A seiche ( SAYSH) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves, and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing the formation of the standing wave. The term was promoted in 1890 by the Swiss hydrologist François-Alphonse Forel, who was the first to make scientific observations of the effect in Lake Geneva. The word had apparently long been used in the region to describe oscillations in alpine lakes. According to Wilson (1972), this Swiss French dialect word comes from the Latin word siccus meaning "dry", i.e., as the water recedes, the beach dries. The French word sec or sèche (dry) descends from the Latin. Seiches in harbours can be caused by long-period or infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, having periods longer than the accompanying wind-generated waves.
Causes and nature 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 periods. The effect is caused by resonances in a body of water that has been disturbed by one or more factors, most often meteorological effects (wind and atmospheric pressure variations), seismic activity, or tsunamis. 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 hydrostatic 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 longest natural period of a seiche is the period associated with the fundamental resonance for the body of water—corresponding to the longest standing wave. For a surface seiche in an enclosed rectangular body of water this can be estimated using Merian's formula:
T = 2 L g h {\displaystyle T={\frac {\ \ 2L\ \ }{\!\!\!{\sqrt {gh}}}}}
where T is the longest natural period, L and h are the length and average depth of the body of water, and g the acceleration of gravity. Higher-order harmonics are also observed. The period of the second harmonic will be half the natural period, the period of the third harmonic will be a third of the natural period, and so forth.
Occurrence Seiches have been observed on both lakes and seas. The key requirement is that the body of water be partially constrained to allow formation of standing waves. Regularity of geometry is not required; even harbours with exceedingly irregular shapes are routinely observed to oscillate with very stable frequencies.
Lake seiches Low rhythmic seiches are almost always present on larger lakes. They are usually unnoticeable among the common wave patterns, except during periods of unusual calm. Harbours, bays, and estuaries are often prone to small seiches with amplitudes of a few centimetres and periods of a few minutes. The original studies in Lake Geneva by François-Alphonse Forel found the longitudinal period to have a 73-minute cycle, and the transversal seiche to have a period of around 10 minutes. Another lake well known for its regular seiches is New Zealand's Lake Wakatipu, which varies its surface height at Queenstown by 20 centimetres in a 27-minute cycle. Seiches can also form in semi-enclosed seas; the North Sea often experiences a lengthwise seiche with a period of about 36 hours.
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