Study On Bed Resistance Law Of Alluvial River

As one of the basic problems of river dynamics and sediment movement mechanics,the study of alluvial river bed surface resistance plays a particularly important role in promoting the basic research of related disciplines.The flow resistance is widely existed in natural river,artificial channels and all kinds of water conservancy projects,and it is a basis for river course regulation and channel engineering design.The study of bed surface resistance law is of great importance to the promotion of basic science,the promotion of water conservancy and the prevention and control of water damage.Based on the resistance characteristics,discusses the calculation method of bed surface roughness,gentle slope sediment starting velocity formula is deduced,from the point of view of single and compound impact factor analysis of the bed surface morphology change law of bed surface morphology in the current typical methods is analyzed and put forward some new methods,finally drag coefficient calculation formula is deduced by the roughness,and the rule of the resistance of the different forms of bed surface.The main conclusions are as follows:(1)The bed surface roughness varies dynamically,which is related not only to the bed surface sediment gradation,but also to the flow conditions.At present,the method of determining roughness by means of multiple relations of representative particle size determination is still to be discussed.Based on the characteristics of bed surface resistance,a formula for calculating bed surface roughness is established,which takes the mean particle size,non-uniform coefficient,hydraulic radius and bed surface slope as variables.Through the verification of the measured data and the comparison of the calculation accuracy between the method of mean variance analysis and the existing formula of uniform sand and the formula of non-uniform sand,it can be seen that the formula is applicable to both uniform sand and non-uniform sand,and the calculation accuracy is higher than that of other scholars.The precision of roughness calculation method based on non-uniform sand is obviously higher than that based on uniform sand.(2)The roughness calculation method is introduced into the logarithmic velocity distribution formula of near bottom velocity,and the sediment rolling starting mode is adopted to deduce the sediment starting velocity formula of gentle slope by combining the force analysis and the characteristics of water flow pulsation.(3)From the perspective of single and compound influence factors,the changing patterns of bed surface morphology were analyzed,and the results showed that: the formation of sand grain is mainly but not completely related to the near-wall flow layer.The shields number of bed surface morphology is consistent with the threshold value of non-viscous sediment proposed by scholars of Sha Yuqing.The upper limit critical value of the flat bed surface is put forward because the shape of the flat bed surface will change under the condition of mid-motion.Most of the bed surface in low energy region is formed by slow flow.In the low energy region and the transition region,there is an obvious trend of increasing relative to the mean water depth,and the transition region reaches the maximum value.The relative mean water depth fluctuates in the high energy state area and reaches the lowest value in the sand wave shape area.(4)The formula for calculating the resistance coefficient is derived from the roughness,and the functional relationship between the resistance coefficient and froude number is determined according to the regression analysis,and then the resistance law of different bed form is obtained.