Hidden Levees: Efficacy of Small-scale Barriers to Sea-level Driven Marsh Migration on Rural Coasts

Abstract:

The ability of coastal wetlands to migrate inland is a primary determinant of global salt marsh area in the face of sea level rise. Large scale levee systems designed to protect urban coasts from flooding are known to impede salt marsh migration in many regions of the world, but little is known about the extent and effectiveness of small, privately owned levee systems along rural coasts. Here, we explore small-scale earthen levees in the Chesapeake region of Virginia, which have been constructed for centuries by individual landowners and can reach up to a meter in height. LIDAR elevation datasets and aerial imagery were used to identify these levees, and 40 km of levees with at least 5 km² of potentially protected land were mapped in Gloucester County. Impacts on marsh migration were evaluated on a subset of these levees, in which we measured elevation, soil salinity, vegetation typeand organic matter accumulation on transects spanning the landward and seaward sides of each levee. Preliminary results suggest that, in some cases, levees may actually enhance marsh migration by increasing subsidence and inhibiting tidal sediment deposition on the landward side of the levee. These effects lead to relatively low elevations which encourage inland ponding and marsh development. Rather than inhibiting marsh migration, as has been observed in larger levee systems on developed coasts, our results suggest that privately owned levees on rural coasts may in some cases encourage inland marsh migration.