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SECTION 2 - RECOMMENDED IMPROVEMENT PLAN pond behind it, according to "Stability Analysis for <'In Existing R<'Iilro<'ld Embankment Proposed Flood Detention D<lm", prepared by Chen <'Ind Associates, Inc., dated June, 1981. This :Jtabilization will consist of providing additional emban).;;m(mt material to flatten out both the upstream and downstream slopes and an impervious cutoff wall on the upstream side. 2.1 Plan Section The selection of the recommended improvement plan was based primarily on the cost analysis of the various alternatives. In addition to cost, the selected plan h<lS other advantages compared to the other alternatives. Important advantages include minimizing the relocation of mobile home park residents "nd m"int"ining Spring Creek and its riparian habitat between College Avenue and Arthur's Ditch. ^ detailed analysis of the varIous alternatives IS presented in the Technical Addendum. Improvements downstream of College Avenue consist primarily of crossing improvements. The inadequate Stuart Street and Stover Street crossings will be replaced. In addition. channel improvements will be constructed from upstream of Stuart Street to downstream of Stover Street. Due to insufficient capacity, the Welch Street bridge will also to be replaced. The C & S Railroad crossing will be improved to eliminate runoff overtopping the tr/lcks and Timberline Road. These improvements downstream of College Avenue bring the channel up to a IOO-year event, developed basin capacity. 2.2 Summary Description The recommended plan consists of a system of improvements throughout the drainageway Which will achieve a developed basin, IOO-year recurrence interval level of protection. Improvements west of College Avenue consist of construction of a now IOO-year design flow channel extending due west from the College Avenue bridge to a new multiple culvert crossing under the Burlington Northern Railroad (BNRR) embilnkment. The exi,;t.illg BNRR culvet.t located west of Johnson Mobile Home Park ...ill be restricted, thereby allowing only low flows to pass; flows within the capacity of the storm sewer under the mobile home park. The new culverts under the BNRR, to be located some 800 feet south of the existing culvert, will be sized to restrict flows through the emb"'ll).;;m~nt to those currently carried by the existing culvert. Consequently, flows downstn;>...m ::>f Coll"<),, Avpnup wi 1.1. T/"m'l i n i1pprQxi mi'ltf>) y the same as existing flows. Improvements upstream of Shields Street conGist of those called for in the original Spring cr"ek Dr"illa<Jeway Plan. These <Ire, improved crossings at the New M",cer Ditch, r,drlmer County Canal No.2, Drake Hoad, T1lft Hill Road, and Pleasant Valley and Lake Can<'ll and the construction of a regional detention pond ",bove Taft Hill Road along with berming in t_,;o locations to protect existing structures. 2.3 Preliminary Design 5 The prelimin3ry d"si<Jn for thp ,.."omm..nrl..rl pl'ln i" h,1""d on the IOO-year recurrence interval flow fer developed basin conditions, incorporating the proposed improvements. The preliminary design of the recommended improvements is presented on Plates 28 through 35. The design data is suml~arized in Tdble 1. A schematic of the proposed improvements is presented in Figure 2. Koy elements uf the pl-elimindr:y design Collow. 5 hs flo'.-I will be rc:::tricted through the embankment, runoff will continue to pond upstream of the BNRR. This necessitate~ stabiliz,ltion of the BNRR embankment, which In its present condition has an una<":cf'pt_"bl,' f"ctar of s,~fl~ty if '.',Olter ,Iere to