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3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community, The hydrology used for this Flood Insurance Study was taken directly from a USACE, Omaha District, Flood Plain Information report (Reference I). The hydrologic analysis for The Slough consisted of determining the magnitude of the 10-, 50-, 100-, and 500-year floods. Because stream-gaging data for The Slough were not available, the Environmental Protection Agency Storm Water Management Model (Reference 3) was used to develop a rainfall-runoff model of the basin. Rainfall input for the hydrologic model was derived from National Oceanic and A1mospheric Administration NOAA Atlas 2, "Precipitation-Frequency Atlas of the Western United States, Volume III - Colorado. (Reference 4), Values for the 500-year flood were extrapolated. Infiltration losses of I inch per hour were used, based on results of the Missouri River Basin Interagency Committee "Missouri River Basin Comprehensive Framework Study. (Reference 5), Storage-discharge relationships were developed for the Black Hollow and Loop Lake Reservoir systems. These relationships were hased on storage and discharge capacity data for these dams and reservoirs contained in the "Inventory of Dams of the United States. (Reference 6). These relationships showed that Black Hollow Reservoir has a significant effect on runoff from storms centered over the entire basin of The Slough. Because of this pheoomenon, the final hydrologic analysis assumed a storm centered over the 16-square-mile drainage area between Black Hollow and the Town of Severance. The effect of the John Law Reservoir on flood flows was determined by routing flood events through the reservoir. Information on this dam was also taken from the "Inventory of Dams of the United States. (Reference 6). Peak discharge-ilrainage area relationships for The Slough are shown in Table I, "Summary of Discharges. . 3,2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals, A hydraulic model of The Slough floodplain was developed using the USACE HEC-2 computer program (Reference 7), The HEC-2 program is used for calculating water-surface profiles for steady, gradually varied flow in natural or man-made channels. Channel Cross Sections A through I were established by photogrammetric methods from flights made on April II, 1978, supplied by the Colorado Water Conservation Board and Weld County, Channel Cross Sections J through P were established by ground surveying that was completed in 1996 by the Colorado Water Conservation Board. Culvert information was provided by the USACE. 3