PhD Defence Jos Muller | Stability Of Salt Marshes Under Extreme Storm Conditions | Detailed observations of hydrodynamics and erosion mechanisms

Stability Of Salt Marshes Under Extreme Storm Conditions | Detailed observations of hydrodynamics and erosion mechanisms

Jos Muller is a PhD student in the department Civil Engineering and Management. (Co)Promotors are prof.dr. S.J.M.H. Hulscher, dr.ir. B.W. Borsje & dr.ir. J.J. van der Werf from the faculty of Engineering Technology (ET), University of Twente.

As climate change intensifies storms and accelerates sea-level rise, interest in integrating salt marshes into coastal defence systems has increased. However, their stability during storm conditions remains uncertain, particularly during extreme storm events.

This thesis addresses this knowledge gap by identifying wave-driven flow patterns and quantifying near-bed velocities to investigate salt marsh erosion under a range of storm conditions. Two series of wave flume experiments were conducted. The first examined flow patterns near the seaward edge of the salt marsh, where erosion can lead to the formation of a vertical cliff. The second was carried out in a true-to-scale wave flume with transplanted salt marsh blocks harvested from a field site. Erosion and near-bed velocities were quantified along the salt marsh under a range of storm conditions, including an extreme case with an offshore significant wave height of 2.0 m and a local water depth of 4.0 m.

The combined experiments demonstrate a shift in dominant erosion mechanisms during storms. At the cliff, the abrupt transition generates complex wave-driven flow patterns, which intensifies near-bed velocities on top of the cliff and promotes local scouring rather than lateral retreat. On the interior, erosion occurred primarily through uprooting of vegetation and attached soil, which was observed when near-bed velocities exceeded 1.7 to 2.0 m/s. These critical velocities were only reached under the most energetic wave conditions and after above-ground vegetation had been manually removed, representing a substantially damaged vegetation state. Total erosion after 40 hours of cumulative exposure was limited to 2% of the salt marsh and no erosion was observed near the dike toe.

Overall, this thesis demonstrates the stability of well-consolidated salt marshes and provides new insights into the processes controlling salt marsh stability during extreme storms, supporting their integration into hybrid, nature-based coastal flood defence strategies.