Experimental field study on the formation process of debris flow dam at channel confluence: Implications for early identification of river blockage

River blockage induced by tributary debris flow is a common hazardous chain in mountainous areas, which could pose a serious threat to human lives and infrastructures. Especially, the failures of landslide dams in tributaries will amplify the debris flow’s magnitude, which could increase the probability of river blockage. However, the dynamic process and criterion of the river blockage induced by such debris flow have not been well understood. Here, we modeled the entire process of river blockage induced by tributary debris flow through large-scale field experiments. The formation process of the river blockage can be outlined into four stages according to the dynamic characteristics at the junction: erosion and entrainment of the river, expansion of the debris flow deposits, damming of the main river, and the dam overflowing and breaching. The erosion rate of the main river and the momentum of debris flow dominate the first and latter processes of the river blockage respectively, and the shift is accompanied by the increasing momentum of the debris flow. Additionally, the erosion rate of the river and the momentum of the debris flow play differential importance in each type of river blockage. The river’s influence is significant for the formation of major blockage and partial blockage if the momentum of tributary debris flow is relatively small. Once the momentum of debris flow is high enough, dynamic characteristics of the tributary are vital for the type of blockage. Then, we established a criterion based on the process of erosion and deposition in the confluence zone. The criterion was verified by 5 river blockage events in the field. It is found that the coupling of the two indicators will improve the accuracy of identification of river blockage compared with existing criteria. In particular, it is possible to better distinguish the type of blockage. This study would advance the understanding of a debris flow dam’s formation, and it is meaningful for the early identification of river blockage.