Reports
Shoreline erosion as a source of sediments and nutrients, northern coastal bays, Maryland
2002, Wells, D.V.; Hennessee, E.L.; Hill, J.M.
File Reports, Coastal and Estuarine Geology, File Report 2002-05
Abstract
The Maryland Geological Survey (MGS) began a multi- year study to determine the flux of sediments and nutrients eroding from unprotected shorelines bordering Maryland’s coastal bays. The first year of study focused on the northern-most bays: Assawoman and Isle of Wight Bays and the St. Martin River.
Sampling locations were selected on the basis of linear rates of shoreline change, as well as geology and geomorphology (marsh, bluff, or beach). At each of the 16 sites, bank heights were measured. Sediment samples were collected from marshes and beaches and from distinct geologic horizons within banks. Samples were analyzed for grain size composition, bulk density, total organics, total carbon (TC), nitrogen (TN), phosphorus (TP), and a suite of trace metals. The analytical results were then used in conjunction with coastal land loss estimates to determine sediment and nutrient loadings to the bays. Annual land lost was based on a digital comparison of two historical shorelines dating from 1942 and 1989.
Based on geomorphologic variability and differing rates of shoreline erosion, the study area shoreline was divided into 23 reaches, ranging in length from about 600 m to 45,000 m; most were less than 9,000 m long. A template of irregular polygons was constructed to demarcate the reaches, and total land loss (m2) during the 47-year period was determined for each polygon. These “land loss” polygons provided a structure for organizing the results of the physical and chemical analyses. Each sampling site was associated with one or more of the land loss polygon. Mean bank heights and concentrations of the measured constituents (i.e., TN, TP, TSS, etc. in kg/m3), averaged for each of the sampling sites, were used to calculate annual loadings (kg/yr) for each polygon.
During the 47-year period, shoreline erosion contributed an estimated 11.6 x 106 kg/yr of total sediments into the three-bay system (Table ES-1). Of the total sediment load, approximately 42%, or 4.9 x 106 kg/yr, were total suspendable solids (TSS). That amounts to about one-third of the TSS load from upland (surface) run-off. Annual total sediment loadings were greatest in the St. Martin River (6.9 x 106 kg/yr), due in part to high bank elevations and relatively dense bluff material. Bulk densities of sediments collected from bluffs averaged 1.4 g/cm3. Total sediment loading from shore erosion in Assawoman Bay was about half that of the St. Martin River (3.2 x 106 kg/yr). Sediment loadings from Isle of Wight Bay shorelines were even lower (1.5 x 106 kg/yr). Much of the shoreline bordering Assawoman and Isle of Wight Bays is low-lying marsh, composed of sediments with average bulk densities of 0.4 g/cm3.
Sand-sized sediments account for approximately 57% of the total sediments contributed from shoreline erosion. The sand contributed from erosion about half of the sand coming into the bays. More than one-third of the sand is eroded from the St. Martin River shoreline.
Shoreline erosion is a significant source of nutrients, contributing up to 8.5% of the total nitrogen and total phosphorus delivered to the system. Nutrient contributions from shoreline erosion slightly exceed input from point sources. In addition to nutrients, erosion also contributes significant amounts of Pb and Zn, accounting for 4% and 9.5%, respectively, of the total loadings of those metals to the bays.
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