The 1977 Toccoa Flood
Report of Failure of Kelly Barnes Dam Flood and Findings
by Federal Investigative Board
December 21, 1977
PROBABLE CONDITION AND CONFIGURATION OF DAM UP TO THE TIME OF FAILURE
1. General. No design, plans, or construction cross sections for any of the various stages of construction and/or repair have been discovered. The maximum section of the dam was destroyed by the failure. It can only be presumed that the cross sections of the dam, as seen on the two faces of the breach, continued across the creek. The dam was generally crescent shaped, concave upstream, and apparently followed a low ridge.
Though it is impossible to fully reconstruct the dam condition and configuration prior to the failure, the following is considered a reasonable reconstruction of recent conditions in the vicinity of the failure. Figure 4 is a probable configuration based on site inspections, historical data, and old photographs.
The embankment in the area
of the failure was approximately 38-feet high with a crest width of
about 20 feet. The upstream slopes were approximately 1V on 1H and
the downstream slopes were about 1V on 1H. The main or newer
embankment was constructed over portions of the old rock crib dam, all of which
may or may not have remained beneath the downstream toe. There was
probably a berm approximately
halfway up the downstream slope.
Examination of the cross section that remains discloses four feet
of different colored soil below the crest. Whether this is part of
the 1939 or 1948 additions, or was added by later maintenance, is
not known. Therefore, it can be concluded that the dam was built in
two to four stages commencing with a rock crib dam in 1899.
There were apparently two steel plate lined masonry inlet
structures. (Photo 3) One structure, at some point in time,
controlled water into a welded steel penstock to the Toccoa Falls
Powerhouse and was located on the right side of the embankment with
the outlet pipe running sub-parallel to the dam axis.
The other structure was essentially a low-level spillway inlet structure and had
a pipe extending through the dam in the direction of the old creek
bed. These inlet structures and pipe lines are shown on Figures 1
and 2. The embankment slopes were heavily vegetated. There is no
evidence of heavy surface erosion. There is no evidence to
indicate either structure was functional in recent years. Evidence
indicates that the main earth spillway was nearly always in use and
the low-level spillway through the dam was blocked or plugged.
Photographs taken in 1973 show that the area surrounding the
low-level spillway outlet pipe had experienced seepage for a number of years and
that a slope failure had
developed in the vicinity of the pipe on the downstream face of the
dam (Photos 12 and 13). Historical accounts support the
photographic evidence of long term seepage in the vicinity of this
pipe.
2. Site Geology. The dam site is
located in the Piedmont
Province. The rock type at the site is a biotite gneiss of the
Carolina gneiss series.
The bedrock is exposed over the
stream channel and in the banks at, and just downstream of, the dam
site. The rock exposed at the site is a strong, competent, biotite
gneiss with minor, shallow weathering. The primary joint sets include two near
vertically dipping sets, one striking N80°E and the other
striking generally North-South. Another primary joint set is
nearly horizontal. A rather extensive root system has penetrated
the near horizontal joint set along the creek bank near the
downstream toe of the dam.
A layer of soft alluvium, from
two to five feet thick, was observed in both faces of the breach of the dam. This material
extends from the proximity of the old rock crib dam upstream--an
undetermined distance. The layer appears to be quite extensive in
the foundation of the earth dam. The lower six inches of the layer
of the alluvium contains an extensive root mat. The alluvium
appears to be composed principally of soft, plastic clay and/or silt with some layers of fine silty
sand.
3. Meteorological Conditions. Before rain began the ground
was already wet from heavy rainfall of 3-1/2 to 4-1/2 inches which
fell on October 25-26. The rainfall began on Wednesday morning,
November 2, and ended by midnight on Saturday, November 5. A
strong high pressure area, centered over New England, was bringing
Atlantic moisture into Georgia on Wednesday and this produced the
rainfall at the beginning of the storm period. Meanwhile, a more
important development was taking place in Texas where a strong low
pressure area was developing at upper levels in the atmosphere.
This intense upper level low was located near Shreveport, Louisiana
on Thursday morning, November 3; New Orleans, Louisiana on Friday
morning, November 4; Mobile, Alabama on Saturday morning, November
5; and Centreville, Alabama on Sunday, November 6. As this
slow-moving upper level storm moved closer to Georgia, more and
more moisture was brought in on southerly winds from the Gulf of
Mexico. Lifting of the air by the higher terrain of north Georgia
accentuated the lifting processes provided by the atmospheric storm
patterns. A plentiful supply of moisture, lifting of the air, and
a slow-moving storm system are the ingredients of a long, heavy
rain event.
