Chesapeake Bay Shore Erosion
| contact: Stephen Van Ryswick ([email protected]) | ||
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ESTIMATED SEDIMENT LOSSES |
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Within days after the storm, Baltimore County surveyed its shoreline by plane. Erosion had occurred along roughly 3.5 linear miles, or 1.5% of the total length of the county's shoreline. Applying that percentage to the length of shoreline bordering Western Shore coastal counties, MGS calculated that approximately 32.7 miles of shoreline experienced erosion during Isabel. Two other variables needed to determine area and volume losses, shoreline retreat and bank height, varied widely fromsite to site. MGS assigned a value of 5 ft to both variables. That is, MGS assumed that along eroded reaches, a 5-ft high bank retreated 5ft. Based on that assumption, the area of fastland eroded by Isabel equaled about 20 acres, and the volume of eroded sediment, 4.3 million ft 3 (122,000 m 3). On average, those 20 acres, lost in a single day, account for about 15% of the acreage lost from the Western Shore in any given year. To convert the volume of eroded sediment to sediment mass, MGS multiplied sediment volume (m3) by 1.30 metric tonnes/m , the mean dry bulk density measured previously for Western Shore bluff samples [12]. A total of 158,8000 metric tonnes of sediment were eroded during the storm. Generally, when fastland sediments erode, only the finer-grained constituents (silt andclay) remain suspended in the water column. Coarser-grained sands and gravels form a lag deposit near the toe of the bluff. The average Western Shore bluff consists of nearly equal parts fine-grained (51%) and coarse-grained (49%) sediments [12]. The fine-grained fraction is of particular interest. Of the 158,800 metric tonnes of eroded sediment, 51%, or 81,000 metric tonnes, is the estimated suspended sediment load contributed by storm-induced erosion of the Western Shore. As a point of comparison, during Hurricane Agnes (1972), a storm noted for torrential rainfall in the Bay watershed, the Susquehanna River alone discharged over 31 million tonnes of suspende dsediment into the Bay [13]. |
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| Seen below, examples of sediment loss | ||||
 MGS surveyed a reach of shoreline at the mouth of the Choptank R. a year before and immediately after Isabel. Erosion rates varied. Todds Pt., ChoptankR., Dorchester Co. |
![Based in part on images like this, MGS assumed that, along eroded shorelines, a 5-ft high bank retreated 5 ft. Annapolis Roads, Chesapeake Bay, Anne Arundel Co. [2]](/images/aacocinder.jpg) Based in part on images like this, MGS assumed that, along eroded shorelines, a 5-ft high bank retreated 5 ft. Annapolis Roads, Chesapeake Bay, Anne Arundel Co. [2] |
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| 2. J. Stein (photographer),
2003, Anne Arundel Co. Soil Conservation District 12. J.M. Hill, G. Wikel, D. Wells, L. Hennessee, and D. Sailsbury, D., 2003, Shoreline erosion as a source of sediments and nutrients, Chesapeake Bay, Maryland, MGS, Baltimore, MD, 24 p 13. J.R. Schubel, 1976, in The Effects of Tropical Storm Agnes on the Chesapeake Bay Estuarine System The Chesapeake Research Consortium, Inc. (ed.), The Johns Hopkins University Press, Baltimore, MD, pp. 179-187 |
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