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<br />'1',~BLg TWO <br /> <br />E,d.sting Condit..ionl'3 Manning "n" valuE!B <br /> <br />"n" Value <br /> <br />Land USI~! DI~SGJ:'iption <br /> <br />0.035 <br />0.040 <br /> <br />Sou-th Plat1:e main channel <br />Old 50ll1:11 Platte river chutes with <br />light v8qetat:ion <br />lrarrn E'iE:!ldB <br />li'arm li'iE:!lda wi.th rClada and fl:Hlcen <br />li'arm F'iBldl::1 wi.th occasional houses, <br />t:reE~s, cmd roa_ds. <br />Heav"ily ll.rbanized a.reas <br /> <br />0.050 <br />0.060 - 0.065 <br />0.070 - 0.080 <br /> <br />0.100 <br /> <br />3.1.1 Comparison with Flooel Plain Irlformation Study. Ftlr this <br />analysis, contraction and expansion coe:~f.icien1:B of 0.1 and O~ 3 were used <br />except at the bridges where 0.3 and a.!; were used for the Norma.l Bridge <br />model routines. This differs from the l'lood Plain Information Model where <br />the bridges were rated by harld calculation". III the Flood Plai.n Informati.on <br />Study, flow was allowed into the ov'erbanks. .:ind hand calculated ratin9 curves <br />were used to Bet the u:l'Btream water surfHc~~ elevations. <br />To model .the U.S. Highway 6 bridge and 'che Burlington Nort.hern Rai.lroad <br />bridge ueing 'che Normal Bridge routine, thr~ effective flow areas had to be <br />determined. If the l)verbanks were all()\<J'f~d al3 an effective flow area. for <br />the lOa-year discharge, the water surface (~levation upstream elf the bridges <br />remained belmoJ' the railroad and road surfacE:! grades and the entire discharge <br />flowed throug;:t the bridge openings. Whem thE~ Hffec.tive areas were set for <br />all the flow -::'0 pass -through the bridge opEmings, the upstream water surface <br />elevations w(~re high enough t.o alla",r flow over the railroad and road <br />profiles. The Flood Plain Information ,::tudy, howeve_r:, documents a flood in <br />1969 with a discharg(~ greater than 'the lOO-RYE!ar event 1:.hat did not flow <br />across the Highway 6 road grade. The st:udy E!xplains t:hat obstructions <br />upstt"eam of the road grade limit the amouut: of flow that could actually <br />overtop the road gradE!. In 'the final.i ZE~d existing conditions model, the <br />10-,50-, and laO-year flood events con:finod t:ne floi-ls t:hrough both bridges <br />entLrely to 1:.he channnl to provide conservative water surface elevations. <br />For .the 500-YE!ar even1: flm" was allowed ever -the road. <br />Other differences include changing the frici:ion loss calculation method <br />from the geor:letric meeLn friction loss t.e the average conveyance me.thod and <br />diffl~rent "n" value dE!signations. The ma~ or difference between the Flood <br />Info:::-mation st:udy watel. surface profilE!s and -':.hiB study' fOl profiles i.s in the <br />bridqe loss calculation met:hods used. <br /> <br />5 <br />