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Drainage Areaa Description Drainage Runoff Outfall <br /> Area acres <br /> 05 Strain Gulch upstream of 330 Conveyed beneath the north <br /> Site quarry and processing area in an <br /> existing 72-inch culvert <br /> 06 North quarry. The hydrologic 80 Contained in Reservoir Site Ilc <br /> boundary at its far south end <br /> was adjusted slightly where <br /> mine construction activity has <br /> occurred. <br /> 07 Processing area 28 Outfall005 <br /> 08 Main quarry haul road 12 Outfall 001 <br /> 09 Bottom of haul road below 1 Outfall 004 <br /> sediment basin <br /> a. As shown on Proposed Conditions Drainage Map B-2 in Appendix B <br /> b. Runoff will be pumped to an outfall if it has not infiltrated within 72 hours <br /> c. As final reclamation is completed, drainage diversions will be installed to convey runoff from a portion of this <br /> drainage area through a detention basin and to an outfall. Not part of this report. <br /> 3.2 Runoff Modeling <br /> As discussed in Section 2.2, the peak flow was calculated for each of the drainage basins using <br /> the Workbook and the built-in CUHP calculator. The watershed area, slope, length, and length <br /> to centroid were estimated using a geographic information system (GIS) and a digital elevation <br /> model of the Site provided by Holcim-WCR, Inc. In practice, there will be some amount of <br /> storage along non-reclaimed benches; however, for the purpose of sub-basin delineation, it <br /> was assumed that there is no flow capture or pooling along the existing benches. This approach <br /> will result in conservative peak flow and volume estimates at project concentration points. <br /> Watershed imperviousness values were selected based on landcover using similar values to the <br /> stormwater evaluation performed by Apex, summarized in Table 3-2. <br /> Table 3-2. Landcover Imperviousness <br /> Landcover Percent Impervious (%) <br /> Active mining area 50 <br /> Reclaimed area 25 <br /> Undisturbed area 2 <br /> The soils map indicates that the project subbasins lie within soils 123— Ratake-Cathedral-Rock <br /> outcrop complex, 25 to 60 percent slopes and 124 - Ratake-Cathedral-Rock outcrop complex, 25 to <br /> 60 percent north slopes both of which fall within hydrologic soil group D. The U.S. Department of <br /> Agriculture (USDA) soil map for the area is included in Appendix D. <br /> The hydrologic inputs for sub-drainage basins 02A, 0213, and 03 are summarized in Table 3-3. <br /> 1670 Broadway,Suite 3400,Denver,CO 80202-4824 hdrinc.com <br /> (303)764-1520 <br />