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    Re: Question on currents and waves
    From: Greg Rudzinski
    Date: 2009 Dec 19, 14:02 -0800

    John,
    
    Any influence which slows a wave will cause the steep face in the
    direction of motion. Possible slowing influences would be continental
    shelf, sand bars, reefs, density differences(temperature inversions,
    salinity), eddies, boundary of underwater turbulence. Add to this the
    coincidence of wave heights from different directions and you have a
    formula for a very bad water sport day.
    
    Greg
    
    On Dec 19, 12:54�pm, Apache Runner  wrote:
    > Yes, I read the post. � Maybe I wasn't clear - what I'm seeking to
    > understand is how this might affect the steepness of the waves on one side -
    > that is to say, they seem to be steep on the side facing the current, and
    > not on the side away and I don't quite understand what causes that.
    >
    > The convergence in the presence of a gradient I can understand, and also the
    > patchiness. � One of my colleagues did some work on freak waves and has a
    > fairly simple model that does a better job of predicting the frequency of
    > these than some computer models. � It's based on what you're describing
    > about currents - so yes, I understand that part.
    >
    > It's the steepness on the side "impacting" the current that I don't quite
    > understand. �That seems to be the common refrain. � I'll ask my colleague,
    > he may be able to give me an explanation on the steepness issue.
    >
    > If I missed that in the post, my apologies.
    >
    > Best,
    >
    > JH
    >
    > On Sat, Dec 19, 2009 at 2:52 PM, George Huxtable  wrote:
    > > John Huth wrote-
    >
    > > "I think already the current gradient effect of focusing the waves that
    > > Frank mentioned is one distinct culprit, but I don't quite understand how
    > > this would steepen them, although I could be convinced. � I'll let you know
    > > what I find out."
    >
    > > If John returns to my posting [11219], he will find a suggested explanation
    > > which seems at least plausible, although I can not claim it to be certainly
    > > correct. I wrote-
    >
    > > "But when the wavefronts meet these varying currents, they get refracted
    > > accordingly, as the net speed of surface propagation varies. So
    > > unpredictable patches occur, where wavefronts converge and their local
    > > energy adds; others where they diverge and the water becomes smooth. It is,
    > > of course, the adding bits
    > > that matter, and create the danger to small craft.
    >
    > > In just the same way, wave patterns, generated away from the locality of
    > > the
    > > Gulf Stream, can get refracted by those powerful local currents when they
    > > meet it, and give rise to characteristic and dangerous wave-patterns, as
    > > they get refracted toward local focus-points.
    >
    > > But all these effects are, in my view, related to local surface velocity
    > > gradients."
    >
    > > It's somewhat similar to the way that sunlight shines through one of those
    > > dimply bathroom windows, and falls on a surface behind it. The varying
    > > slopes of the glass cause the light to refract in different ways, so that
    > > at
    > > some points it adds, and concentrates, creating a dappling of the light
    > > pattern.
    >
    > > The disordered surface velocities of the current-flow cause varying
    > > refraction of the waves, giving rise to transient concentrations of energy,
    > > varying with position and time. For a given wavelength of a surface wave,
    > > there is only a limited amount of energy that it can carry before it
    > > spills,
    > > and the menace to small-craft is of those spilling seas tumbling aboard.
    >
    > > On top of that, waves generated elsewhere, where surface current is low,
    > > will find that they have to carry an increased energy-density when meeting
    > > an adverse current, which also adds to the tendency to break.
    >
    > > And waves generated elsewhere, which meet a more organised surface gradient
    > > at an angle, such as at the edge of the Gulf Stream, could well be subject
    > > to rather more geometrical bending and focussing, compared with the more
    > > chaotic turbulence that I've been discussing above. That could result in
    > > larger, and more predictable, danger-patches.
    >
    > > If John, or anyone else, comes across a decent explanation of such effects,
    > > on the web or in print, I would be very interested to learn about it.
    > > Somehow, I doubt if that will be a work on theoretical fluid dynamics,
    > > though. But you never know...
    >
    > > George.
    >
    > > contact George Huxtable, at �geo...@hux.me.uk
    > > or at +44 1865 820222 (from UK, 01865 820222)
    > > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    >
    > > --
    > > NavList message boards:www.fer3.com/arc
    > > Or post by email to: NavList@fer3.com
    > > To , email NavList+@fer3.com
    
    -- 
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
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