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<br />Flow widths for the surveyed cross-sections across the Pawnee Creek floodplain ranged from <br />about 1,600 feet at the upstream end, to several thousand feet at the downstream end, The flow <br />depth in the confined channel at the upstream cross-section was just over II feet, while the flow <br />depth closer to Atwood was much more wide and shallow, with depths on the order of 4 feet. <br /> <br />A landowner who witnessed the entire flood flow on Pawnee Creek several mile upstream of <br />Atwood (and downstream of Colorado Highway 14), was able to assist the team in re-creating a <br />flood hydrograph. <br /> <br />IV. FACTORS THAT INFLUENCED THE PAWNEE CREEK OVERFLOW <br /> <br />The City of Sterling lies approximately 4 miles northeast of the conflut:nce of Pawnee Creek and <br />the Sout.h Platte River. The community of Atwood lies about 1.5 miles to the southwest of the <br />confluence. The Atwood area is located within the natural floodplain of Pawnee Creek, however <br />Sterling is not. The majority of flood flows from the Pawnee drainage into Sterling are mainly <br />due to man-made obstructions. The U.S. Highway 6 embankment and the Union Pacific <br />Railroad embankment run parallel to each other along the west side of the South Platte River. <br />These two embankments act as diversion structures because flood flows on the Pawnee are not <br />able to pass through the existing bridges and over the embankments directly to the South Platte <br />River. When flow in the Pawnee is high enough, a portion of the flow is diverted to Sterling <br />along the embankments toward the northeast. This was the case in 1935, 1965, and 1997. This <br />situation is known as the "Pawnee Overflow." The estimated overflow in 1965 was 5,000 cfs, <br />while the estimated overflow in 1997 was 3,800 cfs. <br /> <br />Existing floodplain mapping for Sterling indicates that the Pawnee Overflow would travel along <br />the highway and railroad embankments toward Sterling, and would then flow toward the western <br />side of Sterling, rather than the eastern side. The assumption of the westerly flow was based on <br />available documentation for the actual flood event of 1965. During the 1997 flood event, the <br />Pawnee Overflow continued in a northeasterly direction and inundated most of eastern Sterling, <br />including the downtown area and central business district. The flooded portion of Sterling in <br />1997 was generally mapped in the FEMA 500-year floodplain, while the unflooded portion of <br />Sterling was generally mapped in the FEMA 100-year floodplain. There are some qualitative <br />explanations as to why the 1965 flooded area (and the mapped FEMA floodplain) is different <br />than what actually happened in July of 1997. The explanations are summarized below. <br /> <br />. Development has occurred on the southern end of Sterling that has changed the topography <br />and the drainage patterns. <br /> <br />. The 1965 flood occurred in mid-June when the corn stalks were not very tall. The 1997 flood <br />occurred at the end of July when corn stalks were over 6 feet tall. Hundreds of acres of corn <br />fields south of Sterling diverted water by acting as barriers to the flow. Debris, hay, and <br />other materials were caught in the corn fields, thus diverting water toward the northeast. <br /> <br />6 <br />