Study On Scouring Armored By Clear Water In Wide-graded River Bed

Non-uniform bed coarsing is a common scouring phenomenon in natural rivers,which usually occurs when the sediment flow in the upper reaches decreases and the sediment transport in the lower reaches is unbalanced.The coarsening of river bed surface often causes great changes in hydraulic sediment factors such as bed surface resistance,water depth and sediment transport rate.In order to fully understand the coarsening law of the bed surface,the distribution law of sediment particle position parameters,the probability of sediment initiation,the flow resistance of the coarsening bed surface and the calculation of coarsening gradation on the non-uniform sand bed surface were studied by combining theoretical analysis with experiment.The main achievements are as follows:(1)Based on the randomness of the location distribution of sediment particles on the non-uniform bed surface,five sets of sediment particle location tests on the non-uniform bed surface were designed and completed by replacing sediment particles with non-uniform glass spheres.Using image processing method to deal with test data,we obtain the particle location data,statistical analysis after the exposure Angle,non-uniform sediment particle movement direction Angle alpha and plane Angle beta distribution of probability density function;The relationship between particle position parameters,sand weight percentage and median particle size was analyzed,and the formulas for calculating the mean value and standard deviation of the particle exposure Angle under any bed sand grading were obtained.(2)Considering the sediment condition,flow condition and bedload transport,the flow resistance in the process of bed coarsening can be divided into sand resistance and sand transport resistance.Based on the variation law of bed load sediment transport rate in the process of bed surface coarsening,the coarsening degree coefficient of bed surface was introduced,and the calculation formula of sand resistance of flat bed surface was established.The stress of rolling bedload on the bed surface was analyzed,the stress equilibrium equation of rolling particles was established,and the energy consumed by moving rolling bedload by water flow was obtained.According to the ratio of rolling and saltation of different flow intensities,the formula of sediment transport resistance and the formula of water flow resistance suitable for bed coarsening process are established.Verified by the measured data and compared with the calculation formulas of other scholars,the calculation formulas of resistance in this paper have higher calculation accuracy.(3)Based on the location randomness and flow randomness of sediment particles on the bed surface,combined with the distribution law of sediment particle location parameters,a probability model of rolling starting of non-uniform sediment was constructed.Based on the stochastic theory of birth and death process in Markov process and the starting probability of non-uniform sediment particles,the function relation of time of bed surface coarse-gradation under clear water scour condition is derived.Based on the theory of bed surface active layer and the stochastic theory of birth and death process,the multi-step prediction model was used to discretization the bed surface coarsening process for iterative calculation,and the multi-step calculation model of bed surface coarsening hierarchy and scour depth was improved.The results of the two methods are in good agreement with the measured coarse hierarchical distribution curve,and the multi-step calculation model based on the random theory of birth and death process has higher accuracy.