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<br />Water-surface elevations of floods of the selected <br />recurrence intervals for streams within the detailed <br />study areas were computed through the use of the <br />U. S. Army Corps of Engineers' HEC-2 Water-Surface <br />Profiles computer program (Reference 1). Starting <br />water-surface elevations for the Purgatoire River <br />were computed using the slope-area method option of <br />the HEC-2 program. Starting water-surface elevations <br />for Powell Arroyo, Raton Creek, and Reilly Canyon were <br />taken from the HEC-2 output for the Purgatoire River <br />at the confluence of each stream. In many cases, <br />control elevations were shifted upstream to bridges <br />or culverts. Profiles were determined for the 10-, <br />50-, 100-, and SOD-year floods. Flood profiles were <br />drawn showing computed water-surface elevations to <br />an accuracy of 0.5 foot for floods of the selected <br />recurrence intervals. <br /> <br />Locations of the selected cross sections used in the <br />hydraulic computations in the detailed studies are <br />shown on the Floodway and Flood Boundary Maps (Exhibits <br />2 and 3). Channel roughness factors (Manning's On") <br />for these computations were assigned on the basis <br />of field inspection of the flood plain areas and engi- <br />neering judgment. Bridge geometry and elevation <br />information was obtained from the Colorado State Highway <br />Department, when available, and measured in the field. <br /> <br />All elevations are referenced to the National Geodetic <br />Vertical Datum of 1929. Elevation reference marks used <br />in the study are shown on the maps. <br /> <br />Detailed cross sections for the Purgatoire River and <br />Raton Creek were obtained from the Albuquerque office <br />of the U. S. Army Corps of Engineers. Detailed cross <br />section data for Reilly Canyon and Powell Arroyo were <br />field surveyed in December 1974 and were located at <br />close intervals above and below culverts and bridges <br />to compute the effects of backwater. The Reilly Canyon <br />cross sections data were supplemented by Colorado <br />State Highway Department survey data. <br /> <br />The hydraulic analyses completed in this study did <br />not account for probable debris blockage of bridges <br />and culverts. The potential for blockage exists and <br />planning in or near the flood plain should recognize <br />the limitation of this data, if structures become <br />inoperable and fail. <br /> <br />21 <br />