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<br />CR. J3 to CR. 31 - A~ in the lower rem:hes. the channel has limited capacity and cannot
<br />handlc f1ow~nfevcnthe IO-yellrmagnitude The lOO-ycartloodplain isgencrally wide
<br />with shallow JeplM and has flows splilting off from the main cbannel. In the I'pstream
<br />area adjacent to CR. 31, the t100dplain is very wide (approximately 4,600 feel), The
<br />existingchannelanJbriJgeerossingarel,lndersi7.ed.andtheroadaclSasaweirspreading
<br />out the flood waters. Alongthesouthovcrbank,flowssplitinasoutheastcrlydireetion
<br />away from Pawnee Creek toward HWY 6 near the Town of Town of AtwQ<)d. The
<br />highway and milroad r~xlitect the /lows northeast towards the Pawnee Creek main
<br />ebwlncl. A ~rm h<t<! ~ell constructed approximately one mile south of tbc Pawnee
<br />Creek crossing. This berm blocks some of the flows and forccs a portion of the water
<br />over the highway and railroad embankmcnt. The remaining flows overtop the herm and
<br />continues north to the higbway bridge where tbey combine with tbe flows along the main
<br />stern or Pawnee Creek. In tbe lower portion of this reach at CR 33. the floodplain is
<br />confined by the channel banks south of the bridge crossing, as well as by a gravel
<br />drivewaytolhenorthandCR.33t(lthecast.
<br />
<br />channel and tra~el sO\lthwcst towards the Town of Tov>'ll of Atwood wcr~ considered
<br />valid fDr prop<lscd conditions aml were subtracted out of the main effitnnel at thcs~'
<br />lo~ations, ^ portion of thcse nows cvcntually drains hack to th~ main channel at IIWY 6
<br />andtbe Pawnee Crcekcrussing.
<br />
<br />A field survey was completed hy thc Colomdo Water Conservation Hoard in the area
<br />along llWY 6 south of Pawnee Creek to detemlinc the amount of flo,,:s, which gelS h~ck
<br />to the main channeL The flows in this area arc controlled by an ex,stmg berm, wh,ch
<br />blocks the now path, Estimates were madc hy the Colorado Watcr Conscrvation Board
<br />as to how mucb of the flow overtops the berm and how much would be forced over HWY
<br />6 and the UPRR embankment at Town of Atwood prior to the July 1997 nood evenT
<br />
<br />~I}'drllolies
<br />
<br />Upstream of CR. 31 No tlnodplain mapping is available upstrcam of CR. 3 I.
<br />However, hydrJ.ulic analysis wus completed by tbe SCS fot a distance of about 1.000
<br />feel. Based on discussions with the City of Sterling and the Colorado Watcr
<br />Conservation Board, the floodplain is very wide in tbe upstream reaches. There is the
<br />p<ltcntial for nooding of some loc<ll tcsidcnccs, as well as damal1:c to a~ricultural
<br />l,,-rmland, No 11H~Or now splits have hecn deT,'rm,ned in this area which \V(luJd tmpact
<br />the City of Sterling.
<br />
<br />IIydrat.llie anal;,c~< were completed lor hel" the existing channel conditions and for
<br />ptoposed improvements. The Ill";:' r~"~m v"r~ion (4.6.2; May 1991) ofth~ U.S. Army
<br />Corps of Engincets' HEC-2 computer program wa:, "'::::! ,(\ determine watct surface
<br />ptofilcs for thc IO-ycar, 50-year, and laO-year flood events. Th~ rnodelllsOO for tbis
<br />study "':;c Ih~ :;::;;'uc model devek'j:;.d h;, the SCS as prc~ent~'d in (bcir Apj;~ 1992 Report,
<br />For exi,ting channel conditions, tbis modd was duplieat<.xl and the tcsulul1;;- waler
<br />surface profiles matches Ibose pteSl:nted by the SCS. For proposed improvements. some
<br />CfQ5s-section's and ~p!itl1ow;lreaswcrc moditted in order t(l;lccuratcly depict proposed
<br />bridge, Citd""cl, and levee impr,'vell\ellls, A mor" dellliled discussion of thc~e
<br />modilicalions is ineluded in thc Project Formulation Scetion of this report, lligh water
<br />marks fWlll th.,J"ly lQQ7 f100d wcrealso used in the hydraulic analysis,
<br />
<br />HyllrolllJ.:Y.
