The Role of Bank Collapse and Root Mat on Marsh Edge Retreat

Salt marshes are dynamic coastal ecosystems characterized by unstable edges. Root mats reinforce upper banks while exposing the underlying soil to erosion, leading to cantilever failure. This study presents a process-based model of marsh edge retreat that integrates collapse and root mat effects, validated with data from the Shangsha Wetland in the Yangtze Estuary. The model evaluates the impacts of root mat thickness, porewater seepage, and hydrodynamic erosion on marsh retreat dynamics. Results reveal that marsh collapse overhang angles increase with root mat thickness, leading to higher tensile failure along the failure plane and decreased shear failure. This results in root mat thickness having a nonlinear effect on the collapse width and frequency. There is an optimal root mat thickness that balances the benefits of edge reinforcement with the risk of forming overly large cantilevers, thereby minimizing collapse frequency and retreat distance. As root mat thickness increases, the contribution of edge collapse to retreat rises from 65% to 88% due to reduced bare soil exposure and limited hydraulic erosion. Low hydraulic conductivity improves edge stability by dampening stress–strain oscillations and reducing collapse frequency, contribution, and retreat distance. However, when the root mat thickness exceeds the optimal value, it increases the collapse width. Hydrodynamic forces, including alongshore currents, high tidal amplitudes, and wave action, elevate collapse frequency but underscore the significance of optimal root mat thickness in mitigating retreat. These findings suggest that targeted interventions, such as vegetation management to optimize root mat thickness and implementation of wave or drainage barriers, can reduce the rate of marsh retreat.