Reports
Shoreline erosion as a source of sediments and nutrients, middle coastal bays, Maryland
2003, Wells, D.V.; Hennessee, E.L.; Hill, J.M.
File Reports, Coastal and Estuarine Geology, File Report 2003-07
Abstract
The Maryland Geological Survey (MGS) is engaged in a multi- year study to determine the flux of sediments and nutrients eroding from unprotected shorelines bordering Maryland’s coastal bays. The first- year study focused on the northernmost bays – Assawoman Bay, Isle of Wight Bay, and the St. Martin River. The second- year study, summarized here, focused on the middle coastal bays – Sinepuxent Bay, Newport Bay, and the northern third of Chincoteague Bay.
The 19 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 site, MGS measured bank heights and collected sediment samples 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), total nitrogen (TN), total phosphorus (TP), and a suite of trace metals. The analytical results were then combined with coastal land loss estimates to determine sediment and nutrient loadings to the middle bays. Annual land loss 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 1,000 m to 67,000 m; most were less than 8,000 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 polygons. 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.
From bulk density measurements, the sediments eroding from the shoreline in the middle coastal bays are twice as dense as those in the northern coastal bays. In the middle coastal bays, average dry bulk density values for bluff sediments and marsh sediment are 1.62 g/cm3 and 0.76 g/cm3, respectively. Average bulk density values for bulk and marsh sediments from the northern coastal bays are 1.39 g/cm3 and 0.43 g/cm3, respectively. However, the overall sediment loading per meter of shoreline in the middle coastal bays is less than that reported for the northern coastal bay shoreline. The difference is attributed to the lower average bank heights in the middle bays (0.61 m for middle bays vs. 0.79 m for northern bays).
During the 47-year period, shoreline erosion contributed 11.4 x 106 kg/yr of total sediments (solids) to the study area basins (Table ES-1). Of this total, approximately 61%, or 6.9 x 106 kg/yr, are total suspendable solids (TSS), an amount equal to about half of the TSS load from upland runoff. Annual total sediment loadings are greatest in Sinepuxent Ba y (5.8 x 106 kg/yr, or 75.7 kg/yr per meter of shoreline), due in part to higher bank elevations and relatively dense bluff material. The rate of sediment loading from erosion in Newport Bay is 62.7 kg/yr per meter of shoreline; 75% of those sediments are suspendable solids.
In the study area, sand-sized sediments account for approximately 40% of the total sediments eroded from the shoreline. About half of the sand was eroded from the mainland shoreline of Sinepuxent Bay, certain reaches of which have undergone some of the highest rates of erosion in the study area. Thus, shoreline erosion accounts for approximately 1/4 of the sand entering the middle coastal bays.
Shoreline erosion is also a significant source of nutrients, contributing 4% of the total nitrogen loading and 9% of the total phosphorus loading to Maryland’s middle coastal bays. In addition to nutrients, erosion contributes significant amounts of lead (Pb) and zinc (Zn), accounting for 12% and 24%, respectively, of the total loadings of these metals into the bays.
"