This article is the second of three parts on the diving impact of tides and tidal currents. Please refer to part one before reading this section as we are expanding concepts explained there. We repeat that this article is NOT INTENDED to be universal, but general in nature and in NO WAY should be construed to be representative of any particular site. Users are advised to seek local information regarding differences between the strength and timing of tides and tidal current maxima and slack waters.
ANGLE AND DISTANCE OF THE SUN AND
MOON
Not to get too boring with technicalities, but let's remember that
the sun and moon are not always in the same celestial plane.
When they are, we get an ECLIPSE
and the points of maximum centrifugal and
centripetal force coincide. Then, we really are talking
about extremely high HIGH TIDES and
extreme low LOW TIDES ...... and its
consequential extremely strong TIDAL CURRENTS.
Of course, eclipses occur on new or
full moons ...... SPRING TIDES
...... only.
COULD WE COMPLICATE MATTERS MORE ??? Of course !!!
We could indicate that the moon, at its closest distance (perigee)
from Earth,
the stronger its influence. By the same token, the
farther the distance, the lesser the effect over the tides.
With a decreased moon effect, the lesser the change of tidal height.
Remember, the higher (or lower) the tide, the stronger
(or weaker) the tidal current ...... and this is what divers must
understand to avoid a face to face encounter with a strong tidal current.
We could also add meteorological effects on currents, like
strong wind blowing or water accumulating against land masses
and rushing back or rivers and
estuaries discharging unusual amounts of water causing currents.
But things are stuffy enough the way they are to add those irregular
and highly unpredictable events. Let's stick to tides
and its currents.
This was the case of our last "Wreckers Delight"
Liveaboard trip to the Florida Keys (see window in main menu) ......
where we visited the Duanne, the Bibb and the Eagle
Wrecks (all deep wrecks) in one weekend. We just had an eclipse the
day before departure, under a gorgeous harvest full moon which was
just in its perigee the day before ...... and believe it,
all those factors made the current streak by really howling!!!
HOW TO USE TIDE TABLES
Divers don't need a PhD to tell them that currents are critical to a
person jumping from a boat and swimming underwater several
hundred feet away. A diver's average speed is about 100 ft/minute,
which is around one knot or one mile per hour
(give or take). A diver can usually cope with
currents less than 60 ft/minute ...... anything above that would
cause over-stress and exertion (depending on stamina,
equipment and physical conditions).
Tidal currents can vary from zero to well above 200 ft/min ......
that's why dive planning MUST include
the tidal current factor in it.
Let's get into the nitty gritty of the article. When a region
in the ocean is at high tide, we assume water is no longer
coming in (otherwise the level would continue to rise). That
means the incoming current is ZERO. The same logic follows for
low tide. When at low tide time, no water is outgoing
or the level would continue to fall. The TIDAL
CURRENT AT THE TIME OF HIGH OR LOW TIDE IS NEAR ZERO,
or at least so weak (less than 20ft/min), we call it
SLACK water. Returning to our
original point of high tide, ...... now the water will begin to flow
toward other sites which have rising levels and a tidal
current starts. Slowly at first ...... then stronger and
stronger ...... until a maximum strength is reached at about
halfway to the next (low) tide time ...... roughly three hours (since
we have six hours between high and low tide). From
that maximum at the midway point, it begins to weaken until it
reaches a ZERO again at the next tide time.
In the tide tables, the information you need is the time of the
high and / or low tide. We will consider these
times as the slack (near zero) current. We will then estimate
that halfway between the slack times, the current will be at
its maximum strength.
EXAMPLE :
With a high tide at 13:03 (1:03pm) and a low tide at
19:21 (7:21pm) ...... we start with slack current (near zero) at 1:00pm.
Then, there will be a weak out-going movement getting
progressively stronger until 4:10pm, when it reaches its maximum
speed, then weakening until it is back to near ZERO at 7:20pm
(time of low tide). It will now reverse the flow and
a weak incoming current will start rising the water level becoming
stronger and reaching maximum strength halfway to the
next high tide ...... at which time it will zero again and
start the cycle all over.
We now know when the currents are going to be stronger or
weaker, but how strong??? The answer is in the moon phase.
At SPRING TIDE ...... new
or full moon ...... that maximum current is bound to be very
strong and should be avoided by divers ...... unless,
of course, you are doing a drift dive. If you are planning
a reef dive that day, make sure you're in the water very close to
1:00pm with the SLACK current
and out of the water before 2:30pm when the increase in current
speed becomes sudden and dangerous. On the other hand, if
that dive was planned during a NEAP tide
...... first or last quarter moon ...... the dive could be
carried out even at 4:00pm since its strength is NOT likely to be extreme.
AGAIN, THESE ARE GENERAL CONCEPTS OF TIDAL CHANGES
AND DO NOT APPLY UNIVERSALLY TO ALL PLACES AND
AT ALL TIMES. CONSULT YOUR LOCAL DIVING AUTHORITIES WHEN IN DOUBT
OR TO LEARN LOCAL FACTORS INFLUENCING THESE CONCEPTS.
Check your local tide tables, notice the height of the high tide
and of the low tide; open ocean differences of three
(3) ft or more usually cause currents well above 60 ft/min.
In the above example, dive near the time of high
or low tide ...... when the current is SLACK (high tide,
because it brings clean water from the deep sea is usually clearer
than low tide which normally brings dirty water from nearshore).
During NEAP tides, the difference from high tide to low tide is
small (less than 1.5 ft) and the water rush is NOT too
strong, so your timing is not really critical. Of course,
if your swimming skills are not fully developed, don't
take chances, dive when the currents are slack whether it is
SPRING or NEAP tide.
Where can you get tide information ??? That's the easiest one.
Check your local newspaper, your TV weather reports
or even radio. Even yesterday's data is good, just
add one hour to that time for as good approximation.
To quickly summarize what we have discussed here :
ONE......Tidal currents are SLACK near low or
high tide time. That's four times a day. Maximum
current speed is halfway between tide times.
TWO ......Strong maxima during SPRING TIDES
( full or new moon). Weak maxima when NEAP tides
(first or last quarter moons).
THREE ....Your local newspaper carry the tide time
information or local TV weather reports.
Now that we know how tides relate to current, its effect on divers
and the times to avoid strong tidal currents ...... and
just as important ...... how and where to get this information,
we can make it an integral part of our dive plan. It is only
to your own benefit and safety. Many books emphasize
this importance. Now you know how to use it!!!
On the third part of this article, "PART TROIS",
we discuss deviations from these generalities and
quantifying some of the strong or weak currents.
Then a knowledgeable diver can better prepare to cope with strong currents
or weak ones as the case might be.
Any comments, complaints, gripes, grumbles,
grief, laments or accusations can be addressed
to our webslave ......
Wally Barnes