Ice jams: Learning to tame an icy riverBy BEA LEWIS
Sunday News Correspondent
December 02. 2017 10:04PM
PLYMOUTH - Hurricanes aren't the only cause of catastrophic flooding.
Huge chunks of ice can jam up a river, wreaking havoc.
In the past 25 years, they have caused serious destruction in a half-dozen U.S. states, from Maine to Idaho, and have sent smaller floods backing into towns in 36 states - resulting in $125 million in damage annually. This might seem minor compared to the billions in damage a single hurricane can inflict, but for the tiny northern towns that ice jams most often affect, it can be calamitous.
As New Hampshire girds for what weather models are advising will be a frigid December, a team of diverse agencies working together to reduce flood risk at the state level gathered for a workshop. They learned about river ice and the efforts underway to understand and predict ice-jam occurrence and severity.
The N.H. Silver Jackets, who work to advise and assist local communities in their efforts to become more resilient and less vulnerable to flood hazards, sponsored a presentation by a hydraulic engineer with the U.S. Army Corps of Engineers' Cold Region Research and Engineering Laboratory (CRREL) based in Hanover.
They have a giant refrigerated laboratory equipped with a reshapeable "riverbed" complete with chunks of floating ice in which they can test the most promising designs for ice-dam control structures.
A training session was held Nov. 28 in Plymouth, an area that has experienced recurring flooding from the Pemigewasset and Baker rivers, sometimes caused by ice jams. Similar sessions were held earlier in November in coordination with the towns of Franconia, Conway and Lancaster that have also experienced ice-jam induced flooding in recent years on the Gale, Saco and Connecticut rivers.
Shane Csiki, who works for the N.H. Geological Survey at the Department of Environmental Services and is chairman of the N.H. Silver Jackets, said the team is made up of people from both federal and state agencies who focus on New Hampshire's flood risk management priorities and offer technical expertise and resources.
Last December, the Silver Jackets partnered with CRREL to enhance the team's understanding of ice-jam science and mitigate the problem.
That project focused specifically on the Gale River at Franconia and Sugar Hill, which experienced ice jams and related flooding in 2012 and 2016. The team completed an assessment of existing conditions and recommended channel modifications and ice observer training, among other things.
Traditionally, different agencies wear different colored jackets when responding to emergencies. FEMA personnel wear blue and Army Corps members don red. The name Silver Jackets is used to underscore the common mission of the diverse agencies involved.
Joe Rocks, a research hydraulic engineer with the Army Corps, explained that while static bodies of water like lakes and reservoirs form surface ice when the temperatures drop, frazil ice is formed in turbulent water with flow velocity and wind mixing, which creates super-cooled water.
Unlike solid ice, however, frazil ice doesn't float. The small crystals give it what experts call "ineffective buoyancy," meaning that water currents can carry it to the bottom very easily. There, the crystals can quickly increase in number, and will start to adhere to objects in the water, or even the river bottom and become what is known as anchor ice.
The slush-like frazil ice can flow beneath solid ice and cause problems downstream. Ice jams most commonly occur when ice is forming in early winter and continue up until a January thaw.
"It is insidious because there is no way to stop the arriving ice," Rocks said of the ice dams that cause rivers to overflow their banks. They are unlikely to break up until the air warms, and if temperatures drop, the top layer can refreeze, camouflaging the frazil ice below.
Flood-inducing jams also occur as the ice breaks up at winter's end. Mechanical breakup is linked to flow and is prompted by rain and snow melt. Jams happen when ice flow exceeds the capacity of the river channel. Common spots for ice jams are on sharp bends in a river, areas where there is a change in slope of the river bed, or in areas of constriction such as around bridge piers.
Once a jam occurs, water can exceed flood stage in less than an hour and more than one jam can occur in multiple sequences.
"One and done is not always the case," he warned.
As part of its efforts to understand what factors contribute to ice jams forming, Rocks said, CRREL developed a database in the early 1990s to provide timely information to the Army Corps of Engineers with the goal of coordinating a response as well as to help with long-term planning for ice-jam flooding.
The database, which is available to the public on-line, contains information on 22,692 ice jams, including 685 that have occurred in New Hampshire at 181 locations on 85 different rivers dating from 1835 to 2017.
The Pemi River, whose flood stage is 13 feet, is ranked No. 2 on New Hampshire's list of most frequent ice-jam prone spots.
Trained observers can track pre-event ice conditions and can keep tabs on both upstream and downstream conditions when jams and flooding occur, providing helpful information to developing an after-action solution, Rocks said.
Ice motion detectors, which sound an alarm alerting local emergency management officials that ice is on the move, can provide the needed lead time to evacuate flood-prone areas and close roads that will be affected. Ice cutting and drilling holes in the ice can mechanically weaken the ice and can aid Mother Nature. Some communities have spread leaf litter on the ice or even sand to speed thermal weakening.
Communities also should consider positioning equipment such as an excavator or bulldozer ahead of time, which can be used to clear the river channel of ice downstream of the toe of a jam, a move than can allow floodwaters to recede. A supply of unfrozen sand also should be stockpiled for use in sandbagging, he advised.