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.

Basic historical data of Shihmen Reservoir watershed was collected and analyzed. An extensive suspended sediment concentration (SSC) monitoring program which includes automatic monitoring and manual sampling was established at the inflow and all outflow locations. SSC hydrographs were obtained for four storm events, including Kalmaegi, Fung-Wong, Sinlaku, and Jangmi. The sediment loads and desilting results for individual storms events were calculated accordingly. In addition, the monitoring station at section 24, featured by floating installation and multi-point measurements at depths, provides data for analyzing transportation velocity and thickness of venting density current. Based on the comprehensive data collected this year and limited data collected between 2005 and 2008, interpretations and discussions were made in four aspects, including the relationship between discharge and 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 were analyzed. The user interface of 2D model was preliminarily established. On the other hand, 3D water-sand transportation model was developed to model venting density current and SSC distribution. The simulated results were compared with field measurements and results from physical model tests. Although the falling velocity of sediments has not been included, the simulated results generally agree with the trend observed. The 3D model will continued to be improved and applied to evaluation of desilting plans. The simulated flow behavior and SSC distribution will also provide relevant information for planning or modifying the location of monitoring station.