Experimental Study On Incipient Sediment And Bed Load Transport Under Unsteady Flow

In nature,the occurrence of natural disasters such as tsunamis,storm surges and other natural disasters will generally be accompanied by severe sediment movement.After the disaster,a lot of sediment deposition and even scouring will cause great loss to human life and property.The unsteady of this kind of water is very strong,and there is non-uniform sediment in natural river and ocean.At the same time the current theoretical research is focused on the uniform sand and non-uniform sediment under the condition of steady flow,and sediment movement under the condition of unsteady flow is much more complicated than steady flow.Therefore,it is necessary to study the uniform and non-uniform sediment movement under unsteady flow in the background of the Belt and Road and marine power strategy.There is important significance of the research of sediment movement theory to the basic theory of coastal engineering,the construction and development of port,coastal and water conservancy infrastructure.In the theoretical research,the dynamic characteristics of unsteady flow are analyzed based on the equation of Saint-Venant.The interaction relationship between the flow structure and dynamic factors such as unsteady parameters,friction velocity and bed shear stress are analyzed,and the lag of the propagation speed,discharge and water depth of unsteady flow are studied.Secondly,in the research of bed load theory,based on the mechanism of the uniform sediment under the steady flow,the unsteady flow characteristic parameter is used to characterize the unsteady flow based on the sediment particle force model from the dynamic point of view.The inertia force of sediment particle is characterized by adding the characteristic parameters of unsteady flow under the condition of Morison equation.The tangential motion equation and normal motion equation of sediment particle are introduced.The rolling motion equations of the uniform sand and non-uniform sediment particles under different unsteady flow conditions are established.And the kinetic characteristics of the sediment particle movement are analyzed,the analytical solution of the incipient velocity and the moving step under the action of the unsteady flow is deduced.The relationship between the incipient velocity and the movement step of the moving particles under unsteady flow is related to the velocity skewness,particle size and exposure.When the acceleration flow is in the same direction as the sediment movement,sediment particles are more likely to start and move farther away;otherwise,the movement of sediment particles is opposite.Based on the system of wave flume,the experimental study on the mechanism of incipient and transport bed load under unsteady flow is carried out.Flat sediment transport is used in the experiment.The sandbox is placed behind the experiment area to measure the transportation.The water level change of the sediment transport process is measured by the water gauge,and the incipient and transport process of bed load are observed in the experimental area.Five different velocities of skewness are generated through five different flow step curves.In order to distinguish the sediment movement process of different grains,the sediment particles of different grains are stained,and the four kinds of non-uniform sediment are obtained by the ratio of 1:1:1 of equal mass.By drying the sediment in the sandbox,weighing,the bed load sediment transport rate under different conditions is obtained.According to the results,the motion process and results of the moving mass of the five uniform and four non-uniform sediment are analyzed and compared.The transport formula of uniform and non-uniform sediment under unsteady flow is explored.The influence of velocity skew and particle size on the sediment transport rate was analyzed.It was found that the sediment transport rate was significantly smaller than that of unsteady flow when the unsteady flow was converted to constant flow velocity at the same time by numerical integration.The flow intensity parameters,unsteady flow parameters and sediment transport show a good logarithmic relationship.At the same time,through the final sediment transport results of non-uniform sediment particle grain screening,the sediment transport rule of grouping grains is analyzed.It was found that the representative particle size of non-uniform sand could not be used as a single bed size.The representing particle size is changing when the distribution of sediment varies.