Current State of Coastal Subsidence in Europe Derived from the European Ground Motion Service

While the understanding and modelling of sea level rise (SLR) due to ocean density and mass changes have greatly improved over the past few decades, relative SLR contributions due to vertical land motions (VLMs) remain a major source of uncertainty. It is critical to downscale global and regional sea-level rise to local relative sea-level change as this is what causes coastal impacts and adaptation needs. In particular, land subsidence can strongly exacerbate coastal flood risk, saltwater intrusion, erosion and loss of wetlands and damage to infrastructure.

Here, we present the first analysis of pan-European coastal subsidence based on the European Ground Motion Service (EGMS) Ortho product. First, we perform a comparison between EGMS Ortho (Level 3) vertical velocity estimates and GNSS stations vertical velocity. This comparison reveals that the geodetic reference frame used to calibrate EGMS affects the vertical land velocity estimates and needs to be accounted for carefully, especially for the vertical land motions – including sub-millimetric/year velocities – that could affect local SLR estimates. After adjusting the EGMS calibrated product to the International Terrestrial Reference Frame (ITRF2014), we performed an assessment of VLM in European coastal flood plains. Our results show that half of the European area located in coastal flood plain is, on average, experiencing subsidence at a rate stronger than -1 mm/yr. More importantly, we find that urban area and population experience almost a -1 mm/yr subsidence on average (if we discard the uplifting regions due to Glacial Isostatic Adjustment) and for coastal airports and for harbours, the average land motion drops is even larger with -1.5 mm/yr subsidence rate. Finally, while our analysis allows identifying already well-known coastal subsidence hot-spots (e.g. Northern Italian coastal plain, Netherlands), we demonstrate with few examples that EGMS analysis also paves the way toward the identification of subsiding local scale coastal zones that have been ignored so far and for which flooding risk may become a major concern.