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This report was authorized under an "Agreement" with the Urban Drainage and Flood Control District, the City of Broomfield, and the City of Westminster. The report is in partial fulfillment of Agreement Number 84-08.06 with the aforemen- tioned local governments. This report addresses the objectives of the established work program for 1983 (Resolution No. 49, Series of 1982) which includes master planning and delineation of flood hazard areas. I I I I I I I I I I I I I I I I I I I AUTHORIZATION FlOOD HAZARD AREA DELINEATION QUAIL CREEK AND TRIBUTARIES McKAY LAKE BASIN SCOPE OF WORK The Scope of Work for this study was prepared under the current format of the District's Flood Hazard Area Delineation Reports. This study consists of a floodplain analysis of the 10-year and 100-year events along Quail Creek, four of its tributary drainageways and the McKay lake Basin,- all located in the Broomfield vicinity. The flood profiles and flooded areas presented are based on hydrologic analyses assuming fully developed basin conditions and future channel conditions. The floodplain hydraulics are based on the peak flows generated from the future conditions hydrology and the existing stream channel configurations. The study i denti fi es floodprone areas along Quail Creek and four tributary drainageways as well as a portion of the adjoining McKay lake basin. These drainageways are contained within incorporated areas of Westminster and Broomfield as well as unincorporated Boulder County and Adams County. The locations of Quail Creek, its tributaries and the McKay lake watershed are shown on Figure 1. The study reach descriptions and lengths are indicated in Table 1. Name Quail Creek Tributary A Tributary B Tributary C Tributary D McKay lake Basin TABLE 1 DRAINAGEWAYS ANALYZED Description From the confluence with Big Dry Creek to South 124th Street From the confluence with Quail Creek at Station 78+00 to Zuni Street From the confluence with Quail Creek at Station 167+50 to 1000 feet upstream from 144th Avenue From the confluence with Tributary B at Station 8+10 for 4600 feet upstream From the confluence with Quail Creek at Station 243+80 to South 124th Street From the confluence with Big Dry Creek to 400 feet upstream from West 144th Avenue A floodway was also computed to designate those areas that must be kept free of encroachment to convey the 100-year flood event along each of the drainageways. The floodway is defined as the channel and that part of the adjacent floodplain that must be reserved in order to discharge the 100-year flood without cumulatively increasing the water surface elevation more than an acceptable amount. In this case, encroachment into the floodplain was considered until the elevation of the energy grade 1 ine increased by no more than one-half foot. The areas left free from encroachment are labeled as the floodway. length 5.20 miles 0.56 miles 0.91 miles 0.85 miles 0.54 miles 1.99 miles 1