The Sedimentation Models And Their Formation Mechanism In A River With A Widening Boundary

The widening of the river boundary is an important factor affecting sediment deposition.With the increase of the width of the river boundary,the flow slows down and the sediment carrying capacity decreases.The river channel continuously adjusts itself to form sediment deposition to reduce the cross section area,and then improve the sediment carrying capacity promoting a new balance.Therefore,the evolution process of river channel is more complicated under the condition of widening boundary.Qinglan Lake diversion project is planned to build a re-diversion section from west to east to introduce the flow of Fuhe River into Qinglan Lake,so that the flow of Fuhe River flows to the downstream through Qinglan Lake and the exit of Qinglan Lake and the junction of Fuhe River.In other words,the location of the lake and the relationship between the river and the lake have a fundamental change forming a typical widening channel into the lake.At present,Qinglan Lake diversion project has been completed and the preliminary results of numerical simulation have been obtained.In order to further study the feedback adjustment law of sediment deposition under the widening boundary condition,the law and mode of sediment deposition under the widening boundary condition are discussed by using the numerical simulation method.Firstly,a 2D flow numerical model of Qinglan Lake diversion project was established baseing on MIKE software and the water level of Qinglan Lake diversion project model was calibrated according to the measured hydrological data.Then,a series of annual sediment deposition numerical simulation studies were carried out and the numerical simulation results were analyzed.Secondly,the law and mode of sediment deposition under the widening boundary condition are discussed based on the experimental results of existing physical models.Finally,a generalized model of broadened boundary channel was established to analyze the formation mechanism and characteristics of different deposition modes.The final conclusion is as follows:(1)The results of the mathematical model of Qinglan Lake diversion project show that under the condition of widening the boundary,the sediment deposition plane develops from top to bottom and the sediment body develops quickly in the early stage and slowly in the late stage vertically.In the neck of Qinglan Lake,there are belt sedimentation and lateral dike sedimentation,main stream branching and main channel swinging.The water surface line,gradient and velocity along the passage show the same change rule before and after the deposition of the numerical model.The vertical development trend of local siltation body and the total siltation development trend are the same,both of which increase rapidly in the early stage and tend to be stable in the later stage.In the process of transect sedimentation in the lake area,the fluvial facies coefficients of the main channel along the cross section are similar,but the relative position of the main channel and the elevation of sedimentation scour are different.(2)There are two sedimentation modes which are dominated by flow inertial force and sedimentation feedback under the widening boundary condition.The mode dominated by flow inertial force is affected by inertia force of upstream flow.In the process of sedimentation,the main stream position basically remained unchanged.The sedimentation of the non-main stream position in the sedimentation process played a role in restricting the water flow and increasing the flow velocity of the main stream,so that the main channel continued to develop downstream under the promotion of the main stream,and the inertia of the main stream in the upstream played a leading role in the shaping process of the main channel of the widening channel.The mode dominated by sedimentation feedback is affected by alluvial feedback deposition.In the process of sedimentation,the sedimentation at the main channel reduced the flow velocity at the main channel and the flow velocity on both sides of the main channel was greater than that of the main channel which forced the main channel to branch or swing.Therefore,the sedimentation feedback was the dominant factor affecting the whole process of sedimentation adjustment.(3)Under the condition of heavy water and sand,the sedimentation in the lake area is developed in a sedimentation mode dominated by the flow inertia force and the plane sedimentation in the lake area is relatively simple,without the phenomenon of main stream branching and main channel swing.The change of roughness and particle size of sediment does not affect the change of flow along the river and the development of section siltation in the lake area.(4)Under the condition of small water and small sand,the sedimentation in the lake area is developed in a mode dominated by sedimentation feedback and the plane sedimentation changes in the lake area are complicated,such as main stream branching and main channel swing.Roughness,particle size and other factors will affect the variation of water flow along the river and the development of section siltation in the lake area.