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I I I I I I I I I I I I I I I I I I I Roughness coefficients (Manning's "n") for the channel and overbank areas were estimated by field inspection of the study area. Typical ranges for these values are as follows: - FUTURE FLOODS - Overbank Channe 1 .04 to. 20 .025 to .065 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. Likewise, the 10-year flood has a ten percent chance of being equalled or exceeded during anyone year. The IOO-year flood is also commonly referred to as the base flood, regulatory flood, and inter- mediate regional flood. In certain areas the higher "n" values were used to account for obstructions caused by existing floodplain development. In other areas the effective flow limits were accounted for by using encroachment options with the HEC-2 model. At certain locations, flood flows diverge from the main flow path of the stream and become hydraulically disconnected from the flows in the channel. These areas are identified on the floodplain mapping. The extent of such flooding was determined by estimating the hydraul ic properties of the conveyance area. A description of the flooded areas and hydraulic as.sumptions is detailed later in this report. The computed flood profiles and 1 imits of the lOO-year flood- plain are shown on the Flood Hazard Area Delineation Plan and Profile Drawings. Tabulated data regarding cross sections, stationing, flood elevations, and other pertinent hydraulic information is found in Table V. Specific information regarding the hydraulic calculations is available from the Urban Drainage and Flood Control District. The discharge probability profiles in Figures 2 and 4 graphically represents 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 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 floodplains and for the desi~n of improvement facilities. In Colorado, the lOO-year flood- plain represents an area of state interest as defined in H.B. 1041. Flooded Areas The Appendix of this report contains the Flood Hazard Area Delineation plan and profile drawings showing the limits of the 100-year floodplain and the water surface profil es for the 10- and 100-year floods. Table V, "Fl oodpl ai nand Floodway Reference Data", presents the computed 100-year flood elevations, along with other pertinent floodplain information and descriptive data. The following paragraphs contain a general description of flooding for specific study reaches. 17