NavList:

Guy:

A simple question with a not-so-simple answer.

I've been sailing on tidal waters (first New England and now SF Bay) for
30 years.  I've also taught the Power Squadron's courses that touch on
tides and currents.   And to make things even worse, I'm an engineer, so
I think I know how things ought to work in theory.

Let's take two or three cases:

1.  A simple bay or harbor in the ocean.   The ocean's tide comes up,
water flows into the bay, the ocean's tide goes down, water flows out of
the bay.   If you plot water level at the mouth of the harbor, it looks
pretty much like a sine wave.   The most rapid rise or fall in the
height of water is at the midpoint between the high and low.  Since the
water rushing in or out of the bay is driven by these changes, it is
logical to conclude that the current is most strong at the midpoint of
the tide.    At the same time, remember it takes time for water to flow
into this hypothetical bay, so the time of high and low tides will
likely be later and later as one goes up-bay one, similarly for the
times of maximum flood/ebb for currents.  By the way, all these
arguments are true even in areas with diurnal inequality, the technical
term for the asymmetry in height of the two daily highs and lows we
experience on the west coast.

But most navigation texts warn that there is not necessarily such a
correlation between tide and current.  Why?

1a.  My argument above ignores resonances.  Just as one can get the air
in a soda bottle to resonate by blowing across the top of it, the water
flow in a bay can "resonate" due to the shape of the bay.  The most
spectacular example of this is the Bay of Fundy, where the 40' tides are
a result of the shape and size of the bay resonating with the ocean's tides.

2.  More important, many bays (San Francisco Bay is a notable example)
drain rivers.  One third of the entire state of California drains into
SF Bay.  If a bay is also draining a river, current is a result of both
the ocean sloshing in and out, and the feeder river(s) constantly
creating an outgoing current.  Think of it as a tide-driven current as
in the first example superimposed on a constant downstream flowing
current.  As one goes farther up-bay, two things happen:  (1) Tidal
highs and lows occur later and later (ignoring resonances) for the
reasons given in example 1, but (2) the time period during which
currents are flooding (flowing against the river current) becomes
smaller and smaller.   It's in these mixed currents (tidal current plus
river flow) where the time of maximum flood/ebb and high/low tide are
not as clearly correlated as in the first and simplest example.

Let's use an example: For today, Sunday, July 2nd, here's the data for
the Golden Gate:

Tides        0415 High        1041 Low        1756 High      0012 Low

Currents             0926 Max Ebb    1612 Max Flood    2211 Max Ebb
                 0623 Slack       1307 Slack       1933 Slack

Two things are evident from this:

1.  For the Golden Gate, at least, the time of maximum current is much
nearer to the next high/low than the preceding one.

2.  The time of slack water is significantly off from the times of
high/low tide.

Bottom line, it's probably wise to keep both tide and current tables
handy.   Hope this helps instead of confuses.

Lu Abel

Guy Schwartz wrote:
> 
> ----- Original Message -----
> *From:* Guy Schwartz <mailto:guyschwartz@sbcglobal.net>
> *To:* Navigation Mailing List <mailto:NAVIGATION-L@listserv.webkahuna.com>
> *Sent:* Saturday, July 01, 2006 3:18 PM
> *Subject:* A little off topic _ Ebb and Flow
>
> This may not be the correct group for the following question, however
> there is both sailing and math skills present here.
> The question:
> Is there a time relationship between High Tide and Max flood and
> converserly low tide and max ebb?
> For instance if high tide is 0118 what time was / is max flood. I
> suspect the number will vary with the height of the tide. I tried to put
> the information from the tide tables into a spread sheet, but could not
> detect a patern. To complicate the issue, on the west coast where I live
> we have to highs and two lows per day with one of the two being a higher
> high and a lower low.
> Thank you,
> Guy
>
> >

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