Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 100

The intensity of prehistoric tropical cyclones
Amethod to determine the intensity of the prehistoric cyclones respon-
sible for building ridges and eroding terraces in raised gravel beaches was
introduced by Nott and Hayne (2001)and Nott (2003). The height of these fea-
tures is assumed to represent the minimum height of the storm inundation
during the event responsible. The elevation of these features is accurately sur-
veyedtodatum, and samples of coral and/or shell radiocarbon dated to deter-
mine the minimum height and times of inundation, respectively. The height
of this inundation is then related to the intensity of the palaeocyclone which
is determined through the use of numerical storm surge and shallow water
wave models. The models are used to determine the relationship between surge
height and central pressure for each location containing evidence of palaeo-
cyclones. Also, the relationship between the surge height and the translational
velocity of the cyclone, the radius of the maximum winds and the track angle of
thecyclone as it approaches and crosses the coast are determined. Model results
are compared to measured surge heights from recent or historical cyclone events
near the study sites. The central pressure of the cyclone responsible for forma-
tion of the ridge or terrace is determined by modelling the magnitude of the
surge plus wave set-up, and run-up and tide required to inundate the ridge or
terrace.
Nott and Hayne (2001)describeaseries of lithic gravel terraces at Red Cliff
Point, north of Cairns in northeastern Australia. The terraces contain occasional
clasts of deposited coral and sit at a height of 6.1 m above Australian Height
Datum (AHD), or mean tide level (Figs. 4.10 and 4.11). The terraces have been
eroded by surge and wave action during tropical cyclones as they sit well above
thelevel of normal wave action and any slightly higher sea levels during the
mid-Holocene on a tectonically quiescent coast. Wave run-up across the gravel
beach at this site was measured during recent cyclones and wave set-up was
accepted as 10% of offshore significant wave height. By modelling the magnitude
of storm surge and offshore wave environment for a range of tropical cyclone
scenarios, Nott and Hayne (2001) calculated the central pressure of the cyclones
necessary to produce marine inundations (surge, plus tide, wave set-up and wave
run-up) toreachthelevelof each of these terraces. Nott and Hayne estimated
that the cyclone responsible for eroding the highest terrace would have had
acentral pressure of approximately 900 hPa and generated a surge of 4 m,
offshore significant wave height of 5 m, wave set-up of 0.5 m and wave run-up
of 1.6 m. The tide height at the time of the prehistoric event is unknown but
can be estimated (at the 95% confidence level) to have occurred within the 2
probability tidal range of the frequency distribution nodal tide curve for each