| Gravity currents,a special mode of sediment transport,widely exists in the heavily sediment-laden river,and reservoir.The flow is caused by the density variance between upper and lower fluids.The maximum flow velocity for gravity currents occurs in the layer of gravity current where upper fluid velocity is close to zero in the reservoir.The fast-flowing gravity current is not only carrying substantial sediment over a long distance but also scouring out the sediment on bed,where mud is getting cleared.During the decay process of the gravity current,the vast sediment left on the bed will cause a loss of reservoir capacity.Therefore,the high concentration of gravity current plays a key role in impacting both the deposition volume and form for the reservoir,which has been an important research subject with long-term concern in hydraulic engineering.The current research for the transport mode of gravity current is still insufficient.Moreover,there is much to improve for the theory of particle-laden gravity current plunging process as well as the study on the stability of stratified flow in the moving.In this paper,the rheological characteristics of the high concentration particle-laden gravity currents were analyzed to determine the constitutive relationship.Then the impact from kinematics of gratity current to the pattern of erosion and deposition is summarized after the flume experiment is performed,which provides the experimental data of validation for the theory.Lastly,a multi-layer transport model of gravity currents based on suspended sediment concentration stratification is proposed by theoretical analysis.Based on this model,the theoretical calculation method for the upper boundary of the gravity currents was modified according to the difference of the distribution characteristics of suspended sediment concentration between muddy water and the gravity currents section.By simplifying the theoretical model of multilayer transport,a three-dimensional model of the particle-laden gravity currents is created.Stability of stratified flow is illustrated with the stability eigenvalues by Muller.The main contributions are listed in the following part:（1）The rheological properties of particle-laden gravity currents under steady state shear were studied.In view of the high sand concentration in gravity currents layer,the rheological characteristics of the sediment were tested to reveal the solid-liquid two-phase transformation process of the high-density viscous sediment.Therefore,the rheological response of the sediment samples under different shear loads was illustrated.In addition,the Power-Law model was used to describe the rheological relationship for the sediment samples with D5013.6 to 65.7μm,and the concentration from 371 to 1240 kg/m3.（2）The flume experiment of particle-laden gravity currents was designed and performed to simulate and the plunging and migrating process of particle-laden gravity currents.With the experimental data obtained and recorded,hydrojump mixing process,the velocity distribution,and the head velocity variation are explained.According to the law of bed erosion and deposition distribution,the influence of different flow characteristics of gravity currents on sediment erosion and deposition is analyzed.（3）A theoretical calculation method of particle-laden gravity currents plunging line is proposed.Through comparing the commonness and differences between muddy water and gravity currents,meanwhile,considering the uneven vertical distribution of suspended sediment concentration and the influence of turbulence term,muddy water motion equation and gravity currents layer motion equation in different states are derived.（4）A theoretical calculation method of particle-laden gravity currents plunging line is presented.Based on the continuous condition of viscosity and the distribution characteristics of suspended sediment concentration in fluid,the method that describes the upper boundary curve of gravity currents plunging process was modified so that the theoretical prediction curve of gravity currents plunging process was established.Furthermore,it is found that the increase of inlet sediment concentration,water depth,and bottom slope degree accelerates the plunging process of muddy water,which explains the promoting effect of water,sediment,and topographic conditions on the plunging of gravity currents.（5）The stability condition of particle-laden gravity currents is confirmed.A layered three-dimensional motion model was established based on the continuous and dynamic motion conditions for the interface of gravity currents.The eigenvalues of stability obtained by Muller explored the influence of physical characteristic parameters as well as spanwise disturbance of the gravity currents layer on the stability of the stratified flow.According to the shape function of disturbance growth rate and velocity,the characteristics of stratification instability were analyzed. |
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