Intermittent viscous debris flow formation in Jiangjia Gully from the perspectives of hydrological processes and material supply

Debris flows are among the most destructive natural processes to affect areas of mountainous terrain. Debris flow formation involves watershed-scale processes of hydrology and material supply. This study explored both processes based on long-term monitoring data from the Jiangjia Gully in southwestern China. Based on hydrological simulations, we found that the debris flows formed as "normal" or "abnormal" hydrological processes. The former, with normal time lag tau and normal peak discharge Q(max) formed as a typical hydrological process in which slope failure material was mixed instantaneously with channel runoff. Conversely, the latter, with longer time lag tau and/or abnormal discharge Q(max), formed through blockage and outburst associated with landslides. We found that the different types of debris flow required similar rainfall conditions and that the threshold could be expressed as I = 6.25 D-0.73 (0.5 <= D <= 14.5 h). Furthermore, we established that rainfall pattern influences debris flow occurrence, that is, such events are more likely during short-duration rainfall. The findings of this study highlight the significance of soil supply to the process of debris flow formation via random disturbance of the normal hydrological process.