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1997 Toccoa Flood

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The 1977 Toccoa Flood
Report of Failure of Kelly Barnes Dam Flood and Findings

by Federal Investigative Board
December 21, 1977

CONCLUSIONS

While it is not possible to conclude the specific cause(s) of the dam failure, the investigations and available evidence support the development of certain probable failure hypotheses and at the same time throw out consideration of some possible modes of failure. Information received and records available indicate that the dam and/or spillways were modified, repaired, and/or extended over the years. Some witnesses referred to previous spillway and/or dam failure. These activities and the causes leading to them could not be specifically documented. However, documentation shows there was construction about 1939-1940. Additional construction or repair occurred about 1948-1949. The reservoir was essentially empty in January 1955 for some unknown reason. Reports were received that sandbags were used in the past to prevent overtopping of the crest. The crest elevation at that time is not known. A responsible source reported that the main earth spillway on the left abutment was constructed in 1965-1967 and sandbagging was not needed thereafter. This may have been an enlargement of the spillway since the 1955 aerial photos show a left abutment spillway. The 1949 and 1955 photos and the 1954 survey plot show two upstream inlet structures. Information from fishermen and visitors with recent knowledge of the dam and lake refer to only one structure. The conditions of these structures at the time of failure is undetermined.

The Board addressed its attention to any possibilities of failure and circumstances that could have contributed to the failure. These possibilities are addressed individually below.

Tornado. Severe storms occurred in the area including a small tornado the night of the failure, but the possibility of a twister in the area of the dam which could have caused trees to fall and open the fill to failure appears very remote. Inspections of trees in the vicinity of the dam indicated no tornado effects.

Animal burrows. No evidence was found in the remaining sections of the dam to indicate that animal burrows contributed to the failure.

Lightning. Although severe electrical activity apparently occurred prior to the time of failure, there is no evidence to indicate that lightning caused any structural damage.

Earthquake. A check with earthquake monitoring sources at the Georgia Institute of Technology, the University of South Carolina, and the National Earthquake Center in Denver indicates that the only disturbance in the Toccoa area was limited to thunderstorms.

Sabotage. There was no evidence of sabotage.

Overtopping. Site inspections and measurements of the high-water marks in the reservoir and the crest of the dam indicate that overtopping did not cause the initial dam failure.

Dispersive soil. Laboratory tests on foundation and embankment soils show that these materials are not dispersive clays, and did not cause the failure.

Liquefaction. Laboratory tests indicate that the materials, while having a low density, have sufficient cohesive strength and plasticity so that liquefaction was not a cause of failure.

The following mechanisms, conditions, failure modes, other than those above, were examined more extensively because of their more likely being factors contributing to the failure.

Piping. Piping through the embankment, and the subsequent removal of material from the embankment in the vicinity of the low-level spillway pipe, is a possible cause. If not the actual cause, it may have been a contributory factor. In addition, seepage in the area of the old root mat may have caused piping that contributed to the failure.

Structural collapse of Intake. Structural collapse of the low-level spillway intake structure, flashboards, and/or other parts of the structure would allow water to flow under full head into the partially collapsed and blocked pipe. This could have caused an opening of the pipe at a joint or weak spot resulting in further slope destruction on the downstream face.

Slope failure. This appears to be a distinct possibility, particularly on the downstream slope when the previous slope failure is considered along with the possibility of the development of tension cracks upslope of the previous failure together with a computed factor of safety that is marginal. The long period of rain would have saturated tension cracks, if they existed, and the entire downstream slope would have become essentially saturated and even more susceptible to failure. A local downstream slope failure similar to that observed in 1973 could have caused limited breaching allowing localized overtopping. This concept would corroborate the hydraulic computations.

Seepage. A local slide would have foreshortened the seepage paths and could have resulted in rapid erosion, and/or piping, or both.

Local Failure/Pipe Rupture. Since the outlet pipe was observed in the slide area in the 1973 photo, an additional similar slide could have further damaged the pipe sufficiently to cause internal piping and/or erosion that subsequently resulted in a collapse of the embankment. This concept would also corroborate the hydraulic computations.

Dam Condition. The dam was in generally poor condition. It can be concluded that in the vicinity of the breach, the dam was heavily vegetated and root systems extended deep into the embankment. The embankment slopes were steep. A pipe from a low-level spillway penetrated the dam in the downstream direction. In the vicinity of the pipe the embankment had experienced a local slope failure and seepage for some period of time. This pipe was reported to have collapsed during construction and to have been repaired at that time. The welded steel penstock pipe, exposed by excavation after the dam failure, had previously collapsed, ripped, been repaired, and apparently partially collapsed again. Other contributory factors that cannot be disregarded was a design, or lack of same, that included a rock crib dam within the earth embankment, together with several feet of lake sediment and old roots, open ended spiral riveted pipe in the earth embankment, a concentration of three pipes in a localized area, pipes through the dam that were not permanently and/or only partially plugged, and a lack of repair of local slope failures.

The Board could not determine a sole cause of failure. It does conclude that a combination of factors caused the failure. The most probable causes are a local slide on the steep downstream slope, probably associated with piping and/or an attendant localized breach in the crest followed by progressive erosion, saturation of the downstream embankment, and subsequently a total collapse of the structure.

/s/ROBERT L. CRISP, JR., Chairman, U.S. Army Corps of Engineers, Department of the Army
/s/WILLIAM E. FOX, National Weather Service, Department of Commerce

/s/ROBERT C. ROBISON, Soil Conservation Service, Department of Agriculture
/s/VERNON B. SAUER, U.S. Geological Survey, Department of the Interior

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