SHORT-TERM SEDIMENT DYNAMICS AND LONG-TERM SHORELINE EROSION IN CHESAPEAKE BAY

In Chesapeake Bay, as in other coastal plain estuaries, shoreline erosion is a significant contributor to the sediment budget and a leading cause of high nearshore turbidity. This is especially true in the middle reaches of the Bay, where other sediment sources are relatively small and where many shorelines are exposed to episodically high storm waves. Reliable, detailed estimates of long-term shoreline erosion rates have recently become available. However, there is still only a limited understanding of how erosion rates vary through time, how they relate to short term sediment dynamics, and how they relate to nearshore turbidity levels. To address these issues, we carried out a focused study of sediment dynamics, shoreline erosion, and bathymetric change along the exposed shoreline of Todds Point in the lower Choptank River, Chesapeake Bay. This location experienced long term erosion of 1.81 m/yr for the period between 1847 and 2001. The shoreline at this site is comprised of semi-consolidated mid-Pleistocene estuarine sediments of the Kent Island Formation. The sediments are 70% silt plus clay and 30% sand, while the offshore bottom sediments are of similar grain size and consolidation with a thin veneer of sandy lag deposits. We carried out detailed site surveys and month-long sediment dynamics field experiments in the late autumns of 2002 and 2003. Between these two time periods, Hurricane Isabel passed over the Chesapeake and caused massive flooding with associated shoreline erosion, such that the erosion rate we observed (2.47 meters) was roughly 35% greater than the long term erosion rate. Bathymetric changes were difficult to discern over the same period, however. Our sediment dynamics results show a strong, nonlinear response of suspended sediments to wave forcing and sea level variations, with high turbidity plumes advected both alongshore and offshore. We have used our wave observations to calibrate a hindcast wave model for the area. Combining wave estimates with nearby tidal elevation data and our detailed geomorphological data allows comparisons between observed responses, projected sediment dynamics, and the predictions of available shoreline erosion models. These comparisons will be discussed in our presentation.