An oscillation or sloshing of water in a partially confined body of water such as a bay or an estuary. The period of time required for the oscillation is determined by the physical size and shape of the basin.
A seiche (pronounced say'sh) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, bays and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing natural phenomena to form a standing wave.
Causes and nature of seiches
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 natural period for a seiche in an enclosed rectangular body of water is usually represented by the formula:
where L is the length, d the average depth of the body of water, and g the acceleration of gravity.
Seiches around the world
Seiches have been observed on both lakes and seas.
Lake seiches
Small rhythmic seiches are almost always present on larger lakes. On the North American Great Lakes, seiche is often called slosh. Harbours, bays, and estuaries are often prone to small seiches with amplitudes of a few centimeters and periods of a few minutes. the North Sea often experiences a lengthwise seiche with a period of about 36 hours.
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 .
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.
Sea and bay seiches
Seiche have been observed in seas such as the Adriatic Sea and the Baltic Sea, resulting in flooding of Venice and St. Petersburg respectively. Seiche induced flooding is common along the Neva river in the autumn. The seiche is driven by a low pressure region in the North Atlantic moving onshore, giving rise to cyclonic lows on the Baltic Sea. As the cyclone continues inland, long, low-frequency seiche waves with wavelengths up to several hundred kilometers are established in the Baltic.
Seiches can also be induced by 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. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. Seiche waves may continue for several days after a tsunami.
Underwater (internal) waves
Although the bulk of the technical literature addresses surface seiches which are most readily observed, seiches are also observed beneath the lake surface acting along the thermocline in constrained bodies of water.
Engineering for seiche protection
Engineers consider seiche phenomena in the design of flood protection works (e.g., Saint Petersburg Dam), reservoirs and dams (e.g., Grand Coulee Dam), potable water storage basins, harbours and even spent nuclear fuel storage basins. Seiche calculator Seiche acoustics page Bonanza for Lake Superior: Seiches Do More Than Move Water
On aquatic monsters:
The unmuseum on seiches & their relationship to aquatic monsters Geological page on seiches & ^ As an example, the period for a seiche wave in a body of water 10 meters deep and 5 kilometers long would be 1000 seconds or about 17 minutes, while a body about 300 km long (such as the Gulf of Finland) and somewhat deeper has a period closer to 12 hours.
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