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Subiect: "Flood Hazard Areas Delineation." Big Dry Creek. Westminster Area. Urban Drainage and Flood Control District (UDFCD). November 1986, revised December 1988. Greiner Engineering. This flood hazard study and report for UDFCD was prepared for the purpose of floodplain delineation and regulation downstream of Standley Lake and Great Western Reservoir. It is a report for which the substantive data and findings have been approved by resolution of the Colorado Water Conservation Board. Prior to this approval, the report was approved by resolution of UDFCD and by the staff of the City of Westminster. The report was provided to the Federal Emergency Management Agency for use by the National Flood Insurance Program. The subject report was first completed in 1986 and revised in 1988 to better meet UDFCD policy regarding detention storage. This report is meant to be used in conjunction with a parallel study for Big Dry Creek entitled "Outfall Systems Planning Big Dry Creek (ADCO) and Tributaries" (Muller Engineering, Inc., 1987). Data Included 2. The two drainage basins upstream of the reservoirs were studied for fully developed basin conditions, however, the authors state that the subject basins are ". . . presently undeveloped range lands. Due to the expected permanent nature of the Rocky Flats Plant, the areas tributary to the reservoirs should remain essentially undeveloped." This is in conflict with the approved land use map of Jefferson County (Jefferson County, 1989) and with the planning of the City of Arvada. Arvada was not a sponsor in the December, 1988 revision. 3. Future proposed reservoir improvements for Great Western Reservoir and Standley Lake were assumed for routing flood flows. The improvements included increased reservoir capacity and improved enlarged emergency spillways. 4. Both reservoirs were assumed full before inflow from a 100-year flood. The report was prepared by experienced engineers and the work was subject to detailed review and approval of three governmental agencies. Comprehensive displays of design precipitation and resulting design floods are presented consistent with the thoroughness of approved Flood Hazard Areas Delineation reports for UDFCD. For the basins upstream from and including the two reservoirs, data provided include the following: Assumotions Greiner Engineering assumes in this report that the Great Western Reservoir Spillway will be enlarged. The present spillway is too small to pass the 100-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. For purposes of use at Rocky Flats the following limitations apply: 1. The tributary basins upstream of Great Western Reservoir and Standley Lake were analyzed hydrologically as single units to provide design flood flows into the respective reservoirs. 1. 2-, 5-, 10-, and lOG-year flood discharges from Standley Lake and Great Western Reservoir. 2. Point rainfall depths for five recurrence intervals and storm durations from 2 to 6 hours. 3. 100-year rainfall point distribution for basin areas less than 5 square miles, which include Walnut Creek upstream of Great Western Reservoir. Areal rainfall corrections were likely used for basins over five square miles in area. The maximum 5-minute rainfall depth occurs at the mid-point (30 minutes) of the I-hour design storm.