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<br />SECTlONTWO
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<br />Pruuoscd MfUuaUon AhernaUvo
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<br />SECTlONTHREE HYdrologlc/HYdrau"C 10Vlew 0' nood Mlllga"oUAlt1lmaOVos
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<br />As the result of tile historical flooding along Pawnee Creek. flood mitigation alternatives wen:
<br />proposed as part ora H=d Mitigalion G=I Program (HMGP) application submitted by the
<br />City of Sterling and Logan County, Colorado. In conjunction with the Colorado Waler
<br />ConselVation Board (GWeB), the communities of Logan County and the City of Sterling have
<br />requested funding from FEMA to implement the proposed flood mitigation alternatives.
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<br />Several altematives were evaluated as part oflhc HMGP application. These original set of
<br />alternatives included levees and conveyance system improvements. The proposed flood
<br />mitigation ultrntalive inc!udesconstruction of water conveyance Slructuresa t Highway 6 and the
<br />UPRR The applicant has selected a preferred altemative, which would generally limit the
<br />backwatercTCatcd by the road and railroad embankments. Alternative 5, as presented in the
<br />HMGP application, was determined to be the preferred n1temative for flood mitigation. This
<br />altemativeconsists of the foUowing:
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<br />I. A new 5,000 cfs convC)'ance structure for the Atwood overflow at the Highway 6 and UPRR
<br />crossings (currently, no conveyance Slructures are present); ,
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<br />2. Upgrading the existing conveyance structures at the main stem to pass approximatcly
<br />8,200 cfs at both the Highway 6 and UPRR crossings and a proposed levee downstream of
<br />the UPRR designed to protect adjacent houses from flooding up to the SO-year frequency
<br />floodevcnt;
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<br />3. A ptoposed lcvee (5 feet high) and floodwall at the Pawnee Creek overflow at the Riverside
<br />Cemetery; lowering of County Road 24; a toad closure structure at Highway 6; a new
<br />5,000 cfs conveyance structure at the UPRR crossing; a floodwall between the Highway 6
<br />road closure and UPRR conveyance structures; a new 5,000 efs trapezoidal flood channel to
<br />the South Platte RiVCl'; and a siphon structure for the Sterling No. 1 ditch. Theproposed
<br />project components do not include a conveyance Slructure beneath Highway 6up strearnof
<br />the proposed UPRR culvert location. The feasibility study indicates that a road closure
<br />structure would bc iru;talled and that the roadWllY would overtop during eXU'eme flood event s.
<br />The road closure consists ofa manually installed barrier between the levee and floodwalls
<br />across Highway 6.
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<br />4_ Channel clearing, debris removal, and selected slllbilization measures on the Pawnce Creek
<br />main stem from Highway ~ to County Road 29.
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<br />The peak dischMges utilized for the design of the proposed structures were based on information
<br />from existing hydrologic studies lIIld peak discharge measurements madef orsignifieantnouds
<br />occurringinI935,1965.andI997.
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<br />3.1 HYDROLOGY
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<br />The current effective FIS's for the communities of Sterling and Logan County were bas.'o:! ;,,1 the
<br />hydrologic evaluation~ performeu bjt th~ USACE (USACE, 1<l7~). Ille basis for the hydrologic
<br />study was a Massachusetts Institute of Technology Hydrologic Catchment (MITCA 1)
<br />hydrologic model of the upstream watershed. The peak IOO-yeardischarge for Pawnee Crttk at
<br />Hig,hway 6 and the Pawnee Creek overflow to Sterling are reported as approximately 41,000 ern
<br />and 5,000 cfs, respectively.
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<br />Detailed H&H analysis for the Pawnee Creek overl1ow lIIld the main stem were also performed
<br />by the Natural Resources Conservation Service (NRCS) (1992). The basis for this hydrologic
<br />analysis is a TR-20 model of the upstream watershed. The peak lOO-year discharge for Pawnee
<br />Creek at Highway 6 lIIld the Pawnee Creek overflow to Sterlul:; <<rc l"i>C~~ as approximately
<br />10,5IScfsand4,300cfs, respectively.
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<br />For comparativepurposes,tegional regression equations Wt:rc also utUized to estim atepcak
<br />lOO-year frequency discharges (CWCS, 1976). The regression equations estimate a lOO-year
<br />discharge I.t Hil!:hway 6 (approximately 645 square mile drainage area) to be approximately
<br />46,500cfs.
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<br />Peak dischMges determined 1.>) ~i'" various me:.hods at several locations within the watcrshed are
<br />;;;"":,,_"Nri7.cd ":l T..1::>le 3-1. As can be se"" :ivm the variOu.~ Tesults, significant differences in
<br />peak lOO-year frequ<'ncy discharges ha,;: ~~Ml T..~rted_ As such, design Jis:l1arges were
<br />~elected for the propo~~d mitigation alternatives based on a combination of the diS<.;;~'l"2es
<br />tabulated. SincethePawneeCr~...,khydrauliesconveyanceeffeetsthehydrologyinthis
<br />particular situation, it is appropriate 10 scleet design discharges whieh refle ct the changes in
<br />hydrologyassociatedwithhydraulicconveyanct'.
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<br />3.2 HYDRAULIC EVALUATIONS
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<br />Hydraulic evaluations wen: performed by the USACE (1978) and NRCS (1992). Current FEMA
<br />floodplain mapping presents detailed information only for the Pawnee Creek Overflow based on
<br />the USACE hydraulic analyses. The HEC-2 hydraulic model developed by the USACE was not
<br />available at il,,,, time (If ..his study. The NRCS analysis u.dudcd hydrc.:.:lic a.'la!yses oft.lJe
<br />overflow at Atwood, the main stem Pawnee Creek and the Pawnee Creek Overflow to Sterling,
<br />As such, the NRCS models werc utilized by the HMGP applicant in the analysis of proposed
<br />mitigation alternatives. Additional hydraulic modeling using the NRCS HEC-2 cross sections
<br />waspetformcdaspartofthisstudytodetennincful'lherimpaetscrcatedbytheproposed
<br />mitigation strucrures for a range of flow discharges as presented in Table 3-1. Appemllx A
<br />inc1udes output from the pre_andpost_projecthydraulie mode1s of the are c.sevaluated.
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<br />3.2.1 Pawnee Creek Main Stem
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<br />11,,: hydr<lulics of the main stem of Pawnee CIcek werc evaluated Ulilizing lbe HEC.2 model
<br />prepare.! as part ofthc NRCS study and modified by the HMGP applicant. The proj'Osed
<br />mitigation plan calls for incrcasing conveyance through the eltisting Highway 6 and UPRR
<br />crossings. It is our understanding that the Colorado Department ofTransportltli\1n (COOT) is
<br />currently petforming preliminary designs of the Highway 6 structures and that the UPRR has
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<br />(IRS Grolner Woodward Clyde
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<br />(11($ Gremqr Woodward Clyde
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