Influence of cohesion on the incipient motion condition of sediment mixtures

Abstract

Previous investigations on initiation of motion and erosion of cohesive sediment mixtures have not included gravel within the mixtures. Experimental results on the incipient motion characteristics of cohesive sediment mixtures containing gravel are presented in this paper. The sediment used consisted of fine gravel mixed with clay in proportions varying from 10% to 50% by weight, and fine gravel with fine sand mixed in equal proportion, with clay proportions again varying from 10% to 50% by weight. The stages of the incipient condition of motion were visually identified. Three modes of initiation of motion of cohesive sediment mixtures were noticed, namely, pothole erosion, line erosion, and mass erosion. The modes of initiation of motion changed mainly with clay percentages in the mixture, its antecedent moisture characteristics, and the applied shear stress. Analysis of the experimental data from the present study and the literature revealed that depending upon the flow and sediment characteristics, the critical shear stress of the incipient motion condition in cohesive sediment mixtures can be up to 50 times larger than the critical shear stress of cohesionless sediments having similar arithmetic mean size as the cohesive sediments. On the basis of dimensional considerations, a relationship is proposed for the determination of critical shear stress of the incipient motion condition in cohesive sediment mixtures also containing gravel. The variables, namely, clay percentage, void ratio, and unconfined compressive strength of the sediment bed, are noticed to be the main parameters controlling the incipient motion condition of the cohesive sediments. In the absence of cohesion, the proposed relationship reduces to the critical shear stress for the incipient motion condition of the cohesionless sediment mixture having the same arithmetic mean size. The condition for the threshold of erosion of cohesive sediment mixtures by the flow is thus parameterized in the present study.