Determination of critical rainfall thresholds for debris-flow occurrence in central Taiwan and their revision after the 1999 Chi-Chi great earthquake

In this study, 28 first-order streams in the Nan-Tou area of Taiwan were chosen as samples for a debris-flow study. After analyses, five factors of the watersheds of the stream samples were found to be comparatively related to the occurrence of debris flows. Using these factors, prediction equations for the critical rainfall threshold for debris-flow occurrence in this area were obtained before the great 1999 Chi-Chi earthquake in central Taiwan. To determine the effect of the earthquake on the occurrence of future debris flows, modifications were made to the debris-flow prediction equations. The modifications included incorporation of peak ground acceleration (PGA) of the earthquake and the effect of increasing slope stability with time after the Chi-Chi earthquake. The modified equations were tested, providing rather satisfactory results using rainfall data of post-1999 typhoons and storm events.