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I I I I I I I I I I I I I I I I I I I EXECUTIVE SUMMARY The results of a master drainageway planning study for the Fossil Creek Basin upstream of Fossil Creek Reservoir are detailed in this report. The report was prepared for the City of Fort Collins, Larimer County and the Colorado Water Conservation Board. The report defines the basic data used in the hydrologic, hydraulic, sedimentation, erosion and economic analyses and presents alternative improvement plans to solve the potential flooding and erosion problems. One improvement plan is selected, based primarily on the economic analysis, as the recommended improvement plan. For this plan, preli- minary design data and a detailed discussion of the plan implementation are presented. Basin Characteristics The tributary area for the Fossil Creek Basin upstream of Fossil Creek Reservoir is approximately 20.9 square miles. Tributaries analyzed as a part of this study are Stanton Creek, 2.5 square miles; Lang Gulch, 6.7 square miles; Smith Creek, 2.4 square miles; and Burns Tributary, 2.1 square miles. The basins are composed of grass covered, rolling hills mostly used for grazing. Some urbanization exists or is proposed in the lower portion of the basins. Hydrologic Analysis A hydrologic analysis was performed to determine peak flow rates and flood hydrograph volumes at design points for the 2-, 5-, 10-, 50-, 100- and 500-year flood frequencies. The hydrologic analysis was performed for both existing and developed basin conditions. Developed basin conditions assumed land uses according to zoning within the Urban Growth Area. Rainfall depths for the 2-, 5-, 10-, 50-, 100- and 500-year storm fre- quencies were from NOAA and assumed to define hydrographs of the corresponding flood frequency. The Stormwater Management Model (SmM) was used for the ana- lysis. The input data were based on subcatchment and channel unit delineation and watershed parameters estimated from a previous U.S. Army Corps of Engineers (COE) study. vi Hydraulic Analysis Hydraulic analyses of Fossil Creek, Stanton Creek, Lang Gulch, Smith Creek and Burns Tributary were made to determine water-surface elevations for the 2-, 5-, 10-, 50-, 100- and 500-year floods for existing and developed con- ditions. The Water Surface Profile Model, HEG-2, was used to determine water- surface elevations at cross section locations. Cross sections that were digi- tized from 1977 mapping were obtained from the COE. Cross sections for Stanton Creek were obtained from City of Fort Collins topographic mapping. The topographic maps used in the report are at a scale of 1"=200', with two- foot contour intervals. The results of the hydraulic analysis defined a relatively narrow flood plain confined to a valley channel except where floodwater ponds behind railroad and roadway embankments. Generally the ponded area is a few hundred feet wide except upstream of the Union Pacific Railroad at Fossil Creek which ponds water over approximately 85 acres during the 100-year flood. Of 19 major channel crossings along Fossil Creek and the four tributaries, nine would be overtopped by one foot or more of water during the 100-year flood. Improvement Plans Improvement plans were developed to alleviate flooding. Alternate impro- vement plans were developed considering (1) leave the system "as is," (2) eli- minate storage behind embankments and channelize for the 10-, 50- and 100-year flood flows, (3) enlarge culverts as necessary to prevent embankments from overtopping and allow three feet of freeboard for the 100-year flood, and (4) raise embankments as necessary to prevent embankments from overtopping and allow three feet of freeboard for the 100-year flood without culvert enlarge- ment. Improvement plans considered improvements along Fossil Creek and the four tributaries. The economic analysis and preliminary design is for Fossil Creek only. Economic Analysis The economic analysis involved estimating flood damages, costs for the alternate improvement plans, and the corresponding flood damage reduction in dollars as a result of the improvements along Fossil Creek. No dwellings are inundated by flooding. Flood damages were associated with replacement costs of roads that were overtopped and assumed to fail.