Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 126

Figure 5.10. Imbricated clasts of rock deposited inland of the beach during the
surge of Tropical Cyclone Vance, Western Australia March 1999. Onshore flow was
from left to right.
opposite to the direction of fluid flow. Imbricated coral clasts were present on
islands affected by the 1998 Aitape, PNG tsunami. Imbricated clasts can also
occur during deposition of gravel by storm waves as was the case in Western
Australia during Tropical Cyclone Vance in 1999 (Fig. 5.10).
The presence of diatoms and foraminifera are common in tsunami laid sand
sheets. Dawson et al .'s (1996)study of diatoms showed that tsunami deposits
often have a chaotic mixture of species from different environmental settings
including marine and freshwater species. They attributed this to tsunamis trans-
gressing a range of habitats as they travel onshore. Diatoms are often found
in a broken state in tsunami deposits with some more resistant species able
to survive and remain intact. Freshwater species, having been transported from
marshes over a shorter distance, can often be found still intact. The same applies
to foraminifera. Dominey-Howes (1996)studied formanifera in tsunami deposits
in Greece and Crete (Dominey-Howes et al ., 2000). He noted that deeper water
species were high in number in the deposits.
Bryant (2001)suggests that tsunami and storm surge deposits will display
different diatom and foraminifera assemblages. It is proposed that only larger