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I I I . I I I I I . I . I I I I I I I -6- Hydraulic Analysis The lOa-year flood event, which can be expected to occur at any time in a given area, based upon recorded historical precipitation and other valid data, has a one percent chance of being equaled or exceeded during anyone year. The laO-year flood event provides a lower risk of failure than the la-year or 50-year flood event, when considering structural flood plain improvements. Hydraulic analyses were completed to determine the water surface elevations for the 100-year storm event. The elevations were computed using the Corps of Engineer's HEC-2 Water Surface Profiles computer program. Valley cross-sections were measured on the 1" = lOa' scale, 2-foot contour topographic mapping provided as a part of this report. Estimates of channel and overbank roughness factors were made after a field investigation of the study area. Typical roughness or Manning's "n" values used in this study are: Channel - 0.030 to 0.045 Overbank - 0.030 to 0.070 The 100-year flood event is considered by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board and the Federal Insurance Administration as the flood magnitude for which flood plains should be designated for regulatory and improvement purposes. In Colorado, the lOa-year flood plain represents an area of state interest as defined in H.B. 1041. The computed flood profiles are plotted on the Flood Hazard Area Delineation map drawings. Also, the computed flood eleva- tions for Granby Ditch and Sable Ditch are tabulated at each reference point in Tables VI and VII. The 10- and 50-year recurrence flood magnitudes have also been calculated at various design points along each creek. These flood magnitudes represent events which can cause damage but would not inundate the area characteristic of a lOa-year event. The peak flows for these events should be used in the planning and engineering of improvements where the higher risk of failure or damage is economically feasible and the hazard to life and property is lower or non-existent. A detailed, technical appendix with all hydraulic calculations is available at the Urban Drainage and Flood Control District. FUTURE FLOODS Flood magnitudes greater than the lOa-year magnitude can and will occur. Land improvement adjacent to the 100-year flood plain limits should also consider the chance for possible flood damage. Flood Frequency and Discharge Floods of the same and greater magnitude as those that have occurred in the past will occur in the future. With development in the basin, flooding will occur more frequently. The discharges reported on Figures 7 and 8 for the 10-, 50- and lOa-year flood frequencies represent the relative extent and impact of each flood event. The discharge information is usable not only fo~ flood plain regulation but also for planning, engineering and flood plain management. Flooded Areas The lOa-year flood plain limits and water surface profiles are shown on the attached Flood Hazard Area Delineation Maps,