Basic Research on Equivalent Sand-Grain Roughness of the Sand Waves of Small-Scale Having Three-Dimensional Configuration

(1)通过河床稳定性分析，研究了小尺度三维沙波的发生条件。应用势流理论。得到了三维沙波无因次波的初始增长速率是kd的函数，参数为1/k、Fr和c。其中，k和1分别为未扰动主流和展向三维沙波的波数；d =平均流深；F_r=弗劳德数；和C =与泥沙颗粒无因次延迟距离相关的倾角因子。详细研究了三维沙波比二维沙波更容易发生的水力条件域。(2)通过分析前人在室内水槽试验和现场试验中获得的约7500个可靠数据，得到了冲积河床的等效沙粒粗糙度值。提出了一种估算冲积河道等效粗糙度的新图解。分析表明，对于任意一种流动能量斜率，相对于平均粒径的无因次等效粗糙度在taau_ *‘ = 0.2时有明显的最大值，其中taau_ *’为无因次颗粒-粗糙度床剪切应力。(3)为了定量掌握小尺度河床形态在其发生和发展阶段的时空变化，设计了一种利用激光缝片照明和计算机辅助图像处理技术相结合的仪器测量方法。

(1) Conditions of occurrence of three-dimensional sand waves of small scale have been studied by means of stability analysis of river bed. Potential flow theory is applied.The initial rate of growth in dimensionless from of three-dimensional and waves has been obtained as a function of kd with parameters 1/k, Fr and C. Here, k and 1 = wave numbers of three-dimensional sand waves in the undisturbed main flow and spanwise directions, respectively ; d = mean depth of flow : F_r= Froude number ; and C = inclination factor relative to dimensionless delay distance of sediment particle. The domain of hydraulic conditions in which three-dimensional sand waves occur more easily than two-dimensional sand waves has been examined in detail.(2) About 7,500 reliable data from laboratory flume and field experiments published by previous investigators have been analyzed to obtain the values of equivalent sand-grain roughness of alluvial channel bed. A new diagram is proposed for estimating the equivalent roughness to be formed in alluvial channels.The analysis made shows that for any energy slope of flow the equivalent roughness in the dimensionless form relative to mean particle size has a distinct maximum for tau_*' = 0.2 about in which tau_*' is dimensionless grain-roughness bed shear stress.(3) In order to grasp quantitatively the time and spatial variation of channel bed configuration of small scale at their occurrence and development stages a new method of measurement has been devised combining instrumentation by the use of laser slit-sheet illumination and computer-aided image-processing technique.