outdoors
Great Lakes Tsunamis
How and why these surprising events take place Story by Deborah K. Frontiera
W
hen most people hear the word “Tsunami,” they think of those mighty earthquake-generated waves that kill hundreds or even thousands of people. “We don’t have earth quakes in this area,” one might state. But huge, destructive waves do happen on the Great Lakes more often than many people might like to admit. In How the Rock Connects Us, a book by Bill Rose and Erika Vye, the authors state: “Waves higher than 40 feet have been recorded on Lake Superior. Waves build to breakers when the water gets shallow enough that the wave feels the lake bottom’s resistance. This occurs when the depth is about half the distance between the wave crests. The distance travelled by wind or waves over open water, known as the ‘fetch,’ is important for lakes with large surface areas like Lake Superior: the longer the fetch, the greater the strength of winds and height of waves.” Two different types of “rogue waves” can, and do, happen on Lake Superior and the other Great Lakes. One is known as a “seiche.” This large wave, or waves,
occur as a result of quick changes in atmospheric pressure accompanied by heavy wind. It is the oscillation in the water level of a lake or large bay. Think about what happens when you blow down and across hot coffee, tea or chocolate in a big mug to cool it before drinking. Your breath is like the wind, and the downward push of that breath, like a drastic change in air pressure on the surface of the hot liquid. A little depression forms in the liquid and ripples or waves flow across the mug and wash up on the opposite “shore.” Blow hard enough, and you’ll slosh some out onto the table or saucer. If you wait a bit, the liquid will come back to your side. Now picture this in nature on the enormity of the Great Lakes. Water on one side diminishes enough to make it seem like “low tide” along that shore. On the opposite side, rising water can cause quite a bit of destruction. Another comparison is water sloshing back and forth in a bathtub. The second type, known as a “meteotsunami,” is also caused by severe weather events, often formed on the western sides of the various Great Lakes because our
weather patterns generally move west to east. High winds, usually in late spring and early summer, (the height of the thunderstorm season), when accompanied by rapid changes in atmospheric pressure, depress the lake on that side, forming a wave which then rolls in one direction across the lake. The wave’s height depends on the speed, intensity, depth of water and the shape of the lake at that particular place. That might seem exactly the same as the seiche. The key difference is that meteotsunamis roll in one direction, rather than “sloshing” back and forth. The resulting wave gains energy as it moves across the lake, especially as it nears the opposite shore. Here are a few examples of Great Lakes Tusnamis from the history books: • July 4, 1929, a 20-foot wave crashed ashore at Grand Haven, Michigan and 10 people drowned. • June 26, 1954, a 10-foot wave swept 10 fishermen off a pier in Chicago and all 10 were lost. • July 4, 2003, a 10-foot wave drowned seven swimmers on the Lake Michigan shore near Sawyer.
September 2021
Marquette Monthly
23
