Assessment of the erodibility of fine/coarse sediment mixtures

Erosion of mixtures of fine-grained sediments with sand is examined in a heuristic treatment relying on laboratory flume experiments under steady flows. Formulas for the critical stress for mixture erosion, iota(cm), and the rate of erosion of the fine fraction are diagnostically evaluated using data on the erosion of selected clay/sand and mud/sand mixtures. The stress, tau(cm), is found to vary non-monotonically with the fine sediment weight fraction, psi. Starting with a sandy bed, as psi increases, at first tau(cm) seemingly decreases from its value for sand and reaches a minimum at a weight fraction psi(sp), then increases. It appears that psi(sp) may be a measure of the space-filling concentration of fine particles within the pores of the sandy bed matrix. The logarithm of the rate of erosion is shown to vary linearly with the logarithm of the excess bed shear stress, and this variation depends on psi. Further understanding of modeling mixture erosion must rely on additional data on the variation of tau(cm) with psi.