Feasibility Study and Technical Report - Pretivm

18.5 GRADING AND DRAINAGEThe site will be positively drained at all times. Existing drainage courses will be preservedas much as possible as this typically leads to the most economical drainage design. Anyminor drainage systems will be designed for the 10-year storm event, but the site willalways have a major storm outlet route to prevent any area flooding. Design flows werederived from a master drainage plan that was developed for the site. Site specifichistorical hydrological information was not available; therefore, intensity durationfrequency (IDF) information was estimated using the Stewart Airport rain gauge for flowcalculations.Detailed grading of the facility pads has not been completed for the feasibility stage.Additional earthworks required for such grading can be significant because some of thepads are large areas. As such, an approximated additional volume of earthworks hasbeen included in the capital cost.Ditches are designed for the 10-year storm event and grades range from 0.3 to 8.0%.Culverts are designed to discharge a 10-year storm event without static head at theentrance, and to discharge a 100-year storm event utilizing available head at theentrance. Culvert corrugation and wall thicknesses were specified in accordance withstructural load-carrying capacity under H-20 highway loading and depth-of-coverrequirements, respectively. Rip-rap will be used for erosion protection at culvert inletsand outlets, and along diversion/drainage channels where required.Storm drainage from PAG rock areas or ore storage areas will be collected and pumpedto the water treatment plant. Currently, this provision has been included for the mill padarea and the pre-production ore storage area.Sediment control during construction will be handled on site as needed by coveringexposed cuts and stockpiles, diversion ditches, silt fences, etc. Maintenance of theseitems will be important. Tetra Tech assumes that these proven measures are acceptableto the regulatory authorities, and that a sediment pond is not required.18.6 AVALANCHE HAZARD ASSESSMENT18.6.1 BACKGROUND ON SNOW AVALANCHESSnow avalanches generally occur in areas where there are steep open slopes or gullies,and deep (more than 50 cm) mountain snow packs. Risks associated with avalanchesare normally due to exposure to the high impact forces that occur, as well as the effectsof extended burial for any person caught in an avalanche. Impact forces vary significantlydepending on avalanche size. Although the smallest avalanches can be insignificant to ahuman, larger avalanches may produce impact forces capable of destroying trucks,buildings, or several hectares of mature forest.Pretium Resources Inc. 18-13 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC