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<br />were based upon water surface elevations computed for incorporated <br />areas, control sections at bridges or criteria depth calculations. <br />AI I elevations are measured from mean sea level datum; elevation <br />reference marks used in the study are shown on the separately <br />printed Flood Insurance Rate Maps (Exhibit 4). <br /> <br />. <br /> <br />Flood water elevations in these steep mountain streams are often <br />unstable due to the very high stream velocities. Another compli- <br />cating factor is the possibi lity of debris collection at bridges <br />obstructing flows. This is a concern because of the heavi Iy <br />forested areas in the County. Although this possibi I ity exists, <br />especially at the smaller bridges, any attempt to include it in <br />the analysis would be speculative. Therefore, the flood elevations <br />as shown on the profi les are valid only if hydraulic structures in <br />general remain unobstructed. <br /> <br />Bear Creek, Witter Gulch, Yankee Creek, Clear Creek from Georgetown <br />to 5i Iver Plume and from 5i Iver Plume to Graymont, the West Fork <br />of Clear Creek below Empire and from Empire to Berthoud Fal Is, and <br />Bard Creek were not studied in detai I because of the lack of <br />current or planned development and avai lable data along those <br />streams. The 100-year flood for these streams was approximated by <br />the use of regression equations to determine depths of flooding as <br />published by the Colorado Water Conservation Board in their regional <br />hydrologic analyses (Reference 12). Manual calculations of bridges <br />were uti lized to supplement the depth figures from the regression <br />equations. <br /> <br />4.0 LAND USE APPLICATIONS <br /> <br />A prime purpose of the National Flood Program is to encourage state <br />and local governments to adopt sound flood plain management programs. <br />This flood insurance study revealed that the 10, 50, 100, and 500-year <br />flood boundaries of the streams studied wi I I not exceed their channel <br />banks except in a few isolated areas. A Flood Boundary and Floodway <br />Map (Exhibit 2) for these areas has been included in this report. AI I <br />other necessary i nformat i on has been incorporated on the Flood Insurance <br />Rate Maps in an effort to assist the County in developing flood plain <br />management measures that wi I I provide the best use of their flood plain <br />lands. <br /> <br />4.1 Flood Boundaries <br /> <br />In order to provide a national standard without regional dis- <br />crimination, the 100-year flood has been adopted by the Federal <br />Insurance Administration as the base flood for purposes of flood <br />plain management measures. The 500-year flood is employed to <br />indicate additional areas of flood risk in the County. <br /> <br />. <br /> <br />17 <br />