Case Study Coastal Erosion on Niue Island During Tropical Cyclone Heta in January

Tropical Cyclone Heta was the first tropical cyclone to form in the RSMC-Nadi area of responsibility during the 2003-2004 tropical cyclone season (Fiji Meteorological Service 2004). The system was first identified as a tropical depression north of Fiji on 28 December 2003, as it moved northeastwards to reach a position just west of Atafu, the northernmost of the Tokelau atolls.

Coastal Erosion Egypt

Fig. 7.6. Changes in the geomorphology of a sand spit, formerly connected to the low limestone island of Yanuca in southern Fiji. The spit was breached at its southern end and disconnected from Yanuca island during Tropical Cyclone Sina in late November 1990. In consequence, the narrow lagoon behind the spit has been infilling with sediment since that time.

Fig. 7.6. Changes in the geomorphology of a sand spit, formerly connected to the low limestone island of Yanuca in southern Fiji. The spit was breached at its southern end and disconnected from Yanuca island during Tropical Cyclone Sina in late November 1990. In consequence, the narrow lagoon behind the spit has been infilling with sediment since that time.

By this time it had reached tropical cyclone status, and was named at around 3 a.m. on 2 January 2004 (Fig. 7.8). Once mature, the cyclone rapidly strengthened while turning slowly southwards, attaining storm intensity around noon on 2 January and hurricane intensity around 6 a.m. on 3 January. TC Heta was a very intense system with maximum sustained winds estimated at 115 knots (213 km h-1) and momentary gusts up to 160 knots (296 km h-1). Peak intensity was reached about midnight on 5 January while

Cyclone Ofa
Fig. 7.7. Coastal erosion east of Apia on Upolu island in Samoa, caused by storm waves accompanying Tropical Cyclone Ofa in February 1990. Source: Rearic (1990).

the cyclone centre was passing 115 km to the west of Savai'i island in Samoa. This strength was maintained over the next 24 h as the cyclone veered onto a southeastward track, its speed accelerating to 15 knots (28 km h-1), and later to 20 knots (37 km h-1). This path took the cyclone centre to within 80 km of Niuatoputapu island in northern Tonga around noon on 5 January and 50 km off the west coast of Niue around 3 a.m. on 6 January.

The hardest-hit country was Niue. Niue is a single isolated island (19.1°S, 169.6°W) located in between Tonga and the southern Cook Islands. TC Heta's hurricane-force winds, enormous seas and associated coastal flooding, caused devastation that was the worst in living memory. Monstrous waves were the chief cause of the destruction. Much of the infrastructure on the island was destroyed and buildings and houses were either demolished or badly damaged, including Niue's hospital complex and parliament. Even houses built atop 30 m high cliffs, and others up to 100 m inland, were flattened. Roads were closed, telecommunications and electricity were cut, and crops were spoiled by the furious winds and torrential rainfall. Alofi, the capital, bore the brunt of the storm, with half of the commercial area wiped out. A woman and her 19-month old child were killed by a giant wave smashing through the house in which they were sheltering. Several other people were injured and many were left homeless. A national disaster was declared by the government and media reports put an estimate on the damage at NZ$50 million.2

2 Niue is an independent nation but uses New Zealand currency.

180°

175°W

5°S-

Tuvalu

2 Jan.

t

* 1

10°S-

31 Dec./ .„„A i WallisX 30 Dec. / • \ 29 Dec Jtf 5 Jan.\

w 28 Dec. \ 27 Dec. y

Samoa

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Cyclone track

Niue

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^ Storm centre at midnight UTC

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Fig. 7.8. Track of Tropical Cyclone Heta near Tokelau, Wallis and Futuna, Samoa and Niue in early January 2004.

Fig. 7.8. Track of Tropical Cyclone Heta near Tokelau, Wallis and Futuna, Samoa and Niue in early January 2004.

Niue island is a large raised coral atoll with a land area of 259 km2 (Murray and Terry 2004). The cliffline comprises hard coral limestones, but the fringing reefs are narrow and afford little protection from rough seas. Cut horizontally into the coastal bedrock is a series of marine terraces at different levels, giving a staircase effect. The terraces indicate the interaction between changing sea levels and tectonic uplift of the island during the Quaternary period. The broadest marine terrace at 23 m elevation is called the Alofi Terrace and encircles the perimeter of the island. Gouged vertically into Niue's coastline are a series of steep-walled chasms and smaller 'sea tracks'. These chasms are straight, sheer-sided, are up to 25 m deep and extend up to 500 m long. They are interpreted as fault-guided solution channels along fault zones that run sub-parallel to the coast (Schofield 1959).

On 6 January 2004, TC Heta's tremendous waves pounded the north and west coasts of the island. Cliffs collapsed and retreated in several places, such as at Makefu on the north coast (Fig. 7.9). The biggest waves were able to dredge coarse debris from the modern-reef surface and overtop the 23 m

Tropical Cyclone Ian
Fig. 7.9. Collapse of a section of limestone cliffs near Makefu on the north coast of Niue island, caused by wave attack during Tropical Cyclone Heta on 6 January 2004. Photo by Michael Bonte.
Niue Island Makefu
Fig. 7.10. Large coral boulder on Niue that was thrown up by gigantic waves during Tropical Cyclone Heta on 6 January 2004. This site is on top of the 23 m elevation marine terrace on the north west coast. Photo by Douglas Clark.
Cyclone Heta
Fig. 7.11. Hikutavake 'sea track' on the west coast of Niue, swept clean and abraded by waves during Tropical Cyclone Heta on 6 January 2004. Photo by David Talagi.

elevation marine terrace. Large quantities of coral rubble were thrown up onto the Alofi Terrace and deposited there, including many angular boulders weighing several tons (Fig. 7.10). In addition, the coastal chasms and sea tracks funnelled the force of cyclone-generated swells on land. The floors of the chasms were swept clean of sediments and heavily abraded as a result (Fig. 7.11). The funnelling effect also amplified the erosive power of waves, such that coastal forest and buildings up to 200 m inland were torn down (Fig. 7.12).

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