The relationships between landslide and sediment discharge in the periods of earthquake and typhoon events along the catchment of the Sinwulyu River

The hydraulic measurement, geomorphologic changes and geomaterial characteristics are utilized to analysis their relationships in their study. The various images of SPOT satellite were selected to count the landslide ratio in six typhoons and two earthquakes events along the catchment of the Sinwulyu River form 1996 to 2006.
The investigated results show that the landslide ratio ranges from 0.84% to 1.94% during the different disaster event. After the Chen-Gong earthquake (ML=6.6), the following Typhoon Mindulle resulted in heavy landsliding, including 1.94% of landslide ratio, 72.3% of new generative ratio and 66.5% of reactive ratio in the catchment. The elevation of most landslides ranges from 1500 to 2500 meters and keep rising after Chen-Gong earthquake. Besides, the landslides converge in the steep slopes ranging from 30° to 50°. The uniaxial compressive strength (UCS) of the Bilushan Formation is the lowest around 34MPa in all rock units, and the landslide ratio in the Bilushan Formation, ranging from 1.74% to 2.56%, is the highest. Comparatively, 102MPa of UCS in the Dananao formation is the highest and the landslide ratio in the Dananao formation, ranging from 0.56% to 1.34%, is the lowest. The Dananao formation has 47.3N/m3 of the joint number that causes the highest reactive ratio, ranging from 57.3% to 72.5%. There is a positive linear relationship and the statistically significance between the reactive landslide ratio and the joint numbers.
The Normalized Difference Vegetation Index (NDVI) in this region ranges from 0.44 to 0.53. There is week significance between landslide ratio and NDVI. The annual sediment discharge ranges from 7.8 Mt/y to 15.2 Mt/y by using the calculation of average method and rating curve method from 1979 to 2005. The sediment discharge in the period of rainstorm events occupied 6.6% to 25.3% of annual sediment discharge. The concentration of discharge increased 2.58 times in post Chi-Chi earthquake, and further increased 1.18 times after Chen-Gong earthquake. This result demonstrates that the colluvial sediments of hillslope resulted from the successive earthquakes not only caused the increasing of landslide ratio, but the rising of sediment discharge increased.