Debris flows are among the most destructive disasters in mountain areas around the world, and they continue to receive substantial attention in the literature. However, factors influencing the collapse of soil mass and debris flow initiation in hollows have not been explored using experimental approaches. In this study, we conducted a series of indoor hollow-flume experiments to exhibit how three factors, namely, initial moisture content, slope angle, and clay content, influence the collapse of soil mass under the condition of surface runoff. Based on experiment results, we deduced that the major processes that result in soil mass failure are internal seepage erosion caused by subsurface runoff, migration of fine particles, development of tension cracks, increase of pore water pressure, and decrease in cohesion. Fluctuations in pore water pressure were characterized by two main trends: (1) an abrupt rise under static liquefaction, and (2) a decrease prior to failure due to dilatancy. In our study, the time of soil mass failure first decreased and then, increased as the initial moisture content increased. The soil mass having initial moisture content ~12.5 % was more critical to failure. For tests with different slope angles, the time required for failure decreased with increasing slope angle, and the test that had a slope angle of 40o was more prone to failure. For tests with different clay contents, the buildup of pore water pressure increased with increasing clay content; soil having clay content of ~7.5 % required the shortest time to trigger slides. Thus, our results provide a thorough insight into the soil mass failure and initiation processes of debris flow disasters in hollow areas and can be used by governments to improve the existing monitoring, early warning, and forecasting systems. Notably, our study can help identify risk-prone source areas, where sensors can then, be installed
