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  On Lake Superior,  we can have up to a 3 ft seiche tide. Our marina
frequently has an 18 inch tide, with a 5 kt current that has to be dealt
with when trying to get into or out of the slip.  I have been pinned to
the dolphin pole and dock end by this tide, engine at WOT, and no
movement until the tide eased.  The tide on Superior (at least in the
Apostle Islands Archipelago) has a 20 min. period of slack water at the
top and the bottom. The change between top and bottom takes 20 min.  We
have seena tree branch traveling at least 5kts.in the tide, returning
the opposite direction at 5 kts.  Figuring the speed of travel for this
water level undulation, I get 500 mph or more!  It's 300+ miles from one
shore to the other.  These changes occur at 20 minute intervals.  We use
our dolphin pole as a guide to the direction and speed of the seiche, as
it is almost always running to some extent.
  When there is a wind driven seiche on top of the barometric
differential driven seiche, then we can see 3 ft of level change with
huge currents.  A few years ago, we had a 100mph straight line wind come
thru. It caused a huge wind driven seiche of 36 in., and one marina had
some problems due to their concrete docks.  There were several boats
with bowsprits that got pushed under the dock (concrete), and when the
seiche came back in, their sprits were snapped off, and lost their
rigs.  5 boats were knocked over on land, toppling into each other.
They were badly damaged with crushed hulls and egg shell like cracking
where they hit the pavement.  Broken masts were all over the place.

Cheers,

Larry DeMers
s/v DeLaMer
Lake Superior

Frank Reed wrote:

>"John Rousmaniere mentions seiche motions  causing currents up to 2 knots on
>the
>great lakes."
>
>This primarily  applies to Lake Erie. Erie has significant seiches more or
>less continuously.  All lakes have natural frequencies and are therefore prone
>to seiches but lakes  aligned with the typical direction of the weather
>(east-west in the Great Lakes  region) with natural frequencies near 12 or 24 hours,
>the basic frequency of  daily weather, experience natural cycles that are
>almost as regular as ocean  tides.
>
>Lake Michigan sees noticeable seiches much more rarely since it  is aligned
>north-south. There was a big one back in 1954 that drowned some  fishermen.
>
>-FER
>42.0N 87.7W, or 41.4N  72.1W.
>www.HistoricalAtlas.com/lunars
>
>
>
>
>

   

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