South Atlantic Water Science Center - Georgia
1997 Toccoa Flood
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The 1977 Toccoa Flood
USGS Hydrologic Atlas 613
|C and C1||4.6||53,140||2 400|
1 Peak inflow to Kelly Barnes Lake estimated from unit-hydrograph computations.
2 Maximum total spillway outflow prior to dam break.
The computed peak discharge of Toccoa Creek at site A near the head of Kelly Barnes Lake, for the flood of November 5 - 6, was 830 ft3/s (cubic feet per second). The peak inflow of 980 ft3/s to the lake was estimated at site B by hydrograph synthesis with a rainfall-runoff model. Based on a regional flood-frequency analysis (Golden and Price, 1976) for small drainage basins, the estimated recurrence interval of this flood is about 10 years. The recurrence interval is the average time interval between actual occurrences of peak flows of a given or greater magnitude. Although the recurrence interval represents the long-term average period between floods of a specific magnitude, such floods could occur at short intervals or within the same year.
At the site of the discontinued U.S. Geological Survey gaging station on Panther Creek (drainage area, 32.5 square miles), 5 1/2 miles north of Toccoa (fig. 1), peak discharge for November 5 - 6 was 6,600 ft3/s. Based on a frequency analysis of 48 years of station record, this flood had a recurrence interval of approximately 15 years. The Panther Creek basin adjoins the Toccoa Creek basin.
The peak outflow of 400 ft3/s from Kelly Barnes Lake before dam failure was estimated by means of slope-area and flow-over-road computations at sites C and C1, respectively. Flow occurred in both the primary earth spillway (site C) near the left end of the dam and the secondary spillway (site C).
Although the dam-break flood undoubtedly moved downstream as a flood wave, it produced a pool- and-riffle type of flow as shown by the water surface profiles in figure 11.
Peak discharges were computed at sites A, D, E, and F using the standard slope-area computation procedure described by Dalrymple and Benson (1967). The procedure assumes steady flow conditions that obviously did not exist immediately downstream from the broken dam. The effects of unsteady flow were minimized by using short reaches for slope-area computations. Immediately downstream from sites D, E, and F, high-water profiles indicate that the flow changed from tranquil to rapid state. At these sites, peak discharges computed by the critical-depth method agree closely with those computed by the slope-area method.
Discharges were computed at Highview Road and Georgia Highway 184 (sites G and H) using a standard contracted-opening procedure as described by Matthai (1967).
Conditions for making discharge computations were considered good at sites A and H; fair at sites B, F, and G; and poor at sites C, D, and E.
Graphs showing the relation of discharge to drainage area for the observed flood, together with the 10-, 50-, and 100-year regional flood frequency curves (Golden and Price, 1976), are shown in figure 12. No attenuation of the peak discharge was evident between the dam and Toccoa Falls College (sites D to E). Extreme attenuation occurred between Toccoa Falls College and Highview Road (sites E to G) due to increased overbank storage and rapid changes in stage. Flow at sites F and G was 60 percent and 27 percent, respectively, of the peak discharge at site E.
Because this disaster occurred at night, eyewitness accounts of the timing of the flood wave were not consistent. According to one witness, the creek was out of its bank for about an hour at Forrest Hall Dormitory, at the upstream end of the college campus. Other witnesses indicated that the flood waters were out of bank for only about 40 minutes. An estimated discharge hydrograph at Forrest Hall Dormitory, 0.7 mile downstream from the dam, based on these reports, is shown in figure 13.
Flood profile.- Water-surface profiles, determined from a field survey of high-water marks left by the flood of November 6, are shown in figure 11. Average depths in the main channel above Toccoa Falls were about 17 feet. Depths in the vicinity of the college ranged from about 21 feet at Forrest Hall Dormitory to about 18 feet at the trailer village. Depths downstream from Georgia Highway 17 averaged about 15 feet.
Controls for pool-and-riffle flow are located near stations 517, 486, 480, and 465 and at Georgia Highway 17. These controls are at obvious valley constrictions, but some of the resulting backwater could have been caused by temporarily lodged debris. Extreme scour was evident at these controls.
Water-surface elevations on the left bank differed from those on the right bank by as much as 10 feet in the curves along the main channel because of superelevation resulting from high velocities. However, even in these high-velocity areas, ponded conditions existed in nooks near the mouths of tributaries.