Density- and viscosity-stratified gravity currents: Insight from laboratory experiments and implications for submarine flow deposits

Vertical stratification of particle concentration is a common if not ubiquitous feature of submarine particulate gravity flows. To investigate the control of stratification on current behaviour, analogue stratified flows were studied using laboratory experiments. Stratified density currents were generated by releasing two-layer glycerol solutions into a tank of water. Flows were sustained for periods of tens of seconds and their velocity and concentration measured. In a set of experiments the strength of the initial density and viscosity stratification was increased by progressively varying the lower-layer concentration, CL. Two types of current were observed indicating two regimes of behaviour. Currents with a faster-moving high-concentration basal region that outran the upper layer were produced if CL < 75%. Above this critical value of CL, currents were formed with a relatively slow, high-concentration base that lagged behind the flow front. The observed transition in behaviour is interpreted to indicate a change from inertia- to viscosity-dominated flow with increasing concentration. The reduction in lower-layer velocity at high concentrations is explained by enhanced drag at low Reynolds numbers. Results show that vertical stratification produces longitudinal stratification in the currents. Furthermore, different vertical and temporal velocity and concentration profiles characterise the observed flow types. Implications for the deposit character of particle-laden currents are discussed and illustrated using examples from ancient turbidite systems.