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Survey and are published as regional flood data (References 15, 16, and 17). Discharges for the 500-year floods were checked by straight line extrapolation of frequencies that were previously determined using the procedure of the u.S. Geological Survey (References 16 and 18) and compared to the u.S. Army Corps of Engineers' Standard Project Flood data when available (References 10, 19, and 20). Peak-disCharge, drainage ~rea relationships for the streams studied " in detail are shown in Table 1. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the county were carried out to provide estimates of elevations of floods of the selected recurrence intervals along each stream studied in detail. . Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of, the selected recurrence interval~ (Exhibit 1). These analyses indicate that flood flow from Little Dry Creek is divided into two flow paths in the area down- stream of the Colorado and Southern Railroad crossing. The Little Dry Creek profiles show the individual elevations for both of these separate flow paths. Flooding from Northfield Creek above Devonshire Boulevard does not necessarily follow the natural stream channel. For this area, flood profiles were developed using hydraulic flow lines, labeled as base line of flow on the maps and profiles. Water-surface elevations were computed through the use of the u.S. Army Corps of Engineers' HEC-2 step-backwater computer program (Reference 21). Starting water-surface elevations for the tribu- taries of the South Platte River were taken from previously computed stage-discharge relationships when available (Reference 10). In many cases, control elevations were shifted upstream to bridges or culverts. Where no other information or control structures were available, the starting water-surface elevations were computed by the slope-area method option of the HEC-2 program (Reference 21). For the approximate studies, elevations were determined by normal depth calculations using approximate cross sections taken from U.S. Geological Survey maps (Reference 22). , All elevations are measured from National Geodetic Vertical Datum of 1929 (NGVD); elevation reference marks used in the study are shown on the maps. 15