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<br />The ~ol.lth levee will extend east from proposed char\lld improvements do",nstream of the
<br />UPRR for about 1,500 feet to a dirt road just east of the limits Qfthe 100-yearfloodplain
<br />of lhe South Platte River. The south levee ",ill prohibit low flows from draimng along
<br />tbeirexisting course. around a 4,400 foot long oxbow formed by the existing channel;
<br />therefore, a new low l10w channel must be constructed to convey storm ,,',atcr between
<br />stations 16-t{)() and 72+00. This channel will be trapezoidal in ~hape utilizing a 30 foot
<br />bottom width and 3:1 sideslopes. the depth of the low 110wchanncl will range from 2
<br />feel at thepoin[wbere it leaves the existing ehanncl oxbow to nbout 9 fect w hcreitmust
<br />daylight back inlo Pawnce Cteek.
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<br />new bridges. Howevct, the material must be tested to determine if its pbysieal properties
<br />will achieve the goalsofa lloodcontrol levee,
<br />In the sununer of 1996 the UPRR completed the improvements to thcit bridge to meet the
<br />proposed I DO-year llow conditions, However, the UPRR constructed the bridgc OVet the
<br />existingchanncleross-sectionsothattheareaunderthenewhridgeisnot inctcased until
<br />thc ehu..'lnel improvements proposed in this study ate completed, Costs have been
<br />included in ,;;c ("~timal,,; for the e::~illlcl i~rrovemetlts and for soil cement erosion
<br />protection under the new railroad bridge,
<br />This alternative includes tbe oonstruction ofaeonveyanee strueture (box culverts) under
<br />the railroad embankment neat the Town of Atwood. This culvert will provide drainuge
<br />directly to the South Plane Rivet, which is in close proximity at this location. This
<br />improvement is required since the right "av,TIee Creek overbank flows ate trapped in this
<br />area, resulting in high flood depths throughout the Town of Atwood. The culverts will
<br />not be sized to accommodate the entire overbank flow in this area as this cost was
<br />determined to be prohibitively high.
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<br />downstream of the bridges. Two levees downstream of the UPRR ranging in height from
<br />I foot to 5.5 feet ate planned. The north levee will extend east from a\lout 300 feet
<br />downSlream of the railroad 10 the defined low flow channel of Pawnee Creek and then
<br />curve n<lrtb and follow tbe channcl to an existing dirt mad just west of the limitsofth e
<br />IOO-year floodplain of the South Platte River. This levee ""ill be approximately 1,400
<br />feet in length. A 300 foot long floodwall will be required immediately cast of the
<br />railroad bridge on the north side of the channel wbere right-of-way is limited by an
<br />existingresideneeandlivestoekpcn.
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<br />Channel improvements are also proposed from a point 100 feet upstream of HWY 6 10
<br />approximately 200 feet downstream of the UPRR, The highway bridges will need to be
<br />improved to accommodate tbe iocrease in discharges, As disellssed earlier. the exi~ting
<br />bridge for the northbound traffic is a woodcn struelute and COOT plans to replace this
<br />bridge in the next 3 to 5 years. For tile somhoound lanes, fOllrcclls of 20' x 8' RCB
<br />would be instalkd just SQuib of the existing bridge. The culverts could be pre-cast which
<br />would minimi7.c the time required to complete the instaUalio!\, thus limiting the impacts
<br />to existing traffic. Wing walls upstream and downstream, as well as riprap protection
<br />would be TeCjllired, Conceptual cross-sections of the channel and bridge improvements at
<br />HWY 6 and the UPRR were prepared to aid in cost preparation. In order to con~truet the
<br />new box culvel1s south of the existing bridge, the existing siphon along tbe Springdale
<br />Ditch which carries irrigation watet under the rail toad and highway may need to be
<br />extended to the south, There is also an existing concrete diteh diversion struclure localed
<br />just riownstremn oithc UPRR which will have to be removed. lhis structure IS presenUy
<br />abandoned and preliminary discussions v.ritb tbe owners indicmed that they ate agreeable
<br />to its removable.
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<br />North Improvement Area: This alterflliLj~'~ will collect the Pawnee Creek ovcrflow and
<br />..!:"mi 'these flows dir.:ctly to the South Platte Ri'.."" ~'l .\".. \1eneral vieinitv of ,~':
<br />Cemelety ju.-;: ~"uth of St':~!:l,g. This alternativc con~ists of ~otlslruetion of a berm
<br />having an approxim"tc height of 5-feet to I;,.; llIHtb and west of the Rivetside Cemetery,
<br />This heull 'oI'otlld collect and prevent !lows from lb" Pawnee "vo;,r!1ow from entering the
<br />City along the HWY 6/Railremd embankments, Included in the alternative is l"wering of
<br />CR. 24, construction of a cbanncl with a concrete lloodwall, a closure structure for IIWY
<br />6, construction of box culvert, under tho railroad, l! sipbon for tbe Slerling No. I Ditch,
<br />and construction ofa trapc7.oidal channel to the Soutb Platte River. AlIimprovements
<br />will be sized to handle 5,000 cfs, Flows will overtop lbe highway, and flow under the
<br />railroad in the new culvert. The wilroad culverts have been si7ed sucb that no increase in
<br />the highway depth of flooding v.rill result. Construction of the impr{)vements will indude
<br />installation of a small storm sewet which ",ill allow draining of a sigoificant ponding area
<br />located to thc west of the highway and north of the cemetery. The wad depres~ion in this
<br />area is to allow 110ws that f\o around the cemetery to the west to drain back into lhe
<br />propose'<.! improvemeots. The lloodwall and road closure will be sized to accommodate
<br />thrce feet of freeboard
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<br />Based on the HEC.2 model prepared by tbe SCS, the existing channel tbrougb the
<br />bridges falls approximately 1.9 teet from an elevation of 3980,0 upstream of llWY 6 to
<br />an elevation of 3978.1 downstream of tbe UPRR. In order to limit the exteot of the
<br />bridge widening at HWY 6. tbe proposed ehannelthrnugh the bridges will he lowered
<br />about 1.5 feet. The propo~ed channel lowering will r.:ql1ire II drop structure upstream of
<br />HWY6.
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<br />Constnlction of the ehunncl cast of the railrolld ",ill require pUlling the Sterling No. I
<br />Irrigation Canal into a siphon under the channel. This will bllve the added benetit 01
<br />limiting flows d,)wn the irrigation canal. which currently serves as a eonvcyanel: channel
<br />forl1oodwalls into the City of Sterling. Ilaving the siphon ",ill also bclp prcwntcanal
<br />spillagc andbrcak outs at uncontrolled loeations
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<br />Idcally. the exccs; matcrial geoerated frolll tho: nlaj<>r chamlel impruvemellt.' ill Iloe
<br />vicinity of H\VY 6 and the UPRR. in addition to the low llow channel further
<br />dov,'IlStream. would be uscd toeonstruet minor training dikes 10 hclp dir eet llows into the
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<br />rhe channel will T<:<./u;re modilicalions tel two pllvute driveway" ami OIl(' miLlol C\'Ullly
<br />[lOad 11,,, modification, will induuc'l:r,.,Jual uc'pI6.<ic"" in th... tClad tirades with le""
<br />flow culverls, Road surfa<:es in the channel bottom will be hordcl1cu, Thi;, f<lad and
<br />drivcwl!yswiUnotbeacccssiblcdmingtloodcvenls
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