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In early 1999, the Tennessee Valley Authority (TVA) approached the City of Chattanooga, Tenn. with a proposal and seed monies to complete a bank stabilization demonstration project at the city's North Chickamauga Greenway near the confluence of North Chickamauga Creek and the Tennessee River. Over the years, water levels and flow direction constantly and rapidly changed, due to hydropower generation at the TVA Chickamauga Dam (just downstream). Water elevations could vary as much as three feet in a short period of time. Also, increased volumes and peak flows of water during storm events - due to urbanization and overland flow from an existing parking lot - had eroded the toe of the 20 to 25-foot high bank. The bank had become inherently unstable, causing several large trees and portions of a split rail fence to fall into the creek. The existing parking lot at the top of the slope was also threatened. In August 1999, the City of Chattanooga entered into agreement with the TVA to complete the project. The TVA supplied all necessary materials and completed the permitting process. Chattanooga supplied the labor and equipment. "Our team, one of 11 in TVA's Resource Stewardship organizations, works to create sustainable watersheds by protecting and improving natural resources for beneficial uses," said Linda Harris, Senior Field Representative of the TVA. "We work cooperatively with partners and coalitions on protection and improvement projects throughout the watersheds of Chickamauga and Nickajack Reservoirs (in the Chattanooga area)." The design called for the creation of a 196-foot toe along the creek's edge. Great care was taken by the city's equipment operators to minimize the amount of disturbance in the creek itself. Prior to excavation and site clearing, a silt boom was placed in the water, adjacent to the shoreline. A silt fence was installed to provide backup to contain any sediment that could discharge into the creek. Existing vegetation from the area was cleared. Trees along the shoreline that had been uprooted from the water's erosive force were removed. Large stones (six to 12 inches) were used to fill the voids left from the uprooted trees and to create the toe of the slope. Stone was chosen to form a foundation to resist the backwaters and was the most logical choice over soil to form a stronger foundation to resist the backwaters. It was a more logical choice over soils, which would have had to have been compacted beneath the water level at some point in the final grade preparation. Considering the original slope prior to site preparation was 1:1 and consisted of many potential failure plains, the project engineer decided to design an embankment consisting of three unified slopes. The lower, or base, slope was cut at 2:1 from the waterline to the anchor trench, which rose to six feet above the lowest known water level. This slope was then smoothed and top-dressed with 3/4-inch drainage stone. A middle, or bench, slope was cut just above it to 10:1. Finally, the upper slope was cut to 2:1, which rose to the parking lot. A decision was made by the City Engineer and TVA to use Armorflex, made by Armortec of Bowling Green, Kentucky. This articulating concrete block mattress system is used for slope stabilization and to protect the subgrade against high velocity flows. Armorflex blocks are cabled together longitudinally with galvanized steel or polyester cables. This provides ease of handling and installation, as well as increases the factor of safety. An anchor trench was created for the mats by using a trackhoe at the top of the 2:1 slope, allowing a minimum of 2 rows of block to secure the system. Mirafi Filterweave 500 fabric was then placed over the drainage layer of stone and held in place with six-inch soil staples. A total of 7,200 square feet of fabric was used on the project and was overlapped not less than one foot, in a shingle effect, beginning from the downstream end. A specially designed spreader bar was used to place the mattresses in 27 foot x 8 foot sections over the filter fabric. Due to the distance from the parking lot to the lower slope, which represented approximately 65 feet and a drop of 13 feet, a 120-ton crane was contracted to accommodate the reach necessary to place each mat. The weight of each full-sized mat was 10,800 pounds, with a combined total weight of approximately 12,500 pounds, including the spreader bar. A total of 24 mats would be installed on the slope. Armortec provides unique features within its system. One of these features combines the use of more than one style of block within the same mat, the other allows for a mat to be custom sized to any configuration. In this case, open-cell block was used on the slope to allow vegetation to establish to the waterline and closed-cell in the same mat beneath the waterline for more weight, stability, and controlled flow protection. Installation of the system began from the upstream end using full-size mats and three custom-designed mats to fit the radius of the slope. During the fabrication of each mat at the manufacturing facility, another unique feature was utilized. Select blocks were removed from each mat to allow for Helix anchors to be installed at the job site. The anchors were screwed in place with a power take-off and eventually connected to the mat cables by grout. Two Helix anchors at the top end of each mat were installed to reinforce the system and the trench was then backfilled with native soils in preparation for the bench slope. Grout was poured between the installed mat at the existing canoe launch, located at the upstream end of the project. Grout was also poured in any seams that connected the customized (angled) mats. The mat located farthest downstream was toed by burying two rows of block from top to bottom of the system. The bank would then be sloped at a 10:1 grade for an additional 10 feet to create a bench, which would function to remove additional weight from the slope. An upper 2:1 slope was eventually established from the bench to the parking lot. Lastly, North American Green C-350 permanent geotextile was installed on the top slope and anchored with soil staples. The top slopes were then hand broadcast with a seed blend customized by Jen-Hill Construction Materials of Hendersonville, Tenn. The site is often excessively wet in winter months and impacted by flooding. It lacked vegetation and organic matter, and consisted of highly erodible soils. The soil is nutrient poor and does not sustain grass well after growth begins. During the summer months it is exposed to extreme heat and drought conditions. To address these problems, the following seed mixture was developed: o Crown Vetch - Legume; good nitrogen fixer. o Red Top - Cool season grass; tolerates wet conditions; requires little fertilizer; provides wildlife habitat. o Common Bermuda - Sod-forming; tolerates heat. o Perennial Rye - Cool Season grass; high moisture tolerance; rapid germination; gradually dies out in about 5 years in this area. o Japanese Millet -Summer annual; rapid germination; high moisture tolerance; used as a nurse grass & wildlife food source. o Sericea Lespedeza -Woody species; roots to 2-feet-long; drought tolerant. "Two different fabrics were required for the project because of the different applications," Harris said. "We selected NAG C-350 because it combines the superior erosion control effectiveness of a coconut fiber blanket with the permanent root reinforcement capabilities of a synthetic matting." Harris added, "Coconut fiber contains lignin, a cell strengthener that keeps the fiber from rotting for several years. The synthetic matting continues to protect after the natural fibers are gone, so scouring of the vegetation is minimized during flood events long after the coconut fibers are gone. The excellent coverage we attained speaks well for this erosion control/turf reinforcement mat and the grass seed mixture that was applied." Finally, an asphalt berm was built on the parking lot to divert water away from the streambank. A fence and picnic tables create an aesthetically pleasing boundary between the parking lot and the embankment, inviting passersby to walk along the waterside or to dock their canoe at the launch. This heavily utilized area has been preserved and enhanced, minimizing future impacts to water quality from the site. David Kees is Mid-South Regional Manager and Director of Marketing and Advertising for Armortec.

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October 15, 2019, 5:25 am PDT

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