Topographical and rainfall factors determining the formation of gully-type debris flows caused by shallow landslides in the Dayi area, Guizhou Province, China

Three groups of factors, topography, geology and hydrology have influence on the formation of gully-type debris flows triggered by shallow landslides. In this paper, a single representative factor (T factor) for the topography, and a single representative factor (R factor) for the rainfall (hydrology) are proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Dayi area, Guizhou Province, China. During a heavy rainfall event on June 5 and 6, 2011, 37 gully-type debris flows caused by shallow landslides were triggered. In some catchments no such debris flows were triggered even though these catchments were in the vicinity of gullies with debris flows. The triggering mechanism for gully-type debris flows is the transport of sediment provided by shallow landslides into the gully. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies under almost identical hydrological and geological conditions and propose a T factor as a topographical indicator which is a combination of the catchment surface area, the ratio of the catchment area with a slope sensitive to trigger debris flows, and the average gradient of the drainage channel in the catchment. Additionally an R factor is proposed as a rainfall indicator which is a combination of the rainfall in 1 h before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. Higher T factor values and higher R factor values are generally related to higher probabilities of debris flow formation. The primary probability factor P, which is the combination of T and R, gives an indication of the probability of debris flow formation. The T factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.