The troubled Lake Forrest Dam finally has two repair designs on the table – and they could cost more than $7 million and require up to 18 months of road-closing construction.
The 60-year-old earthen dam sits directly under Lake Forrest Drive, right on the Atlanta-Sandy Springs border. Since 2009, the state Safe Dams Program has pushed for fixing the aging dam, but that has been delayed due to complex co-ownership among both cities and several private residents. For over two-and-half-years, the cities have been studying repair options, with Sandy Springs taking the lead and splitting the costs with Atlanta.
Officials from Schnabel Engineering have finally narrowed the alternatives to either an upgraded version of today’s dam or a new, smaller dam built farther upstream. A previous, simpler-sounding concept to get rid of the dam completely and replace it with a culvert has been discarded as too expensive, they said.
“Those two alternatives, they basically would bring [the dam] into compliance with the Safe Dams regulations,” said Schnabel engineer Brad Boyer at the Sept. 19 Sandy Springs City Council meeting.
Mayor Rusty Paul and the councilmembers listened with grim expressions, many frowning and gripping their chins in their hands, as Boyer laid out a lengthy construction timeline and early cost estimates ranging from about $5.9 million to $7.5 million.
But they did not question the need to fix the dam, which the state has classified as “high-hazard,” meaning that if it were to fail, the flood would likely kill people downstream.
There are no signs that the dam is about to fail, but it has “structural deficiencies” and cannot handle a worst-case, hurricane-style rainstorm of 15 inches falling in six hours, Boyer said. Last year, the city lowered the water level in the pond behind the dam by about 12 feet to make it “less unsafe,” as Dane Hanson, the city’s Stormwater Services unit manager, put it. But permanent fixes are needed.
“There are significant issues with the dam,” Hanson told the council. He showed an image from a 2013 video camera inspection of a pipe that provides drainage through the dam, pointing out “significant corrosion in that pipe.” Leaks within the pipe could cause the dam to fail internally.
On the exterior, the dam’s approximately 30-foot-high slope is covered in trees that could destabilize it. And the dam’s spillway – the outflow for excess water – simply doesn’t have the capacity to handle the major rainstorm that Safe Dams uses as a measure. In such a storm, the pond could overflow, with water spilling down the dam’s surface, causing erosion and possible collapse.
Schnabel’s two alternative design options amount to either repairing or replacing the dam. Both involve major changes to the dam, such as a new concrete spillway or turning that short section of the 4600 block of Lake Forrest Drive into a bridge over a culvert.
Either option has a similar, lengthy timeline, Boyer said: Nine to 12 months of design and permitting, and 15 to 18 months of construction.
The repair option is known as the “full pool” design and the replacement option is known as the “reduced lake level” design. The early cost estimate ranges are higher for repair and lower for replacement. However, the replacement option has more cost variables, and its high-end estimate is similar to the repair option’s low-end estimate, both around $7 million.
The “full pool” repair option is so called because it would allow the pond to be refilled to around its original water level. The dam’s height would be increased by several feet; all trees would be removed and the slope flattened; and a new spillway and drainage system would be built. Lake Forrest Drive would continue to run along the top of the dam. This option also could involve “acquiring properties downstream” for easements, Boyer said. The “full pool” cost estimates: $7.027 million to $7.622 million.
The “reduced lake level” replacement option is so called because it would create a smaller pond with the water level similar to the current partly drained state. In what is now the pond’s bed, a new, smaller earthen dam would be built to contain the water. Instead of the dam running under Lake Forrest Drive, there would be an open or closed culvert carrying excess water, which could mean building a small bridge. The new dam would need a concrete structure for controlling water flow. The dam’s surface would have what Boyer called “armoring” of stone blocks, allowing high water to spill over it without severely eroding the surface. The “reduced lake level” cost estimates: $5.854 million to $6.953 million.
The options were presented in an information, non-voting City Council “work session.” So no decision was made about either option.
Councilmember Andy Bauman, who represents much of the dam’s neighborhood, said he was leaning toward the “reduced lake level” option.
“My gut reaction is to go with the less expensive one,” barring evidence it can’t handle a major rain event as well, Bauman said. Boyer responded that analysis of that still needs to be done, but that a smaller dam would have lower rainfall capacity requirements.
Boyer also presented early alternative ideas about managing traffic during 15 to 18 months of construction on the two-lane Lake Forrest Drive, a major connector between Buckhead and Sandy Springs.
A program of normal detours, allowing for a total road closure, could cost $120,000. A more radical option is to “build a temporary road in the lake bed” to allow traffic to pass through the dam construction site, with “significant impacts on adjacent properties,” Boyer said. The estimated cost: $738,000.
Councilmember John Paulson urged the engineers to find a way to avoid a road closure for the entire project. He said an 18-month shutdown would be a “pretty severe, significant, tough thing for everybody to swallow.”
Bauman said after the meeting that he doubts that neighbors would approve of a main option for speeding up the timeline: overnight construction work. In the meeting, he suggested pairing the dam work with any scheduled sidewalk or stormwater installations on Lake Forrest Drive to take advantage of closures.
The next steps involve further meetings among the owners and with the Safe Dams Program to go over the alternative designs. Engineers would then return to the City Council for advice on which alternative is preferred, followed by a public open house for input. Design and construction would follow.