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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


1. General. Site investigations consisted of detailed visual inspections and photographs of the dam site, soil sampling, excavations of portions of the remaining embankment, plane table surveys of the site, and investigation of the debris downstream of the dam by the Board members and data gathering teams composed of personnel from all the Federal agencies involved. Discussion will be divided between investigations at the site and inspections of debris downstream of the dam.

2. Observations at the Dam Site.

  1. In order to facilitate site inspections, a plane table survey was conducted and a base line was established. The location of the base line is shown on the plan view (Figure 1). An assumed datum has been used which is 540.46 feet above National Geodetic Vertical Datum (mean sea level). Investigations consisted of detailed visual inspections, measurements, soil sampling, and physical property testing of various materials found at the site.
  2. The dam site is located on Toccoa Creek, approximately 2000 feet upstream of Toccoa Falls, on the northwest side of the City of Toccoa, Georgia. The dam crest width varies but the average width is approximately 20 feet. Side slopes vary but near the failure area appear to be about 1V on 1H. Maximum height is approximately 38 feet. The total length of the dam was approximately 400 feet, although only a section about 200 feet long appears to be higher than 20 feet. This is the section in which the failure occurred (Photos 5 and 6). The embankment is a homogeneous earth section in that apparently no selectivity was made of material coming from the borrow pits. The predominant embankment material is residual soil consisting mainly of silts of varying plasticity and some silty sands and clays. The embankment materials are distinguishable from the foundation materials by their reddish and tan colors. There is no evidence of an internal drain or foundation cutoff. Portions of the embankment were constructed across a layer of alluvial material (silt). This layer is clearly visible in the photograph (Photo 9). The remaining embankment slopes are heavily vegetated and trees up to one foot in diameter are common (Photo 10). Near the ends of the dam the vegetation makes it difficult to distinguish between the fill section and natural ground.
  3. There exists an uncontrolled earth spillway (main earth spillway) excavated through the hillside beyond the left abutment (Photo 7). The spillway is about 60 feet wide at the entrance but quickly narrows to a base width of approximately 11 feet for the major portion of its length. It is about 380 feet long and has side slopes of about 1V on 1H. The invert appears to vary from about 10 feet below the crest, at the upstream side, to about 20 feet at the discharge area. The spillway area is heavily grown up, predominantly by small trees and brush along the spillway sides. One person interviewed stated that this spillway was constructed in the 1960's. However, aerial photos taken January, 1955, clearly show the spillway. In addition to this spillway, there exists a low spot in the reservoir rim, on the right side, in the vicinity of the "Fort Toccoa" sign that has effectively served as a secondary spillway (Photo 11). There is evidence that water has gone over the rim at this location several times in the recent past including the night of the failure. The crest of the secondary spillway is seven feet below the top of the dam.
  4. Debris marks on the upstream face of the dam indicate that there was five feet of freeboard below the earth road at the dam crest prior to failure. The water depth in the main earth spillway was about four feet. There is a report that the fireman who inspected the dam site two to four hours before it failed found no water in the secondary spillway. Another unconfirmed report has the lake level falling.

