Does sand promote or hinder the mobility of cohesive sediment gravity flows?

Abstract

Sediment gravity flows exhibit a large range of flow behaviours, making their flow dynamics hard to predict and the resulting deposits a challenge to interpret. Cohesive sediment gravity flows containing clay are particularly complex, as their behaviour is controlled by the balance of turbulent and cohesive forces. A first set of laboratory lock-exchange experiments investigated the effect of adding 25% very fine sand by volume to high-density cohesive sediment gravity flows with strongly suppressed turbulence. This caused these mixed clay–sand flows to become more cohesive, have shorter runout distances, and have lower head velocities than the original pure-clay flows, despite the increase in density difference and the non-cohesive properties of the sand. Yield stress measurements confirmed that adding the non-cohesive very fine sand increases the cohesive strength of dense clay suspensions. This higher cohesive strength outcompetes the enhanced density difference and reduces the flow mobility. A second set of experiments across a larger range of clay concentrations showed that, for low-density cohesive sediment gravity flows dominated by turbulent mixing, the addition of 25% very fine sand increased the head velocities because of the enhanced density difference and weak cohesive forces. Thus, the addition of very fine sand may increase or decrease the mobility of cohesive sediment gravity flows, depending on the initial type of flow and the balance between turbulent and cohesive forces. In the natural environment, this study proposes that very fine sand can only increase the cohesive strength and reduce the flow mobility of cohesive sediment gravity flows that have a sufficiently strong matrix strength to fully support the sand particles. The contribution of very fine sand to the cohesive strength of high-density cohesive sediment gravity flows may have important implications for flow transformation on submarine fans, especially in distal regions where transient–turbulent, cohesive flows are particularly common.