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<br />SECTION I <br /> <br />5. Evaluate potential flooding hazard areas and associated damage s <br />within each of thes€ floodplains. <br />7. Develop alternative improvement plans and preliminary designs <br />toaccoroodatethelOO-yearstormrunoff. <br />8. Evaluate the economic consequences of the alternatives and develop <br />a cost-benefit analysis for use in selecting an improvement plan. <br />9. Prepare a final report documenting th€ results of the study. <br /> <br />INTRODUCTION <br /> <br />Purpose <br />The City of Fort Collins and surrounding portions of Larimer County have <br />experienced rapid growth in recent years resulting in increased drainage and <br />flooding problems. local government officials are under pressure to manage this <br />urban development in a way whiCh will accomodate this growth while at the same <br />time protecting the health, safety and welfare of the area's citizens. Such a <br />situation now exists in the McClellands and Mail Creek Drainages of Fort Collins. <br />The Purpose of this study is to provide a means for making decisions re- <br />garding development and floodplain regulation in McClellands and Mail Creek <br />Drafnageways. This purpose was accomplished by determining the flooded areas <br />associated with various frequency storms. Once this was done, an economically <br />feasible and practical plan was formulated for mitigating the drainage and flood- <br />ing problems. The information in this report is intended for the use ~f state, <br />county and local gouernrnents involved in planning and landusere gulatlons. It <br />is also intended for use by developers within the basin so that they will under- <br />stand the problems and solutions and thus be able to design and construct their <br />developments in a manner whiCh will utilize the plan and facilities fully to the <br />b€nefitofall. <br /> <br />Summary of Study Findinqs <br />There is no way of predicting when a flood will occur in the Mail Creek <br />and McClellands Drainageways. Should a flood of even minor magnitude occur under <br />existing channel conditions, considerable damage to homes, businesses, railroads <br />and streets could result. lmplementationoftherecomnendedimprovementplan <br />would prevent potential damage caused by floods greater than a 2-year event. <br />In selecting the recommended plan, a syst~ approach was used as well <br />as evaluating isolated improvements. We initially evaluated the cost (including <br />maintenance and operation) of a wide variety of improvements, and combinations <br />of these improvements, in order to find the most effective plan. We then added <br />to this, those major drainage improvements which are likely to be built when un- <br />improvedgroundisdeueloped. Therefore, economically beneficial improvements <br />in conjunction with improvements likely to occur as a direct result of develop- <br />ment are part of the final plan. Continuity of the plan from reach-to-reach <br />was a major consideration, as were tho environmental and aesthetic aspects of <br />the alternatives. <br />Flooding prOblems in Reach I occur only during intense storms because of <br />the well-defined channel. With peak flow reductions affected by detention in <br />Reach 28, an ir.proved spillway at Fairway ~ar. is all that is necessary to elir.i- <br />nate flood damage. Reach 2A and 26 exhibit extensive flood damage potential due <br />to the lack of a well-defined channel for even the 2-year ,torm flows. Because <br />of the limited amount of development in this area, a grass-lined channel with <br />box culuert crossings of Harmony Road. Crest Road and Nordic access road.will <br />effectively pass the 100.year storm event. The proposed upstream detentlon sy,- <br />tem will allow a reduction in the size and construction costs of downstream im- <br />provements. Also, since the recommended detention is in an undeveloped area, <br />final location and configuration can be coordinated at the time ( if conve~ient) <br />of development and possibly be utilized as a multi-use facility. Grass.lined <br />channels between ir.provements are likely to occur at time o~ develo~e~t wit~ <br />final locations established in conjunction with the developer's plan. <br />Reach J, the New Merc€r Canal, has the capacity to carry approximately the <br />25-year storm in add1tion to the ditch's irrigation flows. Peak flow reductions <br />due to det€ntion in Reach 4 enables the canal to carry the IDO-year storm event <br />within its existing banks. An improved overflow spillway section in the existing <br />Larkborough detention pond safely passes the lOO-year storm flows. <br /> <br />Scopeof'"ork <br />The scope of work defined for the study of McClel1ands and Mail Creek <br />OrainaqewaVS follows the guidelines and specifications for basin studies prepared <br />by the'City of Fort Collins. lngeneral,thestudyconsistsofthefollowin<j: <br />1. Research and collection of base data and information pertaining <br />to the study area. This includes contact with the local govern- <br />ment bodies, ditch company representatives and other public.and <br />private agencies along with comments from property owners wlthin <br />the basin limits. <br />2. Define the basins' characteristics by means of available topo- <br />graphical mapping as well as in-field investigation. <br />3. Prepare a flood history for the area. <br />4. Conduct a hydrologic analysis for determining runoff charac- <br />teristics and flow quantities associated with the 2-, 5., 10., <br />25- 50- and lOO-year storm events under both existing and fully <br />dev~loped conditions. Also a brief comparison to historic con- <br />ditions will be made. <br />5. Kydraul1c routing of peak flow. developed in item 4 above to <br />determine flooded areas by water surface profiles. Once deter- <br />mined. delineate the 100-year floodplain on available mapping. <br /> <br />1-2 <br /> <br />1-1 <br />