A plan to reconstruct and restore a stream that feeds into Mamanasco Lake submitted on behalf of the gained faborable review from the Ridgefield Inland Wetlands Board on Tuesday.
As explained by civil engineer John F. McCoy, of JFM Engineering, representing the MLIF, recent storm events caused considerable erosion along a section of the stream that runs adjacent to Third Lane, just after it comes down a hill and before it enters the lake.
During these storm events the high volume of water carved out a large gully, or "gorge" where the stream bends — as deep as five to six feet, McCoy said — and the sediment was carried downstream and deposited into the lake. Once there it formed "a large delta," said Katherine "Kitty" Fischer of 5 First Lane, who along with neighbors heads up the MLIF, which was formed years ago to fund watershed improvement projects around the lake. The non-profit group uses a mix of private and town money to fund its projects.
"This section of the stream, although it conveys silt and sediment from upstream, also of its own erosion has been contributing heavily to the sediment in the lake," McCoy said. "The primary goal is to stabilize the stream, minimize the disturbance in doing so, and to eliminate the erosion problem in that section."
McCoy said the project will be accomplished by rebuilding the stream bed using gabions, "which are basically wire baskets that are filled with rock."
"The advantage of [the gabions] is they can be set with a minimal amount of machine work," McCoy said, adding that this is important as the area is environmentally sensitive. "We looked at doing a rip rap channel, but because the flow is pretty substantial, and the channel is pretty steep, we would have needed boulders of two to three feet in diameter the whole length of the channel, bottom and sides — and it would have been a very slow tedious project to put all that in."
McCoy said the project involves lining the bottom of the stream bed with what are called "gabion mattresses, which are a foot deep, six feet long and three feet wide." These mattresses will be filled with stone before they are laid.
He said preparation of the channel itself will be minimal "as you only need to make enough space for each [gabion mattress]." He added that care will be taken to avoid disturbing trees that line the stream's banks.
McCoy explained that the sides of the stream will be walled off using larger gabions measuring 3 x 3 x 6 feet. These gabions, he said, "will be filled in place."
In two places the gabions on the stream bed will be stacked form 2 foot high "steps," as the stream rises slightly at an incline as it moves away from the lake.
"At first the water will be able to flow through the gabions, during low-flow periods, but once silt and debris fills the voids between the rocks, they will form barriers," McCoy explained.
McCoy explained that near the top of the hill, where it is steep, the gabions that make up the stream bed will be positioned to form a smooth slope.
Board member Michael Autuori wanted to know the life expectancy of the metal elements used in the gabions. McCoy said "40 to 60 years" and added that the metal baskets are coated with plastic to help prolong their life.
Autuori however said he would prefer to see stainless steel used, so as to avoid using a plastic coating that could eventually dissolve and end up downstream. McCoy said he would check to see if stainless steel was an option and added that it would likely make the project more expensive.
McCoy said the Mamanasco Lake Improvement Fund hopes to start the work this summer, "however it's a [partially] job, so we have to go through an approvals process."
During the meeting David Cronin, a member of the Ridgefield Conservation Commission, which had earlier approved the project, said some members of his commission shared the same concerns about protecting trees.
"We think this is a very good solution to fix the problem, which is severe," Cronin said, adding that most of the sediment that went into the lake came from the area where the stream eroded into the land.
McCoy said they have identified four mature trees "which must come down" where a sediment basin is to be constructed — however he said the plan is to protect the remaining trees.
Board member Joe Fossi, who made the motion to approve the application, said it "seems like a reasonable solution to fix the problem — and it has the support of our Conservation Commission, so I see no reason to delay this process."
Board member George Hanlon seconded the motion saying, "I haven't lived up there for 40 years now… but to see the difference between then and now… it's only going to get worse and I think we need to find a solution to this problem as soon as possible."
Toward the close of the hearing, Fischer said she wanted to make it clear that "it has never been determined who owns this lake."
"The state owns a boat ramp on the lake — so we have to follow their rules — and the town is the biggest abutting property owner, with Richardson Park, which borders the lake," Fischer said. "That's why this is just as much a town project as it is a Mamanasco Lake Improvement Fund project."