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<br />The Pawnee Creek/South Plalle River confluence is located approximately 4 miles southwest of the
<br />City of Sterling along U.S. Highway 6 (HWY 6) and the Union Pacific Railroad (UPRR). During
<br />large storm Cl't:ots. storm water overtops the banks of Pa"'nee Creck and flows into the City. These
<br />overflows a", estimated to be 5,000 cf.< during thc I nO-year stonn. This study focuses primarily 011 the
<br />reilch along Pawnee Creek from its conlluence with the South Platte River upstream to Coumy Road
<br />29 (CR. 29), a distance of about 7.5 miles. The Vicinity Map (Figure 2) presents (he lower ponion of
<br />the pa....llee Creek Ba.~in with respect to the Cily of Sterling. The Pawnee Creek Basin Map (figure 3)
<br />shows the overall Pawnee Creek Drainage Basin. along with a tabulation of pertinent basin
<br />eharaclcristic~.
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<br />Previous Report.
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<br />Several studies have been completed to investigate the existing flooding conditions in the City of
<br />Sterling and surrounding Logan County. Some of these studies address alternatives for mitigation to
<br />the floodiTlg problems. Tho City of Sterling is subject to flooding from four major sources which
<br />include the South Platte River, Pawnee Creck, Sand Creek, and the Pioneer Drainage Ditch, One
<br />recent study, "City of Sterling Drainage and Flood Control Implementation Plan" datt'(] June 1997, by
<br />ICON Engineering, Jne. concluded that addressing the flooding problems associated ....ith the Pawnee
<br />Creek should he given the City's highest priority, and would result in the most feasible and economical
<br />plan of improvements l<lr the area. As a result oftlooding events that occurred in 1997 and discussed
<br />in detAil within this report, the City and County have dclernlined that resolution of the Pawnee Creek
<br />drainage prohlems must be appropriately implemented. 1bc follo>Ving reports were reviewed as a part
<br />MthisslUdy:
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<br />RclcreneeReport,:
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<br />I. Flood l-Ia7,ard Mitigation Reconnaissance Study Pawnee Creek Basin, by ICON Engineering. Inc.
<br />dute<lAugust2000.
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<br />2. City of Sterling Drainage An,t Flood Controllmplcmenlation Plan, by ICON Engineering, Inc.
<br />dakdJunel997,
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<br />3. Pawnee Cn.'ek . Logan County, Col"rado: Flood of July 1'1'17 (Oeotl) hy ('olorado Wate,
<br />Conservation Roard.datcdOctobcr 1997.
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<br />4. Flood Plain Infonnation and Drainage Pb.Tl for Sand Creek and the Pawnee Creek Overflow at
<br />Swrling. Colorado by Charles F. Leafmd Resource Consultanls, ]nc.. oak-d May 1983.
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<br />5. Salld Creek Watershed. Sterling, Colorado by the U.S.D.A. Soil Conservation ServIce. dated
<br />AUgu~t 1984.
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<br />0, Summary Report, Flood Plain InformatllJll for Sand Creek and the P,lwne,. Creek Overt1ow at
<br />Sleriin~. l <~jorado hy Ch~rles F. [.eaf and RC'sollrce Consultants, Inc.. dated January 19~7.
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<br />7. I'colK'sed Near-Ierm [mpwvcments and Opt'ratk>n of Pioneer Drainage Dllch. SterllOg. ColoradD
<br />hy Ch"rlesF. [,eafand ResourccConsultallls.1nc.. diltedJuly 198~.
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<br />H. Proposed Near-Tenn Improvement and Operation of Pionccr Ditch, Sterling, Color~do, "Summary
<br />Report" by Charlcs F. Leaf and Resource Consultants, Inc., dat~'<i July 1988.
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<br />9. Flood Insurance Study. City of Sterling, Colorado. Logan Counly h}' Federal Emergency
<br />ManagementAgeney,datedSeptember29.19H9,
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<br />10. Flood Plain Management Study, Pawnee Creek in Logan County. Colorado by U.S. Department of
<br />Agriculture, Soil Conservation Service, dated April 1992.
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<br />11. Section 22 Planning Assistance. Major Drainageway Plan Pioneer Ditch at Sterling, Colorado by
<br />U.S. Artny Corps of Engineers. dated Deeembt:r 1995.
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<br />Ilcscription of Flooding I'roblems
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<br />Maior flood events have occurred on Pawnee Crock in May 1935, June 1965, ilTld July 19<)7, Minor
<br />floodingoceUfred in ]967, 1990 and 1992. Flows in 1935 damaged the railroad between the City of
<br />Sterling and Town of Atwood. Several hundred yards of HWY [4 west of Sterling were destroyed,
<br />and approaches to the Sterling Bridge north of Atwood, HWY 6, and the bridge and traeks of railroads
<br />at the Pawnee Bridge were also damaged. Discharge estimates from the] 935 event are not available,
<br />but the Pawnee Overflow did operate sending flows into Sterling.
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<br />Flooding occurred on June 14 and 15 of 1965 as a result of heavy rainfall on the Pawnee Creek
<br />watcrshc-d. Considerahle damage oecurred Ihrr1\lghoUl the lower reachcs of the watershed. Soulh of
<br />Sterling, near Town of Atwood, Hoodwater overtopped lI\VY 6 and the railroad tracks. Pawnee Creek
<br />tJoodwaters also flowed in a northea.,terly dinxtion along HWY 6 and mundated portions of the
<br />western section of Sterling. A peak flow estimate of 35.200 cfs W:l>; indirectly measured at the
<br />Colorado Highway 14 Bridge approximately 10 miles upstream from Town of Atwood. The estimated
<br />Pawnc<:Ovcr[J"wwas5,OOOcfs.
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<br />During the wcek of July 28, 1997, heavy rains caused significant flo"ding in ~ever.tl areas including
<br />the communitie~ of fort Collins, Weldona. Town of Atwood. and Sterling (See Figures 4 and 5 for
<br />Ph"tos) Rainfull over the Pawnee Creek watershed on the evening of July 2<Jth saturated
<br />aer;r"Ir"",llr~ng~ !~nds and <:aused majar ~torm ;"t!n0ff in the tribll(,J.n:::; ;:.,'l.;! m<lin eh<l,'l,'ld fkmdw"tc;.,
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<br />in Pavmee Cn..'ek crested in Town of Atwood at "pproximately 3 PM on July 30. floodw"ters from the
<br />Pawn~'C ovcrtlow first reaehed the south side of St..Jrling at about 4 PM, and then erestcd in the
<br />downtoWtl arcaaround midnight "n July 30th. Point tainfall amnuntsofupto 15 inehesina4t08
<br />hour lime period were rcjl<Jned in the upper watershed areas of Pawnee Creek. Estimated raintall
<br />d<,;pths over the drainage basin al'eshowll "n Figure 6. Thcsedepths were ba.o;ed upon a rainfall buekCl
<br />survc'y eonductcd l1y the CWCll. Corps of Enginecrs, Omaha District, and Logan Counly staff. The
<br />peak tl11w estimatc on the mainstem above Colorado IIWY 14 is significantly higher than the ]965
<br />T'<'ak !lowestirna.te. l3a:>c-d upon field.surveyed high water marks, flow estimates were made using
<br />indircct !l0w measur<'m<."nt techniques and ~re prcs<,;nted on the (o!l,)wing t!!hle.
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