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<br />. <br /> <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />If levees protecting the subject area do not meet the requirements outlined in <br />Chapter 6, Section 4.0, the 100-year flood elevations of the protected area <br />should be computed as if the levees did not exist. For the unprotected areas <br />between the levee and the source of flooding, the 100-year flood elevations <br />should be obtained from either the flood profile computed with the levees in <br />place or the profile computed as if the levees did not exist, whichever is <br />higher. This procedure recognizes the increase in flood elevation in the <br />unprotected area caused by the levees. This procedure may result in the <br />1 OO-year flood elevations being shown as several feet higher on one side of <br />the levee than on the other. Both profiles should be shown in the final <br />delineation with a line drawn along the levee centerline separating the areas <br />with different BFEs. <br /> <br />If the calculated water surface elevation (WSEL) of the unprotected area for <br />other frequency events (10-, 50-, and 500-year events) is lower than the top <br />of levee elevation, the computed WSEL should be used. If the computed <br />WSEL is higher than the top of levee, than the top of levee elevation should <br />be used as the WSEL for the unprotected area. Floodplain delineation <br />analysis (except the intemal drainage analysis) of the protected area for flood <br />events less than a 100-year event is not required. <br /> <br />. <br /> <br />If levees exist on both sides of a drainage-way, several levee failure <br />scenarios should be considered including simultaneous levee failure, left <br />levee only failure, and right levee only failure scenarios. For more detailed <br />discussion on the levee floodplain and f100dway delineation analysis, please <br />refer to the FEMA publications. <br /> <br />Where credit will be given to levees providing 100-year protection, the <br />adequacy of interior drainage systems shall be evaluated. Areas subject to <br />flooding from inadequate interior drainage behind levees will be mapped <br />using standard procedures. <br /> <br />2.9 AREAS PROTECTED BY DAMS <br />2.9.1 FLOOD CONTROL DAMS <br /> <br />If a publicly owned, operated and maintained dam is specifically designed <br />and operated, either in whole or in part, for flood control purposes, then its <br />effects shall be taken into consideration when delineating the floodplain <br />below such a dam. Full credit should be given to the diminution of peak flood <br />discharges, which would result from normal dam operating procedures. <br /> <br />2.9.2 NON-FLOOD CONTROL DAMS <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br /><IIIIIil.ELlNEATION <br /> <br />~ECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />If a dam is not specifically designed and operated, either in whole or in part, <br />for flood control purposes, then its effects, even if it provides inadvertent <br />flood routing capabilities which reduce the 100-year flood downstream, shall <br />not be taken into account and the delineation of the floodplain below such a <br />dam shall be based upon the 100-year flood that would occur absent of the <br />dam. However, if adequate assurances have been obtained to preserve the <br />flood routing capabilities of such a dam, then the delineation of the floodplain <br />below the dam may, but need not, be based on the assumption that the <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />CH4-214 <br />