Study On Incipient Motion And Erosion Of Cohesive Nonuniform Sediment

Due to the effect of flood or flood and tidal currents, in some strong alluvial river particular in estuary, the erosion in riverbed is severe. The research on the maximum erosion depth of bed is important for the safety design of the project. This paper estimate the maximum erosion depth of Qiantang riverbed by the research of sediment incipient motion, erosion mechanism and flume test.Erosion is a series action of sediment movement after incipient motion. Based on the friction velocity formula established by Sun Zhilin, I further derived the vertical average flow velocity formula which applies to the cohesive non-uniform sediment. The value calculated by the formula is consistent with the experimental value of test section of drill core samples in Qiantang river tunnel. So the formula can reflect the incipient motion of cohesive non-uniform sediment well.The erosion depth of flood depends on the duration and rate of erosion. This paper obtained the relationship between the erosion rate and relatively residual function of shear stress according to previous studies. The exponential of the formula can be determined by the erosion rate by using the test results of drilling samples erosion. The text result shows that for those silt which is not full consolidated, the erosion rate and the relative residual shear stress is proportional to the square. However for the long-mud which is consolidated, there is a linear relationship between erosion rate and the relatively residual shear stress. Meanwhile, the constant M which determined by the results of erosion erosion test, is located between 10-6 m/ s and 10-5 m/s. This value provides a reasonable parameter for the mathematical model of maximum erosion depth of riverbed in estuary and forecasts the maximum erosion depth of the river bed in the extreme flood conditions according to M mutation.The flume model of Hangzhou Metro Line 2 section of the Qiantang river tunnel is established according to the characteristics of Qiantang river estuary flow and sediment. Qiantang river estuary is measured in accordance with the typical water and sediment data to determine the tunnel test program to simulate the phenomenon of erosion. The results show that:by quantitative analyzing the data, I found there is a good correlation between the single-wide discharge and the depth. According to the correlation between the single-wide discharge and the depth in the scholar motioned mathematical mode, and use the data measured in July 1995 as the initial terrain, we can predicate that by the p=0.33% after the flood washed, the maximum erosion depth of the tunnel section is 17.2 m. This prediction is consistent with other scholar's motioned mathematical model which used to calculate the maximum erosion depth of the Qiantang River. It provides a scientific basis for selecting the depth of subway tunnel should be buried.