弯曲河段滩槽形态调整对水沙条件变化响应机制研究

In numerous catchments throughout the world human intervention has strongly altered natural river dynamics, especially in recent decades. Dams and reservoirs constructed to generate hydroelectric power and flow diversions constitute the most common forms of intervention in fluvial systems, and each causes substantial changes to the flow and sediment regimes. Process-response may generate remarkable downstream meadering channel changes, the phenomenon of erosion at convex bank side and deposition at concave bank side commonly appeared in many downstream meadering channel. The most research have focused on the formation and development of meadering channel, which can not provide useful guidance for explain the catastrophic phenomenon appeared in the meadering channel. Insufficient consideration of mechanical mechanism of hydrograph and boundary control factors to shape the configuration of channel and bar are the main limitations in previous research, the response mode of channel and bar configuration to the change of flow and sediment based on the engery balance can not built. In this research project, Using meadering channel in the middle of the Yangtze River as example, with intensive investigation of data gained both from prototype, numerical simulation and generalized model. The main factor to cause the atastrophic phenomenon appeared in the meadering channel is revealed by the progressive way of from macroscopic law and mechanical mechanism to engery balance. The response mode of channel and bar configuration to the change of flow and sediment is built. The result will not only help to develop the fluvial process theory, but also provide useful guidance for meadering channel regulation.

随着人类对河流开发利用程度增强，众多大型水利枢纽的修建使得工程下游的水沙过程发生了较大变化，造成下游冲积型弯道河流的演变特征与天然河道出现不可忽视的差异，普遍出现了凸冲凹淤的现象。已有研究主要是针对天然弯曲河道的形成原因及发育过程，对弯曲河道滩槽调整发生突变的主要原因尚无法解释。其关键原因在于各种水沙过程和边界形态因子塑造滩槽形态的力学机理未能得到系统揭示，基于能量平衡的滩槽形态特征对水沙条件变化响应模式未能建立。本项目拟以长江中游弯曲河段为背景，在充分吸收河流地貌、河流动力学及河床演变学已有成果和研究手段基础上，以宏观规律-局部动力学机制-能量平衡关系这种层层递进的方式来确定造成弯曲河段滩槽形态调整突变的主控因素，构建能够全面描述弯曲河段滩槽形态调整对水沙条件变化的响应模式，预期成果可以促进河床演变理论的发展，为弯曲河道的治理提供理论依据和技术支撑。

众多大型水利枢纽的修建使得下游的水沙过程发生了较大变化，造成下游冲积型弯道河流普遍出现了凸冲凹淤的现象。已有研究主要是针对天然弯曲河道的形成原因及发育过程，对弯曲河道滩槽调整发生突变的主要原因尚无法解释。本项目以长江中游弯曲河段为背景，主要通过实测资料统计、理论分析、数学模型计算和概化模型试验等技术手段相结合，进行了弯曲河段滩槽冲淤调整的主要影响因素研究、水沙过程和边界形态因子塑造滩槽形态的动力学机制研究以及基于能量平衡的滩槽形态特征对水沙条件变化响应模式研究。主要成果有：.① 分析了各个弯曲河段在三峡水库蓄水前后滩槽演变特性的调整规律，三峡水库蓄水前，下荆江存在“凸淤凹冲”、“凸冲凹淤”两类弯曲河型，而三峡水库蓄水后均表现为“凸冲凹淤”的一致性规律。.②指出了弯道段滩槽发生“凸冲凹淤”的调整现象主要受关键驱动流量级持续时间变化及沿程粗沙来量减少的影响，揭示了不同弯道凸岸边滩出现不同冲刷切割形式的动力学机制。水流对凸岸冲蚀力度在平滩流量级附近（20000～25000m³/s）达到最强，平滩流量附近流量级的持续时间超过20天时，弯曲河道发生凸岸切滩现象，而悬沙中造床粗沙的减少加剧了凸岸的冲蚀程度。在平滩流量下，急弯河段上半段凸岸边滩流速、切应力达到最大值，下半段大幅减小，弯道上段可能发生局部切滩；微弯河道凸岸边滩流速、切应力整体大于凹岸深槽，可能发生整体切滩现象。.③ 建立了适用于描述弯道段能耗过程的计算模式；典型弯道段的能耗计算结果显示，弯道滩槽形态的调整总是向着能耗减小的方向发展，以逐渐达到滩槽格局与水沙过程相适应的某种平衡状态。典型弯曲河段的演变趋势成果也显示，未来弯道段将延续近几年的变化规律，各弯道整体表现为“凸冲凹淤”的规律。.研究成果揭示了弯曲河道滩槽形态调整发生突变的机理，丰富了河床演变理论，促进了河床演变理论的发展，同时还为弯曲河段滩槽形态调整趋势的预测及河道治理提供判断依据和技术支撑。