Matting encourages growth of vegetation at dam

Coon Creek Dam, located upstream of Lennox, Tenn., about 75 miles northeast of Memphis, was built to create a 125-acre reservoir on a section of the creek. In recent years, managers had been experiencing erosion problems at the earthen dam’s emergency spillway that were not responding to traditional methods of erosion repair.

The dam is constructed of a porous silt common to the area and is highly prone to wind and water erosion. Engineers from the Obion-Forked Deer Basin Authority, the local agency that maintains the dam, tried for years to stabilize it with vegetation and to protect its back-slope spillway against rain runoff and high water levels by the use of drainage systems.

Because of Lennox’s location downstream from Coon Creek Dam, the dam has been classified as a high hazard-type structure by the state and is thus inspected every year by the Safe Dams Division of the Tennessee Department of Environment and Conservation. In the summer of 1993, the basin authority faced a costly and labor-intensive overhaul of the emergency spillway system after state officials insisted that a permanent erosion control solution be found.

Growing vegetation in the rich, loamy silt at the dam was never a problem, but keeping it there was difficult, according to John Cooke, an engineer with Continental Engineering, Memphis, Tenn. Engineers managed to establish a good vegetative stand, but even the smallest amount of runoff was causing erosive reels, followed by headcutting.

The outlet end of the emergency spillway was steep. Traditionally, as a last resort, rock riprap would have been installed to alleviate the erosion problem in such a case.

At this location, however, riprap would have been expensive, and bringing in truck loads of stone would have been destructive to the rest of the emergency spillway.

“What we had done [at the dam] was try to take vegetation, such as fescue and Kentucky 31, and stablize the slope, but the type of soil there is a Memphis loess silt, which has a very light bonding effect,” says Joe Kerley, project manager with the Obion-Forked Deer Basin Authority.

“When rain hits those fine silt particles, it just floats out and the vegetation is destabilized. We had tried everything we knew, from using grade structures to trying to slow the rate of rainfall in the spillway with vegetation,” Kerley says.

“We looked at studies from other places with similar soil problems, such as Oklahoma and the Mississippi Delta, where the departments of transportation had success in achieving stability by incorporating [nylon matting] with vegetation. The matting gives the roots something to lock onto.”

Engineers thus decided to use a 3-D geomatrix nylon matting at the Coon Creek spillway. Around 3,400 square yards of the matting, manufactured by Akzo Nobel Geosynthetics, Enka, N.C., were installed, stretching 250 feet from near the water’s edge to the back slope of the spillway.

After the mat was laid, it was seeded and covered over with an excelsior blanket manufactured by Erosion Control Systems, Northport, Ala.

“The covering looks pristine,” Cooke says. “There’s no evidence of erosion [or] that [the matting] went in at all. It’s made it through some pretty severe weather.”

The matting is currently being considered for use in other projects, such as wooden wave wall replacements along the shoreline, that have traditionally made use of riprap, according to Kerley.