| Summary: | 碩士 === 國立中興大學 === 水土保持學系所 === 96 === This study attempts to estimate the sediment yield of Shi-Men Reservoir watershed through the applications of soil erosion model and landslide susceptibility potential models. The sediment yield of slope erosion can be calculated by the two modules in IDRISI program: namely, the IDRISI-RUSLE module (or IR module) and the IDRISI-SEDIMENTATION (or IS module). Regarding the determination of sediment yield of landslide, it is suggested to perform by the following processes. Firstly, incorporating the swell reduction factor 33% with the method proposed by IAEG (International Association of Engineering Geology, 1999) to calculate the collapsed soil volume of landslide. Subsequently, taking the Sediment Transportation Ratio (STR) of slope land and the Sediment Delivery Ratio (SDR) of flow channel into account, the sediment yield of landslide can be determined.
Moreover, prior to determining the critical depth of landslide adopted in the method of IAEG (1990), it is necessary to back-analyze the soil material parameters such as cohesion, friction angle, density and the thickness of colluvium using Landslide Susceptibility Potential Analyzing Program SHALSTAB at first. Subsequently, the critical depth of landslide can be calculated by the critical state equation of an infinite slope proposed by Dymond et al. (1999).
The sediment yield of slope erosion can be calculated by IR and IS modules in IDRISI program. Among the calculation, the net erosion or net deposition of a patch is calculated according to the accumulative contribution of those from the adjacent surrounding patches and such calculation scheme is more coincident with the actual erosion mode of watershed. In addition, the calculations indicate the sediment yield of slope erosion is higher than that of landslide and this is deviated from most of previous studies. The main cause to result in such situation can be attributed to the C value (cover-management factor) adopted in the calculation of IR module was determined by the data bank of land use after Typhoon event. As a consequence, numerous landslides triggered by rainfall during Typhoon are encompassed in the data bank of land use and alternately reflect the influence upon the C value. Further this effect can be verified by the fact that most of patches with higher net erosion fall in the landslide area.
Furthermore, the collapsed soil volume of landslide determined by the method of IAGE (1990) was underestimated when compared with those from the previous studies. This is caused by the adoption of swelling reduction factor 33% and the smaller critical depth of landslide in the calculations. The swelling reduction factor is used to reduce the volume of collapsed soil during landslide which swells due to the release of confining stress.
Eventually, the study indicates merely 35% of the collapsed soil volume of landslide is delivered to the Shi-Men Reservoir and become the sediment yield of landslide. Alternately, there approximates 65% of the collapsed soil volume remains near the down slope and the brink of flow channel. Accordingly, it is arduous to calculate the sediment yield of watershed simply count on a single typhoon or torrential event and it is suggested to take the possible influence of the remaining sediment at the last event into account in calculation.
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