What the Heck is Frazil Ice?

This cold-weather phenomenon requires a particular set of variables: a clear night, a slight offshore wind and plunging temperatures. Learn why it can mean disaster for water system intakes.
What the Heck is Frazil Ice?
Here, frazil ice clings to a water intake in Manitowoc, Wis. The image is from January 2008 when the air temperature was a chilly -3 F and the water temperature was 32 degrees.

First it forms. Then it multiplies. Then it attaches to underwater structures like water system intakes.

It’s frazil ice, and it’s led to the frazzled nerves of managers of surface water systems, especially those along the Great Lakes. Municipalities such as Evanston, Ill., Avon Lake, Ohio, and Manitowoc and Sheboygan, Wis., are among many water utilities that have had to deal with frazil ice blocking their water intakes during cold winters.

“It’s something we expect ... like most of our neighbors,” says Joe Trueblood, superintendent of the water system at Sheboygan.

According to scientific sources, frazil ice forms in open, calm water, usually on clear nights when the water and air temperatures plunge to very cold readings. It consists of loose, randomly oriented needle-like ice crystals.

It sinks
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, such as intake structures, especially if the objects themselves are at a temperature below the freezing point of water. The buildup can restrict or even fully block water flow. It’s a cold-weather chain reaction.



Sheboygan has developed a couple of methods to deal with the issue, which Trueblood says generally occurs in February offshore of his plant. The city’s two intake pipes extend 1 mile and a 1/2 mile into Lake Michigan.

“We have the ability to push water back out through the intake into the lake,” he says. That enables Sheboygan to dislodge any frazil ice that might have accumulated on the intake structure.

“We know we’re getting icing when the intake level drops below normal in our intake tank,” he says. “Back flushing is our main tool in dealing with frazil ice.”

Because the icing tends to occur after midnight, Sheboygan might also take in more water during daylight hours and less at night, Trueblood says. “It’s a reverse of our normal practice.”

Manitowoc experience
Just up the Lake Michigan shore at Manitowoc, water systems manager Rob Michaelson’s operation uses the back flushing approach with a slightly different twist.

“We’re located next to a power plant,” he explains. “We can take noncontact cooling water from the power plant [at 60 to 70 degrees F] and pump it back through our intake line.” The utility uses the warm water to heat the intake while dislodging ice.
Manitowoc’s intakes extend 9,000 feet and 4,000 feet into the lake. The shorter one is equipped with wedgewire screens to keep out debris, and Michaelson says the ice tends to develop on the screens and restrict flow.

Frazil ice has also occurred at Evanston, Ill., in recent years, but according to Dave Stoneback, utilities director, a proprietary heating system has been effective in preventing buildup.

“We’ve used the heating system the last three winters on our 54-inch intake,” he says, reporting good results. The system uses electricity to heat rings that are situated inside the intake pipe at its mouth. “It creates just enough warmth to keep the frazil ice from attaching to the intake,” Stoneback explains. “We have a contract out for another heating system to be installed on our 48-inch intake.”

Both intakes extend about a mile into Lake Michigan.

Highland park, too
In Highland Park, Ill., superintendent of water production Dan Jensen pays close attention to the weather forecasts in an effort to anticipate conditions ripe for frazil icing.

“We try to see it coming. Then we can fill up available storage capacity and notify our wholesale customers. Normally we can make it through [an icing incident] that way,” Jensen says.

Highland Park maintains three intakes in Lake Michigan — one 54 inches in diameter and two smaller ones at 20 and 16 inches. Jensen says his utility can rotate among all three intakes as a way to keep pumping in cold weather.

“Normally we’ll see a turbidity spike [ahead of blockages],” he says. He believes ice might form on the top half of the intake first, increasing velocity through the bottom half so much that it picks up sediment.

Drawing water from neighboring utilities that might not have icing is another way of getting through any blockages. “Mutual aid is a benefit,” Jensen says.

Icy research
Bill Soucie, director of operations at the Central Lake County Water Agency in Lake Bluff, Ill., has had experience with icing at this plant and has also conducted extensive research on the issue, including contact with the U.S. Army Corp of Engineers cold-weather facility in New Hampshire.

“We look for frazil ice when the conditions are right,” he says, noting that those include cloudless nights from midnight to dawn, a lack of surface ice, a slight offshore breeze, water temperature at freezing, and air temperatures at or below zero.
After a serious icing event in February 2011, Soucie conducted a survey among 23 municipalities and one industrial water system on Lake Michigan. The survey revealed that 19 of the systems had experienced icing that month, with most reporting one to three icing events, although some had more.

The survey confirmed the temperature and weather conditions conducive to icing, and also found that utilities with longer, larger-diameter intake pipes extending farther from shore and at deeper depths seemed less prone to icing events.

What you can do
Soucie listed pre-emptive measures water systems can take when icing conditions are right:

  • Top off water supplies during the day
  • Remove gratings and crib sections during winter months
  • Clean intakes and inspect for zebra mussels, which can cause bridging
  • Make sure intake heaters are working
  • Fill backflush tanks

He also listed steps to consider if icing occurs:

  • Reduce flow to maintain raw-water wet-well level
  • Stop flow until suction well levels are normal
  • Backflush intake lines
  • Seek alternate water sources
  • Start and stop flow to dislodge ice

He’s also found that frazil ice and slush ice are different and form in different conditions. Unlike frazil ice, slush ice forms in higher seas with winds onshore, and can block intakes equipped with screens.

“It’s like sucking on a straw,” he says. “I’ve heard Eskimos have 15 different words for 15 different kinds of ice. Ice is not all the same.”


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