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Bill wrote:

> Jared
>
> You bring up an interesting memory that I had put aside regarding fetch and
> waves (which would also affect current).  This summer (AKA the "summer that
> never was" on Lake Michigan) a lot of wind from the north.  Sailing out of
> Michigan City on the southern coast. 10-28 kt breezes out of the north for
> several days one extended weekend. 10 statute miles (love those Great Lakes
> charts) big but nicely formed wave trains and swells.  Within 2-3 miles of
> the dunes, mashed potatoes--huge piles of water.  Reading Trevor and
> thinking back, I was "set up."  What we saw was water bouncing of the
> southern shore in depths ranging from 40-200 feet.
>
> In this case, although fetch was several hundred miles and time was in days,
> because we were in near proximity to the shore (an perhaps factor in onshore
> breezes kicked up a notch by the the northerly breeze--exacerbated by
> shallow depths) it was a wild ride.
>
> While fetch related to waves/wind-induced current may indeed be measured in
> shore-to-ship distance for Doug on the high seas, in this case you make a
> strong point that what is leeward of the craft and proximity also plays a
> role.

I'm not quite clear what Bill's "mashed potatoes" looked like but there
are a number of possible causes.

Water depth was likely part of it, with the waves becoming shorter and
steeper as they felt bottom.

With a steep shoreline, there is the possibility of the waves being
reflected, so that two wave trains running across one another -- the
ordinary wind waves and their reflection. That can make for a wicked
sea. However, a shoreline of dunes suggests a sandy beach and beaches
absorb wave energy rather than reflect it, so I doubt that reflection
caused Bill's wild ride.

Refraction of waves around shoals and islands can have interesting
effects too but it sounds like Bill experienced his ride across too
large an area, with no islands and few shoals to windward.

So I'd guess that the the lake water set up against the Chicago
waterfront was escaping to the northeast in a sort of giant rip current,
following the beach. If so, the waves from the north would have entered
a "wind over tide" situation (except that it wouldn't be a tidal stream)
as they also entered shoal water. That would make things pretty exciting
for a small vessel.


Trevor Kenchington



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Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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