Erosion susceptibility of the Virginia Chesapeake Bay shoreline

Abstract

This study defines the susceptibility of differing shoreline types in the Virginia Chesapeake Bay to erosion. There is a high variability of the coastal environments in this area influenced by: (1) the relict Pleistocene high-order dendritic drainage system, resulting in a large diversity of shore orientations; (2) the moderate, but highly variable wave energy in the Bay system and highly variable submergence rates, resulting in a spectrum of shoreline transformation from primary to secondary types; (3) exposures of coastal plain sediments of widely varying composition and volume; and (4) diverse salt marsh development. Eighty percent of the shore is composed of three morphologically distinct beach environments, each reflecting different susceptibilities to erosion: (1) Permeable beaches, composed entirely of sand-sized material, comprise 59% of the beaches (mean erosion = 0.85 m/yr) and have the largest vertical and horizontal dimensions. This provides the largest vertical buffer to the effects of storm surge and waves. (2) Impermeable beaches, composed of a veneer of sand overlying impermeable, pre-Holocene sediments, comprise 24% of the beaches. The combined influence of low swash infiltration, low beach elevation and groundwater effects result in the highest mean erosion (1.14 m/yr). (3) Marsh barrier beaches (17%), composed of a veneer of sand overlying peat, contains a resistant rhizome framework, resulting in the least erodable beach environment (0.66 m/yr). Marsh margins, the remaining 20% of the shoreline, are the least erodable shore environment (0.54 m/yr). There is a large variability in the local subsidence rates along the shoreline. The subsidence highs correspond to the areas with largest nearshore terraces and the main concentrations of salt marsh development. A balance in the shore stability exists as a result of salt marsh development: The marsh retards shore erosion, yet it occurs at the areas of highest local submergence where the highest shoreline retreat is expected.