Thursday, August 29, 2013

Seiches and WInd Set Up on Lake Superior

For today's posting I decided to describe seiches.  Generally the prevailing winds blowing across Lake Superior come from the Northwest. These winds and passing weather fronts push the water in the lake basin to the far shores setting-up the conditions for seiche activity once the wind dies down.

First the wind pushes the entire lake toward one shore that sets up a higher water level on the far side of the lake.

After the water level rises on one end of the lake due to wind set up, a seiche (pronounced “saysh”) forms, which is the rocking motion of water in a lake that develops pendulum-like movements that are “free standing-wave oscillations.” Seiches, or sloshes as they are sometimes called on the Great Lakes, are almost always present on Lake Superior.
Seiches are often imperceptible to the naked eye, and observers in boats on the surface may not notice that a seiche is occurring due to the extremely long wavelengths. The effect is caused by resonances in a body of water that has been disturbed by one or more of a number of factors, most often meteorological effects (wind and atmospheric pressure variations), seismic activity or by tsunamis. Gravity always seeks to restore the horizontal surface of a body of liquid water
Lake Superior Seiches:
  • Create water-level changes ranging from imperceptible to at least three feet
  • Have a period of 7.9 hours
  • Stir nutrients (good stuff) and pollutants (bad stuff) into the water column
  • Reverse the flow of rivers
  • Sustain a “mini-seiche” oscillation.

The National Weather Service issues low water advisories for portions of the Great Lakes when seiches of 2 feet or greater are likely to occur. Lake Erie is particularly prone to wind-caused seiches because of its shallowness and elongation. These can lead to extreme seiches of up to 5 m (16 feet) between the ends of the lake. The effect is similar to a storm surge like that caused by hurricanes along ocean coasts, but the seiche effect can cause oscillation back and forth across the lake for some time. In 1954, Hurricane Hazel piled up water along the northwestern Lake Ontario shoreline near Toronto, causing extensive flooding, and established a seiche that subsequently caused flooding along the south shore.

Lake seiches can occur very quickly: on July 13, 1995, a big seiche on Lake Superior caused the water level to fall and then rise again by three feet (one meter) within fifteen minutes, leaving some boats hanging from the docks on their mooring lines when the water retreated. The same storm system that caused the 1995 seiche on Lake Superior produced a similar effect in Lake Huron, in which the water level at Port Huron changed by six feet (1.8 m) over two hours. On Lake Michigan, eight fishermen were swept away and drowned when a 10-foot seiche hit the Chicago waterfront on June 26, 1954


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