Oceanic Hazards - Natural Hazards - page 161

measured. In the North Sea, the overall height of
waves is increasing, with freak waves in excess of 17 m
being recorded around many platforms. In the Gorm
field, five waves in excess of 8 m were recorded in a
13-hour period on 6 September 1983. All of these
waves are the result of chaos described best by the
non-linear Schrödinger equation. This formula can
predict waves four times the average wave height.
Non-linear interaction can also produce waves of the
same size, often in sets of three to five monstrous
waves. Perhaps the largest recorded wave occurred on
7 February 1933 when the US Ramapo struck a storm
with winds as high as 11 on the Beaufort scale. After a
week of constant storm conditions, the ship leaned into
the trough of a wave and was met by a wall of water
34.1 m high. Other waves of significance were the 26 m
wave that hit Statoil's Draupner gas platform in the
North Sea on New Year's Day 1995, the 29 m wave that
rocked the QE2 crossing the North Atlantic in 1995,
and the 30 m wave that almost downed a helicopter
during the Sydney Hobart Yacht Race on 27 December
1998. Such waves have even toppled oil platforms, as
was the case with the Ocean Ranger drilling rig on the
Grand Banks of Newfoundland on 15 February 1982.
Since 1969, monstrous waves as high as 36 m have
probably been responsible for the disappearance of
more than 200 supertankers.
Regional wave heights can also increase substan-
tially over time. For instance, waves in the north
Atlantic Ocean have increased in height by 20 per cent
since 1980. Largest wave heights offshore from
Cornwall have risen from 11.9 m in the 1960s to 17.4 m
in the late 1980s, while mean wave heights have
increased from 2.2 m to 2.7 m over this period. These
heights represent a 32 per cent increase in wave
energy, an increase that may not have been accounted
for in the engineering design of shoreline or offshore
structures. Intensification of the north Atlantic wind
field has amplified these wave heights.

recreational activity in Australia and many anglers view
rock ledges and platforms as the best spots to fish.
Four factors exacerbate the hazard. First, if people
have walked along a beach to get to a headland, they
can be lulled into a false sense of security because,
due to wave refraction, wave heights can be up to
50 per cent less along the beach than off the headland
(Figure 8.3). Second, wave heights are diminished
5-20 per cent at the coast because of frictional interac-
tion with the seabed. Along rocky coasts, especially in
New South Wales where the continental shelf has a
width as narrow as 12 km, the steep nature of the
offshore contours minimizes this frictional loss to less
than 5 per cent. Third, the term 'freak wave' typically
relates to the interaction of wave spectra rather than to
a single large wave. Waves rarely arrive along a coast-
line with a constant wave period, height, or direction.
It is quite common, for example, for wave sets along
the New South Wales coast to come from two different
directions. In embayments, any directional variation in
wave approach to the shoreline tends to be minimized
because of wave refraction. On headlands, wave refrac-
tion is not efficient because headlands protrude
seaward from the coast and offshore topography is
steep. Here, the directional variation in wave approach
is greatest and the potential for interaction between
wave crests is at a maximum. Thus, wave crests from
two different directions can often intersect, resulting in
exaggerated wave heights due to the summation of the
energy in the two waves. This combined wave will have
a much larger run-up height along rocks than that
produced by waves individually. Finally, waves along a
beach will often break at some distance from shore and
dissipate their energy across a surf zone. While set-up
in the surf zone and run-up onto the beach can signif-
icantly and temporarily raise water levels at shore,
most of the energy has been lost in the surf zone.
Additionally, wave reflection back from the shoreline is
minimal because beaches tend to be flat. Waves on
headlands, however, break by surging at the shoreline.
Energy loss occurs over the rocks rather than offshore
in a wide surf zone. As well, the steep nature of
headlands results in considerable reflection of energy
seaward, a process that can temporarily raise sea
levels several metres from shore. It is possible for
the next incoming wave to be superimposed upon this
supra-elevated water. This process can generate freak
waves that can override rock platforms even when the
tide is low (Figure 8.5).

Wav es as a hazard on rocky coasts

Simple wave theory can also explain how rough seas
can sweep people unexpectedly off rocks. Presently
one person dies each week in Australia as the result of
being swept off rock platforms. This is currently the
most common natural cause of death in Australia.
While tourists - especially those with little knowledge
of a coastline - appear at risk, the majority of
deaths involve anglers. Fishing is the most popular