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    Re: Wind drift, was: DR thread from Nov-Dec '04
    From: Trevor Kenchington
    Date: 2005 Jan 20, 09:50 -0400

     > 1. If I understand it correctly, tides can be amplified by what I
     > will term the "natural frequency" of a basin.
    Tides are totally different because the driving force is periodic (and
    very regular in its period). To grossly over-simplify: The Moon pulls on
    the North Atlantic, dragging a bit of it into the Bay of Fundy. As the
    Moon's pull reverses, the water in the Bay sloshes back following the
    natural period of that basin. It is just ready to slosh back in again as
    the Moon once again drags water that way, so the resonant "in sloshing"
    coincides with the inward flow dragged by the Moon. Hence the
    amplification. A little bit of energy from the Moon twice a day gets
    captured within the natural sloshing of the Bay and the total energy
    within the system builds up, creating some exciting tides.
    Wind acting over a lake is different because the wind won't switch on
    and off, nor reverse direction, with any regular period, so it can't
    switch on and off with the period of any specific lake. Once in a while,
    you might see the wind blow, chop around to the reverse direction and
    then change once more at just the right time to reinforce the seiche in
    the lake but even a couple of well-timed changes would be a very rare
    event. The semi-diurnal lunar tide, in contrast, has been doing its
    thing twice a day for millions of years with only a very slow change in
    its period.
     > 2. The Annapolis Book of Seamanship states regarding seiches, "On
     > tideless lakes,  strong wind-driven currents called seiches may
     > develop.  The effect of wind-driven current is figured like that of
     >  a tidal current."
    Even the best textbooks contain errors.
    The National Ocean Service's "Tide and Current Glossary" starts its
    definition of "seiche" with "A stationary wave usually caused by strong
    winds and/or changes in barometric pressure" and then goes on to
    supplementary details, including the equation for the period of a seiche
    (2L/gd in my copy though I think there is something missing due to the
    PDF lacking some symbol font). That first sentence of the definition is
    a bit misleading, since winds and barometric pressure cannot "cause" the
    stationary wave. The wave is driven by gravity (hence the "g" in the
    equation). The role of wind or pressure is to set up the water so that
    gravity can set it moving.
    The Annapolis statement that the effect "is figured like ... a tidal
    current" is a bit opaque. They may be trying to say that seiches operate
    as standing waves, which is also the form that tides take in most ocean
    basins. [That is the kind of standing wave which has one or more nodes,
    where there is no rise of tide, although there are strong tidal streams,
    with antinodes elsewhere which do have large amplitudes. Not to be
    confused with the standing waves seen immediately below a weir, where
    something that looks much like a wind wave remains fixed in relation to
    the weir as water races past -- which is the same as the stern wave of a
    planing powerboat seen by an observer on the boat.]
     > 3. If I understand your more complete explanation, that wind/current
     >  piles up water at the leeward end of the lake, and there are two
     > probable outcomes once the fluid sets up (piles up?) on the leeward
     >  end of the body.
     > a.  A subsurface current in the opposite
     > direction attempts to "level out" the basin
     > b.  The water remains
     > "set up" (piled up?) at the leeward end of the body.
    I think those are the two alternatives, while the wind continues blowing
    at the same strength. However, I don't recall reading a serious
    discussion of the issue.
    In reality, there will be some intermediate cases, of course, with some
    small subsurface counter-current but not enough to materially reduce the
    set-up. Plus there will be cases where the shape of the shoreline,
    combined with the direction of the wind, encourages a surface
    counter-current at one side of the lake -- much like a rip current
    forming on one side of a cove when wave transport sets up the water
    against the beach.
     > 4.  By your definition of seiche, when the wind decreases/stops,
     > gravity once again prevails and the piled up water on the leeward
     > side of the body rushes to the former windward end to establish
     > equilibrium (level off), setting up a wave.
    Yes. Gravity pulls the water back towards level but that sets the water
    in motion, giving it momentum. The momentum carries the water past level
    and piles it up at the opposite end of the lake. Gravity then pulls it
    back towards level and so the process continues. However, friction
    swiftly bleeds energy out of the system and the wave dies away.
    You can demonstrate this quite easily in a bucket. (I use a small fish
    tank when teaching tides and related phenomena to Power Squadron
    classes.) If you are a real fanatic, you can amuse yourself while
    waiting for a restaurant meal to be delivered by creating seiches in
    your wine glass -- or better still the two-dimensional version called an
    "amphidromic system", which is the form that tides take in most real
    seas. Repeated experiments into the change in period as you reduce the
    depth of the wine by draining the glass can really add to the entertainment.
     > 5.  If the basin is sympathetic, this "sloshing" may be rather
     > dramatic for a tideless body of water.
    No. All basins have their own natural period (based on length and depth,
    as in the equation given above, albeit in corrupted form). If the wind
    sets up the water surface and then dies away, there will be a seiche
    which will follow that natural period.
    "Sympathy" only comes in if you have a periodic driving force, such as
    the tides. When the period of the basin and the period of the force
    coincide, you do indeed get dramatic results, as in Fundy. That is going
    to be very rare in lakes.
    But given the right relationship of lake length and depth, the sloshing
    can be considerable _relative_to_ people's expectations that lake shores
    ought to stay in place. The little 2-metre tides on this side of Nova
    Scotia don't seem much for an ocean coastline but a sudden change of a
    metre either side of the normal level of a lake would cause serious
    problem to many people living along its shore.
     > I feel you have adequately dispelled the heartland folk lore
     > concerning why a seiche occurs, particularly for a lake that is
     > relatively shallow at its southern tip and would have thermoclines.
     > Given I may have made some logical leaps above, but the definition
     > of seiche remains a wee bit unresolved for me. It is either:
     > A two line reference stating it is a wind driven current.
     > A knowledgeable explanation stating it is akin to a standing wave,
     > AKA sloshing in a basin after the wind dies.
     > I apologize for the second degree.  A lawyer would say, "asked and
     >   I am attempting to "get it right."
     > I hope my summary is close to accurate. Friends might claim
     > decreasing my sphere of ignorance is thankless.  They are
     > incorrect.  Thank you.
    No problem.
    Hope I have clarified things a bit further.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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