Reports
Suspended sediment load contributed by shore erosion, Chesapeake Bay, Maryland
2006, Hennessee, E.L., Offerman, K.A., and Halka, J.P.
File Reports, Coastal and Estuarine Geology, File Report 2006-03
Abstract
Recognizing the deterioration of water quality in Chesapeake Bay and the associated loss of such living resources as submerged aquatic vegetation, the U.S. Army Corps of Engineers has developed a numerical model of light attenuation in the Chesapeake Bay (Cerco and Noel, 2004). Water clarity, one key indicator of water quality, is, in part, a function of the amount of suspended sediment in the water column. Currently, the computer model uses a single, baywide value - 5.7 kg/m/day - to estimate the suspended sediment load contributed by shore erosion. That figure is applied to the entire length of shoreline, regardless of the degree of protection or stabilization. However, according to recent estimates, nearly one-third of the Bay shoreline is protected. To improve accuracy in computing water clarity, the Maryland Geological Survey (MGS) estimated the load of fine-grained sediment (silt- and clay-sized particles) contributed by both unprotected fastland and nearshore erosion to the Maryland portion of the Bay. In addition to topographic maps, MGS used several recently acquired data sets: (a) shoreline rates of change calculated for the period 1850-1990 along 250,000 shore-normal transects, (b) a shoreline inventory that identified erosion control structures erected along the Maryland shoreline, and (c) a study that measured the grain size composition (i.e., sand-silt-clay content) and bulk density of the Bay’s eroding bluffs and marshes. MGS found that, in terms of fine-grained sediment, shore erosion contributes 1.34 kg/m/day, considerably less than the value currently used in the model. Moreover, input varies widely throughout the Bay, depending on erosion rate, bank height, shoreline type, and protected length.
