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Basin slopes and channel slopes were taken from the U.S.G.S. quandrangle maps. Channel and overbank cross sections and roughness coefficients were estimated from field inspection. Reservoir information for routing of flows through Douglas Reservoir and Park Creek Reservoir was taken from the U.S.G.S. quadrangles and aerial photography, respectively. This information was supplemented with esti~ates made during field inspection of Douglas Reservoir. The selected basin conditions for this hydrologic analysis were described in the "Scope of Work" for this contract as follows: Hydroloqy Results Reach I Confluence with Cache la poudre River upstream to the north section line of Sections 29, 28, 27 and 26, Range 69 West I Township 8 ~orth. Use fully developed basin conditions. Peak discharges and runoff hydrographs were determined for the 2-year, S-year, 10-year, 50-year, lOa-year and SOO-year frequency discharges at selected locations in the study reach. In addition, thE: laO-year event was determined neglecting Douglas Reservoir. This discharge was determined to show the effect of a bypass channel around Douglas Reservoir on discharges in the Fort Collins area. Reach II South section line of Sections 19, 20, 21, 22 and 23, Range 69 West, Township 8 North, up- stream to the north section line at Sections 31, 32, 33, 34, 35 and 36, Range 69 West, Township 9 North. Use low density--minimum size tracts (2-5 acres). Reach III - All basin landS upstream of Reach II. Use existing basin conditions--Agriculture. For the 2-year and 5-year flood events, the Larimer-Weld Canal (Eaton Ditch) was assumed to intercept all flows reaching its channel. Therefore, for these events, separate runs were made above and below the ditch. The run above the ditch included only tributary area below the Larimer County Canal since this canal would intercept all flows from the area above its Dry Creek crossing. TABLE II For the la-year, 50-year, lOa-year and 500-year events, the model "'as run to includ.. th", entire vasin. Because of uncertainties in the response of tbe c<mal "ysh,m!< to large flood peaks, the canals were ignored in the analysis. The canal embankments are likely to fail under theSE: extreme conditions, and the effect on the flood peak is difficult to predict. That is, the flood peak may be increased as canal flows drain into Dry Creek, or the flood peak ~ay be reduced should flood levels be high enough to continue to discharge flows into the canals. In either case, for the critical lOa-year event, the eUect would be small--perhaps = 20\--am] ",huulJ L", ignored. Land use for each of these reaches was determined from land use maps for Larimer County. Impervious drainage area was then est~matect using aerial photography that showed similar, existing development, and from ~ncal values for the land use described. Examples of values used in this study are shown in Table II. Impervious Area Percent (%) IC'perviolls 90 95 75 - 80 60 20 - 30 10 1- 5 The frequency discharges determined by these methods at selected points vl'ithin the study reach are shown in Table III. Typical hydrogra"h.. for ~he lO-year and 100-year events on Dry Crc~k at its confluenc~ with the poudre River ~nd ~t its juncture with the Larimer-Weld Canal (Eaton Ditch) are sho"'n in Fi<JurE's 1 and 4. A discbaroe orofile for each flood event studied is shown in Figure 5. Land Use Commercial Industry Multi-Dwelling Residential Units Residential Parks Undeveloped (Agricultural) - 13 - 14 -