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Section 2 <br />Is Drainage Control Design <br />2.1 Existing Drainage Conditions <br />The Van 4 Mine has a drainage area of approximately 27 acres. Runoff flows <br />southwest to an earthen berm located at the crest of the development rock area <br />(DRA), which acts as a retention basin for the site. An existing drainage is located on <br />the north side of the mine, which drains runoff in a southwestern direction. <br />2.2 Drainage Plan <br />CDM delineated three drainage basins based on the topography and existing mine <br />surface as illustrated in Exhibits A and B: <br />1. Basin 1 - Surface water run-on from offsite northeast of the surface mine facilities. <br />2. Basin 2 - Surface mine facilities area. <br />3. Basin 3 - DRA slope. <br />2.3 Peak Discharge Estimate <br />CDM selected 100-year as the design storm return interval for surface water control <br />structure design. CDM estimated the 100-year peak discharge for each drainage area <br />using the following USGS regionalized flood-frequency equations for the southwest <br />118.4(A)0.715 <br />Q ioo - - <br />region of Colorado (USGS 2000): <br />where: <br />Qioo =100-year peak flow in cfs <br />A = drainage area in square miles <br />The standard error of the above regression equation is approximately 78 percent. Due <br />to the error associated with the regression equation, CDM increased the calculated <br />discharge from the flood-frequency equation by the standard error (78 percent). Table <br />2-1 presents CDM's estimated 100-year peak discharges for each basin of the Van 4 <br />Mine. CDM used the 100-year peak discharge of Basin 1 to design a diversion channel <br />to convey the discharge from Basin 1. Design of retention basins requires storage <br />volume and therefore a different method was used. The determination of the 100-year <br />peak discharges for Basins 2 and 3 were produced for information only. <br /> <br />2-1 <br />7:164986-Denison Mines\Task Order 1 - Sunday Mines Drainage Study\Van 4 Mine Drainage StudylReportlVan4 Mine Drainage Report.d=