Numerical simulation of turbidity current generation from dense subaqueous debris flows

Abstract

One way to generate turbidity currents from debris flows is from shearing along the top surface of the debris flow, but the amount of sediment put in suspension is unclear. Turbidity currents generated by shearing along the top-surface of subaqueous debris flows are normally thought to be dilute. Small scale laboratory experiments showed that less than 1 \% of the debris flow mass is put into suspension during the transformation process.

The goal of this thesis is to find out whether top-surface erosion of stiff subaqueous debris flows can be an efficient process in transforming a debris flow into a turbidity current. Key questions to be answered are how much of the debris flow is put into suspension, and how dense the turbidity currents become. This is done by performing numerical simulations using a depth-averaged model. The model is made by couple two existing models for simulating debris flow behaviour and turbidity currents, respectively. The model is able to simulate the movements of both a debris flow and a turbidity current, and the interaction between them. Only dense debris flows (1800 kg/$m^3$ -- 2000 kg/$m^3$) on gentle slopes ($\approx 3^\circ$) are considered.

The erosion off the top of the debris flow is modelled by using an equation for cohesive sediment erosion. The sediments eroded off the debris flow increase the sediment concentration of the turbidity current enough to make it independent from the debris flow by the time the debris flow stops.

In the majority of the simulation runs, between 3 \% and 8 \% of the debris flow sediments (by weight) were put into suspension. Turbidity currents generated by debris flows with initial lengths between 60 and 200 metres would get a sediment concentration of 6 \% (by volume) by the time the debris flow stops. Large debris flows (up to 380 metres initial length) generated turbidity currents with sediment concentrations up to 14 \%. Changes in rheology, debris flow concentration and slope angles within reasonable values resulted in minor variations in proportion of the debris flow being transformed and in turbidity current sediment concentrations. Steeper slope angles than 3.5$^\circ$ may increase the transformation efficiency significantly.