A Dam Incident at Sugar Creek L44

Sugar Creek Site L 44 and many other sites in Caddo and surrounding counties received 8 to 10 inches of rainfall within 3 to 4 hours on Sunday, August 19, 2007. Rainfall amounts were reported to exceed the 100-year rainfall for 24, 12, and 6-hour storms and the 500 year rainfall for a 3-hour storm. The rainfall led to very high auxiliary spillway (ASW) flow, erosion of the inside training dike, erosion of the ASW down to the underlying bedrock, erosion of the downstream toe of the dam near the ASW outlet, and instability of the embankment. The Emergency Action Plan was activated in response to the incident and demonstrated the effectiveness of the NRCS, State Conservation Commission, Conservation District, and local authorities. The embankment instability resulted in subsequent slope failure, which ultimately proceeded through a portion of the crest of the dam by the evening of August 19, 2007. A road embankment and culvert with a history of plugging is located immediately downstream. An Engineering Report has been completed by an investigation team consisting of two engineers and a geologist from the Natural Resources Conservation Service. This paper will present the findings of the Engineering Report which includes a brief chronological description of the project planning, design, construction, and site history; the effect of the downstream channel and road embankment on the tail water; tail water effects on the downstream slope of the dam, ASW, and ASW training dikes; and the effect of the bedrock geology in the ASW had on the erosion of the training dike and the downstream slope of the dam. A subsequent paper has also been submitted that will detail the response to this Dam Safety Incident. 13 pp., 10 figures, 5 references.