Effect of clay type on the velocity and run-out distance of cohesive sediment gravity flows

Knowledge of cohesive clay-laden sediment gravity flows (SGFs) is limited, despite clay being one of the most abundant sediment types on earth and subaqueous SGFs transporting the greatest volumes of sediment on our planet.... [ view full abstract ]

Knowledge of cohesive clay-laden sediment gravity flows (SGFs) is limited, despite clay being one of the most abundant sediment types on earth and subaqueous SGFs transporting the greatest volumes of sediment on our planet. Cohesive SGFs are particularly complex owing to the dynamic interplay between turbulent and cohesive forces. The cohesive forces are controlled by the cohesive strength of the clay within the flow, which differs between types of clay minerals.

Laboratory experiments using a lock-exchange flume have been conducted to isolate the effect of clay mineral type and concentration on the flow dynamics of cohesive SGFs in natural seawater. Results so far have revealed that SGFs laden with kaolinite clay (weakly cohesive), bentonite clay (strongly cohesive) and silica flour (non-cohesive) have strongly contrasting flow properties. For each experiment the run-out distance, head velocity and deposit thickness were measured, and the flow properties were recorded using high-resolution video.

Increasing the volume concentration of kaolinite and bentonite above 22% and 17%, respectively, reduced both the maximum head velocity and the run-out distances of the SGFs. We infer that increasing the concentration of clay particles enhances the opportunity for the particles to collide and form clay flocs and gels. This increases the viscosity and shear strength of the flows at the expense of shear-induced turbulence, and reduces their forward momentum.

Moving from flows carrying bentonite via kaolinite to silica flour, a progressively larger volumetric suspended sediment concentration was needed to produce similar run-out distances and maximum head velocities. Strongly cohesive bentonite flows were able to create a stronger network of particle bonds than weakly cohesive kaolinite flows of a similar concentration, thus producing the lower maximum head velocities and run-out distances observed. The non-cohesive silica-flour laden flows required extremely high suspended sediment concentrations of approximately 50% to produce a high enough frictional strength to counteract the excess density driving these flows.

These experimental results can be used to improve our understanding of the deposit geometry and run-out distance of fine-grained SGFs in the natural environment. We suggest that natural high-density SGFs that carry weakly cohesive clays (e.g. kaolinite) reach a greater distance from their origin than flows that contain strongly cohesive clays (e.g. bentonite) at similar suspended sediment concentrations, whilst equivalent fine-grained, non-cohesive SGFs travel the furthest. Additionally, weakly cohesive SGFs may cover a larger surface area and have thinner deposits, with important ramifications for the architecture of stacked event beds.