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<br />SECTlONO NE
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<br />InUOductlon and Backuround
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<br />This pcak discbarge was split between 3 separate overflow areas along Highway 6: I) overflow
<br />lit Alwood (approximately 20,500 efs); 2) the main stem bridg<: and n:lief culverts
<br />(apPwlIimaldy 2,300 ers); and 3) the PaYme<: Creek overflow at Riverside cemetery
<br />(approxirnately 9.300 cfs total, 3,800cfswruch was conveyed east inlo Sterling ).
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<br />1.2 PURPOSE OF ANALYSIS
<br />A review of the bydrologiclhydraulic (H&H) analysis for Pawnee Creek was perfonncd at the
<br />request ofFEMA under Task Order No. 239.- The purpose of the review was to evaluate and
<br />provide additional H&H analyses for the proposed.mitigation alternative to determine pre- and
<br />post.projecl oonditions that can be utilized to facilitate the detennination ofimpac ts as part of the
<br />Environmental Assessment (EA). The EA is being prepared in compliance with the National
<br />Environmental Policy Act (NEPA) to evalllllte alternatives proposed to alleviate flooding in the
<br />City of Sterling and Logan County, Colorado.
<br />Historical flooding on Pawnee Creek has caused significant damages to both agricultural lands in
<br />Logan County and residential structures within Logan County and the City of Sterling. The most
<br />recent event occurred on Pawnee Creek in 1997 and agriculrural land and residential s tructure5
<br />were severely impacted. During this event, flood water was conveyed by the Pawnee Creek
<br />Overflow into the City of Sterling caused extensive damage to residential structures, roadways,
<br />and property, with damage estimates at approximately $19.1 million (ICON 1998). As a result
<br />of the flood, a proposed mitigation alternative WdS proposed to reduce the potential for flooding
<br />and theassoeiated d:unages and impacts to both the City of Sterling and agricultural l:mds within
<br />Logan County. The proposed mitigation alternative generally consists of upgrading aud
<br />constructing new conveyance facilities at the State Highway 6 and UPRR, whieh both traverse
<br />the Pawnee Creek floodplain just north of the South Platte River. Itisestirnatedthat
<br />approximately 4,482 people and $118 million worth of property would be protected as a result of
<br />tlle proposed mitigation alternatives (ICON 1998).
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<br />1,3 TASKS PERFORMED
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<br />This analysis consisted of Ibrec tasks: site visit, data collection, and H&H evaluation. These
<br />t~.harerletailedhelow.
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<br />Task t Site Visit _ A site visit was perfonned as part of the EA to verify existing conditions and
<br />collectadditionaJdata. The site visit consisted ofa pedestrian survey, discussions with loca1
<br />rcprescnlatives, and data collection.
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<br />Additional data necessary to perform the H&H review and evaluations were obtain<.-d from
<br />\'arioussourccs. The information obtain<.-d included:
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<br />. Historiealllnodinfomlation:
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<br />. H&H information generated b;.thc HMUP applicilllt:
<br />. D<.'signplansandsurveyinfomlationforpropos~-d.ncw.andcJ\l,lillgstru":ltifes"nd
<br />wa!'~rv.'ays;
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<br />. Crossse..:lioninforrnatio'l:and
<br />. Existing topographi..: mapping of the drainage area and lloodplains,
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<br />URS Gn>lne' Woodward Clydo
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<br />SECTlONONE
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<br />IntroduCUIID and BackDround
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<br />The infornlation provided by the HMGP applicant was reviewed for the existing conditions and
<br />proposed flood mitigation alternatives. Other existing H&H evaluations were also reviewed as
<br />purl of this analysis. The original hydrologic models prepared by the HMOP applicant were
<br />modified to furthcr evaluate the future conditions. Impacts ll.Ssoeiated with the propo,cd flood
<br />mitigation alternative and reconunendations for future additional evaluations were made.
<br />Available topographic mapping wa~ utilized to prepare a work map to show appro"::;:ulll:e pre-
<br />and post-cunstnKtlon impacts. ;Il\pactsge::.~rallyincludede..",:~::.;r;uodingdepthsand
<br />floodplain boundaries upstream of Highway 6.
<br />To the extent rossible, the information developed by the HMOP appliclInt was utilized in the
<br />evaluation of the H&H [~~ the proposed alternative. Two additional preliminary hydtaulic
<br />models were developed to mood L':.e extent offlooding and the associated impacts in the vicinity
<br />ofthc TOml of Atwood and at the Riverside Cemetery. These models evaluate the effects of
<br />proposed flood mitigation aller:'1.tive components p.'Oooscd for those areas. These models,
<br />w},,(h were not submitted with the orilSi~ HMOP application, were prep...::! as part of this
<br />study in order to adequately detennine pre. and post.project flood inundation impae !s.
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<br />1.4 EXISTING FLOODPLAIN MAPPING
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<br />FEMA Flood Insurance Studies (FIS) M' e b~,<,n prepared for both Logan County (Community
<br />Number w'n I 0) and tl'.c City of Sterling (Communilj' Number 080294). Flood Insurance Rate
<br />Maps (FIRMs) exis: fur both Pawnee Creek and the Pawncc C.-=",k overflows. Pawnee Creek
<br />and the Atwood overflow paths are mapped by approximate mcthods 01.1)' md flood hazard areas
<br />",... identified a.~ eithe, Zo~e A or Zooe X. The Pawnee Creek Overflow into Slo,;:!ing is mapped
<br />by detaJieci llIcthexis and flood ha:l'.ard areas are identified as either Zone AE, Zone AH Ot Zc:!"
<br />AO. 1be IOO-year floodplain mapping ofPu\-\nee Creek is shown with the floodplain mapping
<br />of Sand Crcek throughthc City of Sterling and the two floodplains overlap. )be only published
<br />flow rates in the effective FIS are for the Pawnee Creek overflow into Sterling, The 100-ycar
<br />peak flow talC at the Pawnee Creek overflow is 5,000 cfs.
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<br />UR5 Greiner Woodw..rd Clydll
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