Prevously and by beginning of the need for human communities to build facilities and Structures such as tunnels, caverns, dams, etc. Underground water flow and water seepage is one of the important and great issues in mining and civiling projects specifically in underground exavations, such as, tunnels or caverns. Estimating the seepage flow and the amount of water inflow into a tunnel and the arragemnets to control it are the two main problems in underground constructions. One of the projects that is exposed to water seepage is powerhouse cavern of Roodbar pumped storage dam in iran. This dam project complex is located approximately 109 km South of the Aligodarz city in lorestan province. This cavern is excavated at a depth of 400 meters and is located 110 m under the dam reservoir water level that which despite the grouting operation, the pore pressure behind the support system may cause problem to stability of the support and facilities. Therefore, to reduce the amount of pore water pressure behind it a drainage tunnel is excavated at a lower level of cavern but this method wastes the water reservoir. Hence in this research the water seepage or inflow into the tunnel has been evaluated using numerical method (FDM) under hydromechanical approach by FLAC 3D and the results are validated according to the land data recorded from the unsupported section of the tunnel. Using back analysis, the coefficient of permeability of the concrete segment in a drainage tunnel is estimated to be about m/s and a sensitivity analysis on the effect of EDZ zone and permeability coefficient on seepage and pore pressure was performed. According to the results, in the case of an increase in permeability up to m/s, which is about 5 times higher than the host rockmass permeability is increased the seepage by 380% and decreased the pore pressure by 58%.