VARIABILITY AND CONTROLS ON SEABED ERODIBILITY IN THE YORK RIVER, USA

Resuspension of estuarine fine sediments plays an important role in the transport of contaminants and the degradation of estuarine water clarity. Additionally, convergences and divergences in the transport of resuspended sediments can influence depositional patterns and estuarine geomorphology. The erodibility of estuarine fine sediments is the product of numerous physical and biological interactions taking place above, at, and below the sediment water interface. While variations in the erodibility of cohesive sediments have been attributed to a wide range of factors (salinity, temperature, grain size, solids fraction, chl A, EPS, etc.), for a given system with little variation in grain size, mineralogy, etc., it appears that the dominant contols on erodibility may be interparticle cohesion and biological cohesive influences. Interparticle cohesion is a factor of the volume fraction of solids, determined by an interplay between consolidation and bioturbation. Biological cohesive influences, are often quantified by measuring extra-cellular polymeric substance (EPS) concentration and are implicated in binding sediments together and decreasing erodibility. Obviously, each of these factors influencing erodibility can exhibit dramatic temporal and spatial variability with changes in sediment supply, wave and tidal energy, and biologic activity. Thus, one would also expect sediment erodibility to be quite variable. To assess the variability in fine sediment erodibility and a number of the key factors influencing erodibility, 3 sites on the York River, Virginia, USA were sampled repeatedly at time scales ranging from bi-monthly to weekly. Sediment erodibility was measured with a dual core Gust erosion microcosm in which sediment cores with minimal disturbance were eroded at a shore station. High resolution measurements of sediment porosity (1 – solids fraction) were determined with calibrated digital X-radiograph images. Concurrent to the collection of cores for erosion, surface sediments were collected and analyzed for EPS content and sediment grain size. Additionally, measurements of Eh and 7Be were used to gain insight on the recent depositional history of the seabed.