Similarity ratios of the tests.

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。本研究针对该区域泥石流堵江机制开展野外物理模型试验，明确了泥石流堵江的动态演化过程，并据此归纳出三类典型的泥石流堵江模式：推力型堵江（thrust-type river blocking）、混合流型堵江（mixed-flow-type river blocking）与渐进型堵江（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：仅采用土石混合体的试验组出现了推力型堵江模式，物质组成对该类堵江模式具有主导控制作用。混合流型堵江模式在交汇角为60°的试验工况中出现频次最高，这是由于较小的交汇角更利于泥石流与主河道水流充分混合。由砂质物质组成的泥石流更易以混合流型堵江模式形成堵江，该类工况在模型试验中占比达50%。主河道高流速、大流量与支沟低流速、小流量的水动力条件组合，是控制渐进型堵江模式形成的关键因素。本研究的试验结果可为该区域的泥石流灾害防灾减灾工作提供重要技术支撑。