中小流量下黄河下游游荡段河床调整规律

小浪底水库运用后，进入黄河下游河道的水沙情势发生显著变化，大流量洪水频率大幅减小，中小流量洪水作用凸显。为揭示黄河下游游荡段中小流量下的演变特征，本文开展了500、1000、1500和2000 m3/s流量级下黑岗口—夹河滩段的动床物理模型试验。结果表明，垂向上，研究河段河床冲刷与淤积沿程相间分布，具有典型的游荡性河段的冲淤交替特征；横向上，中小流量下游荡段仍存在横向摆动的可能，且随着流量增大，主流弯曲系数由1.18减小至1.13，河道逐渐由蜿蜒向顺直发展；纵向上，各流量下深泓线调整趋势与冲淤量变化规律基本一致。此外，本文着重分析了河道工程与桥下河段河床的时空调整过程并运用随机模型进行了模拟。通过分析发现：时间上，河道工程附近河床垂向调整速率早期较快，之后随时间快速非线性减缓；空间上，桥下河段累积冲刷深度沿程非线性减小，空间衰减特性显著。应用表明，随机模型对典型局部河段的时空调整预测效果较好，相关系数R2可达0.74以上。本文探讨了中小流量下黄河下游典型游荡段长河段与局部的冲淤演变特征，可为未来黄河下游典型游荡性河段的河道整治与保护提供科学依据。

Riverbed adjustment characteristics in braided reaches of lower Yellow River under small and medium discharges

The regimes of flow and sediment entering the Lower Yellow River (LYR) have changed significantly since the Xiaolangdi Reservoir started to operate, causing a great reduction in high flood frequency and enhancing the role of small and medium discharges. To reveal the bed evolution of the LYR braided reaches under small and medium discharges, this study conducts physical model tests on the Heigangkou-Jiahetan Reach using constant discharges of 500, 1000, 1500, and 2000 m3/s. The results reveal three key conclusions. (i) Vertically, riverbed erosion and deposition are distributed alternately along the channel, manifesting a typical evolution of braided reaches. (ii) Laterally, considerable lateral migration of the main channel can also occur in the LYR braided reaches under small and medium discharges. With the discharge increasing, the bending coefficient decreases from 1.18 to 1.13 and the main channel develops from winding to straight. (iii) Longitudinally, the adjustment trends in thalweg profile under all the test discharges are basically consistent with the variations in the amounts of sediment erosion and silting along channel. This paper examines the spatiotemporal adjustments in the riverbed near training works and a bridge downstream, and presents the results of numerical simulations using a stochastic model. It reveals that temporally, vertical adjustment is relatively fast in early stage and then slows down rapidly; spatially, the accumulated riverbed degradation downstream of the bridge decreases nonlinearly along the channel until it diminishes, exhibiting significant spatial decay characteristics. Applications show that our stochastic model has good adaptability to spatiotemporal adjustments of typical short reaches, and that the correlation R2 between measured and calculated results can be up to 0.74. The results of this study would help the future channel regulation and protection of the LYR braided reaches under small and medium discharges.