Flow-sediment interactions in turbidity currents moving over soft, muddy beds

Field: Earth and Planetary Sciences (Physical Sciences)

Many aquatic environments have soft, muddy substrates, but this important property has largely been ignored \nin process-based models of turbidity currents. Previous turbidity current studies have focussed on \nflows over hard or non-cohesive, movable substrates. However, the flume experiments conducted in this \nstudy show that soft mud beds interact differently with a passing flow. Flow-bed interaction caused deformation \nand erosion of the beds and changed the flowstructure of the turbidity currents. Subsequently, these \nchanges might impact the geometry of the resulting turbidite deposits. Five different interaction types between \nturbidity currents and soft substrates were defined: 1) no interaction, 2) interfacial waves, 3) mixing \nand erosion, 4) severe mixing and erosion, and 5) leading wave formation. The flow geometry and velocity, \nturbulence and concentration profiles of flows with no flow-bed interaction resembled those of flows over \na hard substrate. But with increasing intensity of the flow-bed interaction, the friction at the flow-bed interface \nand erosion depth increased, the geometry of the front of the flow changed from blunt to pointed, \nand leading waves increased in size. This resulted in a decrease in the flow velocity, an increase in the \nheight of the maximum velocity, an increase in near-bed turbulence, and an increase in flow concentration. \nThese changes were strongest in the head of the flows, where the highest intensity of flow-bed interaction \noccurred. To study spatio-temporal changes in the flow structure of turbidity currents, a turbidity current \nmodel was created using image analysis on videos taken during the experiments. Results of the model indicated \na trend in the head velocity of turbidity currents overriding a soft bed, with an initial decrease in flow \nvelocity followed by an increase until a constant flow velocity was reached. The experimental results were \nsummarised in flow-bed interaction phase diagrams which showed a decrease in flow-bed interaction with \nincreasing yield strength of the bed and an increase in flow-bed interaction with an increase in bed shear \nstress. For the flow-bed concentration ratios used in this study, the bed shear stress had the largest influence \non the flow-bed interaction. Fieldwork was conducted in the Grès de Peïra-Cava (France) which focussed on \nthe lower bed boundary characteristics of turbidite deposits with an underlying mud bed. Flat, non-eroded \nbed boundaries were inferred to result from flows with no interaction with the bed, while wavy, non-eroded \nboundaries were believed to be an indication for interfacial wave interaction. Flame and loading structures \nwere interpreted as products of mixing and erosion and sediment injections were related to leading waves, \nalthough no direct evidence for this was found in the field. Predictions of turbidite deposit geometry and \ncomposition based on the results of this study show a large variation in deposit thickness, run-out length \nand composition which is of great importance to the hydrocarbon industry.