Photos and article by BILL BYRD, Jr.
First appeared in FFA Magazine January 2000 issue.
Why would a fly fisher care about dam construction practices? If dams release water into streams and water systems without erosion and sediment control, the downstream areas can be destroyed by sediment!
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 dam 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 completed on a tight time schedule. A beach operation, riverboats, small boat rentals operate on the lake, so this huge project had to be completed in time for the lake to refill to normal pool by spring.
Anytime a local lake is drawn down, take the opportunity to learn all 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. (4) When possible, improve the lake by adding cover to lakes at stragegic spots. This fishy looking ditch (left) goes unseen with the lake at full pool.
The old dam spillway was a straight concrete ramp pitched downhill at about a 35 degree angle on the dam's backside. Conceived in an age devoid of environmental thought processes and concerns, this spillway system delivered varying volumes of water and hydraulic energy to the stream below it. During high water runoff, the spillway blasted the passing small stream with a rush of white water, gouging soil and flushing the silt downstream. Environmentally, the old spillway was a disaster. The sediment it blasted loose was enough to kill the creek below.
NEW GOALS/NEW TECHNOLOGY
It is a new day with a new generation of environmentally conscious engineers and builders. Jimmy Garrison, owner of Development Planning and Engineering in Buford, Georgia, was the lead engineer on the project. Jimmy coordinated with Bob Cowhig, lake overseer; Tom Woosley from Georgia DNR's Safe Dams Program, the entity which inspects dams and enforces maintenance programs; Karl Myers at Piedmont Geotechnical Consultants; and Jim Wilkerson of J.M. Wilkerson contracting of Marietta, Georgia, the contractors for 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" (steps in image right) system creates maximum hydro energy control. These massive concrete steps bleed enormous energy from water flowing down the steep grade.
These steps coupled with large baffles (center of LEFT photo in holding pond) located at the end of the spillway further subdue massive 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 massive downstream siltation. This new design meets operator and environmental goals so well that it is being studied and is expected to change spillway designs for the future. Garrison is already using this unusual spillway design on other 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 hurriedly to complete this project a whopping 40% ahead of schedule! They knew that this project was ground breaking and they worked to make the new spillway not only functional, but a work of beauty. Landscaping and fencing completed the aesthetics of the project. Good rainfall filled the lake for full spring operation.
FOR OUR FUTURE
As fly fishers, we need to be aware of dam released waters and spillways around us. This new technology goes far in reducing the harmful effects of spillway discharge. As more and more dams are constructed with this technology, more and more studies are being completed to better understand and further refine this necessary technology. 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, contact Jimmy Garrison, Development Planning and Engineering, (770) 271-2868 or email to Jimmy Garrison.
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