Seiche

From Free net encyclopedia

For a Fremen fort in Frank Herbert's science fiction, see sietch.

A seiche (pronounced say'sh) or an underwater wave is a standing wave in a body of water.

The term was first promoted by the Swiss hydrologist François-Alphonse Forel in 1890, who had observed the effect in Lake Geneva, Switzerland. The word originates in a Swiss French dialect word that means "to sway back and forth", which had apparently long been used in the region to describe oscillations in alpine lakes.

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Causes and nature of seiches

Seiches are often imperceptible to the naked eye, and observers on the surface may not notice that a seiche is occurring. 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 wind or seismic activity. Gravity always seeks to restore the horizontal surface of a body of liquid water, as this represents the configuration in which the water is in equilibrium. Vertical harmonic motion results, producing an impulse that travels the length of the basin at a velocity that depends on the depth of the water. The impulse is reflected back from the end of the basin, generating interference. Repeated reflections produce standing waves with one or more nodes, or points, that experience no vertical motion. The frequency of the oscillation is determined by the size of the basin, its depth and contours, and the water temperature. The length of the lake is an exact multiple of the distance between nodes.

The period of a seiche in an enclosed rectangular body of water is usually represented by the formula:

<math>\mbox{Period}(T) = \frac{2L}{\sqrt{g d}}</math>

where L is the length, d the average depth of the body of water, and g the acceleration of gravity.

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Seiches around the world

Small rhythmic seiches are almost always present on larger lakes. They often occur between warm and cold-water layers. They have often been observed in Loch Ness, Scotland; some evidence cited for the Loch Ness monster and the Lake Champlain monster (see also Lake monster and Sea monster) may be traced to seiches.

On the North American Great Lakes, seiche is often called slosh. It is always present, but is usually unnoticeable, except during periods of unusual calm. Harbours, bays, and estuaries are often prone to small seiches with amplitudes of a few centimeters and periods of a few minutes. Seiches can also form in semi-enclosed seas; the North Sea often experiences a lengthwise seiche with a period of about 36 hours.

Image:Lake Erie water level.gif

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. On Lake Michigan, eight fishermen were swept away and drowned when a 10-foot seiche hit the Chicago waterfront on June 26, 1954.

Lakes in seismically active areas, such as Lake Tahoe in California/Nevada, are significantly at risk from seiches. Geological evidence indicates that the shores of Lake Tahoe may have been hit by seiches and tsunamis as much as 10 m (33 feet) high in prehistoric times, and local researchers have called for the risk to be factored into emergency plans for the region [1].

Earthquake-generated seiches can be observed thousands of miles away from the epicentre of a quake. Swimming pools are especially prone to seiches caused by earthquakes, as the ground tremors often match the resonant frequencies of small bodies of water. The 1994 Northridge earthquake in California caused swimming pools to overflow across southern California, and the massive Good Friday Earthquake that hit Alaska in 1964 caused seiches in swimming pools as far away as Puerto Rico. The earthquake that hit Lisbon, Portugal in 1755 caused seiches in canals 2,000 miles (3,000 km) away in Scotland and Sweden. Following the 2004 Indian Ocean earthquake, it was postulated that the shock waves could have induced localised seiches as far away as Oklahoma [2].

A related effect is the tsunami, a wave train (series of waves) generated in a body of water by a pulsating or abrupt disturbance that vertically displaces the water column. On occasion, tsunamis can produce seiches as a result of local geographic peculiarities. For instance, the tsunami that hit Hawaii in 1946 had a fifteen-minute interval between wave fronts. The natural resonant period of Hilo Bay is about thirty minutes. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159 inhabitants. Seiche waves may continue for several days after a tsunami.

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