×

Work begins to stabilize Suncook River in Epsom, 11 years after Mother’s Day Flood

  • An excavator puts tree branches and brush into a mulcher while clearing the bank of the Suncook River on Oct. 10, 2017. GEOFF FORESTER—Monitor staff

  • An excavator places sandbags down on the rock dam to divert the Suncook River to avert another flood like the Mother’s Day Flood of 2006. The branches and trees to the left are from past flooding. GEOFF FORESTER—Monitor staff

  • Suncook River Steve Leone—

  • Suncook River Steve Leone—

  • An excavator places sandbags down on the rock dam to divert the Suncook River to avert another flood like the Mother’s Day Flood of 2006. GEOFF FORESTER—Monitor staff

  • A dump truck passes by the row of rocks that will be used to fill in the Suncook River to help avoid another flood situation like the Mother’s Day Flood of 2006. GEOFF FORESTER / Monitor staff



Monitor staff
Wednesday, October 11, 2017

Eleven years after it broke through a bank during the Mother’s Day Floods of 2006, creating a new riverbed and beginning a slow, destructive spread eastward, the Suncook River in Epsom is about to get a $3 million makeover designed to stop it from shifting.

The most obvious part of the makeover involves placing big boulders to protect the Route 4 bridge near the Epsom Traffic Circle, which is endangered by the Suncook’s new route, but just as important is less visible work that involves burying a massive steel wall downstream.

“The wall will be 40 feet deep, hundreds of feet long, and will cross the entire length of the cornfield,” said Steve Landry, supervisor of the watershed assistance section at New Hampshire Department of Environmental Services, regarding the sheet piling being installed as part of a river stabilization project. “It’s a fail-safe measure, in case the river should outsmart and outflank us downriver, and try to come up and undo what we’re doing by the bridge.”

The wall and boulders are necessary because of geology. This portion of river valley is made up of sandy soils, with bedrock existing between 40 and 65 feet down – too deep to serve as anchors for bridge abutments or anything else that shouldn’t be moving.

“This is a beautiful post-glacial valley. It’s great for sand extraction, nearly impossible for river stabilization,” Landry said.

For decades, the sand was of interest mostly for its use to make icy roads safer. About 14 acres once owned by the late Bob Cutter and given to the state in 1947 have long been the source of sand used by the Department of Transportation after snowstorms.

In mid-May 2006, a huge low pressure system stalled over New England and dropped more than 8 inches of rain on Concord in three days, the largest three-day precipitation total ever recorded in the city. Water poured down the Suncook, and the river broke through the bank separating it from this sand pit, creating a new route that bypassed about a half-mile of old riverbed, including Bear Island and two small dams.

There is debate about the role the pit played in causing the river to abandon its bed, a process known as avulsion, with some saying that too much sand was removed, making the change more likely. There has also been disagreement from some property owners and town officials over whether the state has responded strongly enough.

Whatever the cause of what was called by some the most dramatic change of course for a river in New Hampshire’s history, the change is so unusual that people still come to study it – in fact, a class from MIT showed up this week, Landry said.

“An avulsion occurs so rarely that this project area has become kind of a hot spot for people who study geological processes,” he said.

The area has had a couple smaller floods since, and the river has been eating away at the sandy banks and slowly widening itself to the east, threatening cornfields, homes and the Route 4 bridge. Hence the stabilization project.

Lacey said a study after the initial avulsion concluded that it would be too expensive and difficult to force the river back into its original banks.

Near the bridge, the riverbed and banks will be stabilized with boulders around 3 to 4 feet in diameter, covered with what Landry called “fabric burritos,” filled with “nice organic dirty, loamy stuff” that will allow grasses and trees to take root.

The entire construction project is scheduled to wind up by the end of the year and cost $3 million, not including the amount spent over the years for engineering and studies.

The state is using $1.8 million in capital appropriated funds, the Department of Transportation is spending $900,000, and roughly $300,000 worth of stone being used to shore up the riverbeds is coming from a gravel and concrete company in Kingston as part of a settlement agreement following wetlands violations.

In May 2011, the state approved $2 million for nonfederal cost share to buy out and demolish the homes of residents who were likely to be chronically flooded, leveraging up to $6 million more in federal money. In all, by mid-2011, $6.7 million was secured for projects on the river in the aftermath of the flood, while $21.1 million was denied for lack of matching funds or withdrawn, according to DES data.

The river’s destruction of sand banks has revealed some interesting geology.

“There are perfect laminations ... from glacial Lake Merrimack that used to occupy the middle of New Hampshire” after the Ice Age glaciers retreated. “There’s sediment in it – you’re touching it and thinking, ‘This is 16,000 years old.’ ”

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)