Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 102

Wallaby Is.

P. Charlotte Bay

Redcliff

Fitzroy Is.

Normandy Is.

Curacoa Is.

Lady Elliot Is.

880 900 920 940 960 980 1000

h Pa

5 3 2 1

4

Cyclone category (Saffir-Simpson scale)

Figure 4.11. Intensity of cyclones responsible for depositing coral shingle ridges at
locations shown in Fig. 4.10 .Note that the intensity range corresponds to a 95%
uncertainty margin and is determined by the tidal range over a full nodal tidal
cycle, i.e. the most intense values correspond to a cyclone striking at the lowest
astronomical tide, LAT (extreme values at a lower margin are unrealistic because
they likely exceed the central pressures possible, hence the cyclone would not have
occurred at LAT). (From Nott and Hayne, 2001 .)

along the Great Barrier Reef were very intense category 4 and category 5 cyclones
(Fig. 4.10). The prehistoric record shows that at virtually all sites along the Great
Barrier Reef (sevenintotal)(Fig.4.10), the average return interval of these intense
cyclones is between 200 and 300 years over the past 5000 years (Nott and Hayne,
2001).
The possible caveats to the technique for determining the magnitude of
prehistoric tropical cyclones, are the lack of inclusion of the influence of
hydroisostasy on the present-day elevation of the prehistoric cyclone deposits,
and the imprecision in calculating the wave run-up during the prehistoric event.
Nott and Hayne (2001)and Nott (2003) recognise these possible caveats, and note
that sea levels have fallen, due to hydroisostasy, over the period of the prehistoric
record. However, if this is taken into account in the analysis of the intensity of
theprehistoric storms, the older deposits (coral rubble ridges deposited between