Hydrogeological Controls and Analytical–Numerical Prediction of Leakage in a Mountainous Pumped-Storage Upper Reservoir

Mountainous pumped-storage upper reservoirs are commonly affected by high reservoir water levels, adjacent low valleys, fractured rock masses, and local fault-fracture zones, which complicate leakage pathway identification and leakage prediction. This study investigates a two-valley connected mountainous pumped-storage upper reservoir in northwestern China. Groundwater observations, packer tests, analytical calculations, and three-dimensional groundwater flow modeling were integrated to analyze the permeability structure of the reservoir basin, potential leakage pathways, and leakage discharge after impoundment. The results show that the reservoir rock mass is dominated by very weakly to weakly permeable rocks, whereas local moderately permeable zones may form leakage pathways together with dam abutments, ridge saddles, and fault-fracture zones. The summed analytical leakage discharge of the identified pathways was 3069.80 m3/d. Under the normal reservoir water level of 1895 m, the summed analytical leakage discharge of the identified pathways was 3069.80 m3/d, whereas the three-dimensional numerical model predicted a total leakage discharge of 2661.81 m3/d. Both methods indicate that dam foundations and abutments, the western ridge saddle of Reservoir B, and surrounding fault-fracture zones are the main leakage-prone zones. These results suggest that seepage-control boundary determination for similar mountainous upper reservoirs should consider the spatial association among dam-site areas, local permeable zones, ridge saddles, adjacent low valleys, and fault-fracture zones.