山洪输沙及对震后山洪特性调整过程的影响机理

Landslides and debris flows subsequent to a massive earthquake occur more frequently in the earthquake-hit mountainous area. A huge amount of loose solid materials enter the streams, resulting in a distinct change in the following flood properties and a substantial increase in the threat of flood hazard. However, subsequent series of flash flood transport solid materials downstream as bed load and will mitigate the augmented flood hazard. This project aims to explore this process focusing on bed load transport in different regimes and the relationship between flow unsteadiness and bedload inertia. Additional efforts will focus on the temporal variation of the augmented flood hazard. The influence of the original amount of bedload material and the cyclic action of flash flood hydrograph will be investigated. The potential findings and achievements will contribute to improving our understanding of bedload transport in mountain streams, advancing our techniques for modeling and forecast of flash flood, and mitigating the flood hazard raised by landsides and debris flows.

震后灾区频发的滑坡泥石流，带来了溪沟中的大量固体松散堆积物，改变了山洪演进特性，加剧了山洪危害程度。经由后续洪水搬运，固体堆积物在沟道空间输移，改变沟道形貌和洪水演进特性，山洪危害程度将逐渐恢复到震前水平。本项目拟研究震后山洪威胁调整过程中的山洪输沙特性和机理，建立弱、中和高强度水流作用下的推移质运动随机力学模型，揭示推移质输沙对洪水非恒定性的响应机制、周期性山洪作用下的沟道形貌及洪水特性调整机制。以四川龙溪河进行原型观测和模型检验，探讨震后山洪危害的演化趋势。研究成果对于认识山区沟道洪水输沙规律、完善山洪模拟预报理论方法、指导震后次生山洪灾害防治等都具有重要学术意义和实用价值。

本项目以山洪危害由震后向震前状态恢复过程为工程背景，研究推移质输移理论、山区河流山洪输沙与河床调整模型、震后山洪增幅调整衰减特性及影响因素。项目围绕推移质颗粒运动，发展了LES-DEM耦合方法，获得了明渠紊流-推移质颗粒相互作用的丰富细节；建立了推移质随机力学模型，定量刻画了推移质输沙率脉动量的时间三尺度性；发展了推移质遮蔽函数和河段阻力模型，提供了山区河流地貌动力学模型的封闭关系。围绕山区河流，发展了冲积物部分覆盖的基岩河床的地貌动力学模型，揭示了阵性泥沙补给对河床形貌调整的进口边界层效应，发现了进口边界层破坏机制；阐明了河床受扰后的高程调整速率近似满足指数衰减规律，但衰减速率具有空间非均匀性，发现了滞后响应模型的失效机制。以四川龙溪河、京郊红螺谷为例，展示了流域河谷形貌在震后的显著改变，揭示了推移质输移对河床边界和洪水特性的影响特征。项目研究成果对于深化认识山区河流输沙与演变机理，改进山洪灾害的预报与预警技术都具有重要意义。