冲积河流洲滩平面演化动力机制及其汊道兴衰调整响应研究

Island bar is commonly found on alluvial channels. Planform evolutions of island bar due to lateral erosion and the processes of alternating of main and secondary branches in braided channels play an important role on the morphological changes of alluvial rivers, which is difficult to simulate. Flow and sediment regimes on the downstream of reservoirs will be changed significantly. Thus, the morphological changes will be affected. The mechanisms of island bar evolution and its effects on the processes of alternating of main and secondary branches in braided channels will be investigated by several methods in this project. First, on the basis of river dynamics and soil dynamics, the describe expressions of island bar evolution will be developed. Then, aiming at the insufficiency of traditional numerical models which could only simulate vertical bed deformation, a new three-dimensional model for the processes of island bar evolution and alternating of main and secondary branches on braided channels will be developed, and its technical issues will be solved. The variation patterns of three-dimensional flow structure during the processes of island bar evolution will be studied. The morphological responses of island bar evolution and alternating of main and secondary branches on braided channels to changes of flow and sediment regimes will be investigated quantitatively. This investigation will contribute to further study on the river regime changes of alluvial channels.

洲滩是冲积河流自我调整过程的演化产物，弱粘性滩体侧蚀冲刷所引发的洲滩平面形态演化及汊道兴衰交替等河势调整过程，在河流演变中起着关键作用，且不易成功模拟。当前梯级水库建设等因素均会大幅改变下游河道水沙过程，此时洲滩演化及其汊道调整响应将更为复杂。本项目拟采用多种研究方法，深入研究弱粘性滩体侧蚀冲刷所引发的洲滩平面形态演化及其汊道兴衰调整响应特征。首先，基于水、土交叉理论，针对弱粘性洲滩冲刷演变特点（易冲刷、速度快、四周全动边界等），在基础层面上建立洲滩平面演化全动力学过程的数学描述模式。随后，弥补传统三维模型仅考虑河床垂向冲淤的不足，构建洲滩平面形态演化及其汊道兴衰调整响应的三维数值模拟理论与方法。揭示滩体侧蚀冲刷过程中复杂三维水流结构变化特征，定量研究洲滩平面演化及其汊道兴衰调整对典型水沙变异的响应过程，探讨其规律性。研究成果将有助于河型河势变化理论的深入研究，亦可为江河治理提供科技支撑。

洲滩是冲积河流自我调整过程的演化产物，弱粘性滩体侧蚀冲刷所引发的洲滩平面形态演化及汊道兴衰交替等河势调整过程，在河流演变中起着关键作用，且不易成功模拟。当前梯级水库建设等因素均会大幅改变下游河道水沙过程，此时洲滩演化及其汊道调整响应将更为复杂。通过本项研究，主要取得以下成果：（1）基于河流动力学和土力学理论，建立了洲滩平面演化过程的数学描述模式，包括坡脚侧蚀冲刷、上部洲滩崩塌等动力学过程，从而为洲滩平面演化及其汊道调整响应数值模拟提供了力学依据。（2）基于三维水沙动力学模型和洲滩平面演化数学描述模式研究成果，构建了洲滩演化及其汊道调整响应的三维动力学耦合模型。模型不仅能模拟出河道沿垂向的冲淤变化，而且还能模拟洲滩侧蚀坍塌所引起的平面演变过程，较好地模拟了洲滩侧蚀过程中三维水流结构变化特征，从而为进一步研究复杂河道响应及河势变化提供了一有效技术手段。（3）以长江中下游具有典型洲滩的分汊型河段为例（监利河段、东流水道），采用三维动力学耦合模型，针对三峡水库运行后清水冲刷特征，对其洲滩平面演化及其汊道演变过程进行了模拟分析。研究成果可有助于河型河势变化理论的深入研究，亦可为江河治理提供科技支撑，具有重要的理论和实践意义。