Geology Reference
In-Depth Information
Box 13.1
THE 1998 PAPUA NEW GUINEAN TSUNAMI
On 17 July 1998, an major earthquake occurred some
70 km south-east of Vanimo, Papua New Guinea.
It had an epicentre about 20 km offshore and a depth
of focus of less than 33 km. It registered a magnitude
of 7.1. The earthquake stirred up three locally destruc-
tive tsunamis. Minutes after the earthquake rocked
the area, the successive tsunamis, the largest of which
was about 10 m high, buffeted three fishing villages -
Sissano, Arop, and Warapu - and other smaller vil-
lages along a 30-km stretch of coast west of Atape. The
subsequent events were described by a survivor, retired
colonel John Sanawe, who lived near the south-east end
of the sandbar at Arop (González 1999). He reported
that, just after the main shock struck, the sea rose above
the horizon and then sprayed vertically some 30 m.
Unexpected sounds - first like distant thunder and
then like a nearby helicopter - faded as he watched
the sea recede below the normal low-water mark. After
four or five minutes' silence, he heard a rumble like a
low-flying jet plane and then spotted his first tsunami,
perhaps 3-4 m high. He tried to run home, but the
wave overtook him. A second and larger wave flat-
tened the village and swept him a kilometre into a
mangrove forest on the inland shore of the lagoon.
Other villagers were not so lucky. Some were carried
across the lagoon and became impaled on broken man-
grove branches. Many were struck by debris. Thirty
survivors eventually lost limbs to gangrene, and salt-
water crocodiles and wild dogs preyed on the dead
before help could arrive. The rush of water swept away
two of the villages, one on the spit separating the sea
from Sissano lagoon. A priest's house was swept 200 m
inland. At Warapu and at Arop no house was left
standing, and palm and coconut trees were torn out
of the ground. In all, the tsunamis killed more than
2,200 people, including 240 children, and left more
than 6,000 people homeless. About 18 minutes after
the earthquake, the sea was calm again and the sand
bar barren, with bare spots marking the former site of
structures.
Tidal ranges
have a greater impact on coastal
processes than tidal types. Three tidal ranges are
distinguished -
microtidal
(less than 2 m),
mesoti-
dal
(2 to 4 m), and
macrotidal
(more than 4 m) -
corresponding to small, medium, and large tidal ranges
(Figure 13.4). A large tidal range tends to produce a
broad intertidal zone, so waves must cross a wide and
shallow shore zone before breaking against the high-tide
line. This saps some of the waves' energy and favours
the formation of salt marshes and tidal flats. The great-
est tidal ranges occur where the shape of the coast and
the submarine topography effect an oscillation of water
in phase with the tidal period. The tidal range is almost
16 m in the Bay of Fundy, north-eastern Canada. Some
estuaries, such as the Severn Estuary in England, with
high tidal ranges develop
tidal bores
, which are single
waves several metres high that form as incoming tidal
flow suffers drag on entering shallower water. Tidal bores
run at up to 30 km/hr and are effective agents of erosion.
Small tidal ranges encourage a more unremitting break-
ing of waves along the same piece of shoreline, which
deters the formation of coastal wetlands.
Tides also produce tidal currents that run along the
shoreline. They transport and erode sediment where they
are strong, as in estuaries. Currents associated with ris-
ing or flood tides and falling or ebb tides often move in
opposite directions.
COASTAL EROSIONAL LANDFORMS
Erosional landforms dominate rocky coasts, but are also
found in association with predominantly depositional
landforms. Tidal creeks, for instance, occur within salt
marshes. For the purposes of discussion, it seems sensi-
ble to deal with erosional features based in depositional
