非恒定流作用下的阶梯形丁坝局部冲刷特性

阶梯形丁坝是常见的航道整治建筑物,天然河道中的水流多为非恒定流,非恒定流作用下的阶梯形丁坝局部冲刷特性研究,对丁坝结构设计和水毁防护等具有重要意义。采用自回归马尔柯夫模型,将天然来流过程概化为波谷起冲和波峰起冲2种情况,基于平面二维水流泥沙数学模型,探讨非恒定流作用下的阶梯形丁坝局部冲刷特性。结果表明:① 不同流量过程下,坝后主流区和回流区的流速均随上游来流量的变化而变化,波谷起冲时主流区随冲刷历时的增加逐渐分为2个区域,而波峰起冲时主流区在初期便被分为2个区域。② 受主流区变化和冲坑后方淤积区的共同影响,波谷起冲时的冲坑下游边界向下游凸起后回缩,波峰起冲时的冲坑边界向坑内收缩后扩散到下游,坝头最大冲深较恒定流条件可增加25.5%~54.3%。③ 如采用一级丁坝长度L_d1表征冲坑尺度,6 h冲刷历时中,波谷起冲时冲坑长度、宽度最大值分别可达9.2L_d1和2L_d1,波峰起冲时则分别可达11.3L_d1和2.1L_d1。

Local Scour Characteristics of Step-shaped Spur Dike under Unsteady Flow

The flow in natural river is mostly unsteady flow. Researching the local scour characteristics of step-shaped spur dike, a common waterway regulation structure, under the action of unsteady flow is of great significance to spur dike structure design and protection against flood damage. In the present research, the local scour characteristics of step-shaped spur dike under unsteady flow were discussed by establishing a 2-D mathematical model of flow and sediment. The natural inflow process was simplified as trough and peak inflow processes by using autoregressive Markov model. Research results demonstrated that the flow velocity in the main flow area and the recirculation area behind the spur dike changed with the upstream flow regardless of different discharge processes. The main flow area in trough inflow process gradually divided into two regions along with time, while in peak inflow process into two regions in the early stage. Influenced by the change of the main flow area and the siltation area behind the scour pit, the downstream boundary of the scour pit protruded towards the downstream and then retracted in trough inflow process, while in peak inflow process, the boundary of the scour pit shrank into the pit and then diffused to the downstream. The maximum scour depth at dam head can increase by 25.5%-54.3% compared with that in constant flow condition. L_d1, the length of the first-step spur dike, was used to characterize the scour pit scale; during the 6-hour scouring, the maximum length and width of the scour pit can reach 9.2L_d1 and 2L_d1 respectively in trough inflow process, and 11.3L_d1 and 2.1L_d1 respectively in peak inflow process.