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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge,frequency relationships for <br />each flooding source studied by detailed methods affecting the community. <br /> <br />The hydrology used for this Flood Insurance Study was taken directly from a USACE, <br />Omaha District, Flood Plain Information report (Reference I). The hydrologic analysis for <br />The Slough consisted of determining the magnitude of the 10.., 50.., I()().., and 5()()..year <br />floods. Because stream,gaging data for The Slough were not available, the Environmental <br />Protection Agency Storm Water Management Model (Reference 3) was used to develop a <br />rainfall,nmoff model of the basin. Rainfall input for the hydrologic model was derived from <br />National Oceanic and A1mospheric Administration NOAA Atlas 2, "Precipitation,Frequency <br />Atlas of the Western United States, Volume III ' Colorado. (Reference 4). <br /> <br />Values for the 5()()..year flood were extrapolated. Infiltration losses of I inch per hour were <br />used, based on results of the Missouri River Basin Interagency Committee "Missouri River <br />Basin Comprehensive Framework Study. (Reference 5). <br /> <br />Storage-discharge relationships were developed for the Black Hollow and Loop Lake <br />Reservoir systems. These relationships were hased on storage and discharge capacity data <br />for these dams and reservoirs contained in the "Inventory of Dams of the United States. <br />(Reference 6). These relationships showed that Black Hollow Reservoir has a significant <br />effect on runoff from storms centered over the entire basin of The Slough. Because of this <br />pheoomenon, the final hydrologic analysis assumed a storm centered over the 16,square'mile <br />drainage area between Black Hollow and the Town of Severance. <br /> <br />The effect of the John Law Reservoir on flood flows was determined by routing flood events <br />through the reservoir. Information on this dam was also taken from the "Inventory of Dams <br />of the United States. (Reference 6). <br /> <br />Peak discharge-ilrainage area relationships for The Slough are shown in Table I, "Summary <br />of Discharges. . <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the sources studied were carried <br />out to provide estimates of the elevations of floods of the selected recurrence intervals. <br /> <br />A hydraulic model of The Slough floodplain was developed using the USACE HEC,2 <br />computer program (Reference 7). The HEC-2 program is used for calculating water,surface <br />profiles for steady, gradually varied flow in natural or man'made channels, <br /> <br />Channel Cross Sections A through I were established by photogrammetric methods from <br />flights made on April II, 1978, supplied by the Colorado Water Conservation Board and <br />Weld County. Channel Cross Sections J through P were established by ground surveying that <br />was completed in 1996 by the Colorado Water Conservation Board. Culvert information was <br />provided by the USACE. <br /> <br />3 <br />