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<br />The ro:sults of tho: analysis indicate that Altcrnative No, 5 is the most bencficial project to
<br />pursue. This alternative rallhd highest among the six alternatives evaluated becausc it
<br />provictesardativcJy largcbcncfit for the cost.
<br />
<br />E"j,tin~ F1oodill~ C-'onllitiun,
<br />
<br />. PartiulprotectionofTov.llofAtwooll
<br />. fJes1 Ilse of existing Pawnee Creek Channel
<br />. Best]evcJofprotcctionforStcrling
<br />. Partial protection of farmlands bordering HWY 6
<br />
<br />Historically, Pa\'llIee Creek has proven 10 De tbe most significant flooding source for the
<br />City of Sterling and for Logall County. Flooding in tbe City of Sterling from Pawnee
<br />Creek has OCCUlTed mainly as a result of insufficient channd capacity, roadway
<br />structures, and the bridges at HWY 6 and the UPRR. The Soil Conservation Service
<br />(SCS) completed a floodplain analysis of the Pa""nec Creek overflow area in
<br />unincorpotated Logan County in April of ]992 that determined the 100-year floodplain
<br />for existing basin and channel conditions. This report supplemented the prcvious
<br />LeafiResollTCe Consll1tants report of 1983, which was completed for the portion of the
<br />Pawnee overflow irl the City of Sterling. A floodplain map was prepared which
<br />delineated the ]OO-yearlloodplainfor Pawnce Creek through this study reach, Basedon
<br />theSeS Report,adescriptionoftheexistinglloodpotenlialisp,e&cntedbelow:
<br />
<br />Aftcr exlcnsive analysis, a multillld~ ofpresentatiuns to th~ pllblk. and s~...:raJ tt:vi~iuns
<br />of the design clements, Alternative No.5 was selected as th~ preferred solution. Primary
<br />reasons forscleetion of this altemativc incJuded:
<br />
<br />In general, it makes th.' most sense to construct drainage imptovemenls from dO"'Ilstrcam
<br />to upstream, Ine sc!ed<"<i Pav.'Ilee Creek proj<'Ct is no exception bccat.lse upstream
<br />improvements will mcrease flows in tbe lower reaches, If impwvements have not been
<br />completed in the lower roaches to proP<Jrly convey the increased discharges, then
<br />flooding conditions can be made worse. Inercforc, it is recommended that the Pawn",e
<br />Creek project hc constructed starting downstream, lffllndsarctlotlwailablctoeomplcte
<br />tb",,,,ntirc project at one time, then thc projeet can scparated into the fa Ilowing phases anil
<br />c<>nstnlclCd in the fiJllowingorder:
<br />
<br />1. Riversid<: Cemetery Cotl,lru<:tion
<br />2, Cbanllel and Levees downstream ofUPRR
<br />3. Pawnee Creek malnstcm bridges for HWY 6
<br />4. Box culverts tinder UPRR embankment at Town of Atwuod
<br />5, Channel clcamJ1~ and debris removal upstream of llWY 6 mainstem
<br />bridges
<br />
<br />South PlaUe River 10 HWY 6/UPRR - F]ooding in this reach is limited by the capacity
<br />"flht" existing bridge structures. However, the flooding can be sevcrcand inundate mucb
<br />of the highway' ~"d railroad. Flows split dO\'vllstrcam of the UPRR bridge causing a very
<br />wide (app'oximatdy ,1,500 feet). shallow floodplain to the confluence v.ith the South
<br />Platte River. Two residcl':ps are polentially' floodeil in this reach: One residence is
<br />located in the 100.yellr floodplallt.:''ld is on the north bank of Pawnee Cr<'Ck adjacent to
<br />thcmi]road:andthcotherrcsidenceJ' locatcdfurtberdownstreamonahigberplatcau
<br />whi~h cuuld be impacted by putential ,halJ,,~" fluoding, Additionalllooding ,,,-,curs 10
<br />Ihe Sterling :-Jo. 1 Ditch, and adjacent agricultural :,~"d~.
