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<br />. <br /> <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />.L1NEATION <br /> <br />'lllllfECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />2.6.2.4 PREVIOUS STUDIES <br /> <br />A comparison of any proposed 100-year flood profile with all <br />previously designated and approved information shall be provided. <br />Except where a clearly identified change in flooding characteristics or <br />error in the existing data can be shown, the proposed 100-year flood <br />elevations shall agree with those of other contiguous studies on the <br />same stream. Elevations of cross sections shall be computed to <br />match within +/- 0.5 foot of an existing valid elevation; however, the <br />final published 100-year floodwater surface profile shall be drawn to <br />match the contiguous study exactly. Where elevations cannot be <br />reconciled to within +/- 0.5 foot because of changed flooding <br />conditions or an error in the previous analysis, a full explanation and <br />justification for the difference shall be provided. <br /> <br />2.6.3 FLOODPLAIN DELINEATION <br /> <br />The floodplain boundaries shall be delineated based on one of the following <br />methods: <br /> <br />a. Flood Contour Method <br /> <br />A reference line shall be shown down the center of the low flow channel <br />on the floodplain delineation map for all streams studied using the <br />detailed method. Flood contours (BFE lines) derived from the computed <br />water surface profile shall be used to define the boundaries of the 100- <br />year floodplain. <br /> <br />The flood contours shall have a vertical interval equal to the contour <br />interval of the floodplain delineation map if the following criteria are met. <br /> <br />. The average slope of the computed water surface profile between <br />cross sections is flatter than one percent, <br />. or the wicllh of the floodplain is greater than 200 feet, <br />. or there are, in judgment of the engineer, unusual topographic <br />features. <br /> <br />Altemate flood contour interval may be used in lieu of the above flood <br />contour interval if the slope of the water surface profile, in combination <br />with the contour interval and map scale, would result in an average <br />horizontal spacing between flood contours of less than 1 inch. <br /> <br />b. Map Contour Method <br /> <br />A reference line shall be shown down the center of the low flow channel <br />on the floodplain delineation map for all streams studied using the <br />detailed method. For the channel and floodplain cross sections used in <br />the hydraulic analysis, the 100-year floodwater surface elevations and <br />floodplain horizontal limits should be computed. The 100-year flood <br />boundaries shall be delineated between these cross sectional locations <br />by transposing the water surface elevations to the topographic map. <br />These intermediate locations must be correlated to the reference line and <br />the 1 OO-year flood profile as to their specific location and elevation. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-211 <br />