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Wilmington eyes new risk for ice jams

Army Corps of Engineers, town discuss strategies for dealing with another legacy of Irene

WILMINGTON—Wilmington took another step toward post-Irene recovery last week when the Selectboard and residents met with representatives of the Army Corps of Engineers to discuss the potential for ice jams in advance of the spring thaw.

The Corps employees, board members, and town employees boarded a Moover bus to inspect local waterways, including the North Branch of the Deerfield River, Beaver Brook, and Harriman Reservoir (also known as Lake Whitingham).

The officials and engineers viewed damage caused by Tropical Storm Irene’s flooding, which swamped the downtown last summer, to determine how such changes could exacerbate the formation of ice jams.

According to Selectboard members, Irene changed the local rivers through erosion, redepositing sediment and dumping debris. The board hoped to partner with the Corps on studies that would help remove the Irene-deposited debris from the rivers.

According to Corps engineers, in addition to the debris, the rivers have become more shallow, upping the chance of more ice jams and flooding in new locations.

Types of jams

Carrie Vuyovich, a research hydraulic engineer with the Corps, described the characteristics of ice jams and potential remediation.

Vuyovich and colleague Dr. Meredith Carr serve in the Corps’ Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, N.H., one of only a few in the country.

Ice jams occur when river ice breaks up. The chunks pile up and wedge together, preventing the flow of river water, Vuyovich said.

These jams can cause flooding, erode riverbanks, remove vegetation, or impede navigation or commerce.

Jams form either as thermally grown, or as frazil ice, Vuyovich said.

Thermally grown ice freezes from the river’s banks inward into a solid sheet. Engineers can estimate the ice’s thickness based on air temperature, she said. During break-up, thermally grown ice jams break into large, solid chunks of ice.

Frazil ice forms like slush in fast-moving water, she said. When the water and air are very cold, small ice particles form, eventually flocking and creating bigger ice floes.

As the patches of frazil ice move like sediment through the river, they reach the solid ice chunks and the heavier ice deposits eventually grow into an ice jam.

Frazil ice can accumulate into a 12-to-15-foot-deep cluster, said Vuyovich.

Engineers can also determine the amount of time before an ice jam releases based on air and water temperature, she said. With this information, communities can decide how to best manage a jam.

Sometimes doing nothing is the best action, Vuyovich said.

If a community decides breaking the jam is the better measure, it can use two techniques — thermal melt or mechanical break-up — to remove the ice.

Thermal melt raises the temperature of the water by only a few degrees until the ice melts. Drilling holes in the bright, light-reflective ice often works by exposing the darker water, which in turn absorbs more heat.

Breaking the ice with a device for the purpose is an example of mechanical break-up.

Vuyovich explained that the Corps uses its CRREL Ice Jam and Ice Jam Flooding database to track reported jams nationwide. The database tracks historical and real time events, she said.

According to Vuyovich, the database has 918 recorded ice jams in Vermont dating from 1785 to 2011. Of the jams, 298 were break-up jams and 12 were freeze-ups on 96 different rivers at 149 locations.

Wilmington had nine reported events between 1963 and 2011. All of the jams occurred between January and March.

Three of the jams were classified as break-up, with the rest unknown, she said.

Beaver Brook saw six jams, and the rest were on the Deerfield.

According to Vuyovich, the Corps’ engineers respond to municipal calls every winter to evaluate and consult on ice jams.

Next steps?

Robert Russo, a project manager with the New England district of the Corps’ Engineering/Planning division, explained that the Corps responded over 20 years ago to requests from Wilmington for evaluating ice jams and flooding.

Although Corps engineers found areas of concern and offered solutions, those measures did not carry enough benefits relative to the construction costs.

If Wilmington wants the Corps’ help, he said, the town needs to put the request in writing.

The Corps can perform river studies, he suggested.

The town might also qualify for other programs like flood damage reduction, which assists local communities by identifying flooding problems and suggesting construction alternatives to reduce flood damage.

Such “section 205” projects, however, come with a $7 million limit. In addition, the local government must pay 50 percent of feasibility studies exceeding $100,000 and contribute 35 percent of the cost of project plans, specifications, and construction, said Russo.

“We can design anything and make it as cost-effective as possible, but the other end of it is environmental and economic,” he told the public when he was asked about funding sources and permitting. “You’ve got to be creative when you manage a study.”

Audience members repeatedly expressed frustration with the state Agency of Natural Resources (ANR), which authorizes the permits for digging in waterways, saying if ANR would only allow residents to pull debris and gravel from the rivers, the issue could be solved.

When asked by an audience member if the Corps would “go up against ANR” on the town’s behalf, Russo said he didn’t want to use the word “against.”

He did say that if the Corps were to partner with the town on a project, then Corps members would speak about the project with ANR on the town’s behalf.

Selectboard member James Burke told the audience that he viewed the process of working with the Corps as a possible method for accessing inexpensive studies to show that gravel is impeding the town’s rivers.

If the federal government gives the town a slight nod that local rivers had extenuating circumstances, then the nod would give the town “backing to battle the next level.”

“[The Corps is] not the battle. The battle is ANR,” he said. “Let’s see where we go next. We’re a town. We do this together.”

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Originally published in The Commons issue #140 (Wednesday, February 22, 2012).

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