Page 337 - Fundamentals of Geomorphology

320 PROCESS AND FORM

Box 13.1

THE 1998 PAPUA NEW GUINEAN TSUNAMI

On 17 July 1998, an major earthquake occurred some perhaps 3–4 m high. He tried to run home, but the
70 km south-east of Vanimo, Papua New Guinea. wave overtook him. A second and larger wave flat-
It had an epicentre about 20 km offshore and a depth tened the village and swept him a kilometre into a
of focus of less than 33 km. It registered a magnitude mangrove forest on the inland shore of the lagoon.
of 7.1. The earthquake stirred up three locally destruc- Other villagers were not so lucky. Some were carried
tive tsunamis. Minutes after the earthquake rocked across the lagoon and became impaled on broken man-
the area, the successive tsunamis, the largest of which grove branches. Many were struck by debris. Thirty
was about 10 m high, buffeted three fishing villages – survivors eventually lost limbs to gangrene, and salt-
Sissano, Arop, and Warapu – and other smaller vil- water crocodiles and wild dogs preyed on the dead
lages along a 30-km stretch of coast west of Atape. The before help could arrive. The rush of water swept away
subsequent events were described by a survivor, retired two of the villages, one on the spit separating the sea
colonel John Sanawe, who lived near the south-east end from Sissano lagoon. A priest’s house was swept 200 m
of the sandbar at Arop (González 1999). He reported inland. At Warapu and at Arop no house was left
that, just after the main shock struck, the sea rose above standing, and palm and coconut trees were torn out
the horizon and then sprayed vertically some 30 m. of the ground. In all, the tsunamis killed more than
Unexpected sounds – first like distant thunder and 2,200 people, including 240 children, and left more
then like a nearby helicopter – faded as he watched than 6,000 people homeless. About 18 minutes after
the sea recede below the normal low-water mark. After the earthquake, the sea was calm again and the sand
four or five minutes’ silence, he heard a rumble like a bar barren, with bare spots marking the former site of
low-flying jet plane and then spotted his first tsunami, structures.

Tidal ranges have a greater impact on coastal run at up to 30 km/hr and are effective agents of erosion.
processes than tidal types. Three tidal ranges are Small tidal ranges encourage a more unremitting break-
distinguished – microtidal (less than 2 m), mesoti- ing of waves along the same piece of shoreline, which
dal (2 to 4 m), and macrotidal (more than 4 m) – deters the formation of coastal wetlands.
corresponding to small, medium, and large tidal ranges Tides also produce tidal currents that run along the
(Figure 13.4). A large tidal range tends to produce a shoreline.They transport and erode sediment where they
broad intertidal zone, so waves must cross a wide and are strong, as in estuaries. Currents associated with ris-
shallow shore zone before breaking against the high-tide ing or flood tides and falling or ebb tides often move in
line. This saps some of the waves’ energy and favours opposite directions.
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 COASTAL EROSIONAL LANDFORMS
in phase with the tidal period. The tidal range is almost
16 m in the Bay of Fundy, north-eastern Canada. Some Erosional landforms dominate rocky coasts, but are also
estuaries, such as the Severn Estuary in England, with found in association with predominantly depositional
high tidal ranges develop tidal bores, which are single landforms. Tidal creeks, for instance, occur within salt
waves several metres high that form as incoming tidal marshes. For the purposes of discussion, it seems sensi-
flow suffers drag on entering shallower water. Tidal bores ble to deal with erosional features based in depositional