Quantifying Soil Resistance to Sheet and Lateral Erosion Across Different Habitats on a Managed Sandy Back-Barrier Island

Back-barrier islands are understudied, dynamic, soft-sediment coastal landscapes located in fetch-limited environments. They are of key ecological importance; however, they are threatened by global change impacts including increasing storm surges and sea level rise. This study aimed to investigate the soil erosion resistance to hydraulic forces of different habitats across a sandy back-barrier island with characteristic sandy subsoils that has been managed >100 years to prevent its complete erosion and submergence. We quantified sheet and lateral soil erosion resistance in flumes, investigated their correlations with sediment and belowground vegetation characteristics, and discussed how these characteristics may relate to the island's past management and development. Soil cores were collected from the barrier dune ridge, back-barrier marshes and unvegetated sandflats and were exposed laterally to waves (lateral erosion) and horizontally to water flow (sheet erosion). Resistance to sheet and lateral erosion was not correlated. Resistance to lateral erosion depended on the thickness of the cohesive topsoil layer accreted by the marsh vegetation on top of a sandy subsoil. For instance, marsh soils with thin cohesive topsoils were resistant to sheet erosion (<0.5 cm erosion in 3 hr) but not to lateral erosion (collapsed within the first 10 min), which could make them vulnerable to wave attacks if they get laterally exposed during island erosion. Topsoil thickness was related to marsh elevation and age. In turn, the development of the cohesive layer may be related to the past management of the island, as soft engineering measures such as beach nourishments can create sheltered conditions for marsh development.