孔兑入黄数值模拟及分析

Numerical simulations and analysis on flood merging from ten major tributaries into Yellow River

本文利用平面二维水沙数学模型对十大孔兑之一的西柳沟入黄进行模拟并分析。结果表明:在流场方面,高含沙洪水入汇后,壅水区水位升高,流速先降低后升高,不同点的流速升高不同步;河床在支流入汇口处存在突降,形成高流速区,经高流速区后流速迅速降低,泥沙淤落;相较于低含沙水流入汇,高含沙水流入汇的回流区外形更狭长,水流结构更复杂;低含沙水流入汇来沙全部输向下游,高含沙水流入汇则会有部分泥沙向上游输送;在入汇口上下游,上沙坝的形成过程是从两岸向中间发展最终合拢并堵塞主河槽,下沙坝则同时淤升,左岸抬升较为均匀,右岸抬升先快后慢,沙坝最终沿右岸延伸;在实际当中,交汇区的流场及河床变形与洪水过程密切相关。

Flood merging into the Yellow River from the Xiliugou river, one of the ten major tributaries, has been simulated and analyzed in this study using a 2-D water and sediment mathematical model. Results show that the merging of hyper-concentrated flood can cause a certain rise in water level in the backwater zone upstream of the merging location along with a decrease followed by an increase in flow velocity. Increasing in flow velocity at different locations is unsynchronized. A high velocity zone occurs near the tributary mouth where riverbed has an abrupt drop, and starting from this zone, flow velocity decreases rapidly and sediment deposits in the Yellow main channel. The recirculation zone created by the merging is narrower and longer than that by low-sediment flood, and the corresponding flow structure is more complex. In the main channel, all the sediment injected from a low-sediment tributary can be transported downstream while one part of the sediment from a hyper-concentration tributary will be transported upstream. Near the tributary mouth, an upper sandbar and lower sandbar develop in the main channel. The upper bar develops from the banks to the center and finally closes up, forming a blockage. The whole lower bar rises continuously at a roughly constant rate on its left side but a gradually decreasing rate on its right side, and at the final stage it extends downstream along the right bank. Generally, velocity field and riverbed deformation around a flood merging confluence should also be closely related to the time process of floods.