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Subject CC&V nnade by `ACR ~~b 003-2203.200 <br />~ Wl~~ Phase IV Checkad by ,.{y 1/z~~d~ Date 1/29/2002 <br />ASSOC11teS 1 V <br />Temp Diversion Channel Approved by Sheet 1 of 2 <br />OBJECTIVE: <br />Estimate peak flows in the temporary surface water diversion channel and culvert for Phase IV. <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 25-year, 24-hour storm event. In the HEG1 model, hydrographs were routed using the <br />Muskingum-Gunge method. Subbasin areas for each of [he diversions were delineated and measured using a <br />planimeter on a 10-foot contour map provided by CC&V (see attached). A composite curve number was <br />used to determine the runoff for each subbasin. Golder used a spreadsheet based on Manning's equation to <br />size the channels. Methods described in Ref. 5 were used to design the plunge pool required at the culvert <br />outlet for energy dissipation. <br />ASSUMPTIONS: <br />• Curve Number (CN) for reclaimed slopes and grass covered natural terrain = 71. <br />• CN for tree covered natural terrain = 66. <br />• CN for stripped terrain (for construction purposes) = 80. <br />• Precipitation depths used for the 25-yeaz (2.8 in.), 24-hour storm event were obtained from the Colorado <br />NOAA Atlas 2 Volume III (Ref. 4). <br />• The 25-year, 24-hour storm event is assumed to have a SCS Type II distribution. <br />• Times of concentration (t~) were determined using SCS upland methodology (Ref. 3). <br />• SCS Lag time = 0.6*t~. <br />• Manning's n = 0.025 (stability) and 0.030 (capacity) applies to earth-lined channels. <br />• Manning's n = 0.035 (stability) and 0.040 (capacity) applies to riprap channels. <br />• Manning's n = 0.024 was used for corrugated steel pipes. <br />• The eazth-lined channels were designed to allow a maximum velocity of 5.0 fps. <br />CALCULATIONS: <br />Calculation Table 1 contains measurements and related area-weighted curve numbers. Channel sizing <br />calculations aze summarized in Table 2. The plunge pool sizing calculations are included in Table 3, with an <br />example diagram immediately following. The HEC-1 model output for peak flow calculation and the HY8 <br />culvert sizing output, see attached. <br />CONCLUSIONS/RESULTS: <br />The 25-year, 24-hour storm event generates an estimated 61 cfs at the culvert inlet, and requires a 3-foot <br />diameter CMP culvert. <br />1. U.S. Army Corps of Engineers (USACE). 1990. HEC-1 Flood hydrograph Package User's Manzral, <br />September, 1990. USAGE. <br />U. 5. Soil Conservation Service (USSCS). 1986. Urban hydologrforsmall watersheds, 2nd edition <br />(USSCS Technical Release Number SS). Washington D.C.: United States Department of Agriculture. <br />File Name: 1:\pOIOBS\003-?'03\SURFNATEIChannelcalc l-OZAOc <br />