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Dünung und Windstau
These remarks trie to explain the behaviour of local wind effect on the coast under the additional influence of swell approximately running parallel to the wind's direction. Owing to the high speed of long swell waves the wind's tangential shearing strain on the water masses may become less effective than it would do if no coincidence with swell waves would occur. http://www.springerlink.com/content/u54jn41n1417xq16/
Explanation: I haven't found references to lakes, but in case this really is about the sea instead of lakes, it could be the right term.
Swell waves can travel for long distances in deep water without losing the energy they acquired from the wind. But when they travel into shallow water their shape and direction changes. In shallow water, the waves slow down, their crests can bend and change direction, and their vertical profile steepens. They become so steep that they fall over themselves, breaking and losing most of their energy in the surf zone.
A group at the Scripps Institution of Oceanography uses computer models to predict how deep water swell waves will change direction and height as they enter shallow water along the California coast. In southern California, islands and an irregular coastline cause large changes in the swell waves from place to place. http://www.earthscape.org/t1/grk04/grk04a.html
Unimodal in the cited document describes the motion with swell only, versus bimodal motion (two different spectra, orientations, etc.) with combined 'sea and swell'. The orbital motion obviously interacts with the bottom, but only in shallow water.
Ah, thanks Ken. Of course, in this context the water is relatively shallow, although Lake Constance does get fairly deep in places. AFAIK the deepest they went for this experiment was 20 m (whether or not that is shallow enough to qualify, but they took samples at various depths, the others all in shallower water).
Post-grading: IMO the formulation of your source text is unfortunate or the author was confused. Düning is swell or groundswell, which is essentially a surface phenomenon, but it can affect the bottom in shallow water.
en.wiktionary.org/wiki/groundswell
Are you sure that "See" refers to "der See" (lake) here, as opposed to "die See" (sea/ocean)? In the latter case, "Seegrund" would be the same as "Meeresgrund".
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Answers
17 mins confidence:
swell
Explanation: Confirmed by the German and English Glossary of Geographical Terms by Eric Fischer and Francis E. Elliott, published by the American Geographical Society
Asker: Thank you, but as I specified in my question, my context has nothing to do with waves on water ("A long wave on water"). My source is referring to a lake bed:
CONTEXT:
Wie stark sind die Wellen am Seegrund, die so genannte Dünung?
Explanation: I haven't found references to lakes, but in case this really is about the sea instead of lakes, it could be the right term.
Swell waves can travel for long distances in deep water without losing the energy they acquired from the wind. But when they travel into shallow water their shape and direction changes. In shallow water, the waves slow down, their crests can bend and change direction, and their vertical profile steepens. They become so steep that they fall over themselves, breaking and losing most of their energy in the surf zone.
A group at the Scripps Institution of Oceanography uses computer models to predict how deep water swell waves will change direction and height as they enter shallow water along the California coast. In southern California, islands and an irregular coastline cause large changes in the swell waves from place to place. http://www.earthscape.org/t1/grk04/grk04a.html
Kim Metzger Mexico Local time: 20:04 Works in field Native speaker of: English PRO pts in category: 68
Grading comment
Perfect, that avoids the problem of pluiral waves and singular swell too - thank you!
The translation workplace 
