Erosional Processes along Salt Marsh Edges on the Eastern Shore of Virginia

Erosion and landward retreat of marsh edges has led to land loss in many marshes along the Atlantic Coast of the United States. Four salt marshes in a shallow, coastal lagoon on the Eastern Shore of Virginia were studied to determine long-term rates-ofchange along the edges and to investigate the specific processes that contribute to erosion. Analysis of aerial photographs over a fifty-year period (1957-2007) using Geographic Information Systems (GIS) indicated that the marsh edge eroded rapidly at three of the marsh sites. Matulakin Marsh (a mid-lagoon peninsular marsh) experienced the greatest erosion at 1.62 m·yr -1 , followed by Chimney Pole (a large marsh island) at 1.28 m·yr -1 , and Hog Island (a backbarrier marsh) at 0.98 m·yr -1 . Significant variability in erosion rates existed along the edge at each of the sites due to differences in wave energy, local bathymetry, and internal properties of the marsh edge (sediment, vegetation, and invertebrate characteristics). The edge at a fourth, mainland marsh, Fowling Point, remained relatively stable, with an erosion rate of 0.02 m·yr -1 over the study period. This is the result of a large mudflat that shields the marsh from high wave energy. Sediment, vegetation, and invertebrate properties were sampled to make comparisons between sites and evaluate their effects on erosion. While wind-driven waves provide the force necessary to erode the marsh edge, other factors are important in controlling the rates and mechanisms of erosion. Results indicated that both the internal properties of the marsh edge and the specific mechanisms of erosion varied among the three eroding sites. Erosion at Matulakin Marsh was facilitated by widespread crab burrowing, which led to block detachment and slumping along the edge. At Chimney Pole, removal of the dense root mat by waves was followed by erosion of the weaker, iii underlying sediment; this process was documented by a "webcam" installed on the marsh. Two erosion processes were evident at Hog Island. The southern portion of the site experienced significant undercutting and root mat toppling, which was facilitated by the marsh's sandy substrate. The removal of the root zone by waves led to the formation of terraces at the northern end of the site, which are likely to erode gradually over time. Despite their location within a single lagoon system, the erosion of salt marsh edges may be controlled by significantly different processes. Sea-level rise is predicted to increase wave energy in the bay, leading to an acceleration of these erosion processes.

Note: Abstract extracted from PDF text