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I I I I I I I I I I I I I I I II II I I at a specific point on the reference line, the flood elevations can be surveyed on the ground to alleviate any discrepancies on the base map. 3. Flooded Areas: Flood plain maps, sheets 1 through 23, show the boundaries of the 100-year flood. The flood plain boundaries were plotted using flood contour elevations and stationing from the plotted flood profiles. This was done at elevation intervals compatible with the map contour intervals. Flood contours are shown as wiggly lines at 0.25 m interval, perpendicular to the direction of flow. B. Hydrology The watersheds that produce runoff to channels in and around Douglas Washes are relatively small in sizes. Ponding and storage effects were not considered for the small ponds, irrigation ditches, and canals in the watersheds because they were not built for flood control. Snowmelt was not a contributing factor to runoff frequencies of 10- year or greater, which are the primary concern of this study. This implies that significant runoff is coming from rainfall events. As a consequence, only rainfall-generated runoff was considered in developing peak discharge-frequency values included herein. Hydrologic Program Models: The NRCS TR-20 computer program ('92 Version) and u.S. Army Corps of Engineers' Flood Hydrograph Package HEC-1 were used to generate frequency flows for the Douglas Wash watershed. The Natural Resources Conservation Service (NRCS) 24- hour Type II rainfall distribution was selected for use in the model to generate frequency peak discharges. Precipitation (P) - The historical precipitation data from the palisade Climate Station near Clifton was used as input to the TR- 20 model. No areal adjustments were made because of the small drainage areas involved. Precipitation record is complete for a period from 1948 through 1995 for the Palisade climate station near Clifton, Colorado. The greatest one-day recorded rainfall occurred in 1957 (1.80 inches). There were ten occurrences of 1.0 or more inches of rainfall during the period of record. Runoff Curve Number (CN) - Hydrologic runoff curve numbers (CNs) were developed from the NRCS soil map, land use information, and field observations. CN values for the TR-20 watersheds reflect weighted average CNs for a variety of land uses and soils within each watershed. Time of Concentration (Tc) - Time of concentration values were computed by the method described in the Natural Resources Conservation Service Technical Release 55 (Urban Hydrology for Small Watersheds). Another method from the NRCS Engineering Field Manual - Chapter 2, intended for small rural watersheds, was also considered, but the Tr-55 was favored in estimating the Tc's. 7