Geoscience Reference
In-Depth Information
Oceanic Hazards
adjacent to a shoreline. For this reason, sea-ice must
be considered a significant hazard in any water body
undergoing periodic freezing and melting. This
chapter will describe the problem of drifting sea-ice,
its worldwide distribution, and the measures taken to
negate its effects.
It was pointed out in Chapter 3 that waves might not
be important as a hazard if the full intensity of a storm
reaches landfall during low tide. Such was the case
during Cyclone Tracy in Darwin, Australia, in 1974
(see Figure 8.1 for the location of all major placenames
mentioned in this chapter). Even with a 4 m storm
surge, waves did not reach the high tide mark, because
the storm made landfall during low tide in an environ-
ment where the tidal range is about 7 m. More insidi-
ously, low waves can cause considerable damage along
a shoreline if sea levels are either briefly elevated
above normal levels, or rising in the long term. It is
generally believed that sea level is presently rising
worldwide ( eustatic rise) at a rate of 1.0-1.5 mm yr -1 .
The most likely cause of this rise is the Greenhouse
effect . Human-produced increases in CH 4 , CO 2 and
other gases are responsible for a general warming of
the Earth's atmosphere. This causes sea level to rise
because of thermal expansion of ocean waters. Note
that this cause is more significant than melting of the
world's icecaps. Even if recent warming is not due to
the Greenhouse effect, the implications are the same.
The recent and threatened rise in sea level poses a
In Chapter 3, waves were often mentioned as one of
the main agents of erosion and destruction by storms.
In fact, destructive waves can occur without storm
events, and in association with other climatic and
oceanographic factors such as heavy rainfall and high
sea levels, to produce coastal erosion. In this sense,
waves form part of a group of interlinked hazards asso-
ciated with oceans. This chapter examines these and
other oceanographic hazards. Wave mechanics and the
process of wave generation will be outlined first
followed by a description of wave height distribution
worldwide. This section will conclude with an appraisal
of the hazards posed by waves in the open ocean.
Wind is the prime mechanism for generating the
destructive energy in waves. In cold oceans, seas
or lakes, strong winds can produce another hazard -
the beaching of drifting sea-ice. Sea-ice is generally
advantageous along a coastline that experiences winter
storms, because broken sea-ice can completely
dampen the height of all but the highest waves.
When frozen to the shoreline as shorefast ice, sea-ice
can completely protect the shoreline from any storm
erosion. However, floating ice is easily moved by winds
of very low velocity. These winds might not generate
appreciable waves, but they can drive sea-ice ashore
and hundreds of metres inland. The force exerted by
this ice can destroy almost all structures typically found
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