Simulation Study On The Flow And Deposition Of Sudden-release Turbidity Currents

Turbidity currents is a gravity flow that carries suspended particles and moves forward under the effect of its own weight.According to its initiation mechanism,it is divided into those with sustained inflow type and sudden-release type.At present,there are many researches on sustained inflow type,however few attention was paid to sudden release type,especially those with slope break.Here the Reynolds averaged model using the Boussinesq hypothesis was applied to a two-dimensional numerical simulation of the flow and deposition of equi-volume suddenly-release turbidity currents on beds with different slope breaks.Through the density cloud map,cross-sectional velocity and concentration,current thickness,and depth-averaged velocity and concentration of the currents,Richardson number,and the morphology and particle size distribution analyses on low slope break(0.5 °,1 °,2 °)and high slope break(7 °,10 °,12 °),the following conclusions about the sudden-release turbidity currents are drawn:(1)the flow speed of sudden-release turbidity currents increase with the bed slope,but the growth rate is slow.That is because the enhanced entrainment of environmental fluids with the increase of the slope slows down the growth of its migration speed;(2)there exists high similarity of vertical cross-sectional velocity and concentration distribution between sudden-release type and shose with sustained inflow reflecting the consistency of the turbidity currents co-worked by the bottom boundary layer and the upper mixing layer;(3)The thickness,the average velocity and concentration of the sudden-release turbidity currents are very different from those with sustained inflow..The thickness of the sudden-release turbidity currents is the thickest at the head.Depth-averaged velocity and concentration are very similar,both have their largest values in the head and linearly descend towards their tail;(4)The sedimentary morphology of the sudden-release turbidity presents a distinctly different shape on small and large slope breaks.For small slope break(0.5 °,1 °,2 °),the deposits is produced at the slope break,the thickest part of the deposit is nearby the slope break and the average particle size of the upstream and downstream is not much different;however for the large slope break(7 °,10 °,12 °),erosion occurs at the slope break.The erosion become more significant with slope increasing.The thickest part of the deposit is far away from the slope break and the average particle size on the upper side is significantly larger than on the downstream.The deposition thickness linearly decreases from its highest point via the small slope breaks,while it concaves upwards via larger slope breaks.These shall provide us valuable information to assist in the speculation of the ancient deposition environment based on the observation of modern turbidites.