Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 132

theridge was fully exposed to the near maximum force of the winds and waves
(this location was approximately 10 km away from the eye of TC Vance where
the winds were estimated at over 300 km h − 1 ). No new boulders appeared to
have been added to or removed from the ridge during TC Vance. The boulders
in the ridge were originally derived from an intertidal platform of beach rock
that occurs adjacent to the entire length of the ridge. In order for the waves
generated during TC Vance to have supplied new boulders to the ridge, they
needed to first excavate blocks from the platform adjacent to the ridge; there
were no isolated boulders lying on the beach between the platform and ridge.
Boulders on the ridge are encrusted with oysters which were radiocarbon
dated and returned ages between AD 670 and 1820. The ages represent the time
of death of the oysters and their death would have been caused by the excava-
tion of the clast and its removal from the intertidal position. The radiocarbon
chronology shows a variety of ages suggesting that the ridge accumulated over
anumberofevents,i.e.each new event probably excavates and deposits new
boulders to the ridge. The youngest ages came from small boulders encrusted
with oysters at the back of the ridge. These smaller boulders ( a axis = 28 cm)
could have been excavated from the shore platform and carried over the ridge
by atropical cyclone as they are small enough to have been entrained by storm
waves ortsunamis. The dated sample of shell from 8 m elevation in the sand
dunes behind the ridge, also returned a young age, and likewise it is equally
possible for it to have been deposited during a storm or tsunami. The other
ages came from oyster encrusted boulders that appear too large to have been
entrained by storm waves and suggest they were deposited by separate tsunami
events over the past approximately 1000 years.
Anumber of other studies of wave deposited boulders are also published
in the scientific literature. These include, to name but a few, Scheffers (2002),
Williams and Hall (2004)andYoung et al. (1996).
Determining the type of wave responsible for boulder movements:
atheoretical approach
Because boulder movements by waves are relatively rare, it is not always
possible, to have either direct observations or indirect ones through aerial pho-
tos, to ascertain the relative roles of storm waves and tsunamis. Another method
is to mathematically model the forces required to transport boulders from dif-
ferent environmental settings by both storm waves and tsunamis.
Hydrodynamic equations that relate the forces necessary to entrain and trans-
port boulders of various sizes, shapes and densities, by waves were first devel-
oped by Nott (1997), and later more sophisticated versions that consider the