Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 115

acertain velocity into a certain depth of water. Donn and Balachandran (1969)
suggest that the ocean wave will increase in height according to the following
equation;

v 2 / v 2 − c 2

(5.2)

= =

where c the speed of the atmospheric disturbance and v the water wave

speed ( g h 0.5 ).
The mechanism described in equation (5.2)requires a matching of the phase
velocity of the air and water disturbances. A fall of 1 hPa (hPa

mb) in air
pressure will result in a corresponding rise in the ocean surface of 1 cm. But
this size ocean wave can amplify by 20 times or more (Donn and Balachandran,
1969)intheopenocean. Meteotsunamis have been known to reach 3 m height in
theBalearic Islands (Western Mediterranean) (Rabinovich and Monserrat, 1996).
It is also possible that large volcanic eruptions can generate meteotsunamis
through the propagation of an atmospheric shock wave. Lowe and deLange (2000)
postulated that the AD 200 Taupo eruption in New Zealand may have generated
aworldwide meteotsunami, or volcano--meteotsunami.

=

Modern tsunami impacts on coasts

Tsunamis can both deposit sediments on shore and perform substan-
tial erosion of the coastal landscape (Dawson, 1994). Depositional imprints of
tsunamis are often in the form of sheets of sediment that taper landwards from
the shore (Dawson and Shi, 2000). Boulders too can be transported and deposited
as fields, ridges or trains. Transported boulders have been observed following
tsunami impacts in Japan (Sato et al . 1995), New Guinea and Java (Dawson et al .,
1996).
One of the first observations of the impact of a tsunami on the coastal land-
scape was following the 1960 Chilean earthquake-generated tsunami (Wright
and Mella, 1963). Sand and silt were transported inland and deposited as a thin
(2 cm) layer over the ground surface. Other similar deposits have since been
observed in several locations following tsunami events, particularly in Indonesia
(Minoura et al ., 1997; Shi et al ., 1995;Yeh et al ., 1993) and Japan (Sato et al ., 1995).
The 1992 Flores Island Indonesia event deposited coral clasts as well as a sand
sheet up to 50 cm thick that tapered in thickness landwards. The sheet extended
inland for 150 m and the source of the sediment appeared to be the beach and
coastal foreshore which suffered severe erosion (Yeh et al ., 1993). The 1998 Aitape
(Papua New Guinea, PNG) tsunami left a sand sheet up to 1 m thick that extended
inland for 675 m (USGS, 1998; McSaveney et al ., 2000). The sand sheet consisted of
normally graded sediments (coarser-grained at the base becoming finer-grained