CAUGHT ON TAPE - THE FAILURE OF KADASHAN BOTTOMS SITE 2

摘要

The auxiliary spillway on the NRCS flood control dam, Kadashan Bottoms, Site 2 in Wagoner County, Oklahoma, experienced severe erosion during a prolonged flow event following a series of heavy rains in April, 2008. The headcut that formed during the event progressed upstream past the crest of the auxiliary spillway resulting in a technical breach. The erosion is estimated to have removed about 13,000 cubic yards of soil. The entire event started sometime around 6:00 AM and was over by 5:00 PM. First responders recorded the event on video. The auxiliary spillway was designed using bulk length criterion, contained in NRCS Technical Release 52, which was in effect at the time of design in 1977. That criterion indicated that the spillway would perform satisfactorily; damage was likely, but the site would not breach. New technology has been developed that has changed that criteria. The SITES model which uses that new technology, had it been available when this site was designed, predicted a breach when anatyzed with the vegetation in place during the flow event. Technically, the failure resulted from a loss of integrity in the soils in the exit channel of the auxiliary spillway resulting in a headcut that migrated through the entire reach of the spillway. Contributing factors to the failure of the auxiliary spillway were the vegetative cover, small brush growth in the auxiliary spillway, and an apparent lack of understanding of the operation and maintenance requirements and the importance of maintaining adequate vegetation. Post failure evaluation revealed that the vegetative cover that was originally specified, well maintained Bermuda grass, would likely have limited the damage. However, bunch grasses were later planted and although well established on the date of the failure did not have the same integrity as the Bermuda grass. In addition, small discontinuities in flow caused by brush growth in the exit channel of the auxiliary spillway created enough turbulence to start the erosion process. This paper describes the events leading up to, during, and after the failure and describes how technology has evolved in the design of structures today to help analyze the potential and hopefully prevent such failures in the future. Many factors, acting together, resulted in a potentially catastrophic event - caught on tape.