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I I I I I I I I I I I I I I I I I I I - FUTURE FLOODS - The following paragraphs contain a general description of flooding conditions along Lower Boulder Creek: 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. Valley View Road to North 75th Street - This 3.9-mile study reach is characterized by numerous gravel pits, dikes and berms resulting in a very irregular flood boundaries. Numerous island areas and dikes associated with the gravel mining operations are shown on the floodplain maps. Cross Section Nos. 86 to 129 represent this study reach. Floodplain limits are shown on Sheets 2 through 6 and the water surface profiles on Sheets 18 and 19. The discharge probability profiles in Figure 3 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. The stream in this reach flows generally in a northeast direction. Stream crossings include the Union Pacific Railroad, Valmont Drive, North 66th Street, two access drives, and North 75th Street. The average 100-year floodplain widths range from 1500 to 4000 feet. The Union Pacific Railroad generally parallels the southern floodplain boundary. 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. South Boulder Creek and Fourmile Canyon Creek are the major tributaries within this study reach. At the South Boulder Creek confluence a special analysis was performed to determine the divided floodway and flow splitting assumptions for Cross Sections 119 through 128. The 100-year discharge in the channel area of Boulder Creek was estimated at 4,800 cfs as compared with a total discharge of 13,300 cfs downstream of the South Boulder Creek confluence. The user of this report should contact Boulder County regarding floodway requirements for this area. Flooded Areas The Appendix of this report contains the Flood Hazard Area Delineation Maps showing the limits of the 100- and SOD-year floodplains (Sheets 2 to 17). Water surface profiles are shown separately on Sheets 18 to 23. Table III, "Floodplain Reference Data," which follows the report sununary, presents dis- charge information and computed water surface elevations for the 10-, 50, and SOD-year flood events. Table IV, "lOO-Year Floodplain and Floodway Reference Data," whi ch foll ows Table I II, presents i nformati on perti nent to the regulation of the 100-year floodplain. Specific information regarding flood discharges, flood elevations, widths, and other descriptive data is listed in Table IV. As stated previously in this report, a conservative analysis was used for the reach considering the unknown integrity of the diking systems used in the gravel mining operations. The hydraulics assume that the diking system is intact and the low-lying areas landward of the dikes are ineffective for conveying floodwaters. 10