受降雨影响的浅水泥沙颗粒起动规律

山区浅层水流深度极浅且移动缓慢,降雨条件下其泥沙输移现象尚不明确。为探明受到降雨影响下的浅水河流均匀沙起动问题,该研究假设雨滴落入河道后会影响到整个水流区,形成雨滴群与水流混合的流体,从孔隙介质流理论入手,假设当降雨存在时床面泥沙颗粒增加了向上的附加力,进一步分析泥沙颗粒的受力情况,推导出层流水流泥沙颗粒起动关系表达式。从含沙水流流速分布规律得到启发,雨滴落入层流水流的状态类似于含沙水流,当降雨存在时层流水流流速分布仍然满足线性关系。利用无降雨泥沙起动经典试验数据,确定了拖曳力系数以及上举力系数,发现两者都是沙粒雷诺数的函数。利用已有研究的降雨实测数据,求出了8种降雨强度(0.254～152.4mm/h)下的雨滴直径分布概率密度表达式(R~2=0.998),进而求出雨滴的平均直径表达式,并给出受降雨影响的层流水流泥沙颗粒起动切应力计算模型。该研究模型表明降雨的存在使得泥沙起动所需的临界摩阻流速减小。通过与已有研究进行对比分析,该研究建立的受降雨影响的浅水泥沙颗粒起动计算公式具有最高的精度,平均误差仅为14.8%,能够为山区水沙灾害防治提供理论支撑。

Incipient law of sediment particles in shallow water affected by rainfall

Sediment transport is a key link to the river flow in mountainous areas under rainfall conditions. Mountain river flow is usually considered to be laminar, because of the shallow depth and large slope. This study aims to investigate the influence of rainfall on the incipient motion of uniform sediment in laminar open channel flows. A mathematical model was also established for the incipient shear stress of sediment particles under the rainfall when the rainfall fell into the main flows with terminal velocity. Assuming that the rainfall remained the unchanged shapes in a short period, a scenario was developed, particularly when the rainfall drop particles settled in stationary water. As such, the water was assumed as the transport in the porous media with relative velocity. Both scenarios were considered equivalent, in terms of grain size, porosity, and drag forces. A critical shear stress model was derived using the conception of flow transportation in the porous media, where the forces were analyzed when acting on the uniform sediment particles. The analysis results showed that there was little influence of rainfall on the velocity, where the linear velocity profile was still applied. The coefficients of lift and drag forces were determined with the measured incipient shear stress data without disturbance of the rainfall. It was found that both of them were a function of shear Reynolds number. The combined variables were fitted using two force coefficients, thereby obtaining a linear relationship with shear Reynolds number and the power of -1.8(R2=0.996). 8 different rainfall intensities varying from 0.254 to 152.4 mm/h were selected, when fitting to the 224 original measured data, to finally develop the probability density function of raindrop diameters for given rainfall intensity (R2=0.998). Therefore, the average diameter value used in the present study was obtained by integration. The remaining parameters of the model were obtained to compare the present model with Garry Li experimental data. It was found that the critical shear velocity was reduced for the presence of the rainfall. The experiments were performed on a flume with a test section of 3.6 m long and 0.4 m wide. The flume bed was covered with sediments using the median diameters of 0.74 mm, and the inclined angle at slopes of 2.0°. The simulated rainfall was provided by three nozzles located 3.6 m above the center of the flume connected to the water mains, and the rainfall intensity varied from 0 to 162 mm/h. Reynolds number of shallow flow in the experiment was 39-60, smaller than the lower critical Reynolds number of the natural river channels. Comparison with Garry Li measured data, the computation model could more accurately predict the critical shear stress for the laminar flow with rainfall disturbance. To verify the accuracy of the model, the obtained values were compared with the predictions of Li formula (1997) and Zhao Chunhong formula (2013). Specifically, the accuracy of the model was the highest, where the average error was 14.8% and the minimum error was only 3.9%. A mathematical model of incipient shear stress can be used for the sediment particles, where the influence of rainfall can provide theoretical support to the prevention and control of water and sediment disasters in mountainous areas.