Background Debris flow is a widely distributed geological hazard in the mountainous area, and the hazard assessment of debris flow is essential for its prediction, risk evaluation and prevention. The Wenchuan earthquake has disturbed the geological condition of southern Gansu province, which not only produced abundant loose material to new debris flow formation but also triggered the recurrences of some debris flows in idle. The Bailong River Basin is located in the South Gansu province, where severe debris flow occurred during the Wenchuan earthquake in 2008, thus it was selected for this study.

Methods A watershed unit was taken as the evaluation unit, and the evaluation unit was determined using GIS technology with digital elevation model (DEM) data. A debris flow hazard assessment model at watershed-scale was proposed based on the gradient of debris flow gully, the scale of debris flow and the level of potential river blockage. By applying the proposed hazard assessment model for Bailong River Basin, the hazard degree of debris flow was obtained as 5 levels of very high, high, moderate, low, and very low, and a hazard map of debris flows was constructed for Bailong River Basin.

Results The assessment results indicated that there were 2 debris flows at very low hazardous level, accounting for 1.3% of the total debris flows, 31 debris flows (20.7%) at low level, 60 debris flows (40%) at moderate level, 19 debris flows (12.7%) at high level, and 38 debris flows (25.3%) at very high level. The 117 debris flow gullies were at the moderate hazardous level or above, accounting for 78% of 150 total gullies. Validated by the field investigation and historical debris flow events, the debris flows in Luojiayu, Sanyanyu, Yuzi, liangshui and Shimen gullies were at very high hazardous level, which was consistent with the assessment result.

Conclusions It is scientific to take a watershed unit as evaluation unit because it can reflect the features of valleys, topography and landform conditions when debris flow occurs. On this basis, the assessment factors are selected from the debris flow energy, source and disaster degree to establish an index system for assessing the hazard of debris flow. Through field investigations, it is confirmed that the assessment results by model are consistent with the actual situation of debris flow, thus this model may be used to predict the occurrence of debris flow in the Bailong River Basin.