Geoscience Reference
In-Depth Information
Finally, the shape of the coastline accounts for the
largest storm surge. The Bay of Bengal, where storm
surges have drowned so many people, is funnel-shaped
(Figure 3.10). Any surge moving up the funnel, as the
1970 cyclone storm surge did, will be laterally com-
pressed. In addition, the size of the basin can lead to
resonance if its shape matches the period of any wave
entering it. A similar enhancement effect happened in
1770 to the tidal bore on the Qiantang River, south of
Shanghai, China. A storm surge lifted the tidal bore on
the funnel-shaped estuary to heights in excess of 4 m.
The protecting dykes on each side of the river were
overwashed and over 10 000 people drowned within
minutes. The highest tides in the world are recorded in
the Bay of Fundy in Canada because the basin shape
matches, within six minutes, the diurnal tidal period of
12.42 hours. On 4 October 1869, a cyclone, called the
Saxby Gale, moved up the United States coast in a
similar fashion to Hurricane Carol in 1954, but slightly
eastward. The storm traveled up the Bay of Fundy,
moving a mass of water at about the resonance fre-
quency (13.3 hours) of the Bay of Fundy-Gulf of
Maine system. The resulting storm surge of 16 m was
superimposed on a tide height of 14 m. In all, water
levels were raised 30 m above low tide level, almost
overwashing the 10 km isthmus joining Nova Scotia to
the mainland.
Recurrence intervals
The probability of rare events of high magnitude can be
ascertained in one of two ways: by determining the
recurrence interval of that event or by constructing
a
probability of exceedence
diagram. In both methods,
it is assumed that the magnitude of an event can be
measured over discrete time intervals - for example,
daily in the case of storm waves, or yearly in the case of
storm surges. All the events in a time series at such
intervals are then ranked in magnitude from largest to
smallest. The recurrence interval for a particular
ranked event is calculated using the following equation:
Recurrence interval = (
N +
1)
M
-1
(3.3)
where
N
= the number of ranks
M
= the rank of the individual event
(highest = 1)
The resulting values are then plotted on special loga-
rithmic graph paper. This type of plot is termed a
Gumbel distribution
. An example of such a plot is
shown in Figure 3.27 for 70 years of maximum fort-
nightly tide heights in the Netherlands before the 1953
storm surge event. Note that the recurrence interval is
plotted along the x-axis, which is logarithmic, while the
magnitude of the event is plotted along the y-axis.
Often the points plotting any natural hazard time series
will closely fit a straight line. The extrapolation of this
line beyond the upper boundary of the data permits
the recurrence interval of unmeasured extreme events
to be determined. For example, in the century before
the 1953 storm surge in the Netherlands, the greatest
recorded surge had a height of 3.3 m. This event
occurred in 1894 and had a recurrence interval of one
in 70 (1:70) years. The ranked storm-surge data for the
Netherlands fit a straight line which, when extended
beyond the 70-year time span of data, permits one to
predict that a 4 m surge event will occur once in
800 years. The 1953 surge event fits the straight line
drawn through the existing data and had a recurrence
interval of once in 500 (1:500) years. If the dykes had
been built to this elevation, they would be expected to
be overtopped only once in any 500-year period. Note
that the exact timing of this event is not predicted, but
just its elevation. The engineers who designed the
dykes in the Netherlands before the 1953 North Sea
storm cannot be blamed for the extensive flooding
that occurred because they had built the dykes to with-
stand only the 1:200 year event. At present, dykes in
PROBABILITY OF OCCURRENCE
(Leopold et al., 1964; Wiegel, 1964)
The probability of occurrence of a surge height is
highly dependent upon the physical characteristics of a
coastal site. To define this probability, knowledge of the
size of past events and how often they have occurred
over time (magnitude-frequency) is also required. This
information is usually obtained from tide gauges. The
maximum storm-surge values have been calculated
for many tide stations around the United States Gulf
and east Atlantic coast. This type of information can
be used for planning; however, it is fraught with the
danger that one may not have records of the most
extreme events. Take the example of the 1953 storm
surge in the Netherlands. Engineers correctly designed
dykes to withstand the 100-200-year storm-surge
event; but the 1953 event exceeded those limits. The
probability of occurrence and magnitude of this event,
but not its timing, could have been foreseen from a
simple analysis of past historical events.
