Gradual versus episodic lateral saltmarsh cliff erosion: evidence from Terrestrial Laser Scans (TLS) and Surface Elevation Dynamics (SED) sensors

As lateral erosion can threaten valuable saltmarsh habitats and their capacity to protect the hinterland from waves and floods, there is a need to understand the mechanisms of erosion. We monitored lateral saltmarsh erosion at high spatiotemporal resolution during a 2.5 year period. We performed terrestrial laser scans (TLS) with centimetric spatial resolution, ca monthly, as well as before and after 2 wind events to assess morphological change of a saltmarsh cliff (<0.8 m in height), and deployed surface elevation dynamics (SED) sensors at 3 locations to obtain daily measurements of the cliff edge position with 2 mm resolution. This was complemented by wind data, and a pressure transducer on the mudflat to obtain time-series of local inundation duration, water depth, wave height, period, power and energy. TLS shows gradual lateral erosion of the marsh edge and surface, and occasional local slumping, with substantial local variation. SED sensor data also reveal that lateral erosion was mostly continuous, with episodes with stronger and weaker local erosion, at all 3 locations, often correlated in space. Correlations between changes in total marsh area and sediment volume of the mudflat-saltmarsh interface from TLS with particularly inundation duration and wind velocity were discussed. At plot level, correlations were not consistent; correlations between the local lateral erosion rates and hydrodynamics/winds were not significant. Results emphasize the importance of common low magnitude events for long-term lateral erosion.