The Chattahoochee at Vinings spiked sharply on Sept. 21. Below-dam feeder creeks and urban runoff bear part of the blame. But the spike also followed Georgia Power’s opening of eight out of 12 Morgan Falls spillways, the most in the dam’s history.
Georgia Power says the river would have spiked at Vinings whether the dam was there or not.
It also said the company had no choice but to let the swollen Chattahoochee through. If the spillways had remained closed, the river would have topped Morgan Falls long before the flood crested, dragging dam-damaging tree branches and other debris with it, the company said.
Although it sits above the most populated parts of metro Atlanta, the company says the dam has no role to play in flood control. It’s too small, the company says. Morgan Falls has only enough water storage capacity to do what it’s supposed to do, which is make power and smooth out the Chattahoochee’s day-to-day flow.
As Cobb County residents and public officials questioned the dam’s role in September’s floods, the company also rejected suggestions to improve Morgan Falls’ ability to hold back floodwater.
One came from Cobb County Commission Chairman Sam Olens, who asked the company to consider letting out water early, when big rainstorms were headed in north of the dam.
Georgia Power said it can’t do that without jeopardizing the dam’s day-to-day duties. Threatened storms may not materialize, leaving Morgan Falls short of the stored water it needs.
The company also does not support dredging the lake behind the dam. Silted-in from years of development upstream, Bull Sluice Lake’s capacity has dropped by nearly one third since the 1960s. Dredging Morgan Falls has been kicked around for years, although for reasons other than flooding. A 1979 Georgia State study listed it as one of four alternatives for increasing Atlanta’s water supply, for instance.
The company says dredging is impractical and would cost more than the dam is worth and achieve little.
Even scooped out to its 1960s size, the company said, the lake could not have held September’s Chattahoochee for long, given the record-breaking size of the floods, according to Georgia Power.
The volume of water that came into the lake on Sept. 21 was so high that it would have topped even a 1960s-size lake quickly, Georgia Power said. The company estimates that a dredged-out lake would have held back September’s peak floodwaters for only 20 minutes longer.
Hit like a tidal wave
Questions about the Chattahoochee dams began circulating in Vinings even before the river retreated into its banks this fall.
The speed of the flooding was the reason, according to David Cole, a leader with the Vinings Homeowners’ Association.
“It had rained constantly for 10 days. Then, out of nowhere, there’s this 22-foot spike. It hit us as though it were a tidal wave. It wasn’t a minor surge. It spiked 22 to 29 feet depending on how far down the basin you were.”
“That’s a function of somebody opening a dam,” he said.
Longtime Vinings resident Emily Gray also pointed to the river’s fast rise. A Chattahoochee neighbor for more than 20 years, she said the river never rose that quickly before: “It was coming up feet in 15 minutes. And I’m thinking, this is not God-made runoff water. That is spillway water.”
Flooded neighbors first blamed the Buford dam at Lake Lanier.
Corps of Engineers data show that Buford reduced its releases to the Chattahoochee on Sept. 20. But it never cut them entirely, even as the river buried highways in metro Atlanta and despite the fact that Lake Lanier was not yet full.
Corps spokesman Patrick Robbins, in Mobile, Ala., said the dam released only enough water to produce the electricity needed to run the dam itself.
He said the Corps never cuts flow entirely, because it needs to protect water quality between the dam and the next feeder creek downstream.
Corps data also show that Buford Dam held back enormous amounts of water during the flood and that its releases were small, compared to the volume of water that eventually poured into metro Atlanta.
That’s what turned attention to Georgia Power’s Morgan Falls.
Morgan Falls sits just southwest of Roswell, several miles upstream from Vinings. Built more than a century ago to produce power, the dam began playing a role in regulating the river more than 30 years ago, after Buford Dam was built in 1958.
Buford’s releases cause huge fluctuations in water flow.
Morgan Falls smooths out the surges, holding back and then releasing the water coming downstream to create a steady flow past Atlanta’s Peachtree Creek.
That flow dilutes treated sewage from Atlanta.
September’s rains far exceeded Morgan Falls’ capacity to control the river, Georgia Power says.
The fiercest rain from September’s storms hit metro Atlanta late on Sunday the 20th and early Monday the 21st.
United State Geological Survey stream gauge data show the river’s flow picking up volume steadily as it headed south, then jumping sharply at Vinings like the end of a snapped bedsheet.
Between shortly after midnight on Sept. 21 and 7 p.m. that night, the flow coming into Morgan Falls grew from 5,000 cubic feet per second to 31,000 cubic feet per second. Georgia Power opened eight spillways between 6:30 a.m. and 5:30 p.m. that day.
“Some of the operators who’ve been here a long time say they never remember that many gates open,” said company senior engineer Fred Cox.
The flow at Vinings climbed in tandem with the opened gates.
It quadrupled, from 5,000 to 20,000 cubic feet per second that morning, then doubled again between noon and 9 p.m. that night, at 41,000 cubic feet per second.
Georgia Power’s outreach to suspicious Cobb County residents and officials began shortly after the floodwaters fell back.
The company heard from county commissioner Bob Ott, along with chairman Olens.
It took leaders on dam tours and made a presentation to the full county commission last month, two days before Thanksgiving.
To Olens, the company has made its case.
He said he now believes the dam is too small to do other than it did during the floods.
Olens said the company has agreed to alert Cobb’s emergency management agency before opening more than three spillways in the future, so that the agency — and downstream neighbors — can be prepared.
Some who initially blamed Georgia Power have also reconsidered, including Vinings’ Cole.
In an e-mail, Cole said he now believes “the flooding was not created by Georgia Power,” and that “more importantly, Georgia Power was one of the first to respond and an enormous help to our community. I think the real concern is storm water management and how to prepare for the next flood.”
Gray, the longtime river neighbor, is less willing to let Georgia Power and its dam off the hook.
She thinks the company was “cavalier” in opening its spillways. She predicted that Cobb County will have problems again come spring.
“I believe we are not finished with floods yet,” she said.