Effect of biological polymers on mobility and run-out distance of cohesive and non-cohesive sediment gravity flows

Abstract

Lock-exchange experiments were carried out to investigate the effect of biologically cohesive extracellular polymeric substances (EPS) on the mobility of sediment gravity flows laden with physically cohesive clay, non-cohesive coarse silt and non-cohesive fine sand. The results reveal significant differences in the head velocity, run-out distance and deposit shape of these flows related to differences in physical cohesion, particle size, and EPS content. These differences are captured in a three-way coupling model of turbulent forces, cohesive forces, and particle settling velocity. In general, biological cohesion reduces flow mobility, demonstrated most clearly by a progressive decrease in the run-out distance of the silt and clay flows, as the EPS concentration is increased. This reduction in flow mobility is caused by the dominance of cohesive forces over turbulent forces, which comprise turbulence attenuation and the bulk settling of a biologically cohesive gel in which EPS form a pervasive network of bonds between the sediment particles. However, sand-laden gravity flows were found to behave in a markedly different way, in that the head velocity and run-out distance first increase and then decrease, as the EPS concentration is increased. The increase in sand flow mobility is inferred to be caused by a reduction in the settling velocity of the sand particles, as the EPS cause an increase in flow viscosity at EPS concentrations that are sufficiently low to maintain turbulent flow. Once the EPS concentration is high enough for turbulence attenuation, the sand flows start to agree with the silt and clay flows in establishing a negative correlation between flow mobility and EPS concentration caused by gelling. The experimental data also uncovered that deposits formed by EPS-rich, turbulence-attenuated flows are shorter and thicker and have more abrupt terminations than deposits formed by EPS-free or EPS-poor turbulent flows. The larger thickness of these deposits is partly caused by the ability of EPS to retain water and form matrix-supported textures. Earlier work has shown that EPS is common in many sedimentary environments, including those where sediment transport takes place regularly by particulate density currents. Combined with the increasing rate at which man-made structures, such as pylons and communication cables, appear in these environments, we argue that there is a need to incorporate the results of this study in applied models that aim to mitigate damage to such structures by sediment gravity flows.