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'.ro. .~,,-:-' '" '" '" '" N N i""O !'.;:",, ,;'. - <: ~ >- ): .2>:"8 6 Hi ;IIM" , '* t3 {}'~ ~ g <l>""UlO<:U'" 0'<>, W ~%8~~.!8 "-d:!:'d:!:'d:!:' . " . " , " .. - ., - ..- ~ " ~ " - " ,,-u..,,-"-"-"- . ]( ~ ~ 1 , . ~ l , ~ ~ ." SECTIOHFOUR Flood Inundauon Impacts The flood elevation and floodway width changes from pre-proje<:t conditions to future conditions are summarized in Tab]e )-2 md 3-) for the Pawnee Creek main channel. The results of the revised hydraulic modeling for the overllow reaches at Riverside Cemetery near Sterling and through the Town of Atwood are shown on Tables 3-4 and 3-5, respectively. In 1l~CJ.lIi, the proposed mitigation alternative significandy redu~s flooding depth.~..nd extent upsmam of Highway 6_ In addition, the proposed a1ternativ~ v.~ll reduce the duration of flooding. For the ]997 event, it is estimated that the proposed mitigation alternative would have reduced the durotion of flooding from approximately 45 hours to 20 hours. In general, the propose-d project will inctease flood,conveyan~ and decrease flooding upsmam of Highway 6 and the rnilru.ul. '}~er pre_project conditions, the design flood flow (8,200 efs) would overtop Highway 6 and the railroa.:! by approximately 3.2 feet ;md 2.8 fec::l, ~'pectively. Under the proposed project conditions for the design flood flow, a decrease in water surface d~vation from pre-project to post-project conditions so that the peak flows are conveyed within the modeled opening width without overtopping the Highway 6 and railroad embankments was simulated. loc pre-proje<:t and furore conditions results at Highway 6 and the railroad are shown on Tables 3-2 and 3-3 at cross section locations both upstream and downstream of the opening. Core;'1'nng the {Csults of the design flood flow to the higher USACE IOO-year frequency main channel flow indicates that a flaw of 11,500 eis would overtop only the Highway 6 roadWll)' Ly a depth of approximately 0.75 foot. Thisresulthowever,isasignificantreductiol1!:.fiooding severity from the pre-project conditions. Reductions in flood sta~("! ajJstrcam of Highway 6 arc estimated to be l>j:proximately 4 to 5 feet. Th~ conveyance strucrores could be modified in final design to eliminate the ovcrtopping under u.~ lugher USACE lOO-year frequency flows. 111e levee and floodwall sy~,';::TI proposed for thc main ehannel under the project would prevent thc width offloOO flows from excet:':::'''B the floodway width shown for thejunetion of Pawnee Creek and the South Platte River in the FEMA FIRM's during the tOO-year ftequeney flood. An additiolUll HEC.2 model was prepared for the Riverside Cemetery overflow reach that affects the City ofStcrlingas part of this evaluation. The original NRCS study was used as the ba~is of this model, md additional cross sections were used to model the assumed 208.foot wide by 4.foot high box culvert beneath the UPRR. The model results indicate tbat Highway 6 would overtop under tbe IOO-year flow condition. However, the City of Sterling would be protected from flood flows in the Pawnee Creek overllow by a road closure system and levcc. In general, flood stages would increase slightly (approximately 0.1 to 0.4 fOOl) UpSm'l1IIl <.>f HigllW"y 6; bowever, the road closure and levees would reduce the quantity of floodwater that would enter Sterling. A HEC-211loocl of the overflow segmcntdownstrcam of the Town of Atwood that extends In the main Pawnee Creck ch.:u1nel was prepared as part of the NRCS smdy and evaluated by the HMGP applicant. The original model for tbis segment was not appropriate for dctennining the effects uftbe proposed project fcatures. The original NRCS model does not extefld to either the planned Highway 6 and railroad conveyance structures or tv the overflvw reach upstream. Thc hydraulic model wa. revised as part of this evaluation to determinc the effects of the proposcd mitigation structures. A HEC-2 model of this overflow reaeb with tbe conveyance structures preliminarily sized at 208-foot widc' by 4-foot high was assumed fot this evaluation. Tahlc 3-5 sbowsth::lt if the conveyance structures are built to this capacity, they wi II be adcquatc to pass thedesignflowof5,OOOcfs, It is estimated that reductions in flood stages upstream of Highway6wouldtotalabout3to6fcel. F()rdischarges m excess of tbe design discharge, flooding elevations upstream of the ncw conveyance structure will generally deereasc. URS Grein." Woodwanl Clyde ,...,........""",..._..._-"'_,,...,.....,, 4-1