Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 129

Storm waves can also transport and deposit boulders. One of the most impor-
tant differences between storm and tsunami waves is their respective abilities to
transport boulders at the shore. Tsunamis are able to surge across the shoreline
without rapidly reducing their velocity and energy, whereas storm waves break
and begin to rapidly dissipate energy in water depths at least 1.2 times the wave
height. Hence, a 5 m high storm wave will break and begin to dissipate energy
at a water depth of 6 m. In many situations this means that the storm wave will
break many hundreds of metres and sometimes kilometres offshore, depending
upon the offshore bathymetry. Storm waves can maintain substantial energy
close to the shore where the water immediately offshore is deep. It is in these
situations that storms have the greatest likelihood of transporting large lithic
boulders along or onto the shore. Large waves can strike the shore, therefore, if
the adjacent waters are sufficiently deep to sustain that wave. Such situations
occur adjacent to steep sea cliffs (Williams and Hall, 2004). Deposition of coral
boulders atop steep cliffs, tens of metres high, has been noted during rare con-
ditions, e.g. Niue Island in the Pacific during the passage of Tropical Cyclone
Ofa in 1990 and again during Tropical Cyclone Heta in 2004. Alternatively, large
storm waves can occur close to shore if accompanied by a storm surge, which
can temporarily increase the depth of the normal inshore water by up to sev-
eral metres (Table 5.2). In these situations the size of storm waves possible at
the shore is determined by the height of the storm surge and this is in turn
a function of a number of specific characteristics (see Chapter 4)ofthetropi-
cal cyclone involved. Differentiating between boulders deposited by storm waves
and tsunamis is not easy. Table 5.2 describes the different processes occurring
during transport of coastal boulders by these two wave types whereas Table 5.3
outlines the sequence of events required to be ascertained when attempting to
determine which wave type was responsible for boulder deposition.
Studies of coastal boulder movements
One of the earliest studies of boulder movement by waves was under-
taken by Sussmilch (1912). Sussmilch noted the emplacement of a boulder mea-
suring approximately 6 × 5 × 3m 3 on a shore platform near Bondi Beach,
Sydney, Australia, following a temperate storm the previous night. The boul-
der, now known as Mermaids Rock, stands as an isolated feature on the shore
platform. Sussmilch suggested that the rock had been lifted vertically approxi-
mately 3 m after it was dislodged from the seaward face of the shore platform
and transported approximately 20 m across the platform. Other smaller boul-
ders were also transported during the event, but these also appear to have been
left as isolated features, not displaying any imbrication. It is uncertain whether