Study On Flow Structure And Bed Sediment Incipient Motion Under Backwater Flow

Backwater usually happens in the fluvial river and reservoir channel.The mechanism of bed sediment incipient motion and flow structure in backwater condition is much more complicated than that in the uniform flow condition.Formulas available bed sediment incipient motion were obtained from the steady uniform flow flumes or resulted from the nature rivers that similar to the uniform flow,however,the bed sediment incipient motion law in backwater( non-uniform flow that belongs to decelerating flow)is different from that in uniform flow.With the increasing of water depth from the head of reservoir to dam sit,flow nonuniformity will also increase,especially for river-type reservoir like Three George Reservoir.There is obvious different between non-uniform flow and uniform flow about flow structure,and it will cause great deviation when the formulas of bed-load sediment incipient motion for uniform flow are used in calculating that for non-uniform flow,and result will be worse when the nonuniformity is greater.This paper analyzed the difference of backwater flow structure in different conditions,then used the measured date to point out that existing formulas for bed-load sediment incipient motion were not able to be applied to calculate the high backwater condition using the methods like theoretical analysis,observation date analysis,and flume experiments. Verify the difference of Shields curve at high backwater conditions and the conditions under sink, and then observing the various backwater flow structure under indicators flume test variation, and at last observe the bed-load sediment incipient motion, then analysis the incipient motion law from the three methods from flow rate, towing force, power.The main achievements are shown as fllowing:(1)Using the flume test date and observation date to comparative analysis 4 different sediment incipient motion formulas,which showed that the classic sediment incipient motion formulas could not be used to predict the bed-load sediment incipient motion in high backwater condition,and in sink low backwater condition,although formally meet the formula,but the choice of incipient motion particle size is different.(2)Distribution by backwater under conditions of water tank tests under conditions backwater flow velocity is uniform along the vertical flow is more uniform, the greater the degree of backwater, more uniform water distribution along the vertical line, that is, the smaller the velocity gradient. The law of large backwater of Three Gorges reservoir conditions also been verified.(3)By water flume tests under different conditions backwater, observe and analyze the variation of friction velocity and bed shear stress, both of which are increasing with the backwater level decreases. In order to quantitatively describe their relationship, introducing backwater index ?(the ratio of water depth corresponding to the depth and backwater uniform flow conditions), by four methods to calculate the friction velocity, selected the most reliable set of values obtained by fitting the u*/u*0and ? change in the relationship, and the relation of the bed shear stress and the ?.(4)By water flume tests under different conditions backwater, observation and analysis of the velocity distribution of various indicators of backwater under the three-? relative turbulence intensity distribution, uniform flow compared to both have a similar structure and distribution in adv can be measured within the range of flow rates are to meet for the distribution of the number, along with the increasing degree of backwater(? becomes larger) correlation slight fluctuation, but a higher overall number of related rates. To give the corresponding velocity and ? relationship. Three-relative turbulence intensity, fluctuations in the conditions under backwater near the bottom, but under conditions of uniform flow in vertical fluctuation is not obvious.(5)Start test by load sediment, draw for starting standard load sediment sampling selection criteria that bedload cumulative frequency approaches 100%(d95) particle size particle size is determined as the start, with good accuracy. Under different flow, as the index becomes backwater big, start small particle size, will appear along Coarse sand first start start phenomenon. With the increased traffic, water depth under the same conditions, the starting particle size is large.(6)For the exponential velocity parameters in the formula A, with the increasing degree of backwater, a slight decrease, but is a fixed constant on the whole, according to the test results take 1.23. To give the corresponding starting velocity formula and indicators related backwater. Using this formula to Wanxian hydrological stations measured data are verified by a representative particle diameter d95 as a starter starting velocity calculation is more accurate.(7)Flume experiment results validate the use of drag force formula, generally satisfy Shields curve. Backwater index ? becomes larger than the lower J decreases, starting ?0 drag force is reduced, and the critical relative drag force ? showing an increasing trend. It explains, the greater the impact ?0 start the drag force that is lower than the J's. The drag force is relatively critical in a certain range ? positive correlation with the ? index.(8)Starting verified using a power equation, get the standard dimensionless starting power 0.0191 can be used as load sediment starting.