FLOOD05433 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I embankment across the fl oodp 1 ai n causes a severe flow obst ruct ion. Thi s results from its height and the undersized culvert through it. Flow in excess of the cul vert capacity woul d overtop the embankment one-half mil e north of the culvert. One-half of this overtopping flow would return to the Big Dry Creek channel along the reach immediately downstream of the rail road embankment. The remainder would proceed northward over 168th Avenue (the Weld County Line) and would not return to Big Dry Creek within the Study Reach. The flood situation along this reach was modeled in two ways; one representing the existing condition and one representing a possible future condition. The existing situation described above was approximated by distributing the returning flow among those cross sections included in the region of flow return. The second situation mOdeled assumed that the culvert through the embankment would be improved to pass the entire IOO-year flood flow. Both floodplains are illustrated on the plan sheets. The floodplain and floodway data tabulation in Table VII pertain to the latter situation, with the e~ception of the flood discharge and the floodplain elevation, which are tabulated for both situations. - FUTURE FLOODS - Flood Frequency and Discharge The 100-year fl ood is based upon recorded hi stori ca 1 preci pi tat i on and other data and statistically has a one percent chance of being equaled or exceeded during anyone year. Likewise, the IO-year flood has a ten percent chance of being equaled or exceeded during anyone year. The IOO-year flood is also commonly referred to as the base flood, the regulatory flood, and the intermediate regional flood. The discharge probability profiles in Figure 3 graphically represent the relative magnitudes 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, flood protection facilities, and road crossings. The IOO-year flood is identified by the Urban Drainage and Flood Control Di stri ct, the Colorado Water Conservati on Board and the Federal Emergency Management Agency (FEMA) as the flood magnitude used for the regulation of floodplains and for the design of improvement facilities. However, the UDFCD and FEMA differ in thei r defi nit ions of the IOO-year flood. FEMA uses exi sti ng basi n development for hydrology cal cul ati ons, and the UDFCD uses future basin development. Floodplains mapped by both agencies are based upon the conveyance of floods through the channels, bridges and other facilities as those facilities exist at the time the report is prepared, and do not reflect anticipated improvements, except as noted. In Colorado, the IOO-year floodplain represents an area of state interest as defined in H.B. 1041. 13