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<br />Description of Flooding Problems <br /> <br />Major flood events have occurred on Pawnee Creek in Ma.y 1935, June: 1965, and July 1997. Minor <br />flooding occurred in 1967, 1990 and 1992. Flows in 1935 d.amaged the railroad betwet:n the City of <br />Sterling and Town of Atwood. Several hundred yards of HWY 14 we:st of Sterling were destroyed, <br />and approaches to the Sterling bridge north of Atwood, HWY 6, and the: bridge and tracks of railroads <br />at tile Pawnee bridge were also damaged. Discharge estimates from the 1935 event are not available, <br />but the Pawnee Overflow did operate sending flows into Sterling. <br /> <br />Flooding occurred on June 14 and 15 of 196$ as a result of heavy rainfall on the Pawnee Creek <br />watershed. Considerable darnage occurred throughout the lower reaches of the watershed. South of <br />Sterling, near Town of Atwood, flood water ovel:toppt:d HWY 6 and the railroad tracks. Pawnee Creek <br />flood waters also flowed in a northeasterly direction along HWY 6 and inundated portions of the <br />we,tern section of Sterling. A p,~ak flow estimate: of 35,200 cfs was indirectly measured at the <br />Colorado Highway 14 bridge approximately 10 miles upstream from Town of Atwood. The estimated <br />Pawnee Overflow was 5,000 cfs. <br /> <br />During the week of July 28, 1997, heavy rains eaused significant flooding in several areas including <br />the communities of Fort Collins, Weldona, Town of Atwood, and Sterling. Rainfall over the Pawnee <br />Creek watershed on the evening of July 29th saturated agricultural/range lands and caused major storm <br />runoff in the tributaries and main channel flood wah:rs in Pawnee Creek crested in Town of Atwood at <br />approximately 3 PM on July 30. Flood waters from the Pawnee overflow first reached the south side <br />of Sterling at about 4 PM, and then crested in th.e downtown area around midnight on July 30th. Point <br />rainfall amounts of up to 15 inches in a 4 to 8 hour time period were reported. in the upper watershed <br />areas of Pawnee Creek. Estimated rainfall depths over the drainage basin are shown on Figure 3. <br />These depths were based upon a rainfall bucket survey conducted by the CWCB, Corps of Engineers, <br />Omaha District, and Logan County staff. The peak flow estimate on the mainstem above Colorado <br />HWY 14 is significantly higher than the 1965 peak flow estimate. Based upon field surveyed high <br />water marks, flow estimates were made using indirect flow measurement techniques and are presented <br />on the following table. <br /> <br />Table 4. Peak Discharge Estimates - 1997 Jilood <br />Pawnee Creek and Pawnl~e Overflow <br /> <br />Location .' <br />Total Pawnee Overflow into Sterling just <br />northeast of the Riverside Cemetery <br />Portion of Pawnee Overflow into Sterling <br />Between the Riverside Cemetery and the Railroad <br />Embankment <br />Pawnee Creek Mainstream u/s ofHWY 14 --- <br />Pawnee Creek mainstem at CR. 25 <br />Pawnee Creek mainstem at CR. 29 <br /> <br />Estimated Peak Flow (efs) <br />3,800* <br /> <br />2,500 <br /> <br />65,000 <br />50,000** <br />45,000** <br /> <br />* Assumes that the Sterling No.1 Ditch is full prior to Pawnee Overflow peak discharge. <br />** This value is less than the upstream value due to valley storage and flow attenuation. <br /> <br />1II-2 <br />