January 2000

In the fall of 1998 a crane appeared at Stone Mountain Lake, the 360 acre main lake in Stone Mountain Park, near Atlanta, Georgia. From September 14th, I witnessed an enormous, three month spillway reconstruction project unfold. According to Bob Cowhig, Director of Planning and Construction for Stone Mountain Memorial Association and lake overseer, the spillway had to be completely rebuilt, and on a tight time schedule. Many boats operate on the lake, so this huge project had to end in time for the lake to refill to normal level by spring.

Anytime a local lake is drawn down, take the opportunity to learn all that you can: (1) study its exposed shoreline structure; (2) shoot photos of cover and exposed structure; (3) create your own map and include these important features on the map; and (4) when possible, improve the lake by adding cover at strategic spots.

The old spillway was a straight concrete ramp pitched downhill at about a 35-degree angle. Designed in an age devoid of environmental concerns, this spillway system delivered varying volumes of water and hydraulic energy to the stream below it. During high runoff, the spillway blasted the small stream with a rush of whiter water, gouging soil and flushing the silt downstream. Environmentally, the old spillway was a disaster.

New Technology
It is a new day with a new generation of environmentally-conscious engineers and builders. Jimmy Garrison, co-owner of Development Planning and Engineering in Buford, Georgia was the lead engineer on the project. According to Garrison, “Instead of the conventional spillway technology with a long chute and large hydraulic jump at the bottom, in this spillway we step the water down, end with a lesser hydraulic jump, and let it trickle into the natural creek bed. This is unique; it is the first time we’ve built a step- spillway.”
The new “step-spillway” system creates maximum hydro energy control. Massive concrete steps bleed enormous energy from water flowing down the steep grade. These steps, coupled with the large baffles located at the end of the spillway, further subdue the energy in the spillway discharge water. Finally, the concrete settling pond depletes more hydro energy before the water flows over a slight hydraulic jump into a massive area of rip rap below the spillway and gently oozes into the stream.
The lake has a 14 square mile drainage basin, but now even with varying water volumes flowing down this spillway, the energy is dissipated, eliminating destructive downstream siltation. This new design meets operator and environmental goals so well that it is expected to change spillway designs for the future. Garrison is already using this high-tech design on other spillway projects in the Georgia area.
According to Bob Cowhig, this massive three month project consumed more than 5,000 yards of reinforced concrete. Large construction crews worked overtime to complete the new spillway, and (amazingly) brought the project to completion in less than two-thirds of the scheduled construction time! They knew that this was groundbreaking concept in spillway design, and they worked to make it both functional and attractive. Landscaping and fencing completed the aesthetics of the project. Good rainfall over the winter filled the lake, and the new spillway was fully active in the spring of 199.

For your Future
As fly fishers, we need to be aware of the dam-controlled waters and spillways around us. This new technology goes a long way toward reducing the harmful effects of spillway discharge. If you see construction on dams in your area, make construction personnel aware that new spillway technology is available to keep environmental and water resource problems to a minimum. For details they can contact Jimmy Garrison at Development Planning and Engineering at (770) 271-2868.

by Bill Byrd


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