The Impact of Cyclone Isaac on the Coast of Tonga

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.