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The Impact of Cyclone Isaac on the Coast of Tonga

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Title:The Impact of Cyclone Isaac on the Coast of Tonga
Authors:Woodroffe, Colin D.
Date Issued:Jul 1983
Publisher:University of Hawai'i Press
Citation:Woodroffe CD. 1983. The impact of Cyclone Isaac on the Coast of Tonga. Pac Sci 37(3): 181-210.
Abstract:Cyclone Isaac passed through the limestone and sand islands of
the Ha 'apai and Tongatapu groups of the Kingdom of Tonga on 3 March 1982
and was probably the most severe storm experienced in southern Tonga in more
than 100 years, causing extensive damage to buildings and crops. The limestone
islands most affected by the storm are composed principally of reefal limestones,
and the coastal terraces are late Pleistocene in age, many having been dated to the
last Interglacial. There is little evidence of past storms preserved on the coast,
and storm blocks and extensive rubble deposits on reef flats are rare or absent.
The worst hit islands were in the Ha'apai group. Here the raised reefal limestone
cliffsof the eastern shores of islands on the barrier reef resisted the storm, except
for blocks of up to 2m in diameter detached from the upper visor of a wave-cut
notch on Ha'ano. Coastal scrub, however, was stripped, .coconut palms were
felled and Pandanus was broken for more than 50m on Ha'ano, 30m on Lifuka ,
and locally on Foa and 'Uiha. Some regeneration of the dominant shrubs ,
Messerschmidia argent ea, Hibiscus ti/iaceus, and Bikkia tetrandra, was observed
at the time of survey, 11-15 weeks after the storm. Both deposition and erosion
were noted on pocket beaches between cliffs of reefal limestone , with at least
28 m3 per m of sand removed from the beach face on one beach on Lifuka. The
island of Tatafa was severely devastated; the cyclone had passed directly over the
southern end, causing the sandy shoreline to retreat by 200m, scouring a channel
through the island, leaving scour holes on the western side, and destroying much
of the coastal scrub. Tongatapu also sustained much damage to buildings and
crops, and much of the north coast was flooded. The greatest change was
observed to have occurred on sand cays off the northern coast. Four of these
underwent change since a survey in 1969, and much of the change could have
been attributed to Cyclone Isaac. The greatest change occurred on the smallest
island, Manima, with coconut stumps left on the reef flat to the east indicating at
least 12m of shoreline retreat. On Oneata and Pangaimotu deep-rooted individuals
of Excoecaria agallocha were left exposed more than 8m on the eastern
shore by destruction of the coastal scrub . On Makaha'a retreat of the shoreline
was localized , and along much of the coast a sand cliff occurred which had
changed little since it was mapped in 1969. Undercutting of the shore and
formation of a sand cliff appear to be related to the presence of beachrock at the
foot of the beach, and the storm did not cause much shoreline retreat. On the
northern sand cays off Tongatapu patterns of change were variable. On Fafa
recession occurred on the eastern and northeastern shore , but also occurred to
the south , reflecting the configuration of the reef flat. Monuafe and Tufaka lost
some scrub but the lack of woody vegetation meant that little evidence of former
island shape remained. Deposition occurred on Malinoa, the sand cay nearest
to the path of the storm, which is situated on a narrow reef flat and is open
to a long fetch and deep water to the east. Few of the topographic changes
brought about by Cyclone Isaac will remain for long before they are obscured,
except the reshaping of Tatafa. Similarly , rapid regrowth of vegetation implies
that coastal vegetation will soon be restored, but changes in composition may
persist. Though Cyclone Isaac was a severe storm, a type which occurs infrequently,
its effects on the coastal landforms and vegetation of Tonga appear
to have been less than those of similar storms on more isolated atolls elsewhere in
the Pacific.
Appears in Collections: Pacific Science Volume 37, Number 3, 1983

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