Density ranges for different types of materials.

Debris flow is a typical natural disaster in the middle reaches of the Dadu River, Southwest China. Field physical model tests were conducted to reveal the mechanism of river blocking by debris flow in the middle reaches of the Dadu River. The dynamic processes of riHver blocking by debris flows were revealed, and based on which three typical river-blocking modes of debris flow are observed, i.e. thrust-type river blocking, mixed-flow-type river blocking and progressive river blocking. The test results showed that the material composition of debris flows plays an important role in the river-blocking mode, only the tests that adopted the mixed soil and gravel exhibited the thrust-type river blocking mode. The material composition has a controlling effect on the thrust-type river-blocking model. Mixed-flow-type river-blocking mode appears most often in the tests with an intersection angle of 60°, because the small intersection angle is conducive to the mixing of the debris flow and the water in the main channel. The debris flows composed of sand tend to block the river with mixed-flow-type river-blocking mode, accounting for 50% of the occurrences in the model tests. The high flow rate and discharge in the main channel and the low flow rate and discharge in the branch channel are the key factors controlling the progressive river-blocking mode. The test results in this study can support the debris flow disaster prevention and mitigation in this area.

泥石流（Debris flow）是中国西南大渡河中游典型的自然灾害。为揭示大渡河中游泥石流堵江机制，研究人员开展了现场物理模型试验。试验阐明了泥石流堵江的动态过程，并据此观测到三种典型的泥石流堵江模式，即推挤式堵江（thrust-type river blocking）、混合流型堵江（mixed-flow-type river blocking）与渐进式堵江（progressive river blocking）。试验结果表明，泥石流的物质组成对堵江模式具有显著调控作用：仅采用混合土与砾石的试验组，方可观测到推挤式堵江模式，物质组成对推挤式堵江模式起主导控制作用。当交汇角为60°时，混合流型堵江模式出现频率最高，这是由于较小的交汇角更利于泥石流与主河道水体的充分混合。由砂质组分构成的泥石流，多以混合流型堵江，该情形占模型试验总发生案例的50%。主河道高流速、大流量与支沟低流速、小流量，是控制渐进式堵江模式的关键影响因素。本研究的试验结果，可为该区域的泥石流防灾减灾工作提供重要支撑。