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Section 2 <br />Topaz Mine <br />In 2007, Tetra Tech completed an analysis of the drainage patterns for the expanded <br />Topaz mine surface proposed in the Plan of Operations (POO) and designed surface <br />water control structures accordingly (Tetra Tech 2007). CDM reviewed the Tetra Tech <br />drainage report and surface water control structure designs. This section describes the <br />drainage report review and provides recommendations regarding drainage <br />component sizing. <br />2.1 Existing Drainage Conditions <br />The Topaz Mine has a drainage area of approximately 14.4 acres. Ephemeral <br />drainages are located on the east and west sides of the mine site and drain in a south <br />to north direction into the Big Gypsum Creek, approximately 3/4 of a mile north of <br />the mine boundary. Big Gypsum Creek is an ephemeral stream, which seasonally <br />discharges into the Dolores River and ultimately into the Colorado River. <br />2.2 Tetra Tech Drainage Plan <br />CDM reviewed Tetra Tech's drainage plan for the Topaz Mine. As noted in Tetra <br />Tech's Drainage Report (Tetra Tech 2007), storm water runoff from the mine area <br />generally flows north into two ephemeral drainages. Tetra Tech delineated four <br />basins based on the proposed expanded mine surface illustrated in Exhibit A-1 and <br />described each as follows: <br />1. Bassin 1- surface water run-on from offsite southwest of the surface mine facilities. <br />2 Basin 2 - surface water run-on from offsite southeast of the surface mine facilities. <br />3. Basin 3 - surface facilities area. <br />4. Basin 4 - waste rock area (WRA) slope. <br />CDM concurs with the drainage basin delineation and used the same drainage basin <br />areas in the subsequent calculations. <br />2.3 Peak Discharge Comparison <br />Tetra Tech used the SCS Unit Hydrograph method to estimate the 100-year peak <br />discharge for the four drainage basins. In comparison, CDM estimated the 100-year <br />peak discharge using U.S. Geologic Survey (USGS) regionalized flood-frequency <br />equations for Colorado (USGS 2000).. The USGS equations were developed using long <br />periods of stream flow data and statistical methods to define regression equations for <br />estimating peak discharges having recurrence intervals of 2, 5,10, 25, 50,100, 200, and <br />500 years for unregulated streams in Colorado (refer to Appendix A). The regression <br />equations can be applied to sites of interest on gaged and ungaged streams. The <br />regression equations are separated into five regions for Colorado. The Topaz Mine is <br />located within the Southwest Region for which the USGS recommends the following <br />flood-frequency equation to estimate the 100-year peak discharge: <br />2-1 <br />OAS49136 - Danison\Tssk Order 1 -Sunday Minas Drsitmp StudyVt.porpflnsl Repoffinsl Sunday Minas Drainage Report 0522p0Aoe