Erosion on the banks of a channel in Knoxville, TN, was jeopardizing land and adding to an already critical sediment problem in a nearby creek. Flowing between the concrete retaining wall of a restaurant parking lot and a recreational ball field, the channel threatened to encroach on the field and was undermining the retaining wall. The channel emptied into Second Creek, which, because of excess sediment, is on the Tennessee Department of Environment and Conservation’s 303d list-a list established under section 303d of the Clean Water Act to identify waters where required pollution controls are not expected to attain or maintain water-quality standards.
About 300 lin. ft. of the tributary needed reinforcement to prevent further erosion. The most challenging portion of the project included the first 120 ft. of the channel, where a 54-in. concrete storm drain ended and the natural channel began. Directly downstream from the pipe, the channel angled 45º to the left and, 80 ft. farther downstream, made a 90º turn to the right, about 200 ft. before it entered Second Creek.
The city considered three options to stop the channel erosion. The first, riprap, was rejected because it does nothing to filter stormwater sediment, and Second Creek was already receiving excessive amounts of sediment. Reno mattresses-rock-filled wire-mesh structures-were also considered. They weren’t suitable for the 90º bend, however, because the sideslope was nearly vertical at that point and the retaining wall footing was already exposed. The third option, Pyramat high-performance turf reinforcement matting (TRM), was chosen because it can withstand the high stormwater velocities from the concrete pipe after rainfall. It was also the least expensive of the three options.
Pyramat, manufactured by Synthetic Industries Inc. of Chattanooga, TN, is a permanent, UV-stabilized, rolled erosion control product. TRM, such as Pyramat, intertwines with the natural roots and shoots of vegetation, creating a stabilization matrix that keeps soil in place and promotes further vegetative growth. Because natural vegetation is nature’s best filter for sediment and other pollutants, TRM, by promoting vegetation, helps rid stormwater of pollutants.
Preparing the site was a simple process of sloping and smoothing the channel banks to accept the material. The current meander of the stream was not altered. The channel was graded to a 1.5:1 or 2:1 slope.
To begin installation, the area around the concrete pipe was dressed and seeded. Next, a hole the size of the diameter of the concrete pipe was cut into the Pyramat, and the material was positioned over the pipe as a protective collar. In the channel and pipe outlet apron, the matting was placed lengthwise, overlapping on the sides. Sod staples, 1 x 6 in., were used liberally to hold the matting in place, and live willow stakes were planted throughout the matting. Installation took two days, including planting trees, installing the material, and building check dams.
“Early indications are that this solution is successful,” says David Hagerman, stormwater-quality manager for the City of Knoxville. “The erosion has stopped and the vegetation is flourishing.” Long-term benefits of the project include reduction of sediment loading and deposition downstream, elimination of channel migration that was endangering the ball field and retaining wall, and an increased vegetative channel lining that reduces the impact of urban runoff on Second Creek. The same design is currently being used on a larger site along Second Creek near the World’s Fair Park in downtown Knoxville.
