River bed variations due to sediment bypassing from a reservoir

Fong-Zuo Lee, Jihn-Sung Lai, Cheng-Chia Huang, Yih-Chi Tan, Hui-Ming Hsieh

Friday 3 july 2015

11:00 - 11:15h at South America (level 0)

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

Parallel session: 15J. Sediment - Basin

The watershed of a reservoir could generate huge amount of sediment yield during typhoon flood or heavy storm event. Moreover, land development in the watershed would also accelerate soil erosion. As sediment moves into a reservoir, deposition occurs due to velocity decrease and results in reservoir storage reduction. In recent years, serious deposition problems had been observed in the Wushe reservoir especially in the upstream delta part of the reservoir. The Wushe reservoir located in the central Taiwan has its upland watershed of 219 km2. It had been lost two-thirds of its original capacity according to bathymetric survey in 2013. How to ensure sufficient reservoir storage for water supply and power generation is always an important issue. In addition, if the modification of outlet structure is made, sediment sluicing operation should affect downstream river bed variation and water quality at the withdraw intake. The purpose of this study is to investigate desiltation strategies and impacts of sediment releasing on downstream river reach between Wushe dam and Wuchieh dam. Based on the parameters of the turnover rate of water (CAP/MAR=total capacity/Mean annual runoff) and sediment (CAP/MAS=total capacity/Mean annual inflow sediment), the strategy by using a bypass tunnel is adopted to reduce sedimentation impact of Wushe reservoir. The efficiency of sediment bypass obtained from physical model tests is introduced as the boundary condition for downstream river simulation. A 1D mobile bed model is employed to simulate the impacts of downstream river after sediment desiltation operation releasing from the Wushe reservoir. Based on simulated results, desiltation operation by using sediment bypass with other strategies for the Wushe reservoir can keep downstream river bed more stable. The simulated concentration of suspension sediment during bypassing operation does not significantly affect the function of withdraw intake at Wuchieh dam for water supply.