Editing Seiche (section)

===Underwater (internal) waves===
Seiches are also observed beneath the surface of constrained bodies of water, acting along the [[thermocline]].<ref>The [[thermocline]] is the boundary between colder lower layer ([[hypolimnion]]) and warmer upper layer ([[epilimnion]]).</ref>

In analogy with the [https://encyclopedia2.thefreedictionary.com/Merian%27s+formula Merian formula], the expected period of the internal wave can be expressed as:<ref>Mortimer, C. H. (1974). Lake hydrodynamics. Mitt. Internat. Verein. Limnol. 20, 124–197.</ref>

: <math>T = \frac{2L}{c}</math>       with       <math>c^2 = g \frac{\rho_2-\rho_1}{\rho_2} \frac{h_1 h_2}{h_1+h_2}</math>

where ''T'' is the natural [[Frequencies|period]], ''L'' is the length of the water body, <math>h_1, h_2</math> the average thicknesses of the two layers separated by [[stratification (water)|stratification]] (e.g. [[epilimnion]] and [[hypolimnion]]), <math>\rho_1, \rho_2</math> the [[densities]] of these two same layers and ''g'' the [[Earth's gravity|acceleration of gravity]].

As the [[thermocline]] moves up and down a sloping lake bed, it creates a 'swash zone', where temperatures can vary rapidly,<ref>{{Cite journal|last1=Cossu|first1=R.|last2=Ridgway|first2=M.S.|last3=Li|first3=J.Z.|last4=Chowdhury|first4=M.R.|last5=Wells|first5=M.G.|date=2017|title=Wash-zone dynamics of the thermocline in Lake Simcoe, Ontario|journal=Journal of Great Lakes Research|volume=43|issue=4|pages=689–699|doi=10.1016/j.jglr.2017.05.002|bibcode=2017JGLR...43..689C |issn=0380-1330|doi-access=free}}</ref> potentially affecting fish habitat. As the thermocline rises up a sloping lake bed, it can also cause [[benthic]] turbulence by convective overturning, whereas the falling thermocline experiences greater stratification and low turbulence at the lake bed.<ref>{{Cite journal|last1=Cossu|first1=Remo|last2=Wells|first2=Mathew G.|date=2013-03-05|title=The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada|journal=PLOS ONE|volume=8|issue=3|pages=e57444|doi=10.1371/journal.pone.0057444|pmid=23472085|pmc=3589419|bibcode=2013PLoSO...857444C|issn=1932-6203|doi-access=free}}</ref><ref>{{Cite journal|last1=Bouffard|first1=Damien|last2=Wüest|first2=Alfred|date=2019-01-05|title=Convection in Lakes|journal=Annual Review of Fluid Mechanics|volume=51|issue=1|pages=189–215|doi=10.1146/annurev-fluid-010518-040506|bibcode=2019AnRFM..51..189B|s2cid=125132769|issn=0066-4189|url=http://infoscience.epfl.ch/record/264776/files/Convection%20in%20lakes.pdf}}</ref> Internal waves can also degenerate into non-linear internal waves on sloping lake-beds.<ref>{{Cite journal|last1=Boegman|first1=L.|last2=Ivey|first2=G. N.|last3=Imberger|first3=J.|date=September 2005|title=The degeneration of internal waves in lakes with sloping topography|journal=Limnology and Oceanography|volume=50|issue=5|pages=1620–1637|doi=10.4319/lo.2005.50.5.1620|bibcode=2005LimOc..50.1620B|s2cid=55292327|issn=0024-3590|url=https://api.research-repository.uwa.edu.au/files/3235728/Boegman_Leon_2004.pdf|access-date=2020-09-06|archive-date=2019-04-29|archive-url=https://web.archive.org/web/20190429003848/https://api.research-repository.uwa.edu.au/files/3235728/Boegman_Leon_2004.pdf|url-status=live}}</ref> When such non-linear waves break on the lake bed, they can be an important source of turbulence and have the potential for sediment resuspension<ref>{{Cite journal|last1=Boegman|first1=Leon|last2=Stastna|first2=Marek|date=2019-01-05|title=Sediment Resuspension and Transport by Internal Solitary Waves|journal=Annual Review of Fluid Mechanics|volume=51|issue=1|pages=129–154|doi=10.1146/annurev-fluid-122316-045049|bibcode=2019AnRFM..51..129B|s2cid=126363796|issn=0066-4189|doi-access=free}}</ref>