3. Data Revealed by the Failure.

  1. The left side of the breach exhibited tension cracks extending about 30 feet back from the failure scarp. The cracks were perpendicular to the dam axis and decreased in width from the edge of the breach. The face of the breach had a slope of about 1V on 1H (Photo 8). The central portion of the crest of the remaining dam was free of vegetation except for grass. The slopes and outer edges were covered with trees and brush. The brush on the upstream edge of the crest was free of debris. The grass on the crest, extending from the breach back to the spillway, was not knocked down or bent from the vertical. Also noted on the crest was an old campfire site and numerous newspapers, all undisturbed (Photo 5). On the downstream slope of the left embankment, the location of a rotted out tree stump was observed. This had left a vertical hole in the embankment. Also on the downstream side of the left breach, water had washed away the leaves up to about 21 feet above the existing channel. The after-failure survey indicates the high-water marks. The left bank water mark is about 14 feet higher than the high-water mark on the right side of the channel. These marks are indicated on the site topographic plan (Figure 3, 156k). Some old rock and rubble were noted on the downstream toe of the left breach. This material looks as if it was placed, rather than being a natural deposit. Also noted were several logs or timbers, likely the remnants of old cribbing.
  2. Spillway inspections indicate the main earth spillway usually carried approximately one foot of water. The fact that water continually flowed over the main earth spillway is supported by staining on the rock at the spillway outlet and statements of historical witnesses. Topsoil and leaves were scoured off of the spillway sides up to about four feet above the bottom of the spillway; however, three to four feet of spillway freeboard apparently remained during the failure.
  3. The breach exposed the dam's foundation under both sides of the breach (Photo 5). On the extreme right side, the flood had cut to rock. The erosion of the foundation revealed about four feet of alluvial silt beneath the embankment and overlying the rock (Photo 9). The exposed face (scarp) is practically vertical on the right side (Photo 10). From the left edge of the stream to the toe of the left scarp, the exposed silt layer slopes upward. There appears to be two distinct and different layers of silt beneath the fill. The bottom layer, immediately over the rock, is about two feet thick near the embankment centerline and is a tan plastic silt with some sand. (The tan color may be due to some oxidation). Overlying this layer is about two to three feet (at centerline) of grey fat micaceous silt. At the material interfaces, there are remnants of old root systems. Along the root line, seepage was noted after failure. The impression is that the lower layer was natural material and the grey layer the result of later deposition such as would occur in a reservoir. At least in some locations, the embankment was built across both these layers. The layers are less pronounced near the downstream toe of the embankment.
  4. The right side of the breach was considerably steeper than the left. The scarp was practically vertical for a considerable distance and the overall slope is estimated at 1V on 0.5H. As previously observed in the left scarp, a considerable root system is exposed and extended down into the embankment (Photo 10). These roots varied in size up to three inches in diameter and some extended down into the remains of the rock crib dam. No tension cracks were observed on the right embankment crest. The vertical distance from the exposed rock in the streambed to the top of the dam on the right side of the failure is approximately 42 feet. This includes about four feet of silt and 38 feet of fill material. On the downstream exposure of the right breach, there is a flat area similar to a berm. (This is not evident on the left side). This berm appears to be located about half way up the embankment from the toe. An area of rock and rubble is exposed beneath the downstream toe of the right scarp face about 73 feet downstream of base line station 2+60 (Photo 10). The rock has about an 18-inch maximum size. This rubble is similar to the material observed under the left toe except that there is a much larger quantity of material. The area is approximately 12 feet wide at the base and six feet high. The material extends to natural rock. On the upstream side of the material a feature appearing to be a grout filled sandbag was observed. In the vicinity of the rock and rubble, about 78 feet downstream of base line station 2+75, a log about 12 feet long and 18 inches in diameter was found (Photo 10). This log was secured to the underlying rock by a steel pin one-inch in diameter. This log is believed to be a remnant of the old cribbing. Also noted in this area was another steel pin which was sheared off at the top of rock level and appeared to have been abandoned for some time. Other holes, apparently drilled, were observed in the foundation rock.
  5. Immediately above and upstream of the rubble area a 30-inch diameter spiral riveted steel pipe was found with its open flange face roughly corresponding to the face of the failure surface (Photo 9). This pipe is about 60 feet downstream of base line station 2+60. The invert of the pipe roughly corresponded to the top of the underlying silt layer. The pipe runs sub-parallel to the dam axis and daylights in the downstream slope where it appears to have been cut off and abandoned. Metalurgical analyses date the pipe as being installed about 1900. The pipe is partially collapsed inside. Also noted was a vertical pipe about four inches in diameter penetrating the 30-inch pipe from the top and protruding about a foot into the larger pipe. The small pipe was plugged at the top. The flanged end of the 30-inch pipe shows no evidence that the pipe was sheared. Rather, it appears the pipe was unbolted at some time in the past and left open. The upper half of the pipe is surrounded by a piece of concrete about a foot or so from the end. This concrete may have been part of a headwall, seepage collar, or other old structure. At the flanged end of the pipe, wood framing and cribbing is exposed. This material was left in place when the earth fill was constructed.
  6. Approximately 25 feet upstream from base line station 2+20, remnants of what appears to be a concrete sill were found on and in the face of the embankment. The top of the sill was about 30 feet below the embankment crest. On the top of the sill there were remnants of concrete masonry walls. This sill would have been the base of the welded steel penstock inlet for the pipe to the powerhouse. Excavations into the area of the scarp at the location of the masonry sill, revealed a 30-inch welded steel pipe which had a wall thickness of 3/32-inch. The pipe had a square flange of 1/4-inch thick steel plate. The pipe, later exposed by excavation, was uncoated. About half the flange was torn away from the end of the pipe. The pipe was partially collapsed but inspections indicate that this collapse was not recent. The alignment of the pipe is such that it appears to lead down the mountain to the old Toccoa Falls Power Plant. No evidence of piping or erosion was noted around the pipe.
  7. It is important to note that all remaining slopes of the dam were heavily vegetated. Trees in excess of one-foot in diameter are common. The breach surface indicates a significant root system down into the embankment.
  8. Observations on the right downstream slope of the dam revealed the 30-inch spiral riveted steel pipe exposed on the downstream toe about 85 feet from its exposure in the breach. The pipe appears to have been cut off and abandoned. The pipe is about 80 percent plugged with soil, but leaves and topsoil were washed away at the mouth of the pipe. It appears that some water went through the pipe for a short period of time during the failure once the end of the pipe was exposed on the scarp.
  9. Below the dam there exists remnants of two lines of pipe leading to the Toccoa Falls Power Plant. The older line is spiral riveted steel pipe such as exposed in the right embankment toe at the rubble area. The newer line is at a higher elevation and is welded steel pipe. This newer pipe aligns with the pipe excavated near the concrete sill upstream of the base line. The new line also exhibits an air vent tower and gate valve as shown in Figures 2 and 3.
  10. The area below the dam was inspected in order to observe debris. It was believed that the debris observations might give some clue as to the dam history and construction. The following observations were made:
    1. Pieces of metal were found. These included what appeared to be a track, gate wheel, trash grate, metal channel, steel plate, and various pieces of pipe of varying diameters, some being bituminous coated.
    2. Large logs and timbers such as had been observed to be bolted to the rock at the site were found. These were about 12-inches to 18-inches in diameter and contained old spikes.
    3. Several pieces of concrete masonry were found. In general, these pieces ranged from approximately three to four feet in each direction. They appear to be remnants of an old concrete block wall, perhaps a portion of the outlet structure. One piece was curved as if it had fitted around a pipe. The diameter of the curve was about 30 inches.
    4. A steel plate structure was observed which contained remnants of a sliding gate over an outlet pipe. The pipe diameter was about 30 inches. The plate was three sided with dimensions of about three feet by four feet. It appeared to have been some type of liner for a masonry structure due to imprints on the exterior and masonry in the area.

