Turbidity Current in a Settling Tank

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This paper describes the turbidity current in rectangular settling tank. When sediment-laden water flows into settling tank, it flows downstream as the water is heavier than the ambient one. When the turbidity current developed to the large extent, it resuspends or picks up settled particles from tank bottom into the bulk flow and the bead water zone may be produced at the water surface. These phenomena decreases in the efficiency of the settling tank. The objective of this work is make clear the basic properties of this kind of density current.The laboratory experiment are conducted in a model settling tank, which has 0.2m width, 5m length and 1.5m depth. Flow velocity profiles and sediment concentration distributions in a tank are measured under various flow conditions.In the theoretical analysis, the mass conservation for both fluid and sediment, and equation of motion of fluid were employed. To the treatment of the density difference, the Businesque approximation is used and the turbulent diffusivity is assumed to be constant through the tank. Under these assumptions, the turbidity current can be expressed by only two parameters, Fdc : The densimetric Froude number and K : nondimensional kinematic turbulent diffusibility.After confirming that the numerical solution of the theoretical equation can predict the experimental results, the turbidity current are discussed by using the theoretical model. The maximum velocity of turbidity current Um are given as a function of both Fdc and k, and critical values from which the dead water zone appears are presented. These results show that the turbidity current develops with decrease in the value of Fdc and K. For practical use, an approximate simple equation is given which gives the relationship between Um, Fdc and K.As an additional work on this topic, some other work, such as horizontal pipe line flow with suspended sediment and the turbidity current induced by sediment disposal are also presented.