Laserfiche WebLink
<br />FLOODPLAIN REPORT - Tyra Tract D and Sawmill Gulch, Breckenridge, Colorado <br />(D..AFT) <br /> <br />HYDROLOGY CONSIDERATIONS <br /> <br />WWE did not conduct new hydrology work for the evaluation of Sawmill Gulch. Hydrology <br />data was provided by the Town of Breckenridge with the concurrence of John Liou, the FEMA <br />administrator. The 1988 Drainage Master Plan for the Town of Breckenridge provides for a 100- <br />year peak flow for Sawmill Gulch of 248 cfs. This is in contrast to the 1979 FEMA value of 120 <br />cfs. Further, FEMA, the Town of Breckenridge, and FEC have all indicated 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 meta] pipe culvert underneath <br />the roadway. Amounts above 130 cfs will flow eastward alongside Four O'Clock Road toward <br />Park Avenue and the Blue River. <br /> <br />Herein, we provide a brief summary of the hydrology work performed by FLO Engineering for <br />the Drainage Master Plan. Our cursory review of the Master Plan suggests that FLO Engineering <br />did a detailed study for the Town of Breckenridge. The Master Plan utilized the HEC-I <br />computer model to generate flood hydrographs for the major tributaries to the Blue River, <br />including Sawmill Gulch. Rainfall data was from NOAA Atlas 2, Volume III, Colorado, 1973, <br />and the 100-year, 24-hour point rainfall depth was approximately 3 inches. Rainfall was re- <br />sequenced using the Soil Conservation Service (SCS) Type II storm distribution, Rainfall runoff <br />values were based on the SCS curve number methodology. Soils within the Master Plan area <br />typically are SCS hydrologic soil groups B or C, with the predominate group being Group B. <br />Curve numbers were adjusted to have a minimum value of 60, which is slightly above the <br />"published" value for forested or alpine areas of Soil Group B. Those areas with Soil Group C <br />would have curve numbers of approximately 70. For Sawmill Gulch, the report provided the <br />following peak discharges: 5-year peak-6 cfs; 10-year peak-I 0 cfs; 25-year peak-54 cfs; 50- <br />year peak-161 cfs, and 100-year peak-248 cfs. For reference purposes, the Master Plan also <br />provided the comparison to FEMA peak flow values for Sawmill Gulch: 10-year peak-45 cfs; <br />50-year peak-75 cfs, and I OO-year peak-I 20 cfs. <br /> <br />971-055,010 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 5 <br />