Marsh boundary degradation into open-water in living shorelines under high-energy conditions

Abstract

Living shorelines have been recognized for their potential to attenuate wave energy, reduce shoreline erosion, and enhance coastal resilience, and are gaining traction as a preferred method of shoreline stabilization. Yet much remains uncertain about their resiliency and effectiveness in reducing shoreline erosion during high-energy events. This study examines sediment dynamics at a relatively mature living shoreline (constructed in 2007 and comprised of a created marsh and rock sill at the land/water interface) in the Maryland Coastal Bays (USA) during a storm event via field surveys and numerical modeling. Results are evaluated within the context of observations at an adjacent natural marsh. Our study demonstrates that the mechanisms of marsh loss at the edge during these times differ between the living shoreline and natural marsh. Specifically, the living shoreline boundary is degraded through open-water conversion as ponds expand landward of the rock sills, but the natural marsh is eroded by undercutting at the scarp toe, triggering slumping and edge collapse. Sediments in the living shoreline marsh have larger particle sizes and lower organic content than in the natural marsh; vegetation has higher stem heights and lower stem densities compared to the natural marsh. These differences may drive differences in the stability of the marsh boundary. Numerical modeling indicates that bed shear stresses are higher landward of the rock sill at the edge of the vegetation in the living shoreline. This increase in shear stress under high-energy conditions occurs at both flood and ebb tide, potentially contributing to scouring and/or open-water conversion, as well as substantial sediment transport between the subtidal zone and the marsh platform of the living shoreline. Increasing the bank height (both sill and marsh) may reduce erosion at the marsh edge and/or prevent open-water conversion but should be carefully balanced with marsh ecology and processes during other times of the year.