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From
the numerous storms that have struck the southeastern
US in the last decade we have learned that something
like 75% of all severe boat damage can be prevented
by understanding these storms and knowing the right
protective actions to take.
We also learned that
media sensationalism and too many false alarms are
like the little boy crying "wolf" too many
times. After a while, it gets very tiresome and we
cease taking the warnings seriously, particularly
for tropical storms and weak hurricanes. We’ve
heard all the dire predictions about high storm surges
that never materialized, or wind speeds that were
only half what they said they’d be. You react
to two false alarms and when the third one comes,
you ignore it hoping that the result will be like
the others.
The best protection
that can be given to our boats is to move them out
of harm’s way. But evacuation is very time consuming
and requires the help of other people, so before we
think about going to a hurricane hole, we want to
be sure we’re not responding to a false alarm,
or that moving is even necessary. The purpose of this
article is to get you thinking about the security
of your boat’s dock location and to help you
determine if your boat is a casualty waiting to happen.
The effect of hurricanes
is not everywhere the same. You see that ominous looking
thing on the satellite photo, the area called the
Central Dense Overcast, but not all of that area is
subject to hurricane force winds; of what we see visually,
only about half the size of the thing is actually
the storm. Of course, the eye and eye wall area are
the worst, yet often times as little as 50 miles out
from the center, winds are less than hurricane force.
For other storms, fifty miles out is nearly the same
as the center. This kind of information is given out
by the National Hurricane Center as a graphic on their
web site at www.nhc.noaa.gov along with other
graphic prediction charts that are very useful toward
making decisions about what needs to be done.
Storm effect predictions
can fail to materialize for four reasons: (1) the
storm path doesn’t go where predicted, (2) the
storm substantially weakens before landfall, and (3),
because we are not directly in the eye path, where
the storm’s fury is much less, (4) the storm
passes to the east or north of us, thus we are exposed
to the back side only with offshore winds.
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| Storm
surges on bays and sounds. Note the effect of
a double surge as wind moves the water to one
side of the bay. |
These are not false
alarms, but conditions that can lead us to the perception
that there was no need to take serious preparations.
It’s not an easy thing to decide when you should
or should not move your boat to a safe harbor, but
with a little advance research and planning, there’s
a lot we can do to get a better idea of what kind
of conditions we’ll experience, and thereby
reduce the number of times we take unnecessary preparations.
The first step is to
get a general understanding of the geography of your
immediate area. No two areas are exactly the same
and the effects of a storm in one will be different
than in others. For example, it took decades to realize
that storm surge is vastly different off deepwater
coastlines than for off shallow coastlines like many
parts of the Gulf and Carolina coasts. One of the
most important things we can do is to learn the history
of the storm’s effects in your area.
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| Storm
surge graph showing surge height relative to center
of storm. Notice how the surge can actually be
negative on the west or south side. |
Since storm surge has
the most damaging effect on boats, more
than wind and waves, so we need to understand where
it occurs and where it doesn’t. This we do by
understanding the rotating winds around the eye of
the storm along with its effect on water and the land.
For example, 95% of all Atlantic hurricanes approach
from the east to south quadrant. Since we know that
wind rotation is counterclockwise, this makes wind
directions very predictable. And because we can reliably
predict wind direction, we can more reliably predict
where the wind will pile water up on lee shores.
Since most of our dockage
facilities occur within bays and sounds, I’ll
focus on these. One of the worst of these are funnel
shaped bays where wind pushes water into an increasingly
confined space, with the effect that the water piles
up to unusual heights. Along the Gulf and southeast
coasts, most bays are merely separated from the ocean
by barrier islands that are nothing more than big
sand bars topped by dunes. Storm surge and wind may
or may not cause extremely high water levels. It depends
on whether the surge over washes the barrier island,
and whether the inlet is large enough for the surge
to fill the bay or sound fast. With tropical storms
and weak hurricanes, this usually doesn’t happen,
so that the leeward side of the barrier island may
remain fairly safe. But if the inlet has any funnel
shape at all a slow moving storm will have time to
push enough water into the bay raise water levels.
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| Possible
storm approach is usually within a 90 degree
quadrant. At any location on this map, the
effects of storm surge will be very different
depending on direction. If you were docked
in that bay, which directional approach would
be the most dangerous? Would any be relatively
safe? Answer at bottom of page. |
Mobile, Delaware Rhode
Island Sound, Galveston and Lake Pontchartrain are
all good examples of funnel bays. The next important
factor is the wind direction relative to the orientation
of the bay. Water levels will rise the highest when
the wind is going to blow straight down the length
of the bay. Obviously, if the storm approach is perpendicular
to the bay length and inlet, then water isn’t
going to be pushed in, and that bay may actually end
up being quite safe. If the wind blows straight down
the length of the bay, water levels will be higher
at the back of the bay, than at the mouth, particularly
if the bay mouth is wide.
The good news about
storm surge is that it will occur only on the north
east or right side of the eye, with the worst at the
northeast eye wall. When we plot graphs of water levels
along the coast, what we get is an S shaped curve
with a big spike east of the center that falls rapidly
and then tapers off gradually as the wind field diminishes
to the east, or in the case of the Mid Atlantic and
Texas coasts, to the north and east. Various coast
line irregularities will cause minor spikes. To the
south and west, we get negative water levels due to
winds being offshore.
On bays we have two
surges, a macro and micro as shown in the above graphic.
The macro is due to the major oceanic surge while
the micro is the wind pushing water to one side or
end of the bay. The micro may by no means be little
and can equal the levels of the oceanic surge, which
is what makes bays so dangerous. Depressions, bayous
and estuaries along the leeward side of the bay will
have even higher water levels due to the funnel effect.
If sounds are separated from the ocean by only low
lying sandy and narrow islands, wash over from the
main surge will fill the sounds even higher.
Fetch
is the undisturbed distance over water which the wind
blows, thereby resulting in larger waves. The more
fetch, the larger the waves will become as will the
micro surge that can be out of proportion to the main
oceanic surge. Hurricane force winds are capable of
piling a lot of water up on the leeward side of even
a narrow bay. For example, the surge on two mile wide
Choctawhatchee Bay during Opal with 125 mph winds
was 3 feet, on top of a 6 foot macro surge for a total
of nine feet.
Since bays are usually
irregularly shaped, there are often areas that are
sheltered from waves as well as limiting the storm
surge. The trick is to try to guess which ones these
will be relative to the direction of a particular
storm approach. These are usually going to be along
the leeward and eastern shores and judging the fetch
relative to wind direction will generally tell us
where those places will be.
The assessment of exposure
is important since boats in some areas will risk destruction
even in strong tropical storms whereas others will
be safe even in weak hurricanes. It can save you a
lot of work and heartache if you develop a good understanding
of the risks of your boat’s location. If your
area has had prior storms, it helps to talk to people
who have some local knowledge about how storm surge
behaves. And to find out which areas are more secure
if your present one isn’t one of those.
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| With
a storm approaching from direction "A",
boats in the bay would not experience a very
high storm surge. it would be a different
matter with the approach of storm "B"
where the greatest force of wind pushes water
directly into the bay. |
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