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<br /> <br /> <br /> <br />7 <br /> <br />Figure 5 – CFD Model Estimated Stream Power <br /> <br />Case Study: Sugar Creek L-44 <br />Sugar Creek L-44 is a 550-foot long embankment dam <br />constructed in Southwest Oklahoma in 1971. The dam <br />is 64 feet high with an auxiliary spillway width of 40 <br />feet. In August 2007, the area around the dam <br />experienced 8 to 10 inches of rainfall in 3 to 4 hours, <br />exceeding the 100-year, 6, 12, and 24-hour event as <br />well as the 500-year, 3-hour event. The flow through <br />the spillway during the rain event was estimated by <br />SITE analysis afterwards to be 740 cubic feet per <br />second. The flow resulted in erosion of the inside <br />training dike, the spillway down to the underlying <br />bedrock, and the downstream toe of the dam near the <br />spillway outlet (causing embankment instability). The <br />event also washed out a county road 300 feet <br />downstream, inundated a house, and caused activation <br />of the Emergency Action Plan. <br /> <br />Figure 6 – Sugar Creek L-44 Spillway and Dam Erosion <br /> <br />A total of 38 auxiliary spillways flowed in the region <br />due to the rain event but Sugar Creek L-44 was the <br />only site in the area with damage to the embankment. <br />One other site only incurred damage to the spillway. <br />Numerous factors led to the erosion at L-44. The <br />spillway had vegetated silty sands at the surface <br />underlain by sandstone bedrock that dipped towards <br />the embankment. The downstream road had an 18- <br />inch pipe and riser to pass flows that had been <br />reported difficult to keep clear of debris. The flow was <br />observed to be 3 feet over the roadway during the <br />event and led to backing up of water onto the lower 27 <br />feet of the auxiliary spillway. This increase in tailwater <br />at the bottom of the spillway and toe of the dam was <br />determined to have decreased the stability of the soils <br />and increased erosion. However, erosion would have <br />occurred without the increase in tailwater due to the <br />orientation of the spillway. Prior to construction, the <br />design centerline of the spillway was shifted 95 feet <br />towards the embankment, the channel was rotated 9.5 <br />degrees toward the embankment and the exit channel <br />slope was increased from 7.5 degrees to 9.75 degrees. <br />All of the design changes increased the flow velocities <br />on the spillway and at the toe of the dam. Had the <br />tools described above been available to analyze the <br />erosion capabilities of the increased flow on the silty <br />sand material, the design changes may not have been <br />made. <br /> <br />Figure 7 – Sugar Creek L-44 Plan of Erosion Damage <br />