TAKE A MONTH
After hearing the cost estimates, board member Roy Hadden suggested the solution may need to be brought to the voters. But time is of the essence as engineering and permitting the repair/removal will take about a month and while the dam and pond are not currently unstable, a heavy storm could destabilize them.
The town took action to reduce water levels in the dam last month after the outtake pipe from the town pond to a lower pond was submerged because water was not draining out of the lower pond. Additionally, a large sinkhole developed in the earthen dam separating the two ponds. That action earned the town a 1272 violation order from the state for discharging water into a wetland without a permit. That state order required the town to hire an engineer to develop a plan for remediating the wetland and dealing with the pond/dam issue.
Patenaude provided the board with an extensive and clear understanding of the dam, and how it might be fixed or removed or modified. He explained that the outlet pipe was made of corrugated metal which normally lasts between 15 and 25 years. He said that when he crawled inside the pipe to where the intake pipe enters he found that the pipe had separated two to five inches and that there was a void around the outside of the pipe beyond what he could measure. That void led to the sinkhole this spring.
GEOTEXTILE FABRIC
Patenaude also said that he discovered geotextile fabric around the outlet pipe indicating that the ground had been too wet when the dam was created and problematic because the fabric prevented the soil from knitting itself back together.
"As for the break in the pipe, the only way to fix it is to excavate that section of pipe and try to rejoin it. Doing that means exposing most of the pipe and you're nearing or past the design life of that pipe. If you have it excavated, why not replace it along with the riser? The pipe is 80 feet long and it is not a costly or involved process," he said.
PRECAST CONCRETE
He said that digging out and replacing the entire structure with precast concrete was the recommendation of his firm. He urged the board to remove the geotextile fabric as well and suggested that the board have the earthen dam removed and set the material aside to dry out because its moisture content was too high, the entire dam is sodden from the water that leaks through the break in the pipe. After the material dries out, it will need to be mixed with some dry material so the water content in the new dam is right.
If the town does that, he said, the dam's life would be extended by 30 to 50 years.
"Your other alternative is to remove the dam and return the area to a wetland complex. But that depends on how the town classifies the pond, as a recreational resource, or a fire protection resource, or part of a permitted stormwater management plan," he continued. Removing everything would cost between $10,000 and $20,000, he said.
DREDGING
"If and when the pond is dewatered, it's an ideal time to do some dredging where sediment has accumulated," Patenaude added.
"Well we don't need it for fire protection if we're going to put in a water system," board member Paul Hartshorn said.
"Sure, if the water system were being built right now," said Patenaude.
Board member Bill Parker asked how the state would be involved in removing the dam and recreating a wetland and Patenaude said the state would want to be involved, but the town's liability for the dam would go away once wetland restoration had been completed.
SIZE OF POND
Board chair Charlie Hosford asked how the size of the pond would be affected and was told the pond would be reduced from three-quarters of an acre to one-tenth to one-quarter of an acre.
Parker asked if rebuilding the pond and properly maintaining it would prevent this type of emergency and was told that it would. Patenaude said the town would need to dedicate a half a day to assessing the dam every five years and said that if rebuilt with new materials the town could see if and when any deterioration was taking place.
"Your next step, regardless of what you are going to do with the dam, is to get the rest of the water out. You still don't want to drain the pond more than one foot per day, but we're no longer worried about the embankment slumping away. At this point, it's reasonable to think the hydrostatic pressure on the dam that was causing the problem has gone away," the engineer explained.
100-YEAR STORM
He said that a 100-year storm could refill the pond again and recreate the unstable conditions that threatened the dam. Board member Hadden said that if there were a 100-year storm, the pond and dam might be the least of the town's worries and said "I'm not sure the five people sitting at the table should be the ones to make this decision. We may need to take this to the voters."
Parker asked if the town could begin the excavation without having decided whether to repair, remove and reduce the size of the pond and dam. Town Administrator Valerie Capels said the town would need to have the project defined so that it could be put out to bid.
"Over the course of the next few weeks to a month, you've got to be thinking about what way you want to go. The state won't approve a lengthy process and the state wants a discrete, finite release of sediment, which is going to happen when you excavate," Patenaude said.
"I'm not sure if there's a permitted stormwater treatment system that uses that pond, but if there is, you're going to have to keep providing it. Leaving the pond at a quarter of its size will still provide treatment. Reducing the size of the pond would be a good compromise, assuming there aren't requirements that it be a specific size," he continued.
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