Research On The Migration Regular Pattern Of Sediment Laden Flow In Vegetation Patches

The surface soil is transported by water forces to form a sediment laden flow.The sediment carrying capacity of the sediment laden flow causes the sediment to be continuously transported to the downstream of the basin,resulting in serious environmental problems such as water and soil loss and non-point source pollution.Therefore,studying the flow characteristics and influencing factors of sediment laden flow movement has important guiding significance for understanding the soil erosion caused by hydraulic action.Taking the sediment laden flow as the research object,this paper conducted a series of indoor gate flume exchange experiments by setting different sediment particle size composition（10-100μm65%、10-100μm74%、10-100μm81%）,plant morphology（length width ratio L/W=2.67、1.07）,vegetation density（SPF=1%、2%、3%）and flow depth（H=9、12、15cm）.The vegetation patches under all experiments were arranged randomly,A sediment collection device is set at the bottom of the test area every 20cm,with a total of15 devices.During the test,the forward position and corresponding time of sediment laden flow were recorded by high-speed dynamic camera.The collected sediment deposition samples measured the particle composition and particle size distribution by Malvin 3000laser particle size analyzer.The effects of vegetation patches on the forward velocity of sediment laden flow and the law of sediment deposition flux were quantitatively studied,and the prediction equation was established.The main results are as follows.（1）The migration regular pattern of sediment laden flow in vegetation patches can be divided into three main regimes:inertial force regime,drag-dominated regime and viscous force regime.The velocity prediction equations of sediment laden flow in inertia force regime and drag-dominated regime were established.In the inertial force regime,the prediction equation of the sediment laden flow is V=0.45（g′H/4）^1/2（g’represents the effective acceleration,H is the flow depth）.In this stage,when the water depth was constant,the sediment laden flow velocity remained constant.The forward velocity of sediment laden flow had nothing to do with the sediment particle size composition,plant morphology and density,and was only related to water depth.In the drag-dominated regime,the prediction equation of the sediment laden flow is V=0.64(q_og′d/C_Daφ)^1/4?t^（-1/2）.At this regime,the sediment laden flow velocity gradually decreases,and the forward velocity of sediment laden flow is mainly related to vegetation densityand plant morphology were related,but not to sediment particle size composition.（2）The sediments in the sediment laden flow in the vegetation patches showed different depositional laws.In the inertial force regime,there is no particle segregation in the deposited sediment,and the sediment flux of sediment laden flow remains basically unchanged.In the resistance dominated regime,the deposited material became increasingly dominated by fine sediment,and the sediment flux decreased gradually.Vegetation density,flow depth,plant morphology and sediment particle size composition of the sediment laden flow all have significant effects on sediment flux of sediment laden flow.Larger vegetation densityhas strong sediment retention capacity,and the sediment flux of sediment laden flow was small.The deepening of flow depth leads to the increase of sediment deposition flux.As the content of coarse particles in the sediment laden flow increases,the initial deposition flux of the sediment laden flow was larger,and the migration distance in the vegetation patch was shorter.（3）In the inertial force stage of the sediment laden flow,the ratio of the average measured value to the predicted value of sediment laden flow velocity remains unchanged with the increase of vegetation volume fraction ad,and the existence of vegetation patches has no effect on sediment laden flow velocity.In the resistance dominated stage,the velocity ratio of sediment laden flow decreases with the increase of vegetation volume fraction ad.when ad>0.056,the velocity ratio of sediment laden flow approaches 1,and the velocity of sediment laden flow was the same as the predicted velocity.（4）For the sediment laden flow composed of different particle sizes in the vegetation patch,there is a threshold value for the front area ah of the vegetation.When the frontal area of the vegetation ah<0.68,the sediment laden flow transitions from the inertial force regime to the resistance dominant stage,and then the transition position from the drag-dominant regime extended with the increase of the front area of the vegetation.When the frontal area of the vegetation ah>0.68,the area length of drag-dominant regime was constant.The fine particles in the sediment particle size composition were conducive to the forward movement of sediment laden flow.The more the proportion of fine particles in different sediment particle size composition,the area of the drag-dominant regime of the sediment laden flow was longer.（5）In the absence of vegetation patches,there is no significant difference in the velocity of sediment laden flow with different sediment particle size composition,and the flow velocity of sediment laden flow increased with the increase of water depth.