坡面含沙水流水动力学特性研究进展

Progress in the Research of Hydrodynamic Characteristics of Sediment-laden Overland Flow

坡面流是坡面侵蚀的主要动力之一,具有独特的水动力学特性。本文对坡面含沙水流的流态、阻力系数、流速的测量与计算、径流能量以及含沙量对以上参数的影响进行了系统深入的论述。表征坡面流流态的参数有雷诺数与弗汝德数,雷诺数研究的分歧点一般存在于对其"层流"的界定上,降雨扰动是造成其流态特殊性的主要原因,一般认为裸土上的清水坡面流弗汝德数大于1,较少的研究含沙水流流态的资料表明,目前对含沙水流雷诺数的观点不一,但多数学者认为含沙水流属于缓流范畴;不同坡面试验所获得的阻力系数值不同,影响阻力系数的因素有雷诺数、水深、弗汝德数、含沙量等,在含沙水流中,阻力系数与雷诺数、水深的关系复杂,与弗汝德数呈负相关,随含沙量的增加而增大;测量坡面流流速的方法很多,各自存在优缺点,精密仪器暂不适合量测含沙水流,用染色剂法测量坡面含沙水流的流速具有一定的可行性,常采用坡度、流量的幂函数计算坡面流流速,一般认为流速与含沙量呈反比;能量是坡面流水动力学特性的综合体现,一般认为随着含沙量的增加,坡面流能量消耗呈增加趋势。在此基础上提出了目前研究中存在的不足之处,为分析坡面侵蚀机理、完善坡面侵蚀模型提供理论依据。

Overland flow is the main cause for slope erosion, which has unique hydrodynamic characteristics. This paper made a systematic and thorough discussion on the flow pattern, frictional resistance, mean flow velocity measurement and runoff energy of sediment-laden overland flow, sediment concentration as well as the impacts of the above. Reynolds number （Re） and Froude number （Fr） are used to express flow pattern. The branch point of Re exists in the definition of ＂laminar flow＂, and raindrop splash is the main reason for its specificity. Fr of overland flow free of sediment on bare soil is generally considered to be 〉1. Different flow pattern results are also obtained in the meager literature of sediment-laden flow. However, most scholars believed that sediment-laden flow belonged to tranquil flow. There are different values of Darcy-Weisbach Friction Coefficient （f） in different experiments on hillslopes, and the factors that affect f are Re, water depth, Fr and sediment concentration. On the study of sediment-laden flow, f is closely related to Re, but the tendencies are quite different. In addition, f is inversely proportional to Fr, and on the contrary, it is directly proportional to sediment concentration. Many measurements of surface flow velocity have their respective advantages and disadvantages. Exact instrument is characterized by high price, required routine maintenance and harsh work conditions, which is usually used to measure overland flow free of sediment. So, exact instrument is not suitable for sediment-laden flow temporarily. The accuracy of flow measurement with tracers is not high, but the process of tracers diffusing would be slower through the effect of sediment concentration. As a result, using tracers to measure the velocity of sediment-laden flow is still applicable. Formulas for velocity of overland flow are usually simulated by a power function of discharge and slope gradient, and there are a negative relationship existing between flow velocity and sediment concentration. Runoff energy is synthesis of hydrodynamic characteristics of sediment-laden flow on hillslopes. Most scholars use different formulas of water potential energy to express runoff energy. Generally speaking, the runoff energy consumption of shallow flow on slope increases as sediment concentration increasing. On the basis of reviewing recent research at home and abroad, deficiencies from experimental device and skill and experimental content are also discussed. The main problems of experimental device and skill include low-precision of velocimeter and poor comparability of data. Besides, results of every hydrodynamic parameter acquired by experiment are almost about its average values of the whole hillslopes, and most experiment has been done under the condition of overland flow free of sediment on gentle slope. Finally, the corresponding suggestions are put forward to provide a theoretical basis for slope erosion mechanism and slope erosion model perfection.