Landslides affecting sedimentary characteristics of reservoir basin

Typhoons Aere (2004) and Matsa (2005) caused high nephelometric turbidity in the Shihmen reservoir in northern Taiwan, jeopardizing the operation of the reservoir for several days, and ultimately impacting the living conditions and economy of the downstream residents. The torrential rains caused landslides and debris flows in upland areas, and flowed into riverbeds, likely contributing significantly to the suspended sediment yields in the reservoir. This investigation elucidates how upland landslides affect sediment attributes in the reservoir basin. Study methods including field observations, spatial analysis in GIS and aerial photo interpretation are adopted to trace the sediment sources and contributing factors to the landslide. Torrential rains induced landslides and debris-flows upland, causing river incisions and soil erosion in landslide areas lacking vegetation. These factors, together with the conditions of the engineered structures and geologic vulnerabilities of the area, caused suspended sediment yield in the reservoir. The high nephelometric turbidity could potentially reoccur, with masses of landslide-derived sediment remaining upland and in the riverbed.