Quantifying the impact of sea level and morphodynamics on shoreline changes with remote sensing observations

Abstract

Potential causes for shoreline retreat are diverse and very site-specific. These include inundation through absolute sea level rise or relative sea level changes caused by vertical land motions, as well as morphological processes (erosion or accretion). A widespread approximation to quantify and separate the influences of sea level changes and morphodynamics is to use the Bruun Rule. This simplified model has been interpreted in two ways, either by modelling the sediment redistribution along the beach profile, or by assuming a linear relationship where the ratio of sea level change and beach slope relates to the shoreline change. Here we show that the combination of several remote sensing observations from the last 20-30 years with sea level from radar altimetry, shorelines from optical satellite imagery and land elevation from LiDAR can be used to quantify the influence of sea level change and morphodynamics on shoreline changes. In this case study for the barrier island of Terschelling (the Netherlands), we begin by assessing the uncertainties in the individual datasets. First, we compare ALES-retracked altimetry observations with two nearby tide gauges under different tidal corrections and estimate vertical land motion from GNSS height observations. For timeseries of cross-shore changes from satellite-derived shorelines, we show in a sensitivity analysis how different processing choices influence the outcome. Additionally, we intersect profiles of land elevation from a set of yearly coastal topobathymetry observations (JARKUS) with a horizontal plane at sea surface height in different combinations. We find that between 1992 and 2022 passive inundation accounts on average for -0.3 m/year of landward shoreline change at Terschelling, while the total estimated shoreline trend was on average -3.2 m/year. Finally, we will discuss possibilities and challenges to upscale the methodology to a global approach.