The majority of the ditches were in limited spaces, either up against parking lots, roads and even buildings. The first challenge was the constrained spaces the stormwater conveyance ditches reside, scour of the side slopes would compromise road shoulders and even a foundation of a building on one portion of one of the ditches. Given the volumes of water expected to flow through these ditches, EnkaMat 7010 is fully capable to resist soil movement under it, providing a factor of safety of 2. A less durable or degradable matting could lead to soil movement and eventual damage to surrounding failure of adjoining foundations and structures. Another challenge was the late fall installation of the turf reinforcing system. The contractor had included temporary grasses and clover in the seed mix thinking that it may be too late for germination of the specified warm-season species. As it turns out he was correct, in the spring he was able to scalp the temporary grass that did establish, and the open structure of EnkaMat 7010 allowed him to reapply a thin layer of seed, soil amendments, and BFM class hydraulic mulch directly into the 3-D matrix of EnkaMat. By late spring the Bermuda grass and clover were establishing well and were due their first mowing weeks later. Other TRMs dense fillers would have prevented the contractor from effectively reseeding, as the seed would have merely sat on top of the mat and would have likely become bird feed. EnkaMat’s flow rate and open structure seal success.
THE ENKA SOLUTION
The engineer liked the cost benefit over using rock rip rap linings and other hard surfaced materials and really was more interested in the environmental benefits gained from utilizing EnkaMat to extend the permissible shear stresses and velocities of natural vegetation. With EnkaMat 7010, he was able to design the ditch with the same confidence had he lined it with 2’ of rip rap. He also liked the thought that natural vegetation is not only more aesthetically pleasing, but it also permits filtration, infiltration (groundwater recharge), bio-remediation, and reduction of thermal pollution (increase of stormwater runoff temperatures) among other benefits.
INSTALLATION BENEFITS & RESULTS
The ditch lines were cleared and reshaped, anchor trenches constructed and the EnkaMat was laid down the centerline of the ditch. In the deeper ditches, the mat on the side slopes was oriented up and down the slope to eliminate critical horizontal overlaps on the steep side slopes. Once in place and properly secured with sod pins, the mat was filled with an FGM class hydraulic mulch.
With EnkaMat, the city was able to extend the natural limits of vegetation and enhance the vegetation’s hydraulic performance to act more like a hard lining. The ditch shapes and elevations will not change over time like they would if just grass alone were used. And rock rip rap, concrete linings or ACB’s were not required on any portion of these stormwater conveyance ditches to meet their needs. Aesthetics play a great part in this project, as the ditches are in plain sight along busy Chattanooga auto row roads and parking lots.