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<br />. <br /> <br />well-defined floodplains where conveyance capacity <br />must be preserved. By assuming encroachment into <br />the floodplain from both sides, the floodway line <br />may be calculated. Often this is conservative, <br />since development may not take place at every point. <br /> <br />. <br /> <br />Unfortunately, the floodway is more expensive to <br />determine and can involve some political decisions <br />such as the assumption of equal reduction in convey- <br />ance capacity from other sides (as opposed to <br />assumed more filling on one side of the stream or <br />the other). On steep gradient streams in Colorado, <br />encroachment on the floodplain can often appear to <br />lower water surface profiles as the computer solu- <br />tion of flow passes into the super-critical regime <br />of flow (see Glossary) with shallow depths and high <br />velocities. Another limitation of the floodway <br />concept is that the specified but arbitrary rise <br />criterion of 0.5 foot or 1.0 foot may actually <br />increase damage potential by forcing unconfined <br />flows into previously dry areas due to the increase <br />in the water surface elevations. <br /> <br />. <br /> <br />The Hazard Area Concept is based on depth and <br />velocity criteria. Its application, simple to <br />understand, does not require as much detailed engi- <br />neering. This concept is most applicable to the <br /> <br /> <br />-32- <br />