Estimating volume of deep-seated landslides and mass transport in Basihlan river basin, Taiwan

Deep-seated landslides are often caused by severe rainfall, but the displaced potential landslide mass often does not slide downslope entirely in one time. Evaluation of the mobilized volume of the deep-seated landslide and the volume of the remaining potentially unstable landslide mass on the slope is vital for hazard mitigation. In this study, an integrated method was developed for evaluation of the potential landslide volume and mass transport of deep-seated landslides caused by Typhoon Morakot in Basihlan River Basin, Taiwan. A combination of various types of remote-sensing data, including aerial photographs, satellite images, and light detection and ranging (LiDAR) data, from before and after the event were used for locating landslides, and the deep-seated landslides were identified in the Basihlan River Basin, Liuoguey District, Kaohsiung City, Taiwan. A stability analysis and a profile analysis were conducted to determine the critical sliding surface of each deep-seated landslide, and the volumes (masses) of the landslides were estimated. The total volume of all the deep-seated landslides was approximately 6,017,000 m(3) in the Basihlan River Basin. A topographic analysis was performed to evaluate the mass transport of the deep-seated landslides using LiDAR-derived digital terrain models (DTMs) before and after Typhoon Morakot. The mass transport of deep-seated landslides in the Basihlan River Basin was determined to be approximately 3,527,300 m(3), which suggested the volume of the remaining potentially unstable landslide mass on slope of approximately 41.4%. The available field data suggested subsequent reactivations of the deep-seated landslides after Typhoon Morakot, including debris flow generation, which damaged the infrastructure. Thus, a large landslide volume remaining in the Basihlan River Basin can cause more landslides or debris flow hazards in the future. Estimation of the potential landslide volume and mass volume remaining on slopes is essential for both short- and long-term mitigation strategies.