SHORE EROSION AS A SOURCE OF ESTUARINE SEDIMENTS: CHESAPEAKE BAY

Erosion of unconsolidated shorelines concomitant with sea level rise has been recognized as one of the major sediment sources for Coastal Plain estuaries. Additional sources are sediments derived from the watershed, internal production and oceanic input through the estuary mouth. Quantifying these sources is important both for understanding the long-term evolution of estuaries as they respond to a transgressing sea and to provide a basis for informed management decisions designed to improve water quality and preserve a productive natural resource. Recently the federal/state Chesapeake Bay Program partnership has committed to reducing sediment loads to the Chesapeake Bay to achieve water clarity standards and restore critical benthic habitats by decreasing sedimentation.

To address these issues and determine the degree to which shorelines contribute to the overall sediment budget, we have updated the estimates of shoreline erosion for the entire Chesapeake Bay. For the Maryland portion of the Chesapeake, the update includes new shoreline locations spanning approximately the last 50 years and a recent assessment of the degree of shoreline armoring with bulkheads and revetment. In Virginia, only limited portions of the estuarine shoreline have similar information. Available reports from the early 1990's were expanded to the remainder of the Virginian estuary using comparable information derived from the more comprehensive Maryland data sets. We calculated upland erosion on unarmored shorelines and estimated the associated “nearshore erosion” due to progressive downcutting. Armored coasts reduced upland erosion to zero but downcutting continues and this sub-aqueous erosion was estimated from the historical recession rates of the shoreline.

For the entire Chesapeake Bay the sediment derived from the combined fastland and nearshore erosion totals 3.92 million metric tons per year. This value is significantly less than the previous estimate generated in the early 1990's which totaled 12.6 million metric tons per year. This updated value will be incorporated into an overall sediment budget for the Chesapeake estuary, including the watershed sources, oceanic input and internal productivity.