Experimental Study On The Flow And Deposition Of Turbidity Currents With Slope Transition

Turbidity currents are particle-laden,gravity-driven flows appearing in many natural processes.A lot of hydrocarbon reservoirs are discovered in turbidite formed by submarine turbidity currents.It is of great significance to performe this study since there is still a lack of experimental studies on turbidity current and deposition with slope transition.Turbidity current experiment was done within a flume 15 m long with3 degree slope 5 m long and then connected to a 10 m long horizontal bed.The velocity of turbidity current was measured by acoustic doppler velocimeter(ADV).The concentration was measured using a siphon and a vacuum filtration with two methods adopted at tail of the flume to control the backflow.The bedform formed by the turbidity current was observed with two cameras and measured by and a laser range finder.With a comprehensive analyses of depth-averaged velocity,depth-averaged concentration,current thickness,densimetric Froude number,bulk Richardson number,kinetic energy correction coefficient,specific energy,deposit thickness of the experiment flow,the following conclusion are reached.(1)There exists a local maximum cross-sectional velocity at a position far from the bed,below it is the wall boundary layer and above it the mixed layer.(2)The cross-sectional particle size difference decreases in the downstream direction.(3)The critical densimetric Froude number of the experiment flow is 1.13 supporting a previous theoretical analysis based on specific energy of turbidity current with varying discharge and reduced gravity.(4)Small sediment waves appeared only in the range of 1 m from the inlet when inflow volumetric concentration is 0.006.When inflow concentration is 0.02,downstream migrating sediment waves formed on the whole slope.The waveheight and wavelength are within the range of 1-2 cm and 8-12 cm,respectively.(5)The sediment waves collapsed after the discharge of environmental water,and that the wave height is below 1 cm,the Inflow face is flatter than lee face.Size analyses of deposited sediment particles show that wave peak sediment size decreases downstream,and the average particle size of the wave peak is higher than that of the two sides,and that of the longer stow side larger than that at the shorter leeside.These may assist us in the reconstruction of their formation process or environment according to ancient or modern turbidity current deposit in the field.