Flume measurements of sediment erodibility in Boston Harbor

To obtain in situ measurements of sediment erodibility in defined bottom shear stress environments, a portable, straight flume was built, tested, and deployed in the field for six experiments at three locations in Quincy Bay of Boston Harbor, Mass. The flume had a 1.0-m-long inlet section, which included a boundary-layer trip and a roughened, plexiglass bottom; this design prevented erosion of the sediment bed in the boundary-layer-development region. Downstream of the inlet section was a 1.2-m-long sediment test section, which had a laboratory-verified, uniform bottom stress. In the absence of algal mats, our flume experiments on sites exhibiting a range of bed properties indicated quite uniform erodibility, with a critical shear stress tau(sub c) of 0.10 +/- 0.04 Pa and an erosion rate constant M of 3.2 +/- 0.2 X 10(super -3) kg m(super -2) s(super -1) Pa(super -1) (R(super -2) = 0.92, N = 17, where N is the total number of erosion rate measurements made in the absence of algal mats). The measured rates were consistent with those of many other in situ studies. We observed markedly reduced erodibility in early October 1995 when the sediment was covered by a benthic diatom mat, and measured erosion rates were lessened by 50-80%. The possibility of depth-dependent sediment erodibility in near surface (top 3 mm) was investigated by calculating a set of depth-dependent erosion parameters. The parameters obtained suggested that both the critical shear stress and the erosion rate constant were depth-sensitive (both doubling by 1 mm into the sediment).