Abstract: The large number of debris flow events in the Dayi area, Guizhou, China, due to a large rainfall event in June 2011 provided a good opportunity to study the influence of topographical factors and rainfall factors on the triggering of debris flows by shallow landslides. The triggering mechanism for these debris flows is the transport of sediment provided by shallow landslides into the channel. During the heavy rainfall event in 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. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies with almost identical hydrological and geological conditions and propose a new T-factor as a topographical indicator which is a combination of the catchment surface area, the percentage of the catchment area with a slope of 25-45 degree, and the average gradient of the drainage channel in the catchment. Additionally a new R-factor is proposed as a rainfall indicator which is a combination of the rainfall in 1-hour before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. The relationship of the T factor and the R factor gives a primary threshold P for this kind of debris flow. The primary probability factor P is the combination of T and R, which gives an indication of the probability of debris flow formation.