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Whetstone <br />Associates Technical Memorandum <br />Stormwater from the PDW diversion ditch is routed to NE_Chl and then to NE_Ch2 <br />natural channel, then into the OPWR_Div2 diversion ditch. Stormwater runoff from <br />subbasin NE that would actually report to the natural channels NE_Chl and NE_Ch2 <br />is modeled as reporting directly to the upstream end of OPWR_Divl, which <br />conservatively overestimates flow in this reach. At the upstream end of OPWR_Div2, <br />the reach is assumed to receive all runoff from subbasins NF, NE, ND, D8, and D9. <br />ii. Diversion channels NPE Chl and NPE_Ch2 intercept a significant portion of <br />subbasin NPE (reference Figure 34) that is not accounted for in the runoff analysis. <br />Please re- delineate subbasin NPE or add a re- delineated Basin B4, and revise the runoff <br />model to ensure the appropriate contributing area is included in the runof f analysis. <br />Cotter response: As explained above, runoff from the entire subbasin NPE is included <br />in the runoff analysis. The stormwater runoff from the entire 122.62 -acre subbasin <br />NPE is assumed to report to the upstream end of the OWPR_Divl reach. <br />iii. Disturbed area curve numbers (CN). Subbasins delineated within the both the current <br />and planned conditions ' for the open pit and the open pit waste rock pile use CNs to <br />represent vegetated area (sagebrush w/ grass understory and /or herbaceous range). <br />While this may be appropriate to represent the site as it exists today, once operations <br />resume, vegetation on the dump will be buried and vegetation in the pit will be removed. <br />Please revise the C'Ns to be void ofvegetation for the.following subbasin: Dl, D7, D8, <br />D9, D10, DPI, DP7, DP8, DP9, DP10, DPI7, DP18, DPI9, and DP20. <br />Cotter response: The modeling of the current condition is designed to represent the site <br />as it exists today (including routine maintenance of existing structures). During mining <br />operations, no runoff from subbasin D1 will exit the pit. Instead, the pit will be <br />deepened as waste rock and ore are excavated, and there will be no outlet for <br />stormwater. Any stormwater that runs off the pit walls during operations will be <br />collected in the floor of the pit and handled appropriately. Therefore, the CN of 81 <br />used for the pit as it exits today is considered appropriate for the current condition, and <br />would not be applicable during mining operations within the pit. <br />The reclaimed and revegetated open pit waste rock pile was assigned a curve number of <br />80, 71, or 62 corresponding to an herbaceous mix of grass, weeds, and minor brush, in <br />poor, fair, or good condition or 86 for newly graded area (Table 9). These curve <br />numbers also reflect the reclamation practices for the operating mine. Reclamation of <br />the OPWRP will occur in a phased approach. Recontouring and revegetation will be <br />ongoing as the waste rock pile grows. The surface will be contoured by tracking, <br />furrowing, and seeding. Heavy equipment will be used to pock the surface for water <br />retention to promote more successful vegetation establishment. <br />b. Rationale for rungff estimation parameters ... The response is adequate. <br />Cotter response: Comment noted. <br />c. 100 -year, 24 -hour peak flow calculations analyses ... The response is adequate. <br />Cotter response: Comment noted. <br />4149A.140207 2 <br />