Modeling Landslide Area and Sediment Transport in Landslide-dominated Region, Taiwan

Magnified consequence from rainfall intensification, landsliding to subsequent sediment discharge is an important issue in landslide-dominated regions. However, a few works simultaneously synthesize this consequence due to the time-variant sediment supply in the consequence. The sediment supply responded to the landslide area triggered by rainstorms also regulates sediment transport which strongly depends on stream power. Thus, the conditional interaction between available sediment and sediment transport is rarely discussed. In this study, a model which can simultaneously simulate time-series landslide area and subsequent sediment discharge was proposed and applied onto the landslide-dominated Tsengwen Reservoir watershed in southern Taiwan. Our landslide model considers not only rainfall effect but also pre landslide area to explain the unclear relationship between rainfall and landslide area. Furthermore, the sediment transport model incorporates the status of available sediment, thus the stream transport power and available sediment could be coupled. The result shows that the simulated time-series landslide area and the sediment transport agreed with the observation with the EC of 0.88 and 0.89, respectively. Reactivated ratio of previous landslide area was up to 73% which indicated the highly-frequent reoccurrence of old landslide in landslide-dominated regions. Even the small rainstorm can transport considerable sediment when the sediment supply is sufficient. We argued that quantifying sediment transport should couple not only with water discharge but also available sediment, which is rarely considered in calculating sediment transport. Finally, the scenario analysis of the model reveals that moderate typhoon has the high efficiency in sediment export. Although sediment supply and sediment transport significantly increase with the magnitude of rainstorms, the increment of sediment transport was less than the sediment supply during strong rainstorms. Finally, the quantifying the impact of available sediment on sediment discharge could improve our understanding in sediment transport and help to do the assessments when considering climate change.