How sub-basins runoff contribute to debris flow propagation at a basin scale? A numerical study

Runoff significantly influences the propagation of debris flows by transferring mass and momentum. The hydrodynamics of runoff, which are closely linked to contributions from sub-basins, determine the extent of this influence. In this paper, a cascade model is utilized to quantitatively analyze the contribution of sub-basins to runoff and, subsequently, to the propagation of debris flows, using the 2020 Meilong debris flow event as a case study. First, the propagation of debris flows, characterized by their high mobility and sediment entrainment, is well reproduced. This analysis examines how each sub-basin’s generated runoff contributes to debris flow propagation, revealing that both the area and location of a sub-basin are significant factors. Additionally, a series of scenarios with variations in basin features and debris flow types are simulated. The results suggest that as the basin area and internal relief decrease, the contribution of sub-basins to runoff-and consequently to debris flow propagation-also diminishes, aligning with trends observed in field data. Furthermore, the propagation of debris flows with lower viscosity is more effectively facilitated by runoff from sub-basins due to enhanced mixture between runoff and debris flow. This study provides significant insights into the propagation of debris flows, thereby supporting the assessment of this debris flow type.