河床采沙挖槽的溯源冲刷影响规律

Retrogressive erosion induced by sand excavation on a riverbed

针对长距离河段大量人工采沙、挖槽发生的河床溯源冲刷影响因素、发展规律及预测方法进行研究。考虑该问题的清水冲刷特征,建立了一维水动力学方程、河床变形和推移质输沙方程组,用TVD(Total Vartation Diminishing)性质的MUSCL-Hancock时空二阶格式和对水流、河床变形方程分别用有限体积和有限差分法进行数值求解,用水槽试验作了验证并研究了溯源冲刷规律。溯源冲刷长度在初期30%的时间内可达平衡时的80%,冲刷速率随来流强度、跌坎高度和跌水水头差明显增大。平衡溯源长度随流量的线性增长率约为0.8,随跌水高差的增长较缓,冲刷最大厚度约为跌坎高度的0.5倍。采沙挖槽河床一旦形成溯源冲刷,则发展迅速,流量和跌水高差是重要影响因素。在合理考虑输沙方程的基础上,该数值模型可用于溯源冲刷的预测,计算速度快且精度较高。

For understanding the issue of retrogressive erosion induced by excessive sand excavation on riverbed, in- cluding the influential factors, migration and prediction method, this paper provides a numerical model which is based on a set of equations consisted of the one dimensional Saint-Venant equation, the bed load transport equation, and the Exner equation under clear-water conditions. The MUSCL-Hancock method, a type of TVD （Total Variation Diminish- ing） scheme, was used to distinguish the governing equations with space-time second-order accuracy. The finite vol- ume and finite difference methods were used separately to determine the water-flow and Exner equations to speed up computation. The model was validated by laboratory experiments which were also used to investigate the erosion process. Results show that erosion rates appear high in the early scour stage, and the retrogressive length at the early 30% time reaches to 80% of the equilibrium retrogressive length. The erosion rate increases significantly as inflow in- tensity, headcut height and hydraulic drop increase. The linear increasing ratio of equilibrium retrogressive length to flow discharge is 0. 8 which is higher than that to height of hydraulic drop. The maximum erosion depth is approximate- ly half of the height of headcut. Once retrogressive erosion occurs, it develops rapidly and depends on inflow intensity and height of the hydraulic drop. When considering reasonable water-sediment interactions the present model can ac- curately and quickly predict the erosion process.