Reports
Contributions of shore erosion and resuspension to nearshore turbidity in the Choptank River, Maryland
2014, Halka, J.P., and Sanford, L.P.
Report of Investigations 83
Introduction
Suspended sediments are considered to have potentially significant ecosystem consequences in the Chesapeake Bay which include:
- Contributing to light attenuation, thus inhibiting the growth of submersed aquatic vegetation
- Interfering with the functionality of filter feeding organisms, both pelagic and benthic
- Potential burial of sessile benthic organisms
While the consequences of water column turbidity are broadly understood, the sources and spatial and temporal variations of shallow water turbidity are poorly documented and only generally known. Suspended sediments in the water column can arise from local resuspension, advection from nearby sources, erosion of the adjacent shore, and upstream watershed sources. The contribution of suspended sediment to light attenuation in comparison to the attenuation from dissolved materials and planktonic organisms also is not well documented in shallow waters of the Chesapeake Bay.
This project addressed the specific contributions of shore erosion and nearshore resuspension to nearshore turbidity. The work was performed in a tributary system of the Chesapeake Bay, the Choptank River. This tributary exhibits a range of geomorphic characteristics that are common to many of the other tributaries of the Bay as well as the mainstem Bay itself: 1) a broad embayed mouth region where the fetch is large, the water is deep and many reaches of the shore are very exposed; 2) a narrower brackish tidal reach where fetch is more restricted but tidal velocities are enhanced; and, 3) a progressively narrowing upstream portion where fluvial influences are more dominant. The work focused specifically on an exposed headland in the mouth region where rapid shore erosion is an ongoing problem, Todds Point.
This study comprised was comprised of four components to determine the sources of nearshore suspended sediments, including:
- a mapping based estimate of the long-term contribution of shore erosion as a nearshore suspended particulate source,
- a field oriented process study relating wind generated wave forcing and tidal forcing to resuspended sediment concentrations,
- settling experiments to determine the rate at which particles are removed from suspension by settling under various levels of turbulence, and
- a preliminary modeling effort relating wind and tidal forcing to shore erosion rates.
This work contributed to the Chesapeake 2000 agreement’s commitment to reduce Chesapeake Bay sediment loads to support the aquatic living resources of the Bay and its tributaries and to remove these waters from the list of impaired waters under the Clean Water Act by 2010 (http://www.chesapeakebay.net/content/publications/cbp_12081.pdf). The study provides a basis for setting achievable sediment reduction goals under the Clean Water Act through increasing the understanding of sediment loads attributable to shore erosion.
