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Letters 06-20-2016

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Strange Oscillations: Mysterious ‘seiche‘ Drops Bay Water Level by 18 Inches in Two Hours

Sandy Bradshaw - June 27th, 2002
On Tuesday morning, June 4th, 2002, West Grand Traverse Bay‘s water level dropped an astonishing 18 inches in less than two hours. A sailor was getting his boat ready for summer mooring and by the time he was all set to go - too late! His boat was stuck in a sandbar due to the sudden water level drop.
Known as a seiche, it is a phenomenon which occurs frequently on the bays of Northern Michigan. What is a seiche? Pronounced “saysh,“ you may compare it to water sloshing in a bathtub. Seiches are tide-like rises and drops in coastal water levels, (occurring both in fresh water and ocean waters) caused by prolonged strong winds that push water toward one side of the water body. This causes the water level to rise on the downwind side of the lake and to drop on the upwind side. As the wind diminishes, the water sloshes back and forth, with the near-shore water level rising and falling in decreasingly small amounts on both sides of the lake until it reaches equilibrium.
Seiches occur most in waters that are more or less surrounded by land, such as lakes, fjords and gulfs. Otherwise there would not be that sloshing back and forth that characterizes a seiche. Water blown onto a shore will just ebb back into the ocean if there is no opposite shore to reflect the wave. Seiches can also be caused by earthquakes, even very distant ones. Seiches triggered by earthquakes thousands of miles away have been reported.
“Seiches occur all the time on the bay,“ said John McKinney, past Maritime Heritage Alliance board member and Michigan Sea Grant agent. “This one was a bit larger than usual for Grand Traverse Bay though. Atmospheric pressure and wind changes helped contribute to the quick rise and fall of the bay. The western end of Lake Erie experiences large ones quite often - sometimes as high as six feet,“ he noted.
“Historically no one has taken records of the seiche effect,“ McKinney continued. “They are usually so small no one even notices them. A few years back we had a Japanese researcher, Dr. Uehara, take records on the bay with a special monitoring instrument every ten minutes. He conducted those measurements for six month stretches over a period of several years. This was documented on a graph, and shows seiche occurs constantly.“
According to Hans Joerg Rothenberger, a Swiss science and history buff
and skipper of a gaff cutter in Greece where seiches are
frequent, the word was first used in a scientific context by his
compatriot François-Alphonse Forel (1841 - 1912). “He was a professor of
anatomy and physiology, but known in international science rather as the
‘father of ‘imnology,‘ i.e. the science of lakes,“ said Rothenberger.
“Forel used the French word ‘seiche‘ for the first time in 1869 after
watching the phenomenon on Lake Geneva, a lake roughly the size of the
Grand Traverse Bay. He reported a seiche of eight days with about 200
oscillations. By the way, the question of the original meaning of the
word ‘seiche‘ gave rise to various theories, none of which is very
convincing.“
 
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