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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAlNAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />.ELlNEATION <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />2.4 <br /> <br />. Floodplain development pressure <br />. Existing and future floodplain encroachments <br />. Floodway delineation <br />. Watershed area plan <br />. FEMA submittals <br />. Flooding history <br /> <br />The detailed study approach should be used when a new development is proposed <br />within or adjacent to the 100-year floodplain limits. The limited study method may be <br />used to define the 100-year floodplain limits if no encroachment into the natural <br />floodplain is proposed in a foreseeable future. <br /> <br />2.3 <br /> <br />WATERSHED CONDITIONS <br /> <br />The FEMA 100-year floodplain and f100dway boundaries <br />shown on the Flood Insurance Rate Maps (FIRM) are <br />delineated based on the existing watershed and floodplain <br />conditions at the time of the Flood Insurance Studies (FIS). <br /> <br />To be consistent, hydrologic and hydraulic analyses for all <br />new floodplain delineation studies should be performed, at a <br />minimum, to reflect the existing watershed and floodplain <br />conditions. Public works projects in progress that are planned <br />to be completed within 12 months following the study <br />completion should be included in the analysis. Where <br />construction of a publicly owned, operated and maintained <br />flood control facility will not be completed within 12 months <br />following completion of the study, but adequate progress has <br />been made, the impact/benefit of the project may be included <br />in the hydrologic analysis. The project engineer should <br />coordinate with the public agency in charge of the facility <br />design and construction, effected local agencies and <br />Colorado Water Conservation Board (CWCB) to determine <br />whether to include the subject facility in the existing <br />conditions analysis or not. <br /> <br />As new developments occur, the estimated existing <br />conditions 100-year peak flow and associated floodplain <br />limits may change depending on the nature and amount of <br />new developments within a watershed. Therefore, local <br />communities are encouraged to develop future (built-out) <br />conditions floodplain information in addition to the existing <br />conditions floodplains, especially when the area plan <br />indicates substantial amount of future developments. Once <br />developed, communities may request FEMA to show future <br />conditions floodplains on the community's FIRM maps in <br />addition to the existing conditions floodplains. <br /> <br />TOPOGRAPHIC MAPPING <br /> <br />Hydrologic <br />and hydraulic <br />analyses for <br />aI/new <br />floodplain <br />delineation <br />studies <br />should be <br />performed, at <br />a minimum, <br />to reflect the <br />existing <br />watershed <br />and <br />floodplain <br />conditions. <br />Local <br />communities <br />are <br />encouraged <br />to develop <br />future (built- <br />out) <br />conditions <br />floodplain <br />information in <br />addition to <br />the existing <br />conditions <br />floodplains. <br /> <br />For detailed discussions and specifications of the topographic mapping standards, <br />please refer to Chapter 2, Section 1.0. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-204 <br />