What is driving coastline migration in the Wadden Sea? Quantifying the coastal geometry with geodetic and Earth observation techniques

Higher sea levels can affect the position of a coastline by increased inundation and more erosion through higher wave energy. Other morphological processes like sediment transport by currents or river discharge and sediment accumulation by plants or coastal engineering structures may enhance or counteract coastline retreat. Additionally, vertical land motions caused e.g. by extraction of gas, water or oil change the relative sea level with similar effects on the coastline as absolute sea level changes. In order to be able to assess the severity of future sea level rise and the effects on coastlines for local populations and industries, we first need to separate the effect of sea level changes on the coastline positions from the effects of vertical land motions and of morphodynamics. Here, we approach this problem by combining vertical sea surface heights from satellite altimetry measurements with horizontal coastline positions extracted from optical remote sensing images.

In this case study for the barrier island of Terschelling in the Dutch part of the Wadden Sea, we quantify the different parts of the coastal geometry using geodetic and Earth observations covering a maximum time span of 30 years. For the vertical part, we show timeseries of sea surface height anomalies derived from satellite radar altimetry and compare them to recordings of two nearby tide gauges combined with vertical land motions from the available GNSS station. For the horizontal part, we show tidally corrected timeseries of cross-shore changes extracted from Landsat and Sentinel-2 imagery. A yearly Digital Elevation Model for the Netherlands from LiDAR and bathymetry observations (JARKUS) is used to test the sensitivity of the coastline position to sea level height and beach slope. In the future, the combined evaluation of sea surface and topographic heights together with the horizontal coastline positions will increase our understanding of climate change impacts on the coast. The methods designed in this case study serve as the basis for studies at other coasts.