Abstract:

There are three groups of factors related to topography, geology and hydrology which have influence on the formation of gully type debris flows. In this study,a single representative factor (G-factor) for the topography is proposed, which can be used to give threshold values for debris flow formation. The study was carried out in the upper reaches of Minjiang River, which is located in the Wenchuan earthquake area. During some rainfall events after the earthquake, some gully type debris flows were triggered in some catchments. In some catchments no debris flows were triggered even though these catchments were in the vicinity of gullies with triggered debris flows. We isolated and analyzed the influence of the topography on the formation of debris flows because during these rainfall events the hydrological conditions were almost the same in the investigated gullies. We could also neutralize the influence of the geological factors by selecting groups of neighboring catchments located in areas with the same geological characteristics. A new G-factor is proposed in this study as a topographical indicator G,  in which F₀ is the form factor of formation area, J₀ is the average slope of formation area, A₀ is the area of formation area, and A^* is the unit area (=1 km²). Higher G-factor values are generally related to higher probabilities of debris flow formation. The roles of G-factor are analyzed in other areas such as Zhouqu, Gansu, and Chenyoulan River Watershed, Taiwan. It shows that the higher G-factor values, the higher probabilities of debris flow formation are in generally. In the typical areas of Minjian River areas, it is very high probabilities of debris flow formation when G≥0.21. The probabilities of debris flow formation is very low when G≤0.14.

Key words: topographical factor, debris flow, gully type, Minjiang River