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Flood Hazard Area Delineation For Lower Box Elder Creek Watershed The Box Elder Creek 100-year existing condition flow rate at the upstream study area boundary has been thoroughly reviewed as part of the Upper Box Elder Creek Outfall Systems Planning Study (CH2M-HiIl, 1995). That study concludes the following: During project review meetings, representatives of UDFCD, the local sponsors, the Colorado Water Conservation Board, and Federal Emergency Management Agency concluded that the peak discharges established by this study comprised the best available hydrology for the study area. Because this hydrologic analysis is consistent with standard UDFCD hydrologic criteria, is in close agreement with the detailed COE modeling effort, and correlates reasonably well with the 1973 flood discharge estimate, the project sponsors accept the results of this hydrologic analysis for use in subsequent phases of the Box Elder Creek Outfall Systems Planning Study and Coyote Run FHADc The comparison of sub-watersheds less than 2,000 acres in size with the Upper Box Elder Creek study is shown in Appendix Oc There is a good correlation between the results of this study and the hydrology carried out for the upper portion of the basin. The hydrologic modeling carried out for this study is consistent with previous modeling efforts for sub-watersheds smaller than 2,000 acres. Comparisons of peak flow rates from the Lower Hayesmount or Upper Hayesmount tributaries were not carried out because they had significant areas with high percentage imperviousness in the existing condition and much of the Lower Hayesmount tributary is affected by attenuation of peak flow rates from the existing detention pond on DIA propertyc These conditions of imperviousness and detention were not considered representative for comparison purposes. The USACE (1989) calculated a 100-year peak flow rate at Bootleg Reservoir of 8,000 cfs. The modeling carried out for this study shows a peak flow rate of 11,300 cfsc Using anecdotal evidence, the UDFCD estimated flood discharge from the reservoir during a 1973 storm event to be approximately 6,000 cfs (UDFCD personal communication, 1999). From the comparisons with previous studies described above, the hydrology for this study is considered to be consistent with the hydrology for the upper portion of the basin. 4.0 HYDRAULIC ANALYSIS 4.1 Methodology A hydraulic analysis was done for approximately 53 miles of the main stem and tributaries of Box Elder Creek, Bear Gulch, and Lower Hayesmount Creek. The hydraulic analysis was carried out using the DeS. Army Corps of Engineers (USACE) HEC-RAS computer model version 2c2c The UDFCD provided mapping for the portion of the study area lying outside DIA propertyc DIA provided the mapping for the portion of the study area lying within its boundaries. Pacific Western Technologies, Incc combined the two different maps into one digital terrain model and map with 2-foot contour intervals. The UDFCD also provided detailed survey information regarding bridges and culverts in the study area (AzTec Consultants, 1998b). Cross section and channel reach length information for the model was developed by measuring 1"=200' scale, 2' contour interval mapping. The majority of the stream channels in the area do not have a defined channel. The bank stations were, therefore, set at between 10 and 50 feet from the stream centerlinec Manning's n values were determined through field observations, review of USACE recommended values, and consultation with UDFCD staff In the Lower Hayesmount Creek area, channel and over bank n values are the same for any given cross section. They are set at either a value of Oc035 or 0.04, depending on the agricultural land use of the area. The Manning's n values for cross sections between 1281h and 13ls' Avenues are set at Oc05 to acknowledge the flow through the residential development Manning's n values for Bear Gulch and its tributaries were set at Oc035 for channel and over bank, except that reaches of the main stem that had wetland vegetation or other significant vegetation in the channel were given values of Oc04 for the channel. The Box Elder Creek channel was assigned values of between 0.035 and Oc06, depending on the type and density of vegetation. The over bank was generally assigned a value ofO.035c The portion of Lower Hayesmount Creek upstream of 1281h Avenue, which was modeled as one stream hydrologically, was split into two different reaches for the purposc of hydraulic modelingc This portion of the study area is broad and flat and often has less than four feet of relief over a 4,000-foot wide cross section. The lowest portion of the channel moved from one side to the other of the 4,000-foot wide channel, Page 8 991-134.000 Wright Water Engineers, Inc.