Three-dimensional Numerical Simulation Of Influence Of Reservoir Impoundment On Guoundwater Flow Of Reservoir Area

The Minjiang Navigation-Power Qianwei Junction project is one of the six-grade cascaded hydropower projects which is from Leshan to Yibin. It is mainly for shipping, avionics, taking into account the comprehensive utilization of flood control, water supply, tourism and environmental protection. Normal water level of Qianwei hydropower station is 335m, the water level lifting approximately 16m before water storing at the dam site. The water rise provides good hydrodynamic conditions to the seepage of the reservoir. So in this paper, we take the planned Qianwei hydropower station for an example, to study the reservoir area groundwater seepage with the effect of reservoir impoundment. And it is very important to ensure the capacity and the normal operation of the reservoir.After a lot of field geological survey, we find out the topography, lithology and geological structure conditions and so on in the reservoir area. Through the laboratory and field trials, we analyzed the hydrogeological conditions of the area. The strata of the reservoir area mostly consist of Shaximiao (J2s) and Ziliujing (J2z) formation of the Middle Jurasic, and Xujiahe formation of the Upper Triassic (T3xj). The lithology is mainly composed of sandstone, slitsonte, shaly sandstone, sandy mudstone, mudsonte, shale and so on. The stratigraphic occurrence is very smooth, and the angle generally between 4～8°. The underground water in the reservoir consists of the pore-space water, the clastic rock aperture cranny water and the bedrock fissure water. The clastic rock aperture cranny water of the Triassic system and the sandstone fissure water of the Jurasic take up a great proportion. According to the water rich property and the permeability of the strata, we divided the aquifer into five types: strong～medium aqrifer, medium aquifer, poor aquifer, medium～poor aquifer and relative water-resisting aquifer. We find out the relation of the groundwater recharge, runoff and discharge. And we analyzed the chemical behaviors and the dynamical property of the groundwater. The groundwater of two sides of the reservoir mainly accepts precipitation recharge. The groundwater discharge to the reservoir mainly through the cracks channel runoff. The groundwater of the whole area can be divided into 9 underguround water system.On the base of above study, establish the hydrogeology conceptual model of the study area. The three-dimensional numerical model is built after the generalizations of the boundary conditions and the source sink term. We determined the scope of the hydrogeology paramenters through the pumping tests and the information from the injection and water-presure tests of the other well-drilling in the area. We use the groundwarer mumerical simulation software which is the most popular softwares to make adjustments and computer imitation, and finally make sure the hydrogeological paramenters. The numerical simulation of the before and after reservoir built was made, calculated the seepage of the reservoir and the average leakage of the reservoir after the stable water level. And study the relationship between the leakage and the rainfall of every stress period. At the begining of the umpoundment of the reservoir, the level of the groudwater rise temporarily. But the level continues to decline as the rise of the time, the speed of the change decreases gradually until the dynamic equilibrium. The change of the groundwater level which is near the dam site and the terraces on both sides is more obvious than that near the reservoir tail and the area far away from the reservoir. The water of reservoir mainly leak to the reservoir downstream of the dam site and the outside of the reservoir, through the dam site and the first terrace which is low and flat on both sides of ht reservoir. After it steady, the average leakage is 0.048% of the average annual flow of the Minjiang River at this part. It belongs to the weak leakage, and it would not produce significant effects on the normal operation of the reservoir. These results provide a reliable basis for the design of the Qianwei hydropower station.