Theory And Numerical Simulation Of Scour In Estuary

Human activities in estuaries are frequent. There are so many hydraulic structures such as ports, bridges, embankments, rivers or submarine tunnels. The city economy and society will be greatly influenced by their destruction. Scour in estuaries was one of the most significant factors which led to the destruction of structures. The study on estuarine erosion provides a scientific basis with the safety design of engineering. In this dissertation, scour in estuaries is profoundly discussed from the angles of theory and numerical simulation.Scour is essentially attributed to incipient motion of sediment. Firstly, on the basis of both dimensional analysis and previous cross-quartz-thread experiment, the expression for cohesive force is derived, which is dimension harmony, and suited to nonuniform sediment. Incipient probability is regarded as the probability that the sum of drag force moment and lift force moment is larger than that of gravitational moment and cohesive force moment. After statistical treatment with arms of acting force, the formula of incipient probability for both the cohesive and cohesionless nonuniform sediments is obtained from normal distribution of velocity. The formula indicates the incipient probability is a function of flow intensity, related with such factors as the relative size and geometric variance of particle, and the dry density. A formula of incipient frictional velocity is then deduced.Respectively based on incipient probability and shear stress, functional formulae are established for predicting the maximum scour depth under extreme flood. Differential equations are theoretically studied to describe the form of cross-section in the threshold motion status. The functional formulae are used to predict the scour depth in the Qiantang River estuary. The location of maximum scour depth is also discussed. The results demonstrate that both the flow condition and nonuniformity of sediment affect scour depth. The more nonuniform the sediment is, the greater the scour depth becomes.The tidal current field in Qiantang River estuary is numerically simulated with fixed-bed model. As a result, during the extreme flood the flow is entirely controlled by ebb current. A series of numerical tests on the response of tidal current to multi-bridges are also made, and the result is complicated. In the region where effect of multiple bridges occurs, tidal current velocity obviously reduces, and the gradational decrease of velocity at the same point exists along with the increase in bridge number. The influence...