Technical Studies


Impact Evaluation of Sediment Released from Shih-men Reservoir on Downstream River Morphology

I. <!--[endif]-->Introduction

Shimen Reservoir is located upstream of the Tamsui river. The reservoir has been deposited seriously after the occurrence of typhoon Aere in 2004. For the sustainable operation of the reservoir, in recent years, Northern Region Water Resources Office, Water Resources Planning Institute, and other departments develop the strategies actively, including mixture diluted emissions of the surface water and low-elevation water with high turbidity sediment discharged by the tunnel spillway, low-elevation turbid water discharged in Shimen canal, sediment discharged by Shimen power plant and PRO, and upstream flood diversion and hydraulic desilting and other measures. The purpose of this project is to assess the reservoir desilting operation strategy on the bed evolution of downstream reach.

<!--[if !supportLists]-->II. <!--[endif]-->Data Collection and Analysis

Historical hydrologic, cross-sectional, and bed material data were collected in this project. Moreover, size gradations of the bed material in Dahan and Tamsui rivers (25 sample pits) suspended sediment concentrations were measured.

<!--[if !supportLists]-->III. <!--[endif]-->Impact of Shih-men Desilting on Downstream Reach

This study adopted CCHE1D and CCHE2D models as the numerical model tools, and referred to the physical model data obtained from the project entitled is "Shimen reservoir flood control and desilting project planning-hydraulic model tests", and the discharge and sediment concentration data during Fung-wong, JangmiI, Sinlaku, and Morakot typhoons provided by Northern Region Water Resources Office, WRA. In this study, five cases were considered: the existing condition, C1, C1 with extension pipes, D2 and C1 with extension pipes plus modified D2. After the construction of desilting tunnel, downstream aggradation for a given typhoon event depends on the amount of sediment released from the reservoir . Significant river deposition can be found in the reach from Kan-yuang bridge to Chung-hsiao bridge of Tamsui river. Small the amount of deposition is located in Dahan river, and the deposition at the Tamsui estuary can be neglected.

For the long-term simulation (48 years), most downstream reaches the amount of aggradation increases. The more significant aggradation can be found in the cases of C1 with extension pipes and C1 with extension pipes plus modified D2.

<!--[if !supportLists]-->IV. <!--[endif]-->Evaluation on Dike Safety

To evaluate the freeboard being enough or not and the possibility of overtopping of dike, Q100 in Dahan river and Q200 in Tamsui river , and elevation of 4.03m at the downstream estuary were adopted, It can be found that not enough freeboard and dike overtopping occurred in the reach from the estuary to Guan Tu bridge. In addition, the flood stages do not change much after the construction of desilting tunnel. The safety of dike base at some critical location is also evaluated.

<!--[if !supportLists]-->V.<!--[endif]-->Impact Evaluation of Sediment Concentration

Through 2D mobile-bed simulation, the suspended sediment concentration at the intake of Yuanshan weir can be estimated. It can be seen that the duration of high sediment concentration (larger than 6,000 NTU) increases offer the construction of desilting tunnel.

<!--[if !supportLists]-->VI.<!--[endif]-->Education and Training

The theories, numerical schemes, and application case studies of the CCHE1D and CCHE2D models were introduced on December 1, 2010 and November 8, 2011, repectively.