Effects of topography, lithology, rainfall and earthquake on landslide and sediment discharge in mountain catchments of southeastern Taiwan

From 1996 through 2007, several heavy typhoons and earthquakes precipitated landslide hazards in southeast Taiwan. In the present study, we analyze the impact of topography, lithology, rainfall and earthquakes on landsliding and sediment transport by quantifying the landslide ratio (ratio of landslide area to catchment area) and the sediment discharge for the Sinwulu and Luye upstream catchments of the Peinan River. The steep topography in these two catchments causes a large proportion of landslide ratios on slopes to exceeding 50°, a condition which accelerates the delivery of landslide debris to the downslope channel. The landslide ratios of 0.84%-2.30% are obviously related to the rock strength and discontinuity density of the terrain, but we find the magnitude of landsliding and sediment discharge to vary with the occurrence of earthquakes. Typhoon-induced rainstorms generate the bulk of sediment discharge in the study catchments, though the landslide debris is transported by the huge amounts of runoff produced by rainstorms. A contributing factor is the recent occurrence of severe earthquakes which weakened the geomaterial, leading in turn to further increases of fluvial sediment during subsequent typhoons. Additionally, the finding that eventual reduction of unit sediment concentration after high magnitude earthquake events reveals that the downslope transport of landslide debris associated with earthquake activity and subsequent typhoon events precipitates a relatively short term, 1-3. year, increase of erosion rates in these two study areas in southeastern Taiwan.