The role of the sediment budget in understanding debris flow susceptibility

This study proposes a sediment-budget model to predict the temporal variation of debris volume stored in a debrisflow prone watershed. The sediment-budget is dominated by shallow landslides and debris outflow. The basin topography and the debris volume stored in the source area of the debris-flow prone watershed help evaluating its debris-flow susceptibility. The susceptibility model is applied to the Tungshih area of central western Taiwan. The importance of the debris volume in predicting debris-flow susceptibility is reflected in the standardized coefficients of the proposed statistical discriminant model. The high prediction rate (0·874) for the occurrence of debris flows justifies the capability of the proposed susceptibility models to predict the occurrence of debris flows. This model is then used to evaluate the temporal evolution of the debris-flow susceptibility index. The analysis results show that the numbers of watershed which are classified as a debris-flow group correspond well to storage of sediment at different time periods. These numbers are 10 before the occurrence of Chi-Chi earthquake, 13 after the occurrence of Chi-Chi earthquake, 16 after the occurrence of landslides induced by Typhoon Mindulle (Typhoon M), and 14 after the occurrence of debris flows induced by Typhoon M. It indicates that the occurrence of 7·6 Chi-Chi earthquake had significant impact on the debris flow occurrence during subsequent typhoons.