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    Kelds, waves, and currents. was: kelds
    From: George Huxtable
    Date: 2006 Jul 26, 04:21 -0500

    Lu asked about the meaning of the word "keld",; thanks to others who
    have replied with definitions.

    It's sent me to my own bookshelves, but none of my own dictionaries
    has the word either, so I should have explained it as I used it. Not
    even in Smyth's "Sailor's Word Book", where almost anything nautical
    can be found. My wife has found it in our copy of "A Glossary of Sea
    Terms", by Bradford (1954), as- "Kelds: Smooth patches in the midst of
    ruffled water", which seems to cover it.

    It may be of interest to discuss it a bit further.

    Most small-boat sailors in and around the English Channel will, I
    expect, be familiar with the term, and with the phenomenon. Many
    others, who sail in waters where currents flow over an uneven bottom,
    will know it too. I associate it with upwelling of water, from below
    the surface, which shows itself as a spreading, still, oily pool,
    roughly circular, in the middle of the waves. Those patches can be 100
    ft. across, or more.

    But it has long puzzled me, why it should be like that. Why and how do
    waves, travelling along the sea surface, distinguish between this
    patch of "new water" that has recently appeared, and the rest? Why
    don't the waves quickly cover the whole lot?

    I haven't worked it out properly in my head, but suspect it's due to
    the behaviour of waves travelling over water, where the surface
    velocity differs from one place to another, such as where water
    spreads out, radially, from such an upwelling. The velocity of the
    waves, with respect to the water surface, will try to stay constant,
    so their direction of travel will be refracted, in a similar way to
    light getting refracted by a medium in which it travels more slowly.
    If waves are refracted away from such a patch, on each side, that
    could explain the smooth area between. But it doesn't explain the
    details, such as the circular shape, not to my satisfaction, so I am
    not entirely happy about understanding the full picture.

    Of course, if waves are indeed refracted away from some areas, they
    must be refracted together at others, which perhaps accounts for the
    notorious heaping-up of water that you find in many tide races, that
    can make them so dangerous to a small vessel.

    I remember once watching a "textbook" demonstration of the effect of
    surface current on waves, standing on the northern headland at the
    entrance to the Gulf of Morbihan, in SW Brittany. That encloses a
    large area of water, with a narrow entrance, and the tides flow
    strongly, in and out, between that entrance and Quiberon Bay outside.
    It was a fine day, and a smooth succession of regular waves was
    arriving from across Quiberon Bay, to end up on the shoreline. Except
    at one area, where they happened to meet that strong ebb current, in
    the opposite direction, fanning out from the Morbihan. And there, you
    could clearly see the wave-peaks getting closer and closer together,
    as they were slowed by the water flow travelling the other way. Until
    they reached a point when the speed of the water, out through the
    channel, exactly matched the initial speed of travel of those waves
    coming in. At that point, waves simply piled up, producing a sharp
    line of breakers, unpleasant conditions for sailing through. Past that
    point, the surface was smooth as a millpond; there was no way that any
    wave energy at all could propagate further against the speed of the
    stream. I have seen that sort of effect at other times and places,
    usually when trying to pass through, but never demonstrated as clearly
    as it was on that day. It's one of the recognised dangers of bar
    conditions. Not, in this case, the direct effect of a bar itself, but
    of the behaviour of waves where the water speed changes.

    Of course, it couldn't be quite as simple as I described. No doubt a
    gradually shoaling depth, toward the shore, had its own effect on wave
    speed, and added a complicating factor.

    George.

    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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