<br />
<br />IX, DETAILED DESCRIPTION OF SEI.ECTED ALTERNATIVE
<br />
<br />UPRRlHWY 6 to CR. 35 Flooding in this Teach is ,,~sieally muSeu by: (1)
<br />Insufficient capacity of the hr;dges at HWY 6 and thc lJPRR, (2) insuG;~::"'" channel
<br />capacity on Pawnee Creek: and (3) 1100dwaler draining into the Town of Town of
<br />Alwuod, wbich is a result of flow splits from tbe upper reaches oft~e ba~in. The grcatcst
<br />impact of flooding occurs aloog I-IWY6to tbenorth, Flood t1()WS execeding the capacity
<br />of the bridges travel along HWY 6 damaging farmlands, inundating the Riversid",
<br />F'"_",,,_, ""~ ~".'_h'""C' "~__",:... ......... .. .,', " C., c"." -.'
<br />~.~'h~'~'J> ~,~ ~'~"'~hJ "I.."au,,,~ ."" '" "'" I<OIUl"C' J[j 0 lie "y 01 "'<Tllng. lnlS
<br />area is referred to a~ the Pawnee Cre~k Ovallow, and is the driving fnree f<)r the
<br />Community to make flood 1mprovemcnts lot th", Pa\vllee Crcck t100d events,
<br />
<br />PP~;M'" ncn+'n_
<br />,.v..~. ."""..,'"
<br />
<br />Ihis ~lUdy includcs the rcach along Pawnec Creek from it~ C\HlnUenCe with the Soutb
<br />Platte River ]OO-yellr floodplain upstream to CR. 29, a distance of about 7.5 miles, Areas
<br />of conc.cm ltlelllde lo,;ations wheI"C potential floodwaters cxeced the exi~ting Pawnee
<br />Creek channel capa<:ily and damaged farmlands, CJ{s, and houses, Also of concern is the
<br />Pav.l1ce Creck Ovcrflow where floodwater uvertops the channel to th.. north and
<br />eventually inundates portions of the City of Sterling. 10 addilion In the Pawnee Creek
<br />Overflow, there ..re areas adjal'enl to and upstr....am of CR. 31 where nows ovcrtop tbc
<br />Charulel lO the "'''Jtll ~"'! "",'n111111:, f!"w it!t'l the To'.'.'~~ ef Tow" ~( .\:wGod
<br />lmpnWelllcnl.' lilr cOlllinitl1:( split flow lIreas to tbe south were not evaluated fot thiS
<br />s1ud.,.,
<br />
<br />CR. 35 tQ. CR. 3J - Tbe channel in this reach has limited capacity (Ic~s than the IO-year
<br />event). Oneelbenowsexee"dthechaoncleapacity,thcylollowalongtherelativclyflm
<br />topograph}' in a wid,:" shallow !1oodplain. A]on~ the north overbank in an area
<br />approximately 1.200 f<"Ct downstream of Cit. 33. the flows split away from the main
<br />channel and arc conveyed to tll"" north towards the Cily of Sterling contributing to the
<br />Pa\\1lee Creek Owrfln'" Th,~<~ f1"w~ maln!y i~.Llndat:: l'.gric;:!t;:,-,,! ~eld" w::h ~u:nc
<br />p'lt"ntial for llll<l<ling individual SlmelUrcs. Along the south overbank. the Ilood tlow~
<br />arc wide umil the credo: 1i'('1I1.'" large o"bow. At thi~ location, the flood t1o\\"~ overtop
<br />Ih<' bank and follow a ilir<"t alignme11\ cast to CR. 35. ]n the area 01 the OdlOW. tbc
<br />Pawnee Creek channel contains the flows until they reach CR. 35 ...."'tcnded and at this
<br />point combines back with the main channel flows.
<br />
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