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<br />e <br /> <br />e <br /> <br />e <br /> <br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br /> <br />Gulch and the FEC study have a common point at the Four O'Clock Road culvert, where the <br />flow is 130 cfs. The water surface elevation used for Sawmill Gulch is 9,716.0 (NGVD 1929) <br />just upstream of Four O'Clock Road in each study. Attachment A includes a FEC letter <br />regarding this "match" point and the use of 130 cfs continuing downstream of Four O'Clock <br />Road. <br /> <br />HYDROLOGY CONSIDERATIONS <br /> <br />WWE did not conduct new hydrology work for the evaluation of Sawmill Gulch. Hydrology <br />data were provided by the Town of Breckenridge with the concurrence of John Liou, FEMA <br />Regional Hydrologist. The 1988 Drainage Master Plan for the Town of Breckenridge provides a <br />I OO-year peak flow for Sawmill Gulch of 248 cfs. This is in contrast to the 1979 FEMA value of <br />120 cfs. Further, FEMA, the Town of Breckenridge, and FEC have all stated that only 130 cfs <br />will stay in the Sawmill Gulch channel downstream at Four O'Clock Road due to the 130 cfs <br />capacity of the existing 5.5-foot wide by 4.5-foot high corrugated metal pipe culvert underneath <br />the roadway. Amounts above 130 cfs will overtop the right bank and flow eastward alongside <br />Four O'Clock Road toward Park Avenue and the Blue River. The culvert capacity will not be <br />increased by the Tyra Tract D project. <br /> <br />Herein, we provide a brief summary of the hydrology work performed by FLO Engineering for <br />the Drainage Master Plan, The Master Plan utilized the HEC-l computer model to generate <br />flood hydrographs for the major tributaries to the Blue River, including Sawmill Gulch. Rainfall <br />data were from NOAA Atlas 2, Volume III, Colorado, 1973, and the 100-year, 24-hour point <br />rainfall depth was approximately 3 inches. Rainfall was re-sequenced using the Soil <br />Conservation Service (SCS) Type II storm distribution. Rainfall runoff values were based on the <br />SCS curve number methodology. Soils within the Master Plan area typically are SCS hydrologic <br />soil groups B or C, with the predominate group being Group B. Curve numbers were adjusted to <br />have a minimum value of 60, which is slightly above the "published" value for forested or alpine <br />areas of Soil Group B. Those areas with Soil Group C would have curve numbers of <br />approximately 70. For Sawmill Gulch, the report provided the basis for ultimate adoption of the <br /> <br />971-055.010 <br /> <br />Wright Water Enginee~, Inc. <br /> <br />Page 5 <br />