底坎对减缓采沙河床桥墩冲刷的效果及机理研究

Study on Effect and Mechanism of Pier Scour Mitigation by Sill in Riverbed with Sand Excavation

针对采沙河床演变引起的桥墩冲刷问题,通过水力学分析和水槽试验研究底坎设置对上游桥墩减冲的效果,运用粒子图像技术观测水流紊动结构的变化,分析底坎尺度和水流变化与桥墩减冲效率的相关性。结果表明:相同水流条件下,桥墩减冲效率与底坎的相对高度以及底坎与桥墩之间的距离有关;桥墩减冲效率随来流强度的增大而减小;在试验范围内,底坎的高度与水深之比为1/5~1/3、底坎位于桥墩下游1~1.5倍桥墩直径的距离时,对桥墩的减冲效果最佳,桥墩的减冲效率可达20%~30%;底坎的减冲机理是通过降低水流紊动强度、切应力及紊动涡量降低水流的输沙能力,从而实现对桥墩的防护。

Concerning about the issues of pier scour induced by sand excavation in riverbed,hydraulic analysis and flume experiments were conducted to study the effect of sill setting on the scour mitigation of upstream pier.The variation of flow turbulence structure was measured by particle image velocimetry technology to explore the correlation between sill scale,the redistribution of water flow and the efficiency of pier scour mitigation.Results indicate that under the same flow condition,the efficiency of pier scour mitigation varies with the sill height above riverbed and the distance between the sill and pier.The efficiency of pier scour mitigation decreases with the increase of inflow intensity.Within experiment range,if the ratio of sill height and water depth is equal to 1/5-1/3and the sill is located downstream of the pier with a space of 1-1.5times of pier diameter,the scour mitigation effect on pier is the best and the efficiency of pier scour mitigation is up to 20%-30%.The scour mitigation mechanism by bed sill is to decrease turbulence intensity,shear stress and turbulence vorticity of water flow,and to reduce the sediment transport capacity of water flow,so as to realize pier protection.