What is a Seiche?

[Mark Prouty]

WHATS A SEICHE?

There is, rarely, on the Great Lakes a radical, brief change in the water levelcalled a seiche (pronounced saysh). Its technically thought of as an oscillation of the water, which is a good way to imagine itas though Lake Michigan were a giant gold-miners pan filled with water. The giant goldminer rocks and twists the pan so the water oscillates
and sloshes around the edges.

Normally seiches are smallwhere the water level drops a foot or so in an hour and then rises back to its original level in another hour. But much larger seiches have occurred, and they are often disastrous, since shore-side businesses and people never expect it. The worst in recent history was in 1954 when eight people were drowned along the Chicago lakefront by an unexpected giant wave, apparently part of a seiche.

In the early days on the Great Lakes, they were commonly called tidal waves, though the scientists object to that term. The largest measured seiche, according to the Armys Corp of Engineers, was 7.4 feet, in Lake Superiora mild fluctuation for a tide but a major problem when totally unexpected.

Perhaps the best description of a seiche was one of the first ever written, by US Geologists J. W. Foster and J. D. Whitney, at the east end of Lake Superior in the early nineteenth century:

In the summer of 1834, they report, an extraordinary retrocession of the waters took place at Sault St. Marie. The river here is nearly a mile in width, and the depth of water over the sandstone rapids is about two and a half feet. The phenomenon occurred about noon. The day was calm but cloudy. The water retired suddenly, leaving the bed of the river bare, except for a distance of about twenty rods, where the channel is the deepest, and remained so for the space of an hour. Persons went out and caught fish in the pools formed in the epressions of the rocks. The return of the waters was sudden, and resented an imposing spectacle. They came down like an immense surgeroaring and foaming; and those whohad incautiously wandered into the river-bed had barely time to escape being overwhelmed."

Fifty years later, in Lake Ontario, a government surveyor reported another, including strange noises, and the fish rose to the surface as if stunned.

The theory is that the seiches are caused by extreme atmospheric pressure. An intense low-pressure cell, for example, moves across the center of the Lake, forcing the water to rise by the reduction in atmospheric pressure underneath the cell and to lower elsewhere in the Lake to compensate. When the cell hits the shore, the pressure is released and the water sloshes backseiche time. Of course, wind and waves might either augment or diminish the seichethere are probably midget seiches all the time that we dont think of as seiches.

Elsewhere in the world, earthquakes can cause seich-es as well, but the Great Lakes have had few quakesat least in the last 15,000 years.

In contrast to the seiches, the stretched-out rise and drop of Lake Michigan, when it is a foot or two higher this summer than last, seems to be caused mostly by the amount of rainfall and snow melt that takes place in the entire drainage basin of 68,000 square miles.

In the folklore of the ineteenth century, In the folklore of the nineteenth century, the big changes in water level were caused by Paul Bunyans famous blue ox, who drank rarely, every two years or so, but drank big. The story was that the ox would swig down so heavily that the Lakes would be lowered several feet, and it would take several years of rainfall for the Lakes to fill
back up.

[Bill at BOATS 4 SAIL]

If Paul Bunyan had a white ox, instead of a blue ox, the water level would not change as fast.

[Dominic]

Funny, the term always reminds me of the first time I went sailing in Lough Neagh, Northern Ireland.

(The lough is one of the largest freshwater loughs in Europe but is fairly shallow - if you draw more tham 1 metre you will run aground sonner or later - one of the attractions of the Macgregor 26x for this type of water in that the keel can be raised).

The eastern shore of the lough is relatively rocky and the western shore is relatively sandy and I happened to run aground on the eastern shore(not in a Mac and not my fault but that is another story).

After a few hours the water began to rise and the boat floated off which surprised me because it was not a tidal water and no substantial rain had fallen recently.

Back in port a local described the event as the result of seiche whereby the wind, if it persists in one direction for some time, can 'push' the water to one side. When the wind subsides, and/or the body of water is greater than the wind, then it flows back seeking its own level.

I have since illustrated this to school kids by having them lift a large shallow basin of water which they tilt from one side to the other as much as they can without spilling any.

The force of the water (2/3 gallons) hitting the side of the basin often catches them by surprise and leads to a healthier respect and understanding for the forces of water operating on the boat.

[Dave Mason]

I had the good fortune to observe a seische in progress on Slocan Lake in South-eastern B.C. a couple of years ago. I was at the south end of the lake and the water would receed down the beach fast enough to make small stones rattle, then pause, then return. Total time for each cycle was about 2 minutes and the height was about 2 feet. A couple of local residents said they had lived there for over 20 years and never seen it before.
Driving home I remembered hearing the news on the clock radio that morning reporting a small earthquake in North-western BC. Must have been the cause?

[RandyMoon]

Oh I get it. Is this like the party game Balderdash where someone comes up with an obscure term or date and people have to make up a believeable answer? And if you can trick most people at the party that your made up answer is right, you win points?

OK, I'll play. A SEICHE is what?

1) Strange mythical creature which lived in hollow trees and spent days throwing things at passerbys.

2) A bald spot on a bird's body.

3) Gucky sediment that gathers on the bottom of an aquarium.

4) A black mineral indigenous to Finland.

5) The first stomach of a cow.

6) A lobster's leg joint.

7) Pellets of hardened dung attachd to a sheep's back.

[DLT]

Ok, remind me never to play Balderdash against RandyMoon...

[RandyMoon]

OK, I am ashamed to admit it. I went to the closet and dragged out the Balderdash game and copied those things off the cards. I cheated.

[Catigale]

Spock "I exaggerated."

[RandyMoon]

I tried to look intellectually impressive and I was caught. I will return to Vulcan and exponge my soul.

[Frank C]

Though not precisely on-point, I noticed an article in Discover magazine about the fact that the crust of the earth deflects under the weight of water, glaciers ... and even under the varying weight of the atmosphere (high vs. low pressure cells).

This webpage relates this context to Lake Bonneville.

Lake Bonneville:
The tilting of Lake Bonneville shorelines was first noticed by Gilbert (1890) with simple instruments. When shorelines are created they are horizontal but Gilbert (1890) noticed the shorelines near the center of the basin were higher than the shorelines at the periphery of the basin. He theorized the tilting was caused by the load of the lake and therefore hypothesized a liquid substrate beneath the rigid crust. He believed with more knowledge of the chronology of the lake and ability to do the mathematics involved the characteristics of substrate could be determined. Due to better dating and surveying methods, we have an improved hydrograph and accurate elevations on shorelines throughout the basin. Using this data and computer technology, models are created that Gilbert could only theorize about.

The volume of Lake Bonneville was approximately 10,000 km3 at its highest level. The weight of this much water depressed the Earth's crust by about 1/3 the depth of the water (Bills et al., 1999) and when the water left the crust began to rebound appearing to dome the ground. This is apparent by viewing and measuring the elevations of Lake Bonneville shorelines around the basin.

The Bonneville shoreline is as much as 74 meters higher in the Lake Side Mountains, toward the center of the basin, than at Red Rock Pass, on the periphery of the lake (Oviatt, 1997). The Provo shoreline is as much as 59 meters higher (Oviatt, 1997). According to Bills et al. (1994) the crust was deflected downward by 60-70 meters" under the weight of Lake Bonneville.

[Catigale]

I tried to look intellectually impressive and I was caught. I will return to Vulan and exponge my soul.

Perhaps - but if you put a Bullwaga on a bow extension does that make it a BULLY PULPIT??????