DERRY — Emergency work continued this week to repair the Beaver Meadow dam after town crews discovered some “significant undermining” of the structure.
American Excavating crews began preliminary work last week after a resident called the town to report an unusually swift flow of water at the dam.
The dam dates back to 1929. It is accessed off East Derry Road from Sabra Circle and Martha Drive at the southern end of Beaver Lake.
After initial inspections by town and state officials, public works director Mike Fowler said they found a hole in the dam. He hopes it’s an easy fix.
“The hole seems to be less dramatic than we thought,” Fowler said.
The dam’s location is important; it helps hold the water at Beaver Lake.
“This dam can’t come out,” Fowler said. “It needed to be fixed.”
Early work included placing sandbags at the dam to control the water flow and lower the level of nearby Meadow Pond so crews could see potential damage.
The water level in the pond eventually dropped 6 to 8 inches.
“We started a plan to try to isolate the area,” Fowler said.
Water was then pumped and the area dried out so damage could be assessed.
Holes like this often appear in older dams, Fowler said.
“In 1929, the date is etched on the dam, there may have been no footing or the dam was built on virgin ground,” he said.
The dam received significant upgrades in the early 1990s. A gate to control the water level was replaced.
Once there is a breech in dam performance, the state is always notified, Fowler said. Inspections take place prior to repairs and then inspections continue once work gets started. Catching the problem early was key.
“We are fortunate to catch it when we did,” Fowler said. “The (state) Dam Bureau is always concerned about a breach of this nature, when the water velocity picks up. The hole can grow.”
Fowler estimated dam repairs could cost upward of $15,000 to $20,000, depending on the extent of the repairs.
Work may include pressure grouting to fill other cracks and voids found in the dam. Officials can use a waterproof camera to get beneath the dam structure for additional inspection. When water flows so rapidly due to structural damage, soil and landscape can be comprised.
“You don’t know how much soil you’ve lost,” Fowler said. “We want to try to protect what’s left underneath there. We need to see how much erosion there was.”
Fowler said Derry owns another dam at Hood Pond. A lagoon near the transfer station is also considered a town-registered dam.
An inactive dam at Beaver Lake on Pond Road was removed in 2011. Dams that are no longer in use can become an expensive liability for the town. Others dams scattered around town are privately owned.
It’s always good to be prepared for any dam problems that occur, Fowler said. He said the town and residents living near the dam did a good job responding to the trouble.
“This wasn’t something we expected to happen,” Fowler said, “but we had to address it as soon as possible.”