Technical Studies

The watershed of Shihmen Reservoir has characteristics of steep valley and week geology. Large amount of suspended load transports to the reservoir during heavy storms. The large amount of sediment threatens the reservoir capacity and affects the water quality. The purpose of the project is to investigate the transportation behavior of high sediment concentration flow by field monitoring and numerical simulation. These studies should provide useful information for devising suitable desilting plans to extend the life of Shinmen Reservoir.

The project in phase 2 maintenance the exist monitoring equipments and add two monitoring stations at section 7 and section 15. SSC hydrographs were obtained for Morako storm events by an extensive suspended sediment concentration (SSC) monitoring program which includes automatic monitoring and manual sampling station. Based on the comprehensive data collected these two stage project and limited data collected between 2005 and 2008, interpretations and discussions were made in four aspects, including the relationship between discharge and sediment load, reservoir capacity, sediment-related water quality, and pattern of sediment transportation. Furthermore, particle size distributions of suspended sediment during typhoon and basal sediments before and after typhoon events were obtained. Sediment data were further used as input data and calibration data for 2D and 3D numerical models of sediment transportation.

The 2D mobile-bed model was developed mainly to simulate the spatial distribution and long term variation of sedimentation and erosion at Shihmen Reservoir. In addition, the effect of back water at different reservoir water level, diving point of density current, and deposition and transportation characteristics of non-uniform sediments, and Desiltation Strategy at reservoir upstream were analyzed. The user interface of 2D model was preliminarily established. Because the two-dimensional model has still not been able to disply the information of flow field of three dimensional and trend of the density current, it is necessary to establish a three-dimensional density current model. In this project, the team modifies quasi-three-dimensional hydraulic model of Lin and Huang (2008) in consideration of the density variation effect. The mode would be used in the reservoir computation bsae on result of simulation, datas of physical model experiment, and actual survey data. Also, model revising the bottom boundary condition, adding fall velocity effect, and solving elevation problem by using image method for increasing completely and correctly of simulation. Simulation results of operation of sedimentation prevention show that the velocity of the reservoir has increased slightly. But it decreased in the concentration of sediment by certain extent. However, the case of C1 was more obvious in decreasing sediment at reservoir. And the case of D2 combined with the case of C1 could be the best way to decrease the sediment, but if only that we just considered single situation of sedimentation prevention operation that the case of C1 was achieved the best effect.