Rainfall reports are received by the National Weather Service from
radio stations WLET and WNEG in Toccoa. Since these stations are
located some distance from the drainage area above Kelly
Barnes Dam, a rainfall survey was made. Three additional storm
total values were obtained as shown on Figure 5. In addition to
the storm totals, measurements of rainfall at 11:00 p.m. each night
were available from one source while the 24-hour amount from 8:00
a.m., November 5 to 8:00 a.m., November 6 was measured by another
local citizen. These measurements, interviews with local
residents, radar echoes, and hourly values from nearby recording
stations were used in estimating the rainfall distribution for the
drainage basins above Kelly Barnes Dam. Although radar echoes
indicated that the heaviest rainfall probably occurred between 6:30
p.m. and 7:30 p.m., on November 5, the estimated maximum one-hour
value is shown between 6:00 p.m. and 7:00 p.m. since points are
plotted on the hour.
As indicated on Figure 6, the rainfall was light for the first two days with slightly over one inch falling in the Toccoa area by Friday morning, November 4. Rainfall intensity increased somewhat for the next 24 hours, with a storm total of approximately 2-1/2 inches by 8:00 a.m, Saturday morning, November 5. The storm total increased to approximately 3-1/2 inches by noon on Saturday and then the rainfall was apparently light until about 6:00 p.m., when showers moved into the area. From 6:30 p.m. to 7:30 p.m., radar echoes indicated periods of torrential rainfall with frequent intense lightning and thunder. A small tornado apparently touched down about five miles southeast of Kelly Barnes Dam at approximately 8:00 p.m. The damage to trees was limited to only 100 yards in width and about one-fourth mile in length but the tornado was indicative of the severity of the weather. After 8:00 p.m. the showers were less frequent and had practically ceased by midnight. Estimated basin rainfall for the entire period is about seven inches with almost 3-1/2 inches occurring between 6:00 p.m. and midnight on November 5.
Rainfall intensities estimated for this storm are common for the
Toccoa area. The estimated maximum one-hour, two-hour, three-hour,
six-hour, 12-hour and 24-hour rainfall amounts indicate return periods
between two and five years.
A heavy rainfall period occurred on May 27-29, 1976 in the Toccoa
area. The 12-hour and 24-hour rainfall amounts were larger for that
storm than for the November, 1977 rainfall period. However,
rainfall amounts for six-hour and shorter durations appeared to be
heavier in the November 1977 storm.
4. Hydrology and Hydraulics. Information regarding the lake
capacity, drainage area,
inflow and outflow to Kelly Barnes Lake during the period of
November 5-6, 1977, was determined from field surveys, topographic maps,
interviews with local residents, and engineering
computations. Based on this information it is possible to
determine, within reasonable limits, the probable inflow hydrograph for a condition
assuming the dam had not failed. In addition, certain inferences
can be made regarding the dam failure.
The drainage basin above Kelly Barnes Dam is heavily wooded with
pines and deciduous trees (Photo 4).
It is located in the Piedmont Province in northeastern
Georgia. Stream slopes are fairly steep, generally exceeding 100
feet per mile. The drainage area of Toccoa Creek at the head of
Kelly Barnes Lake is 4.10 square miles, and at the dam site 4.57
square miles.
The normal pool level for
Kelly Barnes Lake is about 596.5 feet (assumed datum). The crest of the flood of
November 5-6, 1977, (a 5- to 10-year flood) reached a level in the
lake of approximately 601 feet which is about five feet below the
top of the dam. A hypothetical 100-year flood, assuming no storage attenuation, would cause a
maximum lake level of about 604 feet which is about two feet below
the top of the dam. The storage attenuation would reduce this
level.
The capacity of Kelly Barnes Lake was determined from a topographic
map made by plane table
survey November 25, 1977. At a level of about 596.5 feet,
slightly higher than the main earth spillway, the lake contained
approximately 410 acre feet of
water with a surface area of about 40 acres. At a level of about
599 feet, equal to the elevation of the secondary spillway, the
lake contained approximately 540 acre feet. At the highest level
reached on the night of the dam failure, approximately 601 feet,
the lake contained approximately 630 acre feet of water.
The peak discharge of Toccoa
Creek, at the head of Kelly Barnes Lake, for the flood of November
5, 1977, was determined from a field survey of cross sections,
high-water profiles, and channel roughness coefficients. The
discharge was computed to be approximately 840 cfs (cubic feet per
second). Based on a regional flood-frequency analysis for small
drainage basins, the average return period of this flood
is estimated to be five to ten years.
