山区宽窄相间河段输水能力变化规律

Investigation of Water Transport Characteristics in Converging and Diverging Channels

输水能力变化直接影响河段行洪安全,当输水能力不足时,洪水风险陡增。影响河道输水能力的因素众多,如:床沙粒径引起河床粗糙度变化,植被等障碍物产生拖曳阻力等,但目前对边界变化影响河段输水能力机理和规律研究较少。受到地形地貌影响,山区河段河宽变化较为常见,本文采用平面2维水流数值模拟试验,研究了窄段–宽段河宽比(窄宽比)和下游水位变化条件下山区宽窄相间河段水流运动变化规律。结果表明:宽窄相间河段整体表现为宽段壅水、窄段跌水;当缩窄段弗劳德数Fr>1,河段由急流转变为缓流,在窄段下游附近形成淹没式水跃。下游出口为低水位时,随窄宽比减小,跌水–壅水–水跃分区效应越显著,展宽段水头损失急剧增大,由此降低了宽窄相间河段的输水能力;随出口水位增加,跌水–壅水效应减弱、水跃消失,展宽段水头损失变小,河宽变化影响降低,显著提高宽窄相间河段输水效应。出口水位变化对展宽与缩窄水头损失比峰值随着河道窄宽比的降低而增大,最大值为6;窄宽比变化条件下,出口水位变化对展宽与缩窄水头损失比的影响较大,表现为先增加后减小,且水头损失比峰值相对应的水位随着窄宽比增大而降低。研究从机制上厘清了宽窄相间河段水位及流速沿程分布和水头损失特征,可为山区河流水沙灾害防治提供科学依据。

The change in water transport capacity directly affects flooding risks.Apart from the effects of sediment-induced roughness and obstacle-induced drag on water capacity,the behavior of flow characteristics in diverging-converging channels under the influence of different narrow-width ratios and downstream waterlevels was particularly investigated and analyzed with numerical experiments.We find that the diverging-converging channels are characterized by the hydraulic rise in the wide section and hydraulic fall in the narrow section.When the Froude number in the diverging section exceeds 1,a submerged water jump can form due to the supercritical flow longitudinally shifting to the subcritical flow.Under low water level at the outlet,with the decrease of narrow-to-wide-section ratio,the effect of the hydraulic fall-rise-jump is more significant and the head loss in the diverging section increases greatly,reducing the water and sediment transport capacity of the wide and narrow channel.With the rise of outlet water level,the hydraulic–rise effect weakens,the hydraulic jump disappears,and the head loss in the diverging section decreases greatly,leading to the decrease of the river width variation effect and the improvement of the water convergence efficiency.The peak head loss ratio of diverging-to-converging increased with the decrease of channel narrow-width ratio with its maximum up to 6.In the case of the narrow-width ratio,the change in outlet water level has a great influence on the broadening/narrowing head loss ratio,which increases first and then decreases,and the water level with the peak loss ratio decreases with the increase of the narrow-width ratio.The results clearly demon-strated the distribution of water level and velocity along the converging-diverging channel and the associated characteristics of head loss,provid-ing supportive knowledge for the prevention and mitigation of sediment-related water disasters in mountainous rivers.