Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 89

so that some soil development was able to take place. Ridges often contain only
one storm deposit, but it is possible for two or more storm deposits to occur in
one ridge. Careful excavation of a ridge is necessary in order to determine the
number of storm deposits making up the ridge. Samples collected for geologi-
cal dating from only one storm deposit, when more than one storm deposit is
present, may bias the age determination of that ridge.
Coral rubble ridges often accumulate on the lee side of islands, presumably
because on the exposed sides they are constantly removed by the largest or
most intense tropical cyclones affecting a region. Leeward shores experience
diminished wave energy, but the full effects of the surge. Because the wave
energy is reduced, the likelihood of ridges being removed during subsequent
cyclones is lessened. A number of ridges are sometimes able to accumulate over
time where the preservation potential is particularly high. Curacoa Island on
theGreat Barrier Reef has 22 consecutive coral rubble ridges paralleling the
shore on its north-western or lee side (Fig. 4.3). Individual ridges extend for over
100malong shore and rise to over 5 m above the mid-tide level (the tidal range
here is approximately 3 m). The ridges were deposited by successive cyclones so
that new ridges are deposited seaward of the previously emplaced ridge. The
age of the ridges increases progressively with distance inland. Curacoa Island
is typical of many sites that preserve coral rubble ridges; however, not all sites
retain as many ridges. Elsewhere, only one or two ridges paralleling the shore are
preserved. This is often due to the site's exposure and the frequency of cyclones
in the region.
The time interval between cyclones will determine the extent to which com-
paction and lithification or induration of the ridge can proceed. Over time,
individual coral clasts within a ridge will begin to weather or break down, and
in so doing provide carbonate that will progressively cement clasts together.
Cementation usually begins within the core of the ridge, and it is common for
loose fragments to remain on the crest and sides of the ridge several centuries
after deposition. But, progressively over time, the ridge becomes resistant to fur-
ther wave attack. However, looser fragments on the ridge crest can be removed
and replaced by younger fragments if subsequent storm surges are sufficiently
high to be able to overtop that ridge. In this fashion also, more recent storm
deposits can be superimposed on older ones.
Prior to stabilisation, and depending upon the geomorphic setting, ridges
can move inland by waves washing over the ridges and transporting clasts to
the landward side of the ridge. The 19 km long, 3.5 m high and 35 m wide coral
rubble ridge deposited by Tropical Cyclone Bebe on Funafuti Atoll continued to
move inland and along shore for many years (Baines and McLean, 1976). Indeed,
in some instances on coral atolls, the ridges will move inland across the reef