Reduction of reservoir sedimentation by adopting sediment bypass tunnel.

Jihn-Sung Lai, Fong-Zuo Lee, Tetsuya Sumi, Yi-Jiun Liao, Yih-Chi Tan

Friday 3 july 2015

8:45 - 9:00h at South America (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) Basin-wide sediment management

Parallel session: 14J. Sediment - Basin

High sediment-laden flow from watershed usually causes sedimentation problems in a reservoir. Most reservoirs in Taiwan are lack of sediment sluicing facilities to release sediment during typhoon or heavy rainy seasons. Based on the classification of sedimentation strategies, when the ratio of reservoir life to reservoir capacity is around 10, the implement of a sediment bypass tunnel might be selected to reducing sedimentation. In Japan, the sediment bypass tunnels at the Nunobiki dam and the Asahi dam were completed in 1908 and 1995, respectively. For designing these bypass systems, hydraulic models of tunnel with a diversion weir had been constructed and conducted to investigate sustainable reservoir management (Kashiwai et al., 1997, Lee et al., 2012). Several other successful examples of bypassing systems are valuable for sediment reduction in a reservoir. The Wushe reservoir located in central Taiwan has faced serious sediment problems due to huge sediment yield from the watershed spatially after 1999 Chi-Chi earthquake. According to the bathymetric survey in 2013, it had lost 67% of its original capacity. Therefore, at the upstream river reach the Wushe reservoir is adopted to plan a sediment bypass tunnel and serious deposition problems could be expected to decrease. The intake location was selected be evaluated both construction cost and bypassing efficiency. The shorter distance of bypass line was decided to minimum construction cost. The impacts of bypassing sediment on downstream river bed, sediment concentration, the security of river embankment and aquatic environment are investigated if the bypass tunnel is completed. The designed bypass discharge, 480m3/s, closed to the two-year return-period flood can result in 75% bypass efficiency. However, there is still 25% of total inflow sediment volume with finer materials needed to be removed.