Effects of Bed Structure on Erosion of Cohesive Sediments

Data from two recent papers are used to illustrate the effects of change of structure of fine sediment on erosion rates. The paper by Roberts et al. presents erosion rates as functions of bed bulk density and shear stress. Ranges of densities were obtained by allowing various times for consolidation, and the erosion tests were made where the density was nearly constant with depth. A marked change in the linear relation between erosion rate and density is presented that is attributed to a change of bed structure. The paper by Zreik et al. presents erosion rate data for a thin layer near the surface of a bed formed by sedimentation. The linear relation between amount of overburden removed and bed strength also has a marked change that can be attributed to a change of bed structure. The initial structures are described as aggregates of aggregates, and the changes are shown to be progressive collapse of the larger to smaller interaggregate pores due to thermal motion, biased by overburden pressure, to a nearly homogeneous primary aggregate structure. Small increases of bed stress above that necessary to initiate erosion of this structure have large effects.