Estimation of Sediment Yields from Landslide and its Post Failures Soil Loss in Shihmen Reservoir Watershed
Date Issued
2012
Date
2012
Author(s)
Tsai, Zong-Xian
Abstract
Landslides generate enormous volumes of sediment in mountainous watersheds; however, quantifying the downstream transport of landslide-derived sediment remains a challenge. This study estimated landslide volumes for the Shihmen Reservoir watershed in Taiwan, for six different time periods, using empirical landslide frequency-area and volume-area relationships. The percentage of total landslide volume reaching streams was estimated using empirical landslide runout models. Fluvial transport of landslide sediment to the reservoir was simulated using the Hydrological Simulation Program- FORTRAN (HSPF). Results indicate that landslide erosion rates ranged 0.4 mm yr-1 to 2.2 mm yr-1 during the periods of 1986-1998 and 1998-2003 , but rapidly increased to 7.9 mm yr-1 following Typhoon Aere in 2004. The percentage of landslide sediment delivered to streams decreased from 78% during the period of 1986-1998 to 55% in 2004. Although the delivery ratio was lower, this decrease is not necessarily associated with a decrease in the volume of landslide sediment reaching streams. Model simulations indicate that only a small proportion of the landslide material was delivered downstream. An average of 13% of the landslide material in the river was moved downstream during the period of 1986-1998. In 2004, the period including Typhoon Aere, the annual channel sedimentation accounted for approximately 23% of the landslide material moved to streams. Channel sediment yield in the study area was determined by transport capacity rather than sediment supply, due to a reduction in sediment transport capacity resulting from existing check dams. The imbalance between sediment supply and transport capacity has resulted in a considerable quantity of landslide material remaining in the upper-stream regions of the watershed.
Landslides have both immediate and extended-duration effects on sediment discharge. Examining the role of post-failure soil loss/sediment yield of landslides is essential for basin management purposes. The comprehensive measurement of sediment yield and the production of existing landslides could provide a better understanding of post-failure behavior. This study investigated the post-failure sediment yield of 13 landslide sites in the Shihmen reservoir watershed during the rainy seasons of 2006 and 2007 using a total station. Our study showed that soil loss is dominated by rainfall conditions and that the geometric and geographic characteristics of each landslide can have significant influences on the efficiency of sediment delivery from fields. We observed that rainfall is important in determining landslide retrogression/enlargement, while the recovery of vegetation in the landslide area increases with time. Intuitive knowledge of these phenomena was confirmed by our data. We obtained valuable landslide measurement data, which could be useful for future landslide modeling. In addition, our results provide more general information about the behavior of landslides.
Finally, we suggest two regression models for the estimation of gross soil loss from cumulative rainfall. We estimated gross soil loss of 0.78 to 1.06 m yr-1, which is equivalent to 20,670 to 28,090 t ha-1 yr-1. Using these estimates, we assessed the sources of sediment in the watershed. We found that the ratio of the landslide erosion, soil loss after landslide, and watershed soil erosion was approximately 66:31:3. The primary implication of this study is that remedying existing landslides might be more effective than any other solution in reducing the sources of sediment in the Shihmen reservoir watershed.
Subjects
landslides
landslide frequency-area statistical estimation
sediment delivery ratio
post-failure sediment yields
Shihmen reservoir watershed
Watershed sedimentation
SDGs
Type
thesis
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