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<br />. <br /> <br />e <br /> <br />. <br /> <br />FLOODPLAIN REPORT - Tyra Tract 0 and Sawmill Gulch, Breckenridge, Colorado <br /> <br />following peak discharges: 2-year peak-50 cfs; 5-year peak---61 cfs; 10-year peak-70 cfs; 25- <br />year peak-84 cfs; 50-year peak-I 61 cfs, and tOO-year peak-248 cfs. For reference purposes, <br />the Master Plan also provided the comparison to FEMA peak flow values for Sawmill Gulch: <br />10-year peak-45 cfs; 50-year peak-75 cfs, and 1 OO-year peak-120 cfs. <br /> <br />Generally, the Master Plan values were adopted by the Town of Breckenridge, and they were <br />used by WWE for the hydraulic analyses for Sawmill Gulch. Specifically, the 100-year flood <br />peak for Sawmill Gulch upstream of Four O'Clock Road is 248 cfs. <br /> <br />FLOODPLAIN ANALYSES <br /> <br />Methodology <br /> <br />The floodplain delineation work for the Tyra Tract D site and the upper Sawmill Gulch area was <br />conducted using a combination of computer modeling and hand calculations. Computer <br />modeling utilized resources available from the Hydrologic Engineering Center (HEC): HEC-2, <br />the water surface profile model. (HEC-2 has recently been superseded by the model HEC-RAS, <br />which was used to supplement the HEC-2 data.) HEC-2 utilizes hydraulic energy concepts to <br />provide an estimate for water surface elevations given data inputs of channel cross-sections, <br />channel characteristics such as roughness, and flow amounts. Many computer runs were utilized <br />to perform the evaluation, which ultimately were consolidated into two model runs: existing <br />conditions and proposed conditions. <br /> <br />A nominal floodway run was performed to consider impacts at the Four O'Clock Road area. <br />However, the area downstream is already developed, and even a conservative floodway <br />surcharge does not have hydraulic impact through the Four O'Clock Road culvert. Floodway <br />encroachments were not used for upper Sawmill Gulch, as the Town of Breckenridge does not <br />allow development within 25 feet of the stream banks-the water quality and drainage setback. <br />Generally, the floodplain is contained within these limits, and so a floodway encroachment <br />assessment is not required. <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~. Inc. <br /> <br />Page 6 <br />