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) .- . . Agenda Item lOb March 3, 1995 Page lbree relative hydrologic homogeneity. First the state was divided into eight major drainage basins (Arkansas, Colorado, Green, Republican, North Platte, Rio Grande, San Juan and South Platte). Because of a lack of data points in the Republican River basin, it will have to initially be combined with the South Platte River basin. Similarly, the North Platte River basin will have to be combined with the Green River basin. Figure I shows the major drainage basins. The major basins will then be divided into regions. The first step in subdividing the major basins will be to consider the following factors: - NOAA's maximum annual precipitation; - Drainage area; - Basin slope; and - Topographic features. Preliminary subdivision into regions has already been performed for the Arkansas River basin. From that preliminary subdivision regression equations/curves were developed just to see whether a methodology like the proposed Guidelines envision might work. Initial findings for the Arkansas basin are promising. The Guidelines will be subject to several technical limitations: the regression values will be for the 1 DO-year flow only; the equations are for natural flow streams only, unless an adjustment for effective drainage area can been made where detention structures are present; the regressions were performed on values which have been derived from different methodologies, resulting in a median value between methodological extremes; the publication is for approximate zones only and is notto be used in place of a detailed study or for the design of structures. The fmallimitation in the list above must be emphasized strongly. The use of the Guidelines is restricted to the intended purpose of applying discharge values to approximate floodplain maps that have already been prepared where no engineering is available and designation is necessary for local regulatory concerns. This publication is not to be used for the design of hvdraulic structures or for the preparation of new floodplain studies. If a detailed study is subsequently completed, using its own detailed hydrologic analysis, for a stream reach which has been designated utilizing this publication, the detailed study will supersede the previously designated approximate delineation.