THE PERSISTENCE OF ANTHROPOGENIC TURBIDITY PLUMES IN A SHALLOW-WATER ESTUARY

Increased turbidity and light attenuation (K-d) in the suspended sedim ent plumes created by hydraulic clam dredging were examined in Chesape ake Bay, MD, U.S.A. Turbidity and K-d values were measured along trans ects in areas with and without dredge plumes. The turbidity and K-d of individual plumes were tracked as they returned to background levels using both Lagrangian and Eulerian techniques. Existing aerial photogr aphs and a geographical information system (ARC/INFO) were used to exa mine plume sizes and dredge boat locations in relation to bathymetry. Hydraulic clam dredging produced plumes with significantly higher turb idity and light attenuation compared to background values. Plume chara cteristics were determined primarily by bottom sediment type and water depth. The greatest increase in turbidity and light attenuation occur red when dredges operated in shallow water (<1.0 m) where bottom sedim ents had increased amounts of silt and clay. Plume turbidity and K-d d issipated exponentially over time. The initial change in concentration of suspended sediments was rapid as the coarse sediments settled to t he bottom. The rate of plume decay slowed as diffusion and resuspensio n acted on the finer sediments remaining in suspension. Some Lagrangia n rates of plume dissipation were faster than Eulerian rates due to re suspension in shallow waters. Examination of aerial photographs indica ted that 72% of the dredge boats digitized in the Chester River were o perating in less than 2 m water. The area of a plume measured per boat in the Chester River was highly variable, ranging from 0.01 to 0.64 m (2). (C) 1998 Academic Press.