Discharge of landslide-induced debris flows: case studies of Typhoon Morakot in southern Taiwan

Abstract. Three debris-flow gullies, the Hong-Shui-Xian (HSX), Sha-Xin-Kai (SXK), and Xin-Kai-Dafo (XKD) gullies, located in the Shinfa area of southern Taiwan, were selected as case studies on the discharge of landslide-induced debris flows caused by Typhoon Morakot in 2009. The inundation characteristics of the three debris flows, such as the debris-flow volume V, deposition area A_d, and maximum flow depth, were collected by field investigations and simulated using the numerical modeling software FLO-2D. The discharge coefficient c_b, defined as the ratio of the debris-flow discharge Q_dp to the water-flow discharge Q_wp, was proposed to determine Q_dp, and Q_wp was estimated by a rational equation. Then, c_b was calibrated by a comparison between the field investigation and the numerical simulation of the inundation characteristics of debris flows. Our results showed that the values of c_b range from 6 to 18, and their values are affected by the landslide ratio R_L. Empirical relationships for c_b versus R_L, Q_dp versus Q_wp, Q_dp versus V, and A_d versus V are also presented.