Topographical factors in the formation of gully-type debris flows in Longxi River catchment, Sichuan, China

Abstract

There are three groups of factors: topography, geology and hydrology which have influence on the formation of gully-type debris flows. In this paper, a single representative factor (T-factor) for the topography is proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Longxi River catchment, located in the 2008 Wenchuan earthquake hit area. During a heavy rainfall event on August 13, 2010, 34 gully-type debris flows were triggered. In some catchments, no debris flows were triggered even though these catchments were in the vicinity of gullies with triggered debris flows. The formation mechanism of the triggered debris flows is the runoff-induced mechanism. The influence of the topography on the formation of debris flows in gullies with almost identical hydrological and geological conditions was isolated and analyzed. A new T-factor is proposed in this study as a topographical indicator which is a combination of the form factor, the average gradient of the stream, the catchment size, and the percentage of the catchment area with a slope gradient sensitive to trigger debris flows. Higher T-factor values are generally related to higher probabilities of debris flow formation. The T-factor can be used to predict the formation of debris flows in the sub-catchments of the Longchi River catchment. The T-factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.