Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 96

be assumed that the barrier being regularly overwashed has remained the
same height over millennia. Hence, it is difficult to infer the magnitude of
the cyclones responsible based upon the height of the present barrier unless
adetailed chronostratigraphy of that barrier is undertaken. A chronostratig-
raphy is a quantitative assessment of the age or chronology of the layers, or
various strata that comprise a geological unit. In this case the barrier sands,
usually being sand dunes at the rear or landward side of the beach, will be
composed of a number of units of sand that have accumulated, often due to
aeolian processes, over time. If the dunes have remained well vegetated and
largely undisturbed by wave attack there is no reason why such a barrier system
should not remain at roughly the same height for some considerable period of
time.
The dunes comprising the barrier at Western Lake, Florida form a continuous
ridge, approximately 6 m high with some dunes rising to 9 m. For storm surge
to overtop this barrier Liu and Fearn (2000) determined that the surge must
have been generated by a high-magnitude cyclone (intense category 4 or 5 on
the Saffir--Simpson scale). While there is a linear and proportional relationship
between surge height and cyclone central pressure, it is possible for less intense
cyclones to generate large surges when those cyclones travel at high forward
speeds or have a large radius of maximum winds. Furthermore, the characteris-
tics of the submarine topography and coastal configuration also strongly influ-
ence the height of a surge. These meteorological and geographical characteristics
need to be considered in any study of palaeostorm deposits.
The long-term stability of overwashed barrier dunes remains one of the most
uncertain aspects of this technique. Coastal sand dunes frequently erode when
they are overtopped by surge and waves. The impact of TC Vance in Western
Australia demonstrates clearly that several rows of high dunes can be completely
removed during an intense cyclone. Examination of the impacts of other high-
intensity tropical cyclones along the Western Australian coast between 1999 and
2002 (TC John, 915 hpa; TC Rosita, 930 hpa; TC Chris, 915 hPa) show that the
extent and nature of dune erosion is a function of the height of the marine inun-
dation (surge plus waves) relative to the height of a dune (Fig. 4.9). Dunes com-
pletely inundated by the surge and waves are likely to be removed, as occurred
with TC Vance (Fig 4.9,stage 4). Dunes are reduced in vertical height, but not
necessarily removed, when that dune is overtopped by wave run-up only, and not
completely inundated. In this instance the waves wash over the dune, but the
height of the 'still water' marine inundation (surge plus tide plus wave set-up)
is less than the height of the dune (Fig 4.9,stage 3). Dunes experience recession,
but not a reduction in vertical height, when the wave run-up does not reach
the crest of the dune (Fig 4.9,stage 2). And dunes remain unmodified when the