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Readers Want to Know

What's a seiche?

A seiche, besides being the name of this newsletter, is a little-known phenomenon that occurs in all water bodies but is more noticeable within large lakes. Seiches are a periodic rising and lowering of water levels in an enclosed or semi-enclosed body of water.

"Seiches can be caused by anything that would cause a pressure change," said Steve Bortone, Minnesota Sea Grant director. "That pressure could be a storm that occurs and then stops, or a heavy rain that takes place and then stops."

Although seiches can cause large amounts of water to move in a short period of time, they're not as threatening as other weather phenomena. And sometimes they can be good for the environment, stirring up nutrients in the lake that are beneficial to plants and animals.

"It's not like a tidal wave or a tsunami where giant waves crash into the shore," said Bortone. "Instead, it's a subtle shift of the water."

It's easy to create a seiche in the bathtub, just move your body back and forth in the water and soon, you'll have a rhythm peppy enough to splash water out of the tub.

"But, if you think of the Great Lakes as being a giant bathtub, by the time the water sloshes back and forth from one side to the other, many hours have passed," Bortone said.

In Lake Superior, the period of a seiche traveling between the farthest points in the lake is eight hours. Even though seiches in Lake Superior are typically modest (less than a foot high), sometimes they can cause problems for ships.

"We've seen some very severe impacts on the loading dock in Two Harbors," said Jim Sharrow, facilities manager for the Duluth Seaway Port Authority. "On a couple of occasions, a seiche came through the harbor and created such a current that a thousand-footer loading could not be held at the dock and all the cables let go. In one instance, a seiche took the shuttle right off the dock and caused well over one hundred thousand dollars-worth of damage."

Sharrow says ships now have a better notification system for changes in water levels than they did 15 years ago. Bortone adds that, although scientists can't definitively forecast a seiche, they are identifying conditions where seiches are more likely to occur.

"The oceanic observing system has stations in the Great Lakes and the oceans that measure long-term things like wave height and wave energy. So we're getting basic monitoring data to help examine some of the features of seiches," Bortone said. "Also, the weather service is getting better at predicting conditions that can cause seiches. For example, in Lake Michigan last summer, the weather service actually issued a warning that there may be a seiche near Chicago."

For more information about seiches:

By Sea Grant Staff
December 2008

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