【作者】 郭辉；

【导师】 齐梅兰；

【作者基本信息】 北京交通大学 ， 防灾减灾工程及防护工程， 2013， 博士

【摘要】 桥渡压缩冲刷是桥梁建筑物压缩过流断面而在桥下河床全断面内产生的普遍冲刷,其发展规律受桥位河段水流、泥沙运动及河床变形的控制。本文根据水流、泥沙运动基本理论,考虑桥渡压缩对水流流态的影响、悬移质泥沙运动机理,利用水流、泥沙运动方程和河床变形方程,通过数值计算,研究了桥渡压缩冲刷随水文条件的演变规律、悬沙的紊动猝发机理及应用,并分析了水文随机性特性的影响及断面冲刷的横向分布,主要取得了以下研究成果：(1)从悬移质中的床沙质对水流紊动扩散的抑制作用出发,采用了基于制紊假说的水、沙方程(模型一),用数值方法模拟了桥渡压缩冲刷。模拟时考虑了桥位压缩河段回流边界的概化处理、天然河道断面几何特征的因时变化及泥沙组成的不均匀性。给出分组水流挟沙能力及级配、悬移质及床沙级配计算方法。采用有限差分法对模型进行数值离散并编制计算程序,通过枝城长江大桥压缩冲刷计算实例,验证了模型的合理性与准确性。结果表明,模型可以合理反映桥址及其上下游断面水位变化与河床冲淤变化,在实际流量过程作用下,桥址河床压缩冲刷具有汛期冲刷而汛后淤积的特点。(2)采用基于悬沙紊动猝发机理的水沙运动方程(模型二)模拟桥渡压缩冲刷。根据天然河流床沙质含沙量资料,采用最小二乘回归方法得到无量纲紊动猝发尺度比与颗粒雷诺数满足幂函数关系,又通过模型率定确定枝城大桥河段对应的无量纲紊动猝发尺度比具体形式。仍用枝城长江大桥的压缩冲刷实例进行了验证计算,模拟值与实测值接近。(3)研究了来水来沙特征量变化对桥渡压缩冲刷的影响。通过对桥位河段水文特征的分析,将水沙特征量概括为洪水频率、流量峰型系数、含沙量及悬移质粒径级配,用模型一模拟了各变量对压缩冲刷的单因素影响以及水沙条件同时变化对压缩冲刷的影响。(4)建立了考虑流量过程不确定性的桥渡压缩冲刷随机分析方法。首先提出一类基于FARIMA模型的日流量随机模拟方法,可以较好地反映日流量的统计特性。基于桥渡压缩冲刷数学模型(模型一),结合Monte Carlo方法,研究了枝城长江大桥在不同时间尺度流量过程作用下的压缩冲刷随机性规律。(5)用不同的数值模拟方法研究了压缩冲刷的横向分布。首先采用平面二维模型FESWMS FST2DH研究了矩形试验水槽长压缩段的冲淤分布,结合回归分析方法得到压缩冲刷横向分布与垂线平均纵向流速横向分布的相关关系。其次,利用模型一,对枝城长江大桥压缩冲刷横向分布进行数值模拟研究,给出两类典型断面(U型及V型断面)的垂线平均流速横向分布经验公式,并将其用于冲淤横向分布计算,得到了天然河流典型断面冲淤的横向分布。结果表明,本文得到的U型断面冲淤分布与实测值更为接近,而V型断面尚存在一定差别,有待研究。更多还原

【Abstract】 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.更多还原