Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 90

flat and abut or lap onto existing ridges through normal, non-cyclonic, wave
action. Individual storm or cyclone deposits and ridges will be more difficult
to recognise at these locations compared to sites such as Curacoa Island where
individual ridges often remain separated from each other.
Chenier and beach ridges
Cheniers are ridges composed predominantly, or entirely, of marine
shell that has been deposited onto a mud substrate. They are also separated
from each other by the mud substrate (Chappell and Grindrod, 1984). Beach
ridges on the other hand can be composed of sand or sand and shell, and some-
times isolated coral fragments. Unlike cheniers, beach ridges are separated by
sand swales, and ridge and swale topography together form a distinct and con-
tinuous sand unit which may be deposited onto any type of substrate. Cheniers
and beach ridges are not restricted to tropical regions, and hence can form
independently of tropical cyclones.
It is likely that most, if not all, cheniers are deposited by storm waves (or in
some cases possibly tsunami), and if in the tropics these waves are likely to be
due to tropical cyclones. Beach ridges on the other hand have been recognised to
form byanumber of processes, including deposition by swash during low or high
wave energy conditions, or aggradation above the mean sea level by an offshore
sand bar (Taylor and Stone, 1996)(Fig.4.5). While all of these processes, and the
beach ridges built by them, can occur independently of tropical cyclones, those
ridges that occur well above mean sea level, and contain layers and/or beds of
shell within tropical regions, are likely to have been deposited during cyclones.
Excellent examples of such ridges occur along the shores of the Gulf of
Carpentaria, Australia (Rhodes et al ., 1980). Up to 80 individual ridges, paralleling
the shore, form a beach ridge plain here that extends inland in places for over
3km. These ridges, along the eastern and southern shores of the Gulf, contain
shell rich layers up to 1--2 m thick interspersed within medium- to coarse-grained
sand (Rhodes, et al ., 1980). The ridges rise up to 6 m above mean sea level (tidal
range of approximately 2 m) and extend along shore for over 10 km in places.
Anumber of factors suggest that these ridges were deposited by storm surge
and waves including: the height of these ridges above sea level, the presence
of abundant shell layers within the ridge stratigraphy, that sea levels have not
varied by more than 2 m in the region during the Holocene, and that the Gulf of
Carpentaria is especially prone to the development of intense tropical cyclones
because of its warm, shallow waters. Radiocarbon dating of the ridges by Rhodes
et al .(1980)showedthattheridgesincreaseinageprogressivelywithdistance
inland.