Re: [Paddlewise] Constricted Bay Entrances

Dave Kruger wrote:

>Peter Treby wrote:

>> Is it generally true that where a tide stream runs through a bottleneck,
>> such as a narrow entrance to a widening bay, slack water in the
bottleneck
>> will not coincide with either high water or low water?

>Yes.  Depends on size of the bay relative to the narrowness of the neck.
Slack
>water will occur when the relative levels of the bay and "ocean" outside
the
>bay are the same height (ignoring the effect of momentum of the tidal
stream).
>OTBE, this will be somewhere close to mid-tide, and may only occur for a
very
>short time.

Peter, it is always true. By definition, a bottleneck creates a delay so the
water won't be the same height on each side of the bottleneck (when the flow
becomes "slack") until sometime after high or low tide. What will determine
how long that delay will be will depend on many factors. The flow rate
through the constriction, the shape of the constriction (deep and narrow vs.
high and shallow), the tidal variations due to the orientation of the moon
and sun (neap and spring), the type of tide due to the tilt of the earth and
other factors (semidiunal, diurnal or mixed), the nearness of the moon and
sun (apogee and perigee), the depth of the offshore water (whether there is
a continental shelf or not will greatly affect tide height), the shape of
the coastal configuration (a bay may be subject to seiches or bores), and
the amount of water needed to fill the estuary up to the point when the tide
outside has risen or fallen to the same level.
With only a very slight constriction in the flow, the delay between high (or
low) tide and the "slack current" time will be small. But imagine a
situation where only a trickle can get through a tight bottleneck (such as a
small vertical gap in lock doors). When the trickle through the lock doors
first started (creating a new estuary where one hadn't existed before) the
slacks would only be near low tide and most of the time the water would be
running in. At some point an equilibrium would be reached where the estuary
had finally filled until the water spent about one half the time running in
and half running out. Once this balance was reached then the delay would
mean that slack current most likely occurred somewhere near mid-tide and the
water level in the estuary wouldn't change much at all. However, then
variations in the tides (spring, neap, apogee, perigee, etc) would become an
important factor shifting slack to nearer the high or low tides (much like
when the basin was first filling).
With a different type of constriction, say one where the estuary's opening
is only reached at the highest of tides, then slack could only occur
somewhere near high tide and most of the time water will be running out of
the estuary (or it will have drained completely).
The variables to knowing when slack will be are potentially so numerous that
the best solution for most navigators seeking "slack water" is to use the
current tables (if they are available for the area).
Gee, I planned a short simple explanation but the complexities and variables
kept on rearing their ugly heads and complicating things. I hope this was
understandable (and correct).

Matt Broze
http://www.marinerkayaks.com


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