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ame0 <br />Date: 06/24/2011 <br />Project: CC &V Phase 1 VLF <br />Re: Hydrologic Analysis <br />TECHNICAL MEMORANDUM <br />OBJECTIVE: <br />1. Estimate peak flows associated with 100 -year 24 -hour storm event for sizing of the operational <br />diversion channels for Phase 2. <br />2. Estimate peak flows associated with the 100 -year 24 -hour storm event for sizing of the post <br />closure diversion channels. <br />3. Estimate diversion channel for Phase 1 based on the 25 -year 24 -hour storm event. <br />4. Estimate sediment detention pond sizing for Phases 1 and 2 by doubling the 10 -year 24 -hour <br />storm event runoff volume generated from the overburden storage areas. <br />ASSUMPTIONS: <br />• Hydrologic model based on the SCS synthetic unit hydrograph type 2 distribution. <br />• Loss method based on the Natural Resources Conservation Service (NRCS) curve number (CN) <br />method with the following CN applied to each type of land: <br />o Original Ground: CN — Wooded 66 and Grasses 71 <br />o Road and Stockpile Surfaces: CN -90 <br />o Fill Slopes: CN -50 <br />• 24 -hour precipitation depths were referenced from Cripple Creek & Victor Gold Mining <br />Company, 200, Amendment 8 to Office of Mined Land Reclamation Permit M -80 -244. <br />• Times of concentration (Tc) were determined using methodology found in "Urban Hydrology for <br />Small Watersheds TR -55." <br />• SCS Lag time = 0.6 *Tc. <br />CALCULATIONS AND RESULTS: <br />• Hydrologic analyses were performed for Phases 1 and 2. The following results of the hydrologic <br />analysis are included in this appendix for both phases. <br />o Schematic from HEC -HMS model <br />o Watershed area, overland manning's n and curve number calculations: Weighted CN <br />used for each watershed sub -basin area determined by specific sub -areas of the <br />overburden storage and native ground footprints <br />o Channel sizing and flows: Open channel calculations based on the 25 year storm event, <br />channel <br />o Lag time calculations using NRCS TR -55 method calculations. Tc calculations were <br />based on evaluating the areas and topography for each sub -basin for the longest travel <br />time and dividing this into sheet, shallow and channel flow. <br />o HEC -HMS model results: Peak discharge flowrates and volume results for each model <br />element. <br />o HEC -HMS model input files: basin file and control file. <br />o Graphical HEC -HMS model results: 25 year flow hydrographs for modeled reaches. <br />