4. Sampling and Testing

  1. In an effort to obtain information relative to the history and condition of the dam prior to failure, samples were obtained of embankment and foundation soils and certain pieces of pipe found in the area.
  2. Soil samples were obtained from the site for purposes of testing the materials in order to have an indication of the properties of the dam. Samples of various materials from a number of locations were obtained for classification tests. Two bulk or chunk type undisturbed samples were taken from the two predominant foundation materials. The upper grey silt layer sample was taken from about 38 feet below the crest and about eight feet downstream of base line station 2+26. A sample of the underlying residual foundation soil was obtained from near base line station 2+72, about 40 feet below the embankment crest. Also, undisturbed Shelby tube type samples were obtained from a core boring located at base line station 2+00 on the crest of the right breach.
  3. Testing of soil samples was conducted at the Corps of Engineers South Atlantic Division Laboratory, Marietta, Georgia. Testing was conducted in accordance with Corps of Engineers Manual EM 1110-2-1906, Laboratory Soils Testing. Classification and moisture content tests were performed on all materials. Shear tests were performed on the undisturbed samples. Field density and standard compaction tests were performed on certain of the materials from the Shelby tube embankment samples. Pin hole dispersion tests were performed on the grey foundation silt and embankment materials to determine if the material was highly erodable or susceptible to piping. Unconfined and rapid direct shear tests were performed to compare peak and ultimate strengths, as well as strain at peak strength and at failure.
  4. Tests indicate the embankment was predominantly composed of fine grained soils which classify as ML, MH, CL, CH, and some SM material, as defined in the Unified Soil Classification System. It is noted that some of the embankment samples have a low plasticity; i.e., plasticity index less than 10. The degree of compaction, indicated by the samples tested, varied from about 78 to 86 percent of Standard Proctor maximum density. These densities, though low, are not inconsistent with what would be expected from construction procedures such as apparently utilized in the dam. The pin hole tests indicated the materials were not dispersive. Shear strengths of the soils appear to be generally good despite the relatively low densities. However, it must be noted that these samples were, of necessity, taken from areas outside the failure zone and must not be regarded as being fully reoresentative of embankment strengths in the area of the failure.
  5. Physical Property Tests on Pipe. In an effort to attempt to reconstruct the dam history, representatives of the Corps of Engineers Construction Engineer Research Laboratory (CERL) visited the site to obtain samples of various metal debris in an attempt to date the pipe. Their findings are summarized as follows:
    1. Spiral riveted steel penstock pipe with flange was installed in the early 1900's.
    2. Pipe from the welded steel penstock to the power plant is dated about 1940.
    3. Pipe with bituminous coating was from 10-15 years old.

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