Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 87

coral rubble in the shallow waters of the reef and maximum accumulation of
rubble in the deeper offshore waters below wave base (depth to which waves
will entrain and transport sediment on ocean floor), suggest that the onshore
ridges could have formed from predominantly live coral fragments broken off
during the storm. At other sites, however, there can be little doubt that onshore
ridges were formed from the reworking of existing accumulations of rubble in
the shallow waters offshore.
It is difficult to know whether the rubble ridge is deposited gradually during
thestorm,orasone or a series of, sediment units moving landward from existing
offshore accumulations. Scoffin (1993) has described coral rubble ridge building
as ridges that 'have been transported and deposited like large asymmetric waves
of sediment; material picked up on the seaward side is rolled up the ridge and
dropped down the advancing slope' suggesting that an entire, or substantial
part of an offshore accumulation of rubble is moved onshore as a single unit
during the storm. If on the other hand ridges accumulate gradually, then the
ridge could be assumed to increase in height over time during the storm. In this
instance wave run-up may play a role in their formation.
It is likely that the height of an onshore coral rubble ridge is a function of
themean storm 'still' water level being the storm surge, tide and wave set-up
combined, and possibly wave run-up. Ascertaining to what extent wave run-up is
responsible for the height of the resulting ridge is important, as run-up can equal
or exceed the height of the storm surge depending upon various conditions.
Wave run-up is a function of significant wave height ( H s )and wave period/length,
wave refraction/diffraction, bathymetry, beach slope angle and roughness and
permeability of beach material (Neilsen and Hanslow, 1991). With very rough,
coarse-grained, permeable substrates Losada and Gimenez-Curto (1981)notewave
run-up can be 0.30--0.75 times that of run-up on a sandy, largely impermeable
beach under the same storm conditions.
Observations of historical storm surge emplaced coral rubble ridges suggest
that wave run-up may play an insignificant role. For example, a 3.5 m high ridge
wasdeposited on Funafuti Atoll during Tropical Cyclone Bebe in 1971 (Fig. 4.4).
The inundation accompanying the storm was 5 m above the level of the reef
flat, or mean low tide level, and 1.5 m higher than the elevation of the resulting
ridge crest (Maragos et al , 1973). A similar situation occurred at Mission Beach,
south of Cairns, North Queensland, Australia, where an intense tropical cyclone
struck in March 1918. Here a 4.5--5.1 m high (above Australian Height Datum
or AHD) ridge of pumice was deposited by a surge as the cyclone crossed the
coast. Eyewitness observations and results from numerical storm surge and wave
models of the event, along with knowledge of the tide level at the time, shows