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Flood Hazard Area Delineation For Lower Box Elder Creek Watershed caused flooding of a dog kennel located just upstream of 88th Avenue. Leonard Rice Consulting Water Engineers calculated a flow of 4,500 cfs for that event (Leonard Rice Consulting Water Engineers, 1999). 3.0 BASELINE HYDROLOGIC ANALYSIS 3.1 Hydrologic Models The UDFCD's Colorado Urban Hydrograph Procedure computer model CUHPFIPC Release 2A (32-bit version, September 10, 1998) was used to develop the hydro graphs for each sub-watershed. The calculated hydrographs were combined and routed using the UDFCD stormwater management model UDSWM95 (32- bit version, 1998). 3.2 Watershed Description The study area was divided into four major watersheds for the purpose of hydrologic modeling: Bear Gulch, Box Elder Creek main stem, Upper Hayesmount Creek and Lower Hayesmount Creek (See Appendix A, Sheet A-2.) Sub-watersheds w~re delineated within each of the major watersheds using the criteria defined in the Runoff chapter of the Urban Storm Drainage Criteria Manual (UDFCD, 1969) that the sub-watersheds should, in general, be relatively uniform and less than 130 acres in sizec The delineation was done using USGS topographic mapping for those portions of the study area outside of DIA property and DlA mapping for the airport property. The mapping used for the watershed delineation had a scale of 1- inch equals 2,000 feet with a contour interval of 10 feet. Sub-watersheds were delineated first on the basis of tributaries shown with a blue line (first order streams) on the USGS mapping. These watersheds were further subdivided using other discemable drainages that were not shown with a blue line on the USGS mapping. If the resulting watersheds were still larger than 130 acres (such as flat areas around the Hayesmount Creek tributary or sloping areas around the creeks that had no defined drainage pattern), the watersheds were further subdivided using anticipated future development patterns (generally section lines and quarter section lines). The sub-watershed delineations are shown in Appendix B. The total number of sub-watersheds, the watershed designations and total watershed areas are shown in Table 3-1. TABLE 3-1 Summary of Watershed Delineation Watershed Stream Name Number of Total Watershed Area (sq. mi.) Desil!nation Sub-watersheds BG Bear Gulch 141 19c8 BE Box Elder Creek 125 205' LH Hayesmount Creek (lower portion) 126 18A UH Hayesmount Creek 74 12cO (upper portion) TOTAL 466 70.7 I This includes only that portion of the watershed in the study area. It does not include that portion of the watershed upstream of the study area (approximately 202 square miles in size)c 3.3 Rainfall The design rainfall for the 10-, 50-, 100- and 500-year events was developed using the criteria in the rainfall chapter of the Urban Storm Drainage Criteria Manual (UDFCD, 1969)c The I-hour, 3-hour and 6-hour point precipitation values used for the preparation of rainfall hyetographs are shown in Table 3-2c The rainfall distributions are the same as used for the study of the upper portion ofthe basin (CH2M-Hill, 1 994)c TABLE 3-2 Rainfall Point Precipitation (Inches) Duration Frequency I-Hour 3-Hour 6-Hour IO:j'Car 1.7 2cO 2c2 50:j'Car 2.3 . 2c7 3cO IOO:j'Car H 3.1 3A 500~ear 3.3 3.9 4c2 The point precipitation values are divided into five-minute increments in order to develop a storm hyetograph. The portion of rainfall attributable to each five-minute increment is based upon Table 3-1 of the UDFCD manual's rainfall chapter. An area adjustment of the storm hyetograph is also needed for any design point with a tributary area of 10 square miles or greater. A summary of where area adjustments were needed in the modeling is contained in Table 3-3. The hyetographs for each storm frequency and each storm duration are included in Appendix Fe 991-134cOOO Page 5 Wright Water Engineers, Incc