泥石流动床形态阻力的实验研究

On-spot observation and field reconnaissance of debris flows indicate that inflection points of longitudinal profile of movable channel at which upslope gradient is not equal to downslope gradient are easily turned into unstable points from which strong erosion by debris flows to bed often develops. In addition, debris flows often slow down, and even stop at meandering reach. The flows will remobilize again when subsequent flows arrive at the reach and hence flow depth increases. Peak discharge of debris flow increases greatly by the erosion and accumulation corresponding to the two form changes. And form resistance of movable channel to debris flows is mainly attributed to the profile inflection and planimetric meander. In order to study the effect of form change on flow resistance, flume experiments are designed to examine the characteristics of skin resistance of fixed bed, and form resistance of movable channel to debris flows due to longitudinal profile discontinuity and planimetric meander. Five factors including bed sediment grain size, slope gradient, bulk density, inflection angle, and turning angle are considered in these experiments. The goal is to describe the skin resistance as a function of the former three factors, and form resistance as a function of all the factors. Based on these empirical relationships of boundary resistance and theoretical result of two-phase resistance inside debris flow, a hydraulic model for debris flow routing along channel can be developed. The model can be used to calculate the debris-flow discharge under combination of reaches with different forms and various bed sediment sizes at specific channel, and improve previous empirical methods of calculating debris-flow velocity and discharge etc. This research can highly promote the state of the art of debris-flow prevention engineering.

本项目抓住泥石流沟道运动时的动床形态阻力损失两个重要因素，在野外观测和灾害调查基础上，通过不同条件下的泥石流动床和弯道水槽实验，在已有的明渠流动床阻力模型基础上，主要研究定床条件下泥石流的阻力特性、动床条件下泥石流纵向形态阻力特性和沟槽平面形态对泥石流阻力的影响，建立动床条件下泥石流阻力损失与床沙组成和沟道形态之间的关系。根据这些研究结果并结合成熟的泥石流内部阻力模型，研究不同纵向和平面形态的沟槽组合、不同床面物质组合下泥石流的演进过程，构建泥石流一维沟槽动床演算模型，发展能全面反映泥石流阻力特性的一维沟道水力学演算模型和方法，解释泥石流运动中的流量增大等现象，揭示像舟曲、清平此类大型泥石流的沟道演进机制，为改进现有的泥石流流速和流量等参量的计算方法提供理论依据，提升泥石流防治工程设计的科学水平。

泥石流运动时的阻力或者能量损耗很大一部分归因于沟道动床形态变化。如果仅从定床或者泥石流体本身出发来建立泥石流阻力关系，而忽略动床和床面形态的影响，不可能得到符合实际的泥石流阻力关系。阻力大对应着强侵蚀。现有的泥石流侵蚀理论大多基于无限边坡模型，即认为坡度是不变的。实际的沟道存在许多转折点。沟道纵剖面的形态对泥石流侵蚀有着重要影响。比如，四川清平的文家沟泥石流，在纵剖面转折点处的侵蚀深度远超过其他部位。.项目围绕动床纵剖面和平面的形态变化对泥石流侵蚀率的影响这一核心问题，开展泥石流野外原型调查和室内水槽试验，分析了纵剖面转折角、弯道转角和沟床物质组成等对泥石流侵蚀率和能量耗散的影响。进而将纵剖面形态概化为均一坡度、凹形和凸形三类，研究了泥石流底床的剪切侵蚀和冲击侵蚀机理。前者以层与层之间的剪切力为侵蚀动力，后者表现为泥石流体与底床的正面冲击效应。由此得到非均匀坡沟道中考虑沟道纵剖面凹凸形态影响的泥石流侵蚀率新公式。该公式引入一个曲率半径函数来刻画沟道纵剖面形态变化对侵蚀的影响，并加入了考虑冲击侵蚀影响的项。基于该侵蚀率公式，以泥石流一维运动微分方程为基础，采用TVD类型的数值计算方法，建立了考虑沟道纵剖面形态变化的泥石流沟槽计算模型，开发了相应的计算软件。.项目的研究成果可解释泥石流运动中的流量增大等现象，揭示像舟曲、清平此类大型泥石流的沟道演进机制，为改进现有的泥石流流速和流量等参量的计算方法提供理论依据，提升泥石流防治工程设计的科学水平。相关研究成果近期发表在国际知名学术期刊Journal of Hydraulic Research和Journal of Mountain Science上，并得到多家科技主流媒体的报道。