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4. The 16.75 square mile area upstream of Standley Lake has an existing imperviousness of 19 percent which increases to 23 percent for future development conditions. Initial infiltration loss rates for this basin are 3.1 inches per hour. Other detailed data are provided. "Outfall Systems Planning." Big Dry Creek (ADCO) and Tributaries. UDFCD. August 1987, revised January 1989, Muller Engineering. Subiect: This master drainage plan was prepared for UDFCD to provide preliminary designs for drainage and flood control for Big Dry Creek and tributaries. Tl.~ UTn'_... r'~441, h~<;n "ndr4~m Of Great Western Re-ervol'r has <; 0 square .l.lJ~ .yaJJ1UI. '-'l....."'.n. V~111 Uy.,...l......U...... ...... ~ ... .. ....,0 .. miles of area with both an existing and developed imperviousness of 34 percent. This basin has an initial infiltration loss rate of 3.1 inches per hour. The substantive data and findings of the report have been approved by UDFCD. Sponsoring governmental agencies include Adams County, Jefferson County, Jefferson County Airport, City of Broomfield, and the City of Westminster. 5. The report includes Table 7 which provides sub-basin parameters for Big Dry Creek tributaries as prepared by Muller Engineering Company, Inc. in the parallel study for drainage master planning. However, the Muller data for basins upstream of the reservoirs is not included in the Greiner report. Background data and calculations for the report are included in a Technical Appendix. The report is to be used in conjunction with a 1988 parallel study for Big Dry Creek entitled "Flood Hazard Areas Delineation Big Dry Creek, Westminster Area" by Greiner Engineering. 6. The future conditions lOO-year discharge profile assumes the proposed future spillway configuration for Standley Lake and a base flow from the outlet works of 430 cfs upstream of the confluence with Walnut Creek at the Denver- Boulder Turnpike. A combined base flow of 480 cfs from both Standley Lake and Great Western Reservoir would exist downstream of the confluence with Walnut Creek. Muller Engineering assumes in this report that the Great Western Reservoir Spillway will be enlarged. The present spillway is too small to pass the IOG-year flood. The State Engineer of the State of Colorado has a "hold order" on Great Western Reservoir, restricting its operation as a dam safety precaution. Assumntions 7. Figure 2 of the report shows a discharge below the Standley Lake spillway for the 100-year flood of about 1850 cfs and 100 cfs for the 2-, 5-, and 10-year floods. The study and report were prepared by experienced engineers using accepted and approved criteria ,for the Denver metro area. 8. The future conditions lOG-year discharge profile on Walnut Creek assumes the proposed future spillway configuration for Great Western Reservoir and a base flow from the outlet works of 50 cfs. The configuration is not defined. Limitations 1. The study area extends from the outlets of Great Western Reservoir and Standley Lake to 144th Avenue. It does not include the basins upstream of the two reservoirs. 9. Table 8 of the report is taken from the parallel Muller report. The Muller discharge for the upstream limit for the study of Walnut Creek, as presented in Table 8, is 1840 cfs. 2. The flood discharges out of Great Western Reservoir and Standley Lake were used as input to the upstream end of the hydrologic model. There is little reference to the basins tributary to the two reservoirs. 10. Sheet 3 of the report presents a map of the Walnut Creek and Woman Creek basins upstream of Standley Lake and Great Western Reservoir. Walnut Creek is identified as Basin 672 and Woman Creek is combined with Big Dry Creek as Basin 075.11