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Reaches 4 and 5 are primarily agricultural land with isolated development. A multi-use detention facility with an overflow section can effectively detain and transmit flows through the Larkborough Subdivision at the lower end of these reaches. Drainage crossing at Shields Street and at Pleasant Valley and La ke Canal as well as a grass-lined channel throughout Reaches 4 and 5 will be a re- sult of future development. Ben€fits of uninterrupted traffic flows, water qua- lity and aesthetics will be realized by these improvements. Interpretation and Us€ TheZ-, 5-, 10-, ZS., 50-, and lOO-Yearfloodevents were us€d as th eflood frequencies for this floOdplain analysis. These various flood events have an average occurrence of one in the number of years as indicated. Forexample,the lOO-year flood occurs, on the average, once a 100- year period, and has a one (1%) percent chance of being equaled or exceeded in any given year. The particular uses for the various flood events in addition to those stated above are as follows: Reach 6 is at the present all agricultural except for Reach 6C which is being developed for residential and mul'ti-family uses. The drainage improvements which are proposed as a part of this development can adequately pass the lOO-year design flows. A box culvert under Timberline Road at the lower limit of ReaCh SA effectively passes the lOO-year storm event. A pipe culvert jacked under the Union Pacific Railroad is highly beneficial improvement because of the inadequacy of the existing pipe to pass flows equal to the 10.year storm and greater. Major interruptions to railway service could occur if the existing culvert is not en- larged. A grass. lined channel with varying capacity will be constructed from Timberline Road to Harmony Road once development occurs in this area. Floodplain management will be an effective means to control the layout and scheduling of improvements to provide continuity in this reach. 2.Yearthrough50-YearFloodEvents_ lnformation regarding these lower frequency floods is especially useful for future engineering studies and land use planning purposes related to minor road systems, minor channel improvements, the location of parks and recreational faci- lities, agricultural lands, and appurtenant structures. For structures and use of this type of the smaller tributaries and in areas where the high risk of struc- tural failure is economically feasible and the hazard to life and property non- existent, the use of the lower frequency floods may be considered. lOO-YearFloodEvent_ The lOO-Year flood event may also be used for €ngineering design purposes where a lower risk of failure than the 10- or 50-year flood is desired. However, the most important use of the lOa-year flood event lies in floodplain designation and land use reoulation as set forth in the state statutes. The State of Colorado considers the lOo-year frequency flood as the flood event to be used in designing and protecting structures and dwellings for human occupation. Therefore, all flood- plain regulations are based upon the 100-year flood. BenefitjcostcalculationsfortherecOOIllendedplanarelistedinTablel-1. In addition to reducing flood damage, benefits of the overall plan are that of providing park and recreation opportunities in the floodplain, enhanced land values and improved visual impact of the floodplain, improvement of water quality, preservation of the floodplain ecology, and reduced potential for public incon. venience. Also, several improvement schemes have a benefit/cost ratio greater than one, thus effectively utilizing public funds. Mappinq and Surveys Topographic mapping for this study was furnished by the City of Fort Collins and Larimer County. The scale is 1" ~ 100', and the contour interval is 2-feet. All mapping north of Harmony Road is orthophotogJ'aphy of quarter sections. South of Harmony Road, the topography was taken from the manuscript prepared for the Corps of Engineers' Fossil Creek flood study, verified by field surveys. FlOOdplain information prepared for this report is presented on this topographic mapping at a scale of 1" ~ 100', and copies of these flood plain maps are avail- able from the City of Fort Collins or from Larim€r County. The floodplain maps printed in this report are reduced to a scale of 1 n ~ 200' for easy presentation. Field surveys were completed by Intermill land Surv€ying to supplement the topographic mapping as required. Thes€ cross.sections were taken at stream cros. sing structures, at locations where the channel was obscured by trees or foliage, and at other locations where elevations were critical to the hydraulic computations. In general, field information was consistent with the topographic mapping. 1-3 1-4