Editing Seiche (section)

==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 [[tsunami]]s.<ref>Tsunamis are normally associated with earthquakes, but landslides, volcanic eruptions and meteorite impacts all have the potential to generate a tsunami.</ref> [[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 (physics)|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:<ref>{{Cite book |publisher=Methuen |location=London |title=Dynamical oceanography |last=Proudman |first=J. |author-link=Joseph Proudman |year=1953 |oclc=223124129 |pages=§117 (p. 225) |no-pp=yes}}</ref><ref>{{cite book |first=J. R. |last=Merian |title=Ueber die Bewegung tropfbarer Flüssigkeiten in Gefässen |language=de |trans-title=On the motion of drippable liquids in containers |type=thesis |publisher=Schweighauser |location=Basel |year=1828 |oclc=46229431 }}</ref>

: <math>T = \frac{\ \ 2L\ \ }{\!\!\!\sqrt{g h}}</math>

where ''T'' is the longest natural period, ''L'' and ''h'' are the length and average depth of the body of water, and ''g'' the [[Earth's gravity|acceleration of gravity]].<ref>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&nbsp;km long (such as the [[Gulf of Finland]]) and somewhat deeper has a period closer to 12 hours.</ref>

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.