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[ lV-l SECTlONIV HYDRAULICS AND SEOIMENrATION the April Hi, 191'l1 mapping varied significantly from the chaMel inverts used by the Army Corps of Engineers in their floodplain analysis in 1977. I I I HYDRAULIC ANALYSiS Existing Conditions For reaches 3, 4, 5, 6 and 7 which extend from Oxford Avenue to Sand Creek, a hydraulic analysis was performed todf'termine the water surfaceelevd- t Ions for the 10-, 50-, and lOO-ye~r fl ood events. Hydraulic computat ions "ere not performed for the first two reaches of this study,fromCha tfield Dam to Oxford Avenue. The lOO-year '",ater surface profiles and lOO-year floodolains shown on Sheets 4 to 7 w..f!' has..~ nn F1Mrl lnsl,lr~n<::", ~tud;es for Littleton and Arapahoe County. Where the twu studies over1~pp~d, th~ d~t~ from the most recent st~dy were ~<;ed. Reach 2. frO'1l Coll.llTlbine V~l1ey to Oxford Aven~e. is part of the Army Corps of Engineers channel project and the Corps nas developed the lOO.year profile for this reach. ForReaCh ~, from Sand Creek to Basel ine Road in Brighton, the 10-, 50-. and lOO-year flood profil~ wer" ohtai'1ed frlJ'" the 1977 flood hHard area delineation study. Al 1 hydra~l ic analyses were done on the basis of existinq Channel condi- tions. Estimates of Channel and overbank "oughness factors were,nade by field inspe<::tions and use of photographs. Typical roughness, or Ma'1l1ings "n", used in thIs study are: for the channel, .r13to .035, and for the overbank .04 to .07. It was assWled that none of the bridges would be blocked by debds. Large bridges whiCh were expected to have no effect on flood profiles were not modeled in this study. Most small foot bridges construded as part of the South Platte River tral1 system were not modeled because they arf' df'~ igned t.o hrf'Ak d"'ay in times Of floods. ,~,n e~ception ",as the Bronco footbddqe at 1-25 whiCh creates backwater in a crHical area of the river. I I [ [ [ [ For Reac~es 3, 4, 5, 6, and 7 flood profi les were computed using the IJ. S. iltrmy Corps of Engineers HEC-2 wdter surface profile computer program. Valley cross sections were dev'!loped using digitized cross sections ob. tained from aerial pl1otographyflownonl\pri116, 19B3. SinCf'therewasa significant flow in the South Platte River on that day, the channel bottom elevationafthccrosssectionswasactuallythe\Olatersurfaceelevationon that day, rather than the actual stream thalweg. Because it was believed that in certa in c~ses ~~ much as ? f"et. of differ!:'n,,, could e~ist between the water surface elevation on Apdl 16, 1983 and the adual stre<lm thal- w€9,thethalwegwassurveyed in thefie1doradjustedbyaSSllllingall12 foot depth of flow in critical areas. Critical areas """reconsidered to be tMse locations wher~ the computations show€<! that OVi'rhank f100<1109 W)'lld occur and in those locations ..bere the channel bottom profile obtained r,.om Split Flow Calculations Significant Wlit flows occur between York Street and 1-70. in the central Pl atte '1''111 ey are~, and near Evans Avenue. Sp' it flows occurs where water flows in two distinct and hydraulically separate channels. "here split flow is exo..cterl, the lOO-yearwater surface profile was calculated assurn- ing that all flow would be contained ;n the South 1'1 aUe Riv~r chantlel. Weir flow calculations were made to determine the amount of flow into the overbank. MIJ1tipl e flows were run throug~ tile nverbank areas to determine tailwater elevations. In the Central Platte Valley area {between 1-25 and Cherry Creek) analySis rating curves were developed for both the main channel and the overbank Meas, which, along with a w"ir flow eQuatiM, were u~ed to determine channel and overbank elevations. D~, The fo 11 owi ng dams and drop ~tructure~ "Pre ~<lrv"yed to estab 1 i<;~ eres t elevation,; the York Street drop struct~re, the Burl ington darn, the 3rd