Numerical Study Of Contraction Scour At Bridge Crossings

Contraction scour at bridge crossings is defined as the general scour of riverbed cross section under the bridge due to the constriction of underwater structures, which is governed by flow and sediment transport, and river bed deformation. Numerical simulation of contraction scour is studied in this dissertation using the theory of flow and sediment transport, in which the influence of constriction on water flow and the transport mechanism of suspended sediment are considered. Numerical simulation methods are then applied in the researches such as the characteristics of temporal evolution of contraction scour under different hydrologic conditions, the turbulent bursting mechanism of suspended sediment and its application, the stochastic effects of hydrologic variable on contraction scour and the transverse distribution of contraction scour, etc. Main research achievements are as follows:(1) A mathematical model for contraction scour as the first model, including equations of flow and sediment transport on the hypothesis of turbulence restriction of bed material load near the river bed, is proposed and applied in the numerical simulation of contraction scour at bridge crossings in alluvial rivers. In this model, several key aspects are considered, such as the simplification of recirculation boundary at the constricted reach, the temporal variation of geometrical properties of cross sections, and the sediment non-uniformity. The calculation methods are proposed for the sediment carrying capacity and its gradation, and the gradations of suspended sediment and bed material, respectively. Finite difference method (FDM) is adopted and the program CSBC implemented in Compaq Visual Fortran(?) is utilized for numerical simulation of contraction scour. The verification of the model at Zhicheng Yangtze River Bridge shows that the model is capable of predicting water surface elevation and bed elevation with reasonable accuracy. The results for contraction scour also indicate that the erosion happens mainly during the floods and the deposition happens after floods and mainly in the lower water period.(2) The second mathematical model is proposed based on the mechanics of turbulent bursting for suspended sediment and applied in the numerical simulation of contraction scour. According to the measured near-bed concentration data of natural rivers, form of power function is deduced for the relationship between the turbulent bursting dimensionless ratio and the particle Reynolds number using the least square regression analysis, and its specific form is derived by numerical model calibration for the reach of Zhicheng Yangtze River Bridge due to the absence of measured data. The numerical results of contraction scour indicate that the simulated values are satisfactory with acceptable error compared to the measured values at the bridge crossing.(3) Effects of characteristic quantities of incoming discharge and sediment on contraction scour are studied using the first mathematical model. First, the quantities are summarized as the flood frequency, flood wave type, sediment concentration and grading of suspended sediment according to the analysis of hydrologic characteristics. Second, the independent and mutual effects of these quantities are obtained by numerical simulation, in which the latter mainly considers the simultaneous changes of incoming discharge and sediment.(4) A stochastic analysis scheme for contraction scour is proposed considering the uncertainty of incoming discharge. A stochastic simulation method for daily river flow is put forward by adopting the FARIMA model which is proper for consideration of the statistical properties of daily river flows. The first mathematical model, coupled with the Monte Carlo method, is utilized to perform the statistical analysis of the contraction scour at the bridge crossing with different time scales.(5) The transverse distribution of contraction scour is numerically studied using different numerical simulation methods. First, the distribution of contraction scour in a rectangular flume is simulated by the two-dimensional depth-averaged model FESWMS FST2DH. Results show that the transverse distribution of contraction scour can be expressed in the function of the longitudinal depth-averaged velocity and time in this case. Second, the transverse distribution of contraction scour at Zhicheng Yangtze River Bridge is studied by the first model which incorporates the formulae for transverse distribution of erosion and deposition with empirical relations for transverse distribution of depth-averaged velocity for two typical cross sections (U and V type). The results show that the calculated transverse distribution of contraction scour is basically closer to the measured values for U type cross section and some differences between the calculated and measured results exist for V type cross section which needs to be studied.