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I I I I I I I I I I I I I I I I I I I Reach No.2. Beginning upstream of the lower Arapahoe Avenue cross- ing and ending upstream of 24th Street, or Folsom (Station 1099+80 to 1154+70). - FUTURE FLOODS - Reach No.5. Beginning upstream of 9th Street and ending at the Canyon Mouth, just downstream of the upper Arapahoe Avenue crossing (Station 1216+85 to 1266+80). Flood Frequency and Discharge The 100-year flood is based upon recorded historical precipitation and other valid data and, statistically, has a one percent chance of being equalled or exceeded during anyone year. The 100-year flood is also commonly referred to as the base flood, regulatory flood, and intermediate regional flood. Floods of this magnitude or greater have occurred on Boulder Creek in the past and will occur in the future. Reach No.3. Beginning immediately upstream of Folsom and ending immediately upstream of Broadway (Station 1150+10 to 1199+00). Reach No.4. Beginning upstream of Broadway and ending upstream of 9th Street (Station 1199+00 to 1216+85). The stationing used for this study was tied into the COE's stationing for the Boulder County study currently in progress. The discharge probability profiles in Figure 2 graphically represent, the relative magnitude of the major flood events. This information is useful not only for regulating future floodplain construction, but also for planning and engineering of channel improvements and flood protection facilities. Special Split Flow Analysis A special split flow analysis was performed for the area referred to in this report as the "Arapahoe Avenue Overflow", a reach of left overbank floodplain from 24th Street (Folsom) to just upstream of the new 47th Street (Foothills Parkway) crossing (Sheets 5 to 9). The flood profile for this overbank area is hydraulically disconnected from the main stem profile. The split flow analysis was performed using the HEC-2 model to more accurately estimate flow levels and velocities for this highly developed area. Assumptions for flow splitting which were made for an earlier study (Reference 6) were tested hydraulically and supported by the results of this study. The 100-year flood is identified by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board and the Federal Emergency Management Agency as the flood magnitude used for the regulation of flood- plains and for the design of improvement facilities. In Colorado, the 100-year floodplain represents an area of state interest as defined in H.B. 1041. Flooded Areas Special stationing for this reach begins at the intersection of Arapahoe Avenue and 47th Street, runs along the center of Arapahoe Avenue, and ends just west of 24th Street (Folsom) at Station 63+00. The flood profile for the Arapahoe Avenue Overflow becomes hydraulically connected with the Boulder Creek main stem profile between 47th Street and the lower Arapahoe Avenue crossings. A special analysis was also conducted for the floodplain area downstream of the C & S Railroad crossing. This analysis is described in detail on page 11 of this report in the paragraphs dealing with Study Reach No.1. The Appendix of this report contains the Flood Hazard Area Delineation Maps showing the limits of the 100-year floodplain (Sheets 1 to 12). Water surface profiles are shown separately on Sheets 13 to 17, and bridge sections are shown on Sheets 18 and 19. Table II, "Floodplain and Floodway Reference Data", presents the computed 100-year flood elevations, along with other pertinent floodplain information and descriptive data. The floodway data portion of the table will be completed at a later date, following a floodway selection process and formal adoption of the final floodway by the Boulder City Council. 10