Vertical land motion and relative sea level changes along the coastline of Brest (France) from combined space-borne geodetic methods

Abstract

In spite of the recent advances and the current perspectives of satellite radar altimetry to reach the coastline and monitor coastal sea level changes, this space-borne technology remains geocentric by essence. In contrast, the quantity that matters for coastal management and society is relative sea level. That is, the sea level relative to the land, as measured by conventional tide gauges. However, two important issues are associated with the use of tide gauges. First, supplemental information is required to separate vertical land motion from ocean climate signals in tide gauge records, and hence to understand the causes of future coastal sea level changes and their relative importance. This can be provided by space-borne geodetic techniques, for instance installing a permanent Global Positioning System (GPS) antenna at the tide gauge. Second, vertical land motion can show local spatial patterns, which limits the spatial validity of the point-wise information at the GPS station. Using Interferometry Synthetic Aperture Radar (InSAR) and high-precision GPS data, the above-mentioned limitations were addressed in the case study of Brest (France). Our findings show that, over the past decades, Brest and its surroundings are overall stable, in agreement with the geological setting, except for the embankment areas of the commercial and military harbours. The GPS-calibrated InSAR results were then further combined with geocentric satellite radar altimetry data to obtain relative sea level trends along the Brest coastline with an unprecedented high-spatial resolution of 200 m. Our approach is applicable beyond the case study of Brest, especially in coastal areas where tide gauge data are not available, provided satellite data are.