Signatures of Tsunami in the Coastal Landscape - Tsunami - page 40

reworked deposits. Chronology, based upon the suite of
signatures present along this coast, suggests that the
deposits are only 500 years old. In many cases, the
reworked deposits are overlain by undisturbed midden
devoid of pumice. Storm waves can be ruled out because
many of the sites are situated in sheltered locations or at
excessive heights. Storm waves also tend to erode shell and
sand-sized sediment seaward and do not deposit bodies of
mixed debris several meters thick, at the point of run-up, on
steep backshores at these high elevations. The fact that
many of these disturbed middens still contain a high pro-
portion of midden material indicates that disturbance was
rapid and incomplete with an input of contaminants from a
nearby marine source. Other signatures of tsunami surround
all sites. The importance of these disturbed middens will be
discussed later in Chap. 9 .
Dump deposits have also been described globally
(Scheffers and Kellatat 2003 ) and at specific locations: near
Lisbon in Portugal (Scheffers and Kellatat 2005 ); the south
coast of Spain (Scheffers and Kelletat 2004 ); Cyprus
(Kelletat and Schellmann 2002 ); the Caribbean (Scheffers
2004 ; Scheffers and Kelletat 2004 ); the Bahamas (Scheffers
and Kelletat 2004 ); the southern Ryukyu Islands of Japan
(Ota et al. 1985 ); the North Island of New Zealand (Nichol
et al. 2003 ); and on Molokai Island, Hawaii (Moore 2000 ).
The Spanish deposits consist of shell fragments, rounded
but broken pebbles and cobbles, and rounded boulders up to
several 100 kg in weight. The deposits appear to be related
to the November 1, 1755 Lisbon Tsunami. Those in Cyprus
are similar. Mixtures of sand, shell, rounded cobbles and
boulders can be found up to 15 m above sea level. In the
Caribbean, dump deposits have been identified on the
islands of Guadeloupe, St. Lucia, Grenada, St. Lucia,
Aruba, Curaçao, and Bonaire. On Guadeloupe and St.
Lucia, such deposits reach up to 50 m above sea level and
contain boulders up to 10 tonnes in weight. On these islands
and the Bahamas, the tsunami events responsible for the
dump deposits may be as young as 300-400 years. Often
these dump deposits, especially in the Caribbean region, are
eroded by heavy rainfalls that have removed sand and silt,
leaving behind a lag of exhumed boulders. So pervasive are
dump deposits that they can be considered a universal
signature of tsunami that have overwashed rocky environ-
ments such as cliffs, coastal platforms and coral reefs,
where there is a variety of sediment sizes available for
transport.

modern storm wave activity. None of these deposits can be
explained satisfactorily by ordinary wave processes. Shell
mounds several tens of centimeters high were formed in
Lake Jusan, Japan by a tsunami in 1983 (Minoura and
Nakaya 1991 ). Perhaps the best example of a mound is the
sand dome deposited by the 1854 Ansei-Tokai Tsunami in
the town of Iruma located at the head of Suruga Bay, central
Japan (Sugawara et al. 2005 ). The tsunami, enhanced
fourfold by resonance as it travelled up the narrow v-shaped
bay, exceeded a height of 13.2 m. It deposited, parallel to
the shoreline, 700,000 m 3 of nearshore sand as a huge
mound that reached a height of 11.2 m above sea level.
Backwash exited to the side of the re-entrant and hence did
not remove the mound. Similar, smaller examples exist
along the embayed, south coast of New South Wales,
Australia (Bryant et al. 1992 ). For example, at Bass Point
80 km south of Sydney, Australia, a ridge has been sculp-
tured into mounds standing 2-3 m in elevation. The mounds
consist of chaotically sorted shell hash mixed with rounded
cobble and boulder-sized debris. The mounds extend 100 m
alongshore and lie 5-10 m seaward of a scarp cut into sand
dunes. As the shoreline becomes more sheltered, the
mounds merge into a 10-20 m wide bench consisting of the
same material and rising steeply to a height of 4-5 m above
the present storm wave limit. Slopes on both sides of the
mounds exceed 20, a value that is much steeper than the
7-9 found on equilibrium profiles formed by storm waves
in similarly sized material. An unusual form similar to the
mound deposited at Iruma, Japan can be found at Mystery
Bay. It consists of uniform rounded cobbles and rises
3.25 m above the surrounding beach. While no cobbles or
gravel occur on the modern beach, no sand occurs in the
mound. As at Iruma, backwash exited from the side of the
beach. A second, similar ridge lies landward at an elevation
of 9.1 m.
There are several examples of ridges formed by tsunami.
On the island of Lanai in Hawaii, boulder deposits sup-
posedly deposited by tsunami evince ripple forms 1 m high
with a spacing of 100 m between crests (Moore and Moore
1988 ). On the South Ryukyu Islands in Japan, ridges several
meters high and 40 m wide have been linked to tsunami
(Ota et al. 1985 ; Minoura and Nakaya 1991 ). However,
among the most unusual coastal features are the chenier-like
ridges formed in Batemans Bay on the New South Wales
coast of Australia (Fig. 3.3 a). These ridges are described in
Bryant et al. ( 1992 ). Cheniers are ridges of coarser sediment
usually deposited at the limit of storm waves in muddy
environments. Batemans Bay is a 14.4 km long, funnel-
shaped, sand-dominated embayment that averages 11 m in
depth and is semi-compartmentalized by north-south
structurally controlled headlands (Fig. 3.8 ). This shape is
conducive

3.2.4 Mounds, Domes and Ridges

Chaotic dump deposits can be molded into isolated mounds,
domes and ridges, especially in embayments along rocky
coasts. These deposits rise to heights above the limits of

to resonance and enhancement of tsunami

entering from the open ocean. The basin's resonance