Geoscience Reference
In-Depth Information
water piled up depends upon the speed of the wind, its
duration, and its location relative to the center of a
cyclone. Wind set-up is difficult to calculate, but
Wiegel (1964) gives an equation that shows the magni-
tude of wind set-up in a channel as follows:
similar to that of the storm. The height of the wave is
related to the size of the disturbance. Along the United
States east coast, this long wave can have a height of
several metres, becoming highest where land juts out
into the path of the tropical cyclone. Figure 3.25 shows
the track of Hurricane Carol in 1954 along the United
States east coast, together with records of sea level ele-
vation at various tide gauges. Note that as the cyclone
approaches land, the surge height increases, but it
increases most where land at Long Island and Cape
Cod intercepts the storm path. At these locations the
long wave, which has been moving with the storm,
piles up against the shoreline. The 21 September 1938
east-coast hurricane, moving along a similar path, sent
a wall of water 6 m in height plowing into the Long
Island coastline, where it made landfall. Along the
Cape Hatteras barrier island chain that sticks out into
the Atlantic Ocean, and along the Gulf of Mexico
coastline of Florida, planning dictates that houses must
be constructed above the 6.5 m storm-surge flood
limit. In New South Wales, Australia, most storms
move offshore; so, the long wave is in fact traveling
away from the coast, and there is minimal storm surge
felt at the shoreline. This is the reason storm-surge
elevation during the May 1974 storm reached only
0.7-0.8 m instead of the theoretically possible value of
1.2 m due to atmospheric deloading.
Figure 3.25 also reflects two other factors affecting
surge. As Hurricane Carol moved along the coast, it
began to cross a shallower shelf. The storm-surge wave
underwent shoaling (shallowing) as it moved shore-
ward. This raised its height. Figure 3.26 illustrates the
effect of shoaling on a long wave. The movement of
the storm-surge wave throughout the water column is
dictated by the speed of the storm. As this wave moves
through shallower water, its speed decreases and,
because the energy flux is conserved through a
decreasing water depth, wave height must increase.
Any shallow body of water can generate large surges
for this reason. The Great Lakes in North America are
very susceptible to surges of the order of 1-2 m.
In December 1985, a major storm struck Lake Erie,
producing a surge of 2.5 m at Long Point, Ontario,
coincident with record high lake levels. Lakeshore
cottages were floated like corks 0.5-1.0 km inland. In
the shallow Gulf of Finland towards St Petersburg,
surges 2-4 m in height can be generated. Since 1703,
there have been at least 50 occasions when surges
greater than 2 m have flooded this city.
h
2
~ [(2.5
-1
g
-1
) (
x
+
C
1
)] -
d
(3.1)
where
h
= height of wind set-up in metres
d
= water depth of channel
= 0.0025
U
o
2
= density of salt water
g
= gravitational constant
U
o
2
= wind speed
(
x
+
C
1
) = the
fetch
length of the surge
Equation 3.1 does not help one to easily conceptualize
wind-induced surge heights, because the actual surge
depends upon where you are relative to the movement
of the cyclone and its center. If the wind is moving
away from a coast, it is possible to get set-down. In
general, for a cyclone with 200 km hr
-l
winds, one
could expect a wind set-up of at least 2 m somewhere
around the cyclone. Sea level elevation depends more
upon the weight of air positioned above it. As air
pressure drops, sea level rises proportionally. This is
known as the
inverted barometer effect
. It must be
remembered that some tropical cyclones have a
pressure reduction of up to 13 per cent. Equation 3.2
expresses simply the relationship between sea level
and atmospheric pressure:
h
max
= 0.0433(1023 -
P
o
)
(3.2)
where
h
max
= the height of the storm surge
due to atmospheric effects
P
o
= the pressure at the center of the
hurricane in hPa
From Equation 3.2, it can be seen that the lowest
central pressure of 870 hPa ever recorded for a tropical
cyclone could have produced a theoretical surge
height, due to atmospheric deloading, of 6.6 m. The
theoretical surge height for the North Sea storm of
1953 is 2.47 m, while for the 25 May 1974 storm in
New South Wales it is 1.2 m. The last value is higher
than the recorded tide level difference and reflects
cyclone movement.
If a storm moves in the direction of its wind speed,
then it will tend to drive a wall of water ahead of it.
This wall behaves as a wave and travels with a speed
