Effects of landslide and other physiographic factors on the occurrence probability of debris flows in central Taiwan

This study focuses on the effects of physiographic factors, such as landslide area, on the occurrence of debris flows in central Taiwan. Four physiographic factors were selected for their significance to the occurrence of debris flows, including landslide ratio (the ratio of landslide area over watershed area), average steepness of the streambed, effective watershed area, and form factor. Two quantifying methods of factors were performed and compared: one using genuine values of factors and another one using values converted by degree of membership from fuzzy theory. Then the logistic regression method was applied for building a model to assess the occurrence probability of debris flows from five variables: four physiographic factors and one hydrologic factor. The model is consistent with the mechanism of debris flow occurrence, with all physiographic and hydrologic factors positively correlated with the occurrence probability. In addition, the accuracy of the model was validated with randomly selected historical events and demonstrated fairly satisfactory validity, ranging from 70 to 80 %. It was found that adopting the degree of membership made the model more stable and more reliable. In addition, the model also shows that reducing the landslide area can significantly reduce the occurrence probability of debris flows. The results show that the model built in this study has the potential to be well applied and fully integrated into current or future warning systems.