Additional computations were made using unit-hydrograph techniques
to fit a hydrograph to the computed peak discharge. An estimated
inflow hydrograph to Kelly Barnes Lake was derived by making
appropriate adjustments for the additional intervening drainage
area.
The relationship of the lake level to outflow discharge for the
main earth spillway and the secondary spillway were defined from
field surveys and hydraulic computations. The two relationships
were combined to provide a composite (total) outflow rating. This
rating indicates that at lake level equal to the maximum height
reached on the night of the failure, the two spillways discharged a
total of about 400 cfs.
Streamflow and reservoir routing techniques were used to
estimate the probable lake-level hydrograph, the lake-outflow
hydrograph, and the stream-flow hydrograph just below Dead Man's
Branch in the Toccoa Falls College area for the night of the dam
failure. The computations are based on the assumption that the dam
did not fail, so that the probable normal situation could be
depicted. The stream-flow hydrograph just below Dead Man's Branch
was computed because testimony of local residents provided
information which could be used to roughly verify the computations.
Based on these computations, the lake began a significant rise at
about 6:00 p.m. to 7:00 p.m. on November 5. Outflow at the time
was limited to the main earth spillway of about 50 cfs. At
about 8:00 p.m. to 9:00 p.m. the lake reached a level so that
outflow began through the secondary spillway. Total computed
outflow at this time was about 120 cfs. Had the dam not failed,
the lake would have risen to a level of about 601.5 feet and would
then have started a slow decline after about 11:00 p.m. on November
5. The maximum outflow through the two spillways would have been
about 500 cfs. It would have taken at least six hours following
the crest for the lake to recede to a level below the level of the
secondary spillway, assuming the dam had not failed.
The streamflow hydrograph
computed for Toccoa Creek just below Dead Man's Branch indicates a
peak discharge occurring at about 9:30 p.m. November 5. The timing
is supported by testimony of local residents who state that the
creek overtopped a bridge in the vicinity at about 9:00 to 10:00
p.m., and then began to recede. After this time, local residents
state the stream was "up and down" several times. No
information is available to support the computation of peak
discharge; however, the good agreement of timing indicates the
hydrograph computations are reasonable.
The actual maximum level reached by the lake was 601.1 feet based
on field surveys of high-water marks. This is
only about a half a foot different than the computed elevation, and
is well within the range of error to be expected in such
computations. The computed time at which the lake level rose above
the secondary spillway is significant. It is highly probable that
the water would be flowing through this spillway after 8:00 or 9:00
p.m. on November 5, and throughout the remainder of that night and
the early morning hours of November 6. However, a local volunteer
fireman has stated that he and another person went to the dam at
about 10:00 or 10:30 p.m. and crossed the secondary spillway. He
said there was no water in the spillway and the lake level was "not
even close" to the secondary spillway. Newspaper accounts say the
men went to the dam two hours prior to the break, or about 11:30
p.m. In either case, the lake level should have been over the
secondary spillway. Considering this in light of the hydrograph
computations, it would imply that outflow, in addition to that
carried by the two spillways, occurred to cause the lake to recede
from a crest at about 601 feet, and to a level below the secondary
spillway before the men went to the dam. This would suggest a
partial failure of the dam several hours prior to the main failure.
Additional support for this conclusion is that field observations
of bent grass in the lake bed indicate little or no velocity
occurred between the maximum lake level of 601.1 feet and about 597
feet. Below a level of 597 feet, a definite pattern of high
velocity was noted, indicating a large amount of flow occurred in
the lake in a short time span after the lake receded to that level.
A portion of this drawdown could have occurred after the main
failure began. Furthermore, the statements regarding the "up
and down" nature of the stream in the college area after about
9:00 p.m. suggests something unusual happened before the main
break.
The major failure of Kelly Barnes Dam occurred at about 1:30 a.m.
November 6, 1977. At that time a surge of water was released from
the lake. Field surveys of cross sections, high-water profiles, and
roughness coefficients were used to compute a peak discharge of
24,000 cfs in the channel at a site about 800 to 1000 feet
downstream from the dam.
5. Stability Analysis. A steady seepage stability analysis,
utilizing unconsolidated-undrained tests performed on undisturbed
samples, was performed for the downstream slope of the dam. For
the analysis, the phreatic
line was assumed to exit at a point approximately one-half way
up the downstream slope. The cross section used included the
estimated 12 foot by 20 foot slope failure section shown
by the 1973 photos of the downstream slope. The other assumed
slopes are based on observations of the remaining slopes, topo map,
and reports of witnesses. The sliding circle method of
analysis was used. The stability of the assumed embankment section with the
stated conditions is marginal as indicated by a factor of safety of 1.02.
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U.S. Geological Survey
