淤地坝泥沙淤积高度对宽顶堰水力特性的影响

Influence of siltation thickness on the hydraulic characteristics of the broad crested weir of check dams

为解决淤地坝长期运行,泥沙不断沉积超出设计库容,漫延至溢洪道内并在宽顶堰前堆积从而影响泄流的问题,该研究通过物理模型试验的方法,对不同淤积高度下溢洪道内宽顶堰的水流流态、水面线、流速分布、流量系数等特性进行对比分析。结果表明,随着淤积高度的增加,堰流在保持宽顶堰原有的过流形态同时壅水程度逐渐降低;淤积高度的不同会影响堰前及堰上区域的水面高程和断面流速,堰前水位随着淤积高度的增加略微下降,过流断面逐渐减小,水流流速增大;随着淤积高度的增加,流量系数逐渐增大,即宽顶堰的泄流能力有所增强。因此引入参数相对淤积高度(淤积高度S与堰高P之比),根据试验数据拟合得到了淤积工况下宽顶堰流量系数经验公式,计算流量与实测流量的平均相对误差为2.7%,满足精度要求,可为淤地坝除险加固工程的相关工作提供参考。

Check dams are widely located in the Loess Plateau area of western China.Their siltation elevation can exceed the design value after long-term running.The excess sediment spreads into the spillway,and then accumulates in front of the overflow weir,leading to the normal flood discharge.This study aims to clarify the influence of siltation thickness on the hydraulic characteristics of the broad-crested weir of check dams.The spillway of a typical check dam was also selected as the research object.The physical model test was finally carried out to analyze the flow pattern,water surface profile,velocity distribution,and discharge coefficient of overflow weir under different siltation heights.The test results were as follows:the upstream water surface of the broad crested weir was straight and flat under the condition of free flow.Then,the water level was dropped near the weir wall and gradually flattened just above the weir crest.Finally,the flow was fallen from the weir to the downstream channel.Sedimentation in front of the weir was reduced the degree of backwater,and thus the water depth dropped significantly.The overall water surface basically remained the same as that without sediment,and then gradually developed towards the open channel flow,with the increase of siltation thickness.More outstanding behavior was also found under small flow or low hydraulic head.The influence of siltation on the weir flow was further quantified using water surface profile.Therefore,the slight decrease was observed on the water surface line in front of and above the weir crest under different cases with the increase of sedimentation height,compared with the water depth along the weir.The water depth in the area behind the weir was depended mainly on the downstream channel,indicating no influence from the siltation height.Furthermore,there was an increase in the flow velocity in front of and behind the weir increases,where the reduced water level indicated the reduction of the cross-section area.Consequently,the rising siltation elevation increased the overflow capacity.It infers that the existing calculation formula cannot be applied for the calculation of spillway design in the check dams with the high siltation height.Thus,it is necessary to quantify the impact of siltation to calculate the discharge coefficient.The test data was substituted into the weir flow formula to deduce the test value of discharge coefficient under various working conditions.The discharge coefficient of the broad crested weir increased with the increase of hydraulic head using the general law of hydraulics.The siltation height shared a great influence on the weir flow coefficient under the same flow condition.Anyway,the discharge coefficient increased gradually with the increase of siltation height,indicating the enhanced overflow capacity of the broad crested weir.Therefore,the fitting empirical formula of discharge coefficient was obtained to introduce the relative siltation heightα(Ratio of siltation height to weir height).The average relative error between the calculated and the measured values was 2.7%,fully meeting the accuracy requirements.The new formula can be applied to calculate the discharge coefficient.These findings can provide a strong reference for the check dams reinforcement.