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<br />Detenticn ponds delay the peak flow within the sub-basin tlley serve, there- <br />by upsetting the natural hyelrolic patterns, It is possible tor severai detention <br />ponds, oa.ch serving its own sub-basin, to have peak discharges at approx. <br />imately 1he same time which Gould result in a basin.wide peak discharge larger <br />than the basin.wide peak without detention ponds. It is, therefore, important <br />that any ~iuch facilities be designed only after a basin-wide master drainage- <br />way plan has been developed. This plan will allow local qovernments to see <br />the overall effect of each proposed facility alone and in combination with other <br />facilities. In some cases such plans will need to be a joint effort of several <br />jurisdictions. The results will be a series of facilities which serve the whole <br />basin's rweds rather than a disjointed set of projects that serve individual por- <br />tions of the basin. <br />Permanent storage of stormwater runoff on the development site (retention) <br />is not allc.wable under the terms and conditions of Colorado water law. <br />Watersh13d Treatment <br />Watershed treatment is a method of treating lands to render the soil better <br />able to absorb and retain excessive rainfall until flood h'3ights in nearby or <br />adjacent swollen streams have receded.Treatments include crop rotation, <br />terrace c1Jnstruction, contoul' strrp cropping, and selective planting or <br />reforestation. <br />Changl!S to the Channel <br />Levees and Floodwalls <br />Levees ,Jr floodwalls provide protection by acting as a barrier confining <br />floodwaters to a floodway. The difference between a levee and a floodwall is <br />that leveHs are earth emb8111<ments and f100dwalls are usually concrete. <br />Urban le\lee projects must be designed to the 1 OO-year standard to remove <br />lands froln a designated floodplain. The CWCS and FEMA require that the <br />following criteria be used when designing a flood control project: <br />. a minimum levee freeboard of 3.0 feet above the 1 OO-year elevatin is <br />required, with a minimum 0'14,0 feE>t of freeboard within 100 feet <br />upstream and downstream of any structure on or through the levee u <br />brid\Jes, for example; <br />. a minimum width of 10 to 1;! feet for thE> top of the levee to accom- <br />modate maintenance vehicles is recommended; <br />. an interior drainage system is required to minimize the potential for <br />stormwaters to be trapped behind the levee system; <br />. the placement of riprap on the slope of the levee must be extended two <br />feet below grade at the toe of the slope; <br />. additional design crite.ria SllOUld be adhered to as outlined in the Corps <br />manual entitled, "Design and Construction of Leve,es," EM 1 '110-2'1913, <br />@ . <br />A floodVlal1 is subject to hydraulic loading from floodwaters on one side with <br />little or no earth loading as a resisting force on the opposite siele. Floodwalls <br />must bn <;onstructed as cantilever Hype sheet piling walls, cellular walls, but. <br />tress walls, or gravity walls, Design of floodwalls by a professional engineer is <br />necessalY to assure that proper consideration is given to the hydrostatic and <br />hydrod)fflamic loads involved. <br />Chann<el Improvements- <br />Flood h,eights can be reduced through improvement 01 a natural stream's <br />conveyance capacity or by clearing the stream of obstructions, Tile concepts <br />generally used in designing a channel improvement project are: <br />. straightening the channel to remove undesirable bends; <br />. deepening or widening the channel to increase the waterway size; <br />. cOl1Btructing open span bridges and stream crossings; <br />. lining the channel with concrete to increase efficiency and conveyance <br />capacity. <br />Channel improvement projects that are not designed to the 1 OO-year stan- <br />dard can still reduce flood risks by lowering flood heights. CarE! should be <br />taken in designing channEll improvE>ment projects to understand the whole <br />stream system. Projects that are inappropriately designed or located can <br />cause sE~rious erosion problems upstrE!am or downstream and worsen flow <br />conditions rather than improve them. <br />Summary of Flood Control Measures <br />In communities where there is substantial existing del/elopmE~nt in the <br />floodplain, flood control projects may be the most cost effective tloodplain <br />management option. Flood control projects may also bl3 a particularly <br />appealll1g option in the case wllere undeveloped lands in tile floodplain have <br />a high development potential. Tile value of the proposed development may <br />33 <br />