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The modification was designed too cont:ain the 100---year s.torm wi thout emergency spillway oyer:"lm., from Drainage Area No. 6B, the proposed 72-inch sitorm sewer (capac:ity 303 cfs) from Drainage Area No.7, and the storm ",.atez' released from .the Upper Lowry Detention Pond. Th:, moclification calls for length- ening the emergency spillway from 300 to 900 ft, raisinCf the emergency spillway elevation from :;ci61 ft too 5J6;;~ ft, and raising the embankment. from 536:! ft t:o ~)J67 ft. The modifica- tion was also desiqned to passl:he FHI" without an embankment faL.ure. Dra:cnage Improvements _Down~.::~:'~_2.~~.!.:21,]:y Pond A combination of open channels and Lnde:cground conduits has been designed for the drainage irnprc:v2ments between IlttJ. Avenue and Stapleton Airport. As mentionec in Section II, a det:ail- ed cost-benefit analysis for determining a design frequency was made by the Omaha Dist:rict Corps of Engineer<;. 'l'he cost information including construction, right-of-way, flood proofing, and maintenance of th':l ch,mnel improvement.:" were provided by Camp Dresser & HcKee for :3 torms v1:l th ret.urn frequencies of 5, 10, 25 and 100 years as well as thE' upstream storage improvements according 1:0 lll.t:ernative 3A of t:hE~ Pha~;e A Report. The descrip'tio:Cl of ld.ternative JA was inc~uded in Section II. The damage estimates were revised by t:he Omaha Dist:rict Corps of :Cngineers according to L'leir detailed data compiled from a structure--by-struct:ure land use survey conducted i.n 1974. Dama"les t.O structures and their cont:ents were included in .the cost-'benefi t analysis. From the resul t.S of the cost-benefit: analY:3is, chan':lel improve" mentes with the capacity to handle t:he 5-year frequency design storm proved to be the mos.t cosl:-effecti ve scheme. HOvlever, channel improvemElnts with the capacit:y t.O handle the J.(J...year design storm were recommended by the Omaha District Corps of Engineers and used for this preliminary desiqn because the benefit-cost ratio was very ClOSE' to that for the 5--year storm event. The cost and damage data as well as the results of benefit-cost analyses are presented in Tables 5 to 8. All cost and damage data presented in Table 5 to 8 are based on the Phase A report in order to select a design frequency for the Phase B preliminary design. The detailed descri.ption of thE' channel improvElments is pre[ient:ed in the following paraoraphs. Drainage System Al~llth_J~venY~.___I~~!:er Plan Draw.!:ng No.. 2D): The existing park and detent.Lon area locat.ed at ~corner 'of ]:1 th Avenue and Verbena St.reet does not have enouCjh capaci ty to t.andle the storffiwater from Drai.na.ge Area No. 6D. ThEl detE,ntion area could be improved to detain storms of hiCjh return periods. For storms of low TetuT'l period~;, a storm sewer havinCJ capacity for :i-.year storm event was designed alor.g 11th Avenue to' drain Drainage Area No. 6D into ;~e:3terly CreEk. -26-.