<br />
<br />A detail~d hydrologic analysi~ was not completed for this study. lktaiJcd bydrology
<br />uK'd for this .,tudy was completed by the SCS rmd presented in their April 1992 Report.
<br />flow data for various storm cvent~ generated for that repon W<''r'' used as a basis for
<br />development of existing flOodplain information. as well as fot the evaluation of proposed
<br />improvements. Final flows werc reviewed and approved by the Cotomdo WiIIN
<br />Consetvation Hllard and the City of Sterling for use ill this,<;tudy.
<br />
<br />Comparis()n~ of tbe 1 (H).year water surface elevations were madc for cxisting llild
<br />proposed eonditions in order 10 evaJuatc Jl<ltenti<ll impacts Qfthe pro p<l~cdimprovcm<'nts
<br />The p,oposed improvements will not increase the floodway elevations, except for the
<br />area immediately downstream of the proposed improved HWY 6 etos<;ing. This can be
<br />expected since the proposed improvements will allow more /lows to follow the bistoric
<br />n ,--., ~ . 'L'. '.'. " ,. . ' .., . ,
<br />,;1o,,;O',cc '--,cc~ ",,,,nagc w~y ra"",r\,-""., O<:lJi!; Uj'~' "u ...",ltg UI!: Itl!;"W"l,
<br />
<br />There are ""veral areas where the Hood t10ws exceed the capacity or tbe channel for the
<br />lO-ycar. 50.year.:lJ\d IOO.ycarcvctlls. Flows. which split from the main channel, are
<br />sll\:l!ractcd out and an: accounted for under existing conditions: thcrefore, thc dischargcs
<br />vary signiticantly from downslrcam to upstream. For bwnee CrCl'k Overllow in the Cit>.
<br />of Sterling. IOO-yc'ar!1mH:treestim,lted to l1e 5,000 efs.
<br />
<br />l'roje~I_":tJ!:mullltilJn
<br />
<br />As a result of the improvemellls pruposed in this report. Ilood nnw, ar~" partially
<br />eontaltled in the a"<'aS adjncent t" thc channel in the north overbank TIlerefnre, these
<br />!lows were considered .1 purt of The main chan"'" ami WNe meluded in the analysis. FN
<br />the selected altemalive, impro"ements We'tc!lot pn'p'''ed in the upper re;!clles ofPa;\TIec
<br />Crcek along the southo,crb;rnksinccth<'ydollotdircetlyeol1tribulclothcP<\\\l\cc(reek
<br />Overflow in tbe City of Sterling, Subsequently. the areas where flows split out of the
<br />
<br />Hased on an evalmllion otcxisting studies and input fwmtheproje'ctsponsors.itwl1s
<br />determined that a syslem (If levees and channel Improvements in the vicinity of (be
<br />Riversidc Ccmetcry, in combination witb widening the existin~ bridges at HWY 6 "nd
<br />the UPRR i, tbe most cconomical approach to solving the Pawnee Creek Overflow
<br />problems. Development "j' the Pllwn<'e (reek improvements began with modifications to
<br />tn" "~i~!i,,S If Fr. ~ m,><1d u, ~n"ly~" (h-: imr~c.t~ "I' l'r")l'('~d kvee,. !-ri'.!~e. ~m! c;h~!)~,'~!
<br />improvemcnls. Ihe n,II\'\\ing dcsnihcs the lllodificali"n, mack l<l the c.xisting HFC-l
<br />",,>del.
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