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iii iiiiiiiiuiiiiii <br />Subject CC&v <br />C701~'~ Phase IV <br />Associates <br />Hydrologic Estimates <br />Made by HMM <br />Checked by l/ <br />APProvetl by C~C <br />~Db 993-2099 <br />Date March 14, 2000 <br />Sheet 1 Ot 2 <br />OBJECTIVE: <br />Estimate 100-year peak flows in the surface water diversion channel reaches for Phase N, and estimate the <br />required storage capacity of sediment detention basins at the five locations where surface water is diverted <br />into Bateman Creek, Squaw Gulch, Grassy Creek, Poverty Gulch A, and Poverty Gulch B. <br />METHOD: <br />Use the USAGE HEC-1 model (Ref. 1) and SCS methodology (Ref. 2, 3) to estimate peak flows resulting <br />from the 100-year design storm event. Tn the HEC-1 model, hydrographs were routed using the <br />Muskingum-Gunge method. Subbasin areas for each of the diversions were delineated and measured using a <br />planimeter on a 10-foot contour trap provided by CC&V. The operational azea and overburden storage <br />aeeas (OSA's) were divided into 36 subbasins including 6 subbasins for the Arequa Gulch OSA and 16 <br />subbasins for the Squaw Gulch OSA .The required storage capacity at the downstream most location of the <br />sediment detention basins was designed to contain the runoff generated from two times the 10-year, 24 hour <br />storm event. The runoff volume contributing to each of the five detention basins was estimated using the <br />SCS runoff equation and a composite CN for the entire contributing <br />watershed. <br />ASSUMPTIONS: <br />• A SCS Curve Number of 71 was used for reclaimed slopes and original ground terrain that is grass <br />covered, and a SCS Curve Number of 66 was used for original ground terrain that is tree covered. The <br />values of the curve numbers are identical to those submitted to, and approved by, the Office of Mined <br />Land Reclamation (OMLR) for the Cresson Project: Permit M-80-244: Surface Water Drainage <br />Modifications for Arequa Gulch Overburden Storage Area, dated March 4, 1997. <br />• Precipitation depths used for the 10-year and 100-year, 24-hour storm event were obtained from the <br />Colorado NOAA Atlas 2 Volume III (Ref. 4), and were consistent with those used in Amendment No. 7. <br />• The 100-year, 24-hour storm event was assumed to have a SCS Type II distribution. <br />• Times of concentration (rJ were determined using SCS upland methodology (Ref. 3). <br />• SCS Lag time = 0.6*4. <br />CALCULATIONS: <br />CNs and 4 calculation results for the subbasins are presented in Table 1. <br />CONCLUSIONS/RESULTS: <br />HEC-1 computer model output is attached. Runoff volumes for channel reaches are provided in Table 1. <br />REFERENCES: <br />1. U.S. Army Corps of Engineers (USACE). 1990. HEC-1 Flood hydragraph Package User's Manual, <br />September, 1990. USAGE. <br />2. U. S. Soil Conservation Service (USSCS). 1986. Urban hydologyfor small watersheds, 2nd edition <br />(USSCS Technical Release Number 55). Washington D.C.: United States Department of Agriculture <br />3. U. S. Soil Conservation Service (USSCS). 1972. National engineering handbook, Section 4 Hydrology. <br />• Washington D.C.: United States Department of Agriculture <br />File Nvnc: 1\991D699913WNHaNCr\CCVI\HYOCALC.DOC <br />