“I don’t think we’ll ever have to do a big clean-out here again.” That was the sentiment shared by Willie Vance, stormwater operations manager at Environmental Works, Inc. (EWI), after excavating 25 tons of sediment from a massive stormwater basin, grading a 50-foot length of the basin between an outfall pipe and the central catch basin, and installing a “blanket” of Flexamat® at the mouth of the outfall.
Flexamat® consists of concrete blocks (6.5˝ x 6.5˝ with a 2.25˝ profile) locked together and embedded into a high strength geogrid. There is 1.5˝ spacing between the blocks that gives the mat flexibility and allows for vegetation growth, according to the Flexamat® website.
“We’ve used it on a couple of different sites; spots that are hard to maintain,” said Vance.
EWI identified this particular trench site at a department store in south central Kansas as a candidate for Flexamat® installation after dealing with habitual erosion and stormwater issues in the expansive 300,000 square foot basin.
“It’s one of our largest basins,” said Vance.
EWI’s stormwater team, which visits the site at least eight times a year during the spring and summer for inspections and landscaping, continually observed the outfall pipe that drains the entire western half of the store’s three acre asphalt parking lot holding water and not draining into the central catch basin due to sediment build up.
“That pipe is a 36-inch pipe, and, at any point in the season, it was holding 12 inches of water,” said Vance. “There’s a lot of residual asphalt and sandy type of material that just collects down there (in front of the pipe opening). All of this build up was prohibiting water flow, reducing water quality and making part of the basin fail. You’re talking about mosquito nuisance and erosion problems.”
Vance expects the Flexamat® installation to reduce sediment build up in the basin, and adds that any sediment build up that does occur will be easier to clean and remove from atop the concrete blanket as opposed to removing sediment from “rip rap,” the large man-placed rock commonly used in stormwater basins for erosion control.
Flexamat® is also much more permanent than “rip rap,” which washes away during big rain events over time. Vance estimates that “rip rap” lasts seven to nine years in a typical stormwater basin, while the concrete blanket his team installed lasts 20 years to a lifetime. This saves not only time and effort in the long run, but money for the client. Instead of having to remove the old “rip rap” and haul and install new rock, as well as excavate any sediment build up, Vance and his team will be able to drive a EWI-owned Guzzler Vacuum Truck down to the basin and remove the residual materials when necessary.
“We’re not having to stockpile old rock that we’re trying to dispose of, and then bring the new rock in at the same time,” said Vance. “This is going to be a permanent fix, where we can do occasional cleaning of it at a much lower cost than doing the whole ‘rip rap’ reconstruction.”
And Flexamat® allows for vegetation growth, assimilating with the natural landscape over time.
“We put down the same types of indigenous grasses that are in these basins in Kansas – our fescue and tall grass,” Vance said. “We seeded the graded basin and laid the concrete blanket on top.”
Although this particular job took two and a half days because of the 25 tons of sediment that needed to be removed before the Flexamat® could be installed, Vance said that the concrete blanket “can be laid in a matter of hours.”
And that’s music to a client’s ears.
“The less we’re disturbing their store and how they operate and at the same time doing stormwater repairs and helping them stay compliant, the better,” said Vance. “That’s a good thing.”