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<br />I. t-;o changes were madc to the existing model from the study limits upstream of CR.
<br />31 (erms-:;.elOtion 3839) to just dovvnstrcam ofeR. 31 (cross_section 30.1) since no
<br />irnprovernellls arc proposed in this reach,
<br />
<br />freeboard. The low chotd elevation of exisling and proposed bridges at HWY 6
<br />assumedtnbcthesarne.
<br />
<br />2, The model was revised to eliminale the existing flow splits in the north overbank
<br />downstream ofCR. 33 at cross-section 2728. This wa..~ accomplished by adding the
<br />flows that split out of Pawnee Creek towards the City of Sterling back into !he model
<br />starting at Ctoss-section 26 and continuing to cross-seetion2L The discharges were
<br />also increased upstream of HWY 6 to acoount fot the flow splits draining from the
<br />Town of Atwood and to eliminate the existing flows which currently dnlin past the
<br />bridgcs at HWY 6 and on towards the City of Sterling. The Colorado Water
<br />Conser\'ation Boatd based on field surve)"s and utili:dng a weir flow equation
<br />determined tht' amount of storm water dwining from th", Town of Atwood to Pawl\~'e
<br />Creek on thenortbwestsidcofHWJ:" 6.
<br />
<br />Meetings were held by the CWCB and COOT to discLL~s the feasibility of widening the
<br />existinghigbwaybridges" Becau~elhenorthbollndbridgeisanexist;ngwoodstrueturcit
<br />eannot be INidened: therefore, total replacement would be necessary,
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<br />Two option~ w<:rc invcstigak-d to tesolve the problem of the und<:rsizcd btidges at HW'r
<br />6.
<br />
<br />Opti"H 1 i,,;"c!ves thc mmov<ll <lnd repla~ement of exi~ling timber bridge along the
<br />nortbbound lanes" lbe COUI cui7~nrly has plans to repbee the timber bridge within the
<br />next 3 10 5 years. Fot the southbound Ian,,>, t':c ~A;~'::":;; poncrete bridge would be
<br />v,idcnedtoeonvcythelOO-ycarpeakdischarge.
<br />
<br />3, Cro~~-~~tions 27 and 29 were straigbtened out to refl~t the actl.llil floodplain in the
<br /><lrea wbere the propo~ed bridge si7,c in<:rease will allow gremer flows to follow the
<br />actual drainage way alignment, and limit the floVv'S from splitting out of the creek and
<br />draining to the north towards the CityofStcrling.
<br />
<br />Option 2 involves <ldding box culverts just south ofthc eXlstmg bridges in li<:u <)1
<br />widening or replacing them. Bascd on a cursory cvall.llitiol1, it was dct<:rmin~'d thllt four
<br />20' xWconcreteboxculvcrtswouldbctcquitedinlldditiontotheexistingbridges to
<br />convey the IOO-yearpcnkdischargc, ll\e culverts would bc plac<;d lldjaccnt to and south
<br />oftheexistingbridgcstru<:ture.
<br />
<br />4, Cross-section 21 was added in order to more <lccurately model the proposed
<br />tloodplainupsttellmofHWY6,
<br />
<br />l'lanDcscriptioH
<br />
<br />5, New bridges at HWY 6 and the UPRR w~te modeled. Constn'ction of the UPRR
<br />bridge was completed duting 1997. but downstrcmn channd restrictions prevented
<br />thebtidgetromusingitshydrauliccapacity,
<br />
<br />111" selccted pbn COIlSlM~ v:' ;::,:,mvemcnts ot two ~epar~l~ ~:::dli()ns. 11l<: first arc",
<br />designated as the "South lmprovcment Ate"" is the existing main channel ofPawncc
<br />Creek from the confluence of the creek with the South Plalle Riv<lr, extcnding upstream
<br />of the HWY 6 bridge. This impmvement arca also includes a proposed railtoad bridg<l
<br />nCllr the ToWll of Atwood, to allow l'a\\Tlcc Cteck rightllverbank flows lo.l drain directly
<br />to the river, The second improvemc~( area. designated ru; tbe "North lmprovernmt Ar<:a"
<br />is n",ar tile River:;ide Ccm<:lcry, wh~-re Hood prot<:ction will prevent Pawnec Creek icft
<br />overbank flows from being trapped wcst of tbe HWY 6 and railroad cmbankmcnts
<br />forcing flows into the City of Sterling. Each of tbe two improvements areas <lre discussed
<br />
<br />6. Downstream "f HWY 6, an emirely new HEC-2 model was prep;o"Cd and combined
<br />with tbe revised model upstream in order to determine the <l'fects of the gre<ltet flows
<br />rC,lChillg this location as a result oftbe proposed levees and larger hridge openings
<br />Tbe new model resulted in highet water surface elevations with more widespre<U!
<br />nooding. Therefore, a system of flood walls and lcvees were considered downstream
<br />of the UPRR brid>\c to confine the flood floVv'S dovmslrealll to the South Platlc River
<br />100-yeartloodplain,
<br />
<br />""M.~,,,l),h.,'''''..
<br />"~t'~~.~. u~.~,.
<br />
<br />The revised model WaB used to compute water surface elevations for the [0-, 50-, and
<br />IOO.ycar flood events under proposed projeeteonditions. The ptoposcd water surface
<br />elevations for the lOO-year event were used to determine the size and extent of the
<br />propowd floodwalls, leve.'s. and bridge opcnings, The tops of the proposed
<br />llwJwdlb ",,'; leve<:. "'crt' see al 3 leel "oo\'<: iI,t' lvv+}eal "'<ller ~w-f""e clevali,,1J ilt
<br />order to meet eurr<:m FEM^ criteria. Th<: bridg<: openings were si,.cd <lssuming a
<br />trapelA.liJal d"muel...:tiun with 3:1 siJt'slvp<:s. It w""ussumed thai the JOO-year
<br />discharge must be eonve>'cJ under the propos<:d bridges wirh a minil1l umofl foot or
<br />
<br />South hnprovcment Ar<lac B<lsed on discussions with the City, Logan County. CWCS.
<br />and COOT, and concerned citizens. it W<lS heli<:\'ed es,cntial tllal th<: pmjeet should t<lke
<br /><ldv<lI1Wgeoftllt'exisling drainage way to the degree possible. Cb=dcap<lcitynearthe
<br />ll\VY 6 bridges exceeds thehydrllulic capacity of the hridge. Since the railroad bridge
<br />was te<:ently rcbuilt with a greater hydraul;.; capadty, il \\'as dctcrmined that the highway
<br />hridge should be increased as well. CDOT is planning to upgrade tbe northbound bridge
<br />and work is scbcdule to br:gmm tbe next two )"ears. Costs for construction of the
<br />northhound bridge are nol a project co~t, sinee the wooden bridce is structurally and
<br />fllncliollally obsolde. and currently budgeted for replacement for those reasons,
<br />Improvements to L'Je southbound brid;:!e will indud", inst<llbtiull of supplemental box
<br />culverts Since this work would be done only to increase the hydra.ulic e<lp<lcit>'ofthe
<br />bridge, this cost is a portion of the proposed project. lbe proposed improvements for
<br />i'awn~'C Creck include increasing the hydtaulic capacity of the natural channel
<br />
<br />7. The model of the Pawnee Creek ov<:rflow area was u~cd unmodified <:xccpt that flows
<br />wereincre",edloref1ectthe5,OOncfslOO-yearewnl.
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