This article is the third of three parts on the diving impact of tides and tidal currents. Please refer to Part One and Dois before reading this section as we are expanding concepts explained previously. 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.
In this article we expand on other astronomical phenomena that influence the height of the tide and, therefore, the strength of the maximum current. Please refer to the previous two articles on the subject.
DISTANCE OF THE MOON
The attraction force between our planet and the moon varies
with their distance apart; the farther apart, the less
attraction and vice versa. When the moon is on
its perigee, it is on its closest point to the Earth and
the tidal currents' maximum strength will be intensified.
Upon reaching its apogee, or farthest distance from our
planet, the moon's effect will be decreased and the
maximum current strength will be reduced. The effect of the moon's
apogee and perigee on the tidal current
is irrelevant to whether we have spring or neap tides.
Let's quantify these forces in an oversimplified way. Please remember these numbers do not, in any way, reflect a real current strength. They are used only to help establish a reference point. Local currents may vary significantly. Check your local diving authorities for more appropiate information.
If we assume that the average strength of a tidal current is around 50 ft/min (a normal diver moves at 100 ft/min and this is only for a few minutes), we would expect that:
Max current Perigee Apogee Spring Neap = Total 50 ft/min +10 +25 = 85 ft/min 50 " -10 +25 = 65 " 50 " +10 = 60 " 50 " -10 = 40 " 50 " -25 = 25 " 50 " -10 -25 = 15 "The speeds range from a maximum current so slow that any diver could cope, or so strong that none could. Keep in mind we only refer here to the maximum current occurring halfway between tides (usually about three hours after a tide or before the next one). The minimum or slack time occurs close to the time of the tide and could go as slow as 0 ft/min (no current).
Other currents, like global or wind driven currents are not included in any of these calculations. They have to be determined by the diver before s/he jumps in the water. But, because these currents do not normally reach the strength of the tidal flow, they usually are not accounted for in a dive plan.
ANGLE OF THE MOON
Most of the information offered here works well when the moon is at
or near the Earth's equator. In other words, events
...... highs and lows ...... are symetrical.
While this simplifies explanations, we still have to deal with
the case of the moon traveling to the north and south of
our equator, consequently displacing the high tide sea level bulges
and the low tide depressions.
The moon travels once in a cycle as far as 28.5 degrees north and south of its plane of rotation. In the USA, that would be about the latitude of Cape Canaveral, Tampa and Corpus Christi. An observer at these locations would see a maximum high tide with an overhead moon, meaning a stronger maximum tidal current for whatever moon phase (spring or neap). But 12 hours later, s/he would then find a not-so-high tide because the high tide bulges are not symetrical ...... producing a weaker maximum tidal current. In Figure 1, we see the diver under the moon's zenith in the northern hemisphere with a very high spring tide. Half a day later, s/he is at a not-so-high tide on the other side of the planet. Of course, now the change of tide height is minimum, therefore, the maximum tidal current is weak. So on days when the moon is at or approaching its northernmost or southernmost points in its orbit, it would be smart to dive when the tidal change is minimal and maximum tidal currents are weak. So, rather than a morning dive with a strong current, plan on an afternoon dip with little or no current. You'll enjoy it more and be safer.
Let's get our conclusions together here now :
ONE ......Moon perigees intensify the "normal"
tidal current maximum current. Moon apogees decrease the
"normal" tidal current maximum current. We can find information
on the moon's orbit on any nautical almanac or in the
newspaper clipping on the tides.
TWO ......The tidal current speed is the resulting
combination of forces affecting the main water in (or out) flow
including apogee, perigee, neap or spring
tide.
THREE ....The moon at its northernmost or
southernmost position will produce one very high tide
and one very low tide, then one not-so-high tide followed
by a not-so-low tide ...... so there will be one strong maximum tidal
current and one weak maximum tidal current.
This is regardless of whether it is a spring or neap tide.
Divers must be aware of the forces the moon exert upon our oceans. Very few events will scare a diver more than being pulled away from the return point by an uncontrollable force. From trying to counter an overwhelming current comes fatigue and from fatigue comes despair, from despair, panic, and from hours waiting for a rescue, a very chilled and disgusted diver (at best!!)
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 to low is so small (less than 1.5 ft) and
the water rush is not too strong, so your timing is not
as critical. Of course, if your swimming skills are not
fully developed, don't take any chances, dive when the
currents are slack whether it is spring or neap tide.
Now that we know how tides relate to currents, its effect on divers and the times to avoid strong tidal currents ...... and just as important, how and where to get the information ..... we can make it an integral part of our dive plan. It is only to our own benfit and safety. Many books stress the importance, but fail to explain how you make your calculations to know when to jump in.
Any comments, complaints, gripes, grumbles,
laments or accusations can be addressed to our
webslave ......
Wally Barnes