Flume measurements of sediment erodibility in Boston Harbor

To obtain in situ measurements of sediment erodibility in defined bottom sh ear stress environments, a portable, straight flume was built, tested, and deployed in the held for six experiments at three locations in Quincy Bay o f Boston Harbor, Mass. The flume had a 1.0-m-long inlet section, which incl uded a boundary-layer trip and a roughened, plexiglass bottom; this design prevented erosion of the sediment bed in the boundary-layer-development reg ion. 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 prope rties indicated quite uniform erodibility, with a critical shear stress tau (c) of 0.10 +/- 0.04 Pa and an erosion rare constant M of 3.2 +/- 0.2 x 10( -3) kg m(-2) s(-1) Pa-1 (R-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 observe d markedly reduced erodibility in early October 1995 when the sediment was covered by a benthic diatom mat, and measured erosion rates were lessened b y 50-80%. The possibility of depth-dependent sediment erodibility in near s urface (top 3 mm) was investigated by calculating a set of depth-dependent erosion parameters. The parameters obtained suggested that both the critica l shear stress and the erosion rate constant were depth-sensitive (both dou bling by 1 mm into the sediment).