Author Topic: Internal seiches - underwater seiches (lakes and lochs)  (Read 17169 times)

electrobleme

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« Last Edit: June 25, 2009, 08:08:36 by electrobleme »

electrobleme

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Internal seiches/seiching - Encyclopædia Britannica
« Reply #1 on: June 25, 2009, 06:56:45 »
Quote
Lake hydraulics » Seiches » Internal seiches
Encyclopaedia Britannica

Internal seiching results from thermal stratification. The layers separated by the thermoclines oscillate relative to one another. Observed uninodal periods for Loch Earn, Lake Geneva, Lake Baikal, and Lake Cayuga (New York) are approximately 16, 96, 900 (binodal), and 65 hours, respectively.

Because hypolimnion water is very different from epilimnion water with regard to both thermal and biological characteristics, the massive movements of water and the turbulent exchanges that can occur during internal seiching are very important. Substantial portions of the bottom of shallow lakes can experience periodic alternation of exposure to hypolimnetic and epilimnetic water, and hypolimnetic water can be periodically exposed to the surface.

electrobleme

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Internal seiche pumping between sill-separated basins
Authors: David C. Van Senden,  Dieter M. Imboden a
Affiliation: Lake Research Laboratory, Switzerland
Published in: journal Geophysical & Astrophysical Fluid Dynamics, Volume 48, Issue 1 - 3 October 1989 , pages 135 - 150

Abstract
Despite their close proximity and similar dimensions (~ 200m deep times 10km long times 2km wide) the two eastern basins of Lake Lucerne, Gersauersee and Urnersee, exhibit considerable differences in their internal behaviour, particularly during late winter and spring. The two lakes are separated by a small intermediate basin (~ 120m deep times 4km long times 1km wide) with sills of approximately 90m depth at each end. We report results of a field program conducted over the period February—May, 1988, when observations were obtained from weekly CTD transects and from three thermistor string/current meter moorings deployed for two months, one near each sill and the third at the southern end of Urnersee near Fluelen. During the observation period the stratification, relative surface to bottom density difference, Δ?/?, was 12 times 10-6 in Gersauersee and 4 times 10-6 in Urnersee. Following wind events a large amplitude internal seiche in Gersauersee (vertical excursions of ~50m and period ~60 hours) effectively pumped the heavier Gersauersee bottom water onto the intermediate basin and eventually into the hypolimnion of Urnersee. Temperature spectra show a peak at this seiche frequency at all levels at the Gersauer sill but only near the bottom at the sill in Urnersee. Coherence estimates between the bottom temperatures at Gersauer sill and Fluelen showed a significant peak at period 60 hours suggesting transmission of energy from the Gersauersee seiching motion through the weaker stratification of Urnersee to Fluelen. The phase relationships indicate that the wave phase speed decreases as the wave propagates into the region of weaker stratification. Application of a simple two-layer Defant model which includes topographic variations confirms these observations. The estimated volume exchange due to seiche pumping is only a small fraction of the Urnersee hypolimnion. However, the dissipation of energy transferred from the Gersauersee seiche may be an important contribution to mixing in the deeper waters of Urnersee.



electrobleme

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Underwater seiche's - internal seiching (pdf)
« Reply #3 on: June 25, 2009, 07:15:23 »
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Underwater seiche's - internal seiching

Underwater seiching - internal seiches .pdf

Since the relative density differences within the water column are much smaller than the density difference at the air-water interface, the same amount of energy produces internal seiches of much greater amplitude than the corresponding surface seiche. However, the internal wave speed is much slower than the surface wave...

In the Great Lakes of North America and the long and deep Rift valley lakes of Baikal and East Africa, amplitudes can reach almost 100 m with periods of many weeks to months.


« Last Edit: June 25, 2009, 08:00:50 by electrobleme »

electrobleme

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Internal seiches - Loch Ness and Lake Champlain
« Reply #4 on: June 25, 2009, 07:19:26 »



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Underwater seiching - Loch Ness and Lake Champlain
unmuseum.org

Both Loch Ness and Champlain also have a deep layer of cold water, called the hypolimnion, under a layer of warm water, called the epilimnion. Perhaps these characteristics, suggest supporters of the lake monster theory, are needed for the creature's survival.

The Seiche

The long, deep and layered nature of these two lakes also make them susceptible to another strange phenomenon. A wave called a seiche.

A seiche is a standing wave in an enclosed body of water, such as a lake. The term "standing wave" means that the wave goes back and forth between two fixed boundaries. You can observe a standing wave whenever you watch a guitar string being plucked. The wave goes back and forth from one fixed point on the guitar string to another.

In the case of Loch Ness and Champlain, the fixed points are the farthest ends of the lakes. The wave moves back and forth between them. To picture what is happening in the lakes, imagine what goes on when you sit at one end of a bathtub and suddenly get up. The water your body displaced at one end of the tub comes rushing in to fill the void. As the wave hits the end of the tub, it turns around and heads back to where it originated. This wave goes sloshing back and forth in the tub many times before it loses energy and levels out. By standing up, you have created a seiche in your tub.

The same kind of wave occurs in the lakes. In the case of Lake Champlain, the wave may be from 30 feet to 300 feet high! Why does nobody notice a wave that high in the lake? Because the wave doesn't happen on the surface, but underwater. This huge wave moves along the boundary between the warm water layer and the cold water layer.

Boundary Layer

The warm and cold layers of these lakes do not usually mix. The boundary is very much like the boundary between the surface of the lake and the air above it. In the same way the waves we usually see move along the water-air boundary, the giant seiche wave moves along the warm water-cold water boundary. A giant 300-foot wave might be roaring along underneath the water, while the surface is smooth and placid.

What starts these waves? Scientists think that prevailing winds running the length of the lake can cause a build-up of the epilimnion (warm water) at the end forcing the hypolimnion (cold water) to the opposite end. When the wind stops, the warm water on the surface starts flowing back to its regular position. This is very much like getting out of the bathtub in our example. The cold water layer then suddenly rushes back to the end the warm water vacated. This giant, powerful wave of water then bounces back and forth between the ends of the lake to make a seiche. It can take 4 days for the wave to go the entire 60-mile length of the main part of Lake Champlain.



electrobleme

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Movement in "Still" Waters: Under the Surface (internal seiches)
« Reply #5 on: June 25, 2009, 07:55:10 »
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Fresh water By E. C. Pielou
Google Books

Chapter 7 - Lakes - 7.5 Movement in "Still" Waters: Under the Surface (internal seiches)

Consider internal seiches first; they are also called temperature seiches. As an internal seiche peaks at one end of a lake, the water surface vertically above it sinks slightly: that is, the internal seiche produces a very small "reflection" of itself at the surface of the lake.

In contract to a surface seiche, in which water flows in only one direction at any one moment, it flows in opposite directions simultaneously in an internal seiche: while it is going from left to right below the interface, it goes from right to left above.

« Last Edit: June 25, 2009, 08:01:58 by electrobleme »