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--;~:/:::: .'....~_.~ .,' ..-..---- :"' ~,. , ", ~ t':.~;S,:C:C7?S. inc. . 0- ; ~ MolY 20, ] 992 ::. H:: -..: ,0, "i "'r'ler ::2' .-' ",(-; )>~,-e _ ~:::,',:<'lr ~~--::."~.:: ~~= ~4~ COr';"9 -'::-: :':C-243, ~ "'-< ^;;~: :.:::- :;~.::-S Dr. John Liou, P.E., P.L,S. Federal Emergency Maaagement Agency Re:gion VITI - Denver Federal Center Building 710 - Box 25267 Denver, Colorado 80225-0267 ] 11'. Ken Bruch 4 ':ity Manager I:ity Hall : 00 West Main Hore:nce, Colorado 81226 REF: 8924F - OAK CREEK LMMP - FLOODW A Y D(~ar John and Ken: During OlU' May 13th meeting, the methodology IJsed to determine the FEMA l()()-year floodplain for Oak Creek was presented to representatives from the City of Florence (City), FEMA, the U.S. Almy Corps of Engineers, aad concerned citizens. The methodology used in our aaalysis was generally accepted by all parties. At the time of t e meeting, the floodway had not yet been detennine:d. However, based upon our cursory revil w of the hydraulic model, the floodway for Oak Creek would be significaatly wider in places tha. the, currently regulated floodway. The City expressed some conl:em over the increase in the flo ldway. Thus, as agreed in our meeting, the enclosed drawings show the currently regulated f oodway taken from the 1984 FIS aad aa approximate floodway computed from the LMMP } EC.,z hydraulic model. The purpose of the comparison is to illustrate the potential difference in tI (: two floodways. The LMMP floodway was delineated using the LMM ') HEC-2 modd as a single model as opposed to the tht,:e models presented in the May 13 mee:ting The floodway shown on lh,~ drawings was delineated based on the methodology discussed in th : May 13 meeting, A single model assumes th,: water surface is l,~vel over the width of the floodp ain and that all portions of tl1'~ floodplain are hydraulically connec:ted. Thus, the base water surfa ,:e use-,d in the calculation of the floodway is not the same as that c:omputed using the three separatl models. The floodway critl:Ila for FEMA is a maximum of a one-foot rise in the 100-year watl r surface. The LMMP flO<Jdway shown is simply a first cut estimate using a single model ap )roach. As seen, in some places along the channel, lhe two floodways are significaatly different This info:nnation on the approximate floodway shou d be: used as a starting point for discussions so that an agreement caa be reached between the City of Florence and FEMA as to how to proceed w.th the LMMP study for Oak Creek within the City.