2012-10-01_REVISION - M1977306 (23)

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7.0 Drainage Design 7.1 Engineering Approach The drainage design is focused on preventing erosion and transport of the waste rock stockpile and ore storage areas during operation and long -term protection of the waste rock pile after reclamation. Currently, drainage at the site is partially controlled by berms and swales that route most offsite runoff around existing stockpile areas. The effectiveness of these existing features has been analyzed for selected design storms and subsequent runoff with improvements recommended when necessary to ensure compliance with BMPs to prevent erosion and offsite transport of potential pollution sources. Drainage improvements are proposed for the site. These include grading and installation of an improved collection/diversion ditch along the northwestern edge of the current waste rock pile. This will provide a mechanism for diverting the runoff from the small offsite basin above it (Offsite Basin 10A). Another berm/swale diversion structure is proposed along the northeastern corner of the site, above the portal, to shed offsite runoff from the area north and east of the site. This will divert water toward the equipment building parking area and into the east -side access road ditch which is a bypass for runoff from offsite Basin 20. The other proposed improvement is construction of a retention pond and associated collection ditches to capture runoff from the onsite basin (Basin 30). These improvements are intended to shed offsite runoff and to collect and retain onsite stormwater. The attached drawings in the back of this document illustrate the locations and details of these proposed improvements. In addition, the attached drawings also illustrate: onsite basin topography, boundaries, and BMP components; offsite basin topography and boundaries; and mine permit boundaries The National Resource Conservation Service (NRCS — formerly known as the Soil Conservation Service, or SCS) dimensionless unit hydrograph procedure is one of the most well -known methods for deriving synthetic unit hydrographs and determining runoff peaks and volumes. For drainage basins where no runoff has been measured, the Curve Number Method can be used to estimate the depth of direct runoff from the rainfall depth, given an index describing runoff response characteristics. This method was originally developed by the SCS for storm conditions prevalent in the United States and was the method used in this study. 7.2 Design Storms DRMS requires analysis using 10 -year and 100 -year, 24 -hour storms. The National Oceanic and Atmospheric Administration (NOAA) Atlas 2, Volume III isopluvials were used in conjunction with site locations (latitude and longitude) to determine precipitation amounts for the given storms. Copies of these charts are included in the attachments. 7.3 Runoff /Discharge Calculations Peak flows were determined for the 100 -year storm in all basins to determine required capacities for the diversion channels. SCS Unit Hydrograph Methods were applied utilizing "Haestad Pond Pack" software. Worksheets and results produced by the software are included in the ESWMP -5