Andy Dixon began his talk with an introduction of himself. He’s been with NWS Grand Rapids for four years and before he came to Grand Rapids, he worked in Alaska. His talk titled, “River Ice and Ice Jams: The Basics” is intended to provide some information on how ice and ice jams form on rivers.
River ice begins to form as daily average temperatures drop to around 20F. There are two types of ice that tend to form first: Border Ice and Frazil Ice.
Border ice forms in the shallower and slower moving water. It sticks to the border of the river bank.
Frazil ice forms in the middle of the river as clumps of slush that tumble past obstacles and throughout the turbulent flow.
As the ice begins to form, water levels slowly rise. This is similar in concept to a pipe that has a lot of build-up and begins to clog or at the very least, drain slowly.
A third type of ice that is less common in Michigan is called Anchor Ice. In very cold water, frazil Ice begins to stick to itself and to debris on the riverbed. The sticky-ness is caused when particles of water within the river become supercooled.
Eventually, frazil ice turns into “frazil pans” that are essentially larger clusters of frazil ice that tend to remain close to the surface of the river. As the border and frazil ice formations grow, the open channel becomes smaller and smaller and eventually a solid/stable ice cover forms. At this stage, the water level tends to plateau and the river reaches a more balanced state. A feature known as a “canopy” then forms on the entire surface of the river. This is process tends to be the least impactful for the land surrounding the river.
Moving on to ice jams, there are two types; Freeze-up Jams and Break-up Jams.
Freeze-up jams tend to occur when it’s very cold and ice growth happens too quickly for the normal river flow to keep up. Frazil pans tend to crunch up on top of each other causing an obstruction in the middle of the river. This is common at the Rogers Heights portion of the Muskegon River and happens almost every year. In general, freeze-up ice jams tend to cause minor flooding as there isn’t a large amount of water in the river system when these events occur.
Break-up jams happen when there is a rapid warmup and all of the water that was locked up in the ice is released quickly. The flow of the river can’t keep up and the excess water isn’t carried downriver fast enough. As temperatures warm, the ice begins to melt and flows downriver. When the amount of water rise is approximately equal to 1.5 to 3 times the thickness of the ice, the ice cover will start to fracture into large pieces. These fractures are known as transverse cracks. The large pieces of ice continue to break apart and gradually move downriver. Relatively large chunks get stuck where the river pattern changes (like a bend or a narrow point). When break-up ice jams occur, there is typically an increase in water levels up-stream – sometimes this can be quite drastic. Water levels downstream tend to be lower. Break-up jams can escalate quickly and can quickly flood surrounding areas.
River ice breaks up on a sliding scale between two extremes: Thermal Break-up on the benign side caused by gradual melt as the ice weakens and eventually disappears vs. Mechanical Break-up where the weather fluctuates greatly between extreme cold followed by a drastic warm-up. Most Michigan winters tend to fall somewhere in the middle.
Craig Woods asked about an old practice of breaking up ice jams with dynamite. Andy then talked about a period in history when bombing ice jams in rivers in Alaska occurred. Apparently during one exercise, a bomber missed the target and hit a nearby village instead so that practice ended! Another issue that formed from dynamiting ice jams is that it would break up jams on parts of the river and send a larger mass of water and ice farther downstream causing more issues.
The slide show below shows some of the elements discussed above as well as the URL for the videos