., Dixon, M., Ilyushchenko, S., Thau, D., & Moore, R. (2017). Google Earth Engine: Planetary-scale geospatial analysis for everyone. Remote sensing of Environment, 202, 18-27.

Vos, K., Splinter, K. D., Harley, M. D., Simmons, J. A., & Turner, I. L. (2019). CoastSat: A Google Earth Engine-enabled Python toolkit to extract shorelines from publicly available satellite imagery. Environmental Modelling & Software, 122, 104528.