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<br />SECTlONFIVE
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<br />Conclusions
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<br />Ingeneral,theproposedrnitigulionallerrultivcappeursadequatetoreduec:f100dingdc:plhslUld
<br />extents due: 10 extreme: events associated with backwater from the Highway 6 and UPRR
<br />embankments. The proposed alternative would substantially reduce the conveyance of flood
<br />waters via the Pawnee Creek overflow into the City of Sterling. However, upon final design of
<br />all proposed mitigation components, refinements to the H&H modeling should be perfonned to
<br />incotporate details ofthc: final designs. Additional detalled topographicll1!\PPing is required to
<br />more llCcUl'lltely determine pre- and post-project flooding extents. Appendix B provides a work
<br />map of approximate pre-and post-project flooding l\rl:as (based on the design event). The
<br />overtopping of Highway 6 for the Riverside Cemetery overflow under the proposed mitigation
<br />alternative sbould be re-<::valuated to include flood warning measures and possibly a low flow
<br />conveyance structure below the roadway. In addition to the proposed railroad eulverts.
<br />instailation ofaconveyancc structure undcrHighway 6 would be noccssarylO convey the design
<br />flow of5,000 cfsto the South Platte River to avoid any road overtopping.
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<br />5.1 USE OF HYDROLOGICfHYORAULlC MODELING CONCLUSIONS FOR
<br />FLOODPLAIN MAPPING
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<br />Thc information collected and presented herein is not adequate to revise the effective FIRMs.
<br />The H&H modeling evaluated and prepared as part of this study should be further revised based
<br />on final dcsign of the hydraulic struetures proposed. The hydraulic modeling was based on the
<br />best available information at the time of this study, however thc information is limited in dctail.
<br />Many of the hydraulic structures proposed have only ooeneoneeptuallydesigned.
<br />In addition, the available hydrologic information indicates'alarge di fferencein ealculated
<br />dischatge.frequency relationships. The H&H information used in the existing FEMA floodplain
<br />mapping studies is limited. Further, more detailed topographic information is required in order
<br />to generatc detailed flood maps for the Atwood overflow and the Main Stcm Pavmee Creek.
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<br />URS GreIn..,. WoodW3rd Clyde
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<br />SECTlONSIX
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<br />Beferencesand Previous Stu[fv
<br />
<br />The follo.....;ng studies related 10 the hydrology and hydraulics of the Pawnee Creek watershed
<br />were teviewed as part of this evaluation:
<br />
<br />CWCB. "Pavmce Creek., Logan County, Colotado, Flood of July 1997". Prepared by Thomas
<br />Browning,P.E. Octoberl997.
<br />FEMA. "Flood Insurance Study, City of Sterling, Colorado, LIgan County". Community
<br />Number 080294. Effeo;tive Date: September29, 1989. (Includes Flood Insurance Rate
<br />Maps. City ofSteding, Cnlorado, Logan County. Panels 080294 0001-0002.)
<br />:cMA. "Flood Insurance Study, LIgan County, Coio,;u!t-' (Unincc~ted Areas). Community
<br />Number 080110. Ff:::ctive Date: September 29, 1989. (Includes Flood Insurance Rate
<br />Md.;;:, L02'a" Cuunty, Colorado. Pati::ls 080110 0245, 0325, 0330. 0335, 0340, and
<br />0345.)
<br />
<br />ICON Engineering, Inc. "Flood Hazard Mitigation Feasibility Study, Pawnee Creek". (HMGP
<br />Application for City of Sterling and Logan County, Colorado.) Prepared for the City of
<br />Sterling, L<:lgan County and the Colotado Water Conservation Board. February 1998_
<br />
<br />U. S. Department of Agriculture, Natural Resources Conservation Service (formerly Soil
<br />Conservation Service)_ "Flood Plain Management Study, Pawn~;; Creek it; UJg~n
<br />County, Colorado"_ Prc'parrrl for tbe Co!"t:'.<:!" ~y;::c. Conservation Board, Logan COUnty
<br />andti,~TownofStcrling. April 19;:.
<br />
<br />U. S. Department oflbe Army, Corps of Engineers (USACE). "Flood Plain Information, South
<br />Platte River and Pawnee Creck, Sterling, Colorado. Prepared for Logall County and the
<br />Colorado Water Conservation Board. September 1978.
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<br />URS Grol"er Woodward Clyde
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