The 1977 Toccoa Flood
USGS Hydrologic Atlas 613
The Flood Without Dam Break
Streamflow estimated by rainfall-runoff model. - Computations were made using a simplified version of the U.S. Geological Survey rainfall-runoff model (Dawdy and others, 1972) to develop a hydrograph of inflow to Kelly Barnes Lake for November 5 - 6 (fig. 14).
The model was calibrated by reproducing the observed discharge at site A of 830 ft3/s from Toccoa Creek into the lake. The model, with streamflow and reservoir routing techniques, was used to estimate discharges (fig. 12) in the reach between Toccoa Falls and Georgia Highway 17 that would have occurred if the dam had not broken, and also if there had been no dam.
Profile estimates without the dam break. - Computed water-surface profiles for assumed conditions without the dam break, both with and without effects of lake storage, are shown in figure 11B. The two profiles were computed by means of the U.S. Geological Survey step-backwater routing program (Shearman, 1976) using the discharges obtained from the rainfall-runoff model. Cross sections used in the analysis were surveyed after the flood by the U.S. Geological Survey and the U.S. Army Corps of Engineers. The stream channel at these cross sections was generally scoured by the flood. Thus, the computed profiles represent only the minimum water-surface elevations that would have occurred had the dam not broken, and are not representative of conditions prior to the dam break.
A qualitative reconnaissance of sediment sources and sediment deposition was conducted on November 10 and 11. The operation of earthmoving equipment downstream from Toccoa Falls, however, prevented a qualitative evaluation of sedimentation conditions and limited this investigation to the reservoir and the stream channel upstream from Toccoa Falls. The following is a description of observations made during this reconnaissance.
The breached section of the dam was a major source of gravel (2.0 - 64 mm), sand (0.062 - 2.0 mm), and finer material. Field measurements indicate that about 13,000 cubic yards of material was removed from the dam. Other sources of sediment in this size range include soil eroded from the valley walls and reservoir deposits that were eroded during stream incision. Most of the sand and finer material was transported over Toccoa Falls. One large deposit of sand, however, occurred on the inside of a broad bend about 400 feet upstream from Toccoa Falls. The areal extent of this deposit was not measured because of debris, however, its thickness is evident in figure 15. The relative amounts of gravel transported through or deposited in the reach cannot be reliably estimated. Deposits of gravel-size material are present throughout the reach, generally intermixed with larger material or in association with debris piles.
Material larger than gravel was derived from bedrock exposed on the valley walls and from material stored on the flood plain. In the relatively narrow valley downstream from the dam, evidence indicates that the water had sufficient power to pluck large pieces of bedrock off the walls of the valley. Well-developed jointing in the bedrock seemed to define the size and shape of these large rectangular blocks. Most of these bedrock blocks were deposited about 500 feet downstream from the dam, where the valley widens. This deposit is the first area of large-scale sediment deposition on the flood plain below the reservoir. The deposit formed a partial dam across the stream channel.
Figure 16 shows the flood's capability for transporting large boulders, which probably had been stored on the streambed. The photograph, taken about 0.3 mile downstream from the breached dam, shows a large boulder that was apparently in suspension when it hit the tree.
Sediment transported by the flood can be divided by particle size, source area, and mode of transport. Sand and finer material derived primarily from the breached dam, dissected reservoir deposits, and eroded soil cover were, for the most part, transported through the reach and over Toccoa Falls. These fine materials fonr.ed most of the wash load of the flood wave. Cobbles (64 - 256 mm) and boulders (larger than 256 mm) derived from the streambed and surrounding bedrock were moved only short distances and thus constitute the bed-material load of the stream. Gravel-size material that was derived from both the breached dam and the streambed forms a transition particle size between wash loads and bed-material load.
Approximately nine houses, 18 house trailers, two college buildings, and many motor vehicles were completely demolished. Four houses and five college buildings were damaged by water. Only two houses downstream from Georgia Highway 17 were damaged.
The embankment at Toccoa Falls Drive, the oxidation pond above Georgia Highway 17, and parts of the main channel were scoured. Two bridges on Toccoa Falls Drive and the culvert at County Farm Road were completely destroyed. The highway embankments at Georgia Highway 17 were washed out at both ends of the bridge, and one of the bridge abutments at Highview Road was destroyed. The water-supply pipe for the city of Toccoa was broken and the city's water supply was contaminated for several days.
Dalrymple, Tate, and Benson, M. A., 1967, Measurement of peak discharge by the slope-area method: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A2,12 p.
Dawdy, D. R., Lichty, R. W., and Bergmann, J. M., 1972, A rainfall-runoff simulation model for estimation of flood peaks for small drainagebasins: U.S. Geological Survey Professional Paper 506-B, 28 p.
Federal Investigative Board, 1977, Report of failure of Kelly Barnes Dam, Toccoa, Georgia: U.S. Army Corps of Engineers, National Weather Service, Soil Conservation Service, and U.S. Geological Survey, 37 p.
Golden, H. G., and Price, McGlone, 1976, Flood-frequency analysis for small natural streams in Georgia: U.S. Geological Survey Open-File Report 76-511, 75 p.
Matthai, Howard F., 1967, Measurement of peak discharge at width contractions by indirect methods: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A4,44 p.
National Oceanic and Atmospheric Administration, 1977, Climatological Data, 1977, v. 81, no.11,16 p.
Shearman, J. O., 1976, Computer applications for step-backwater and floodway analysis: U.S. Geological Survey Open-File Report 76-499, 103 p.
Toccoa Falls Institute, 1954, 1954 Survey of buildings, roads, streams, and lakes: Toccoa, Georgia, Toccoa Falls Institute unpublished report.
[U.S.] National Bureau of Standards, 1977 ed., The International System of Units (SI): National Bureau of Standards Special Publication 330, 41 p