顺直型展宽航槽沿程水沙运动规律试验研究

Study of Flow and Sediment Motions in a Straight Widening Navigation Channel

由于山区地势原因,山区卵石航槽几何形态时常由窄深突然转变为宽浅,引起水沙运动特性的改变。本文开展了概化模型试验,研究了单一航槽(窄深段)到复式展宽航槽(宽浅段)水沙运动沿程变化规律并开展了机理分析。在不同上游来流条件下,测量了主航槽中心线沿程水位以及滩槽流速,用于探究水流特性的沿程变化规律。同时,开展了床面切应力测量和推移质试验用于判断推移质在粗化层不发生破坏的山区河道中的运动状态。试验结果表明,航槽几何形态变化明显影响水沙运动,水深在窄深航槽段与宽浅航槽段交界处附近变化最为明显;上游窄深航槽段与下游宽浅航槽段稳定水深的差值随着来流量的增大而增大;主航槽断面平均流速从窄深段逐渐增加,在窄深和宽浅断面交界处附近达到最大值(约为上游窄深段断面平均流速的1.5倍),而后逐渐降低直至稳定(约为上游窄深段断面平均流速的0.8倍);推移质淤积试验结果表明,可通过比较床面摩阻流速与推移质临界起动摩阻流速来判断推移质的运动状态,当满足淤积条件时,绝大部分推移质会停止运动并淤积在下游宽浅航槽段。在山区卵石航槽中,当河床粗化层不发生破坏,可通过直接比较局部航槽段床面摩阻流速与推移质临界起动摩阻流速来预测推移质运动或淤积情况。

In the mountainous region, the geometrical shape of a navigation channel is often changed from narrow-deep to wide-shallow, yielding the longitudinal change of motions of flow and sediment. By performing model experiments, this study investigated the variation of flow and sediment motions in the longitudinal direction, and explained the mechanism. With different upstream flow conditions, the water depth and the velocity in the main channel and foodplains were measured to explore the pattern of their changes in the longitudinal direction. Based on the measurements of bed shear stresses and bedload experiments, the threshold for estimating the movement of bedload had been proposed. The experimental result indicated that the geometrical shape of navigation channel significantly affected flow and sediment motions; The largest change of flow depth was observed at the interface of the narrow-deep channel and the wide-shallow channel; The difference of flow depth in two channels increased as flow depth increased; The mean channel velocity increased from the end of narrow-deep channel, achieved the maximum（about 1.5 times greater than the mean channel velocity in the upstream channel） at the interface of two channels, and finally reached to a steady value（about0.8 of the mean channel velocity in the upstream channel）. The bedload experiment indicated that the motion of bedload could be evaluated from the comparison between the bed shear velocity and the critical shear velocity for the initiation of bedload. Most bedload stops moving and deposits when the condition for bedload deposition is satisfied. In the mountainous navigation channel with an intact bed armor layer that did not break,the proposed threshold can be employed directly to estimate whether the bedload from upstream will move to downstream or not.