Storm Surge and Sea Flooding Wind and Pressure Components

Widespread sea flooding by storm surge around the coastlines of South Pacific islands is a serious hazard during tropical cyclones. For example, on 25 February 2005, Tropical Cyclone Percy, a hurricane-intensity system with sustained winds up to 249 km h-1, had a severe impact on the atoll nation of Tokelau (Fig. 5.14). Tokelau consists of three atolls: Fakaofo, Nukunonu and Atafu. The storm surge generated by TC Percy inundated all three atolls. The high surge allowed the powerful winds to send waves sweeping across the low-lying atoll islands (Fig. 5.15). Waves swept over from both the ocean and the lagoon sides of the atolls, clashing in the middle of the islands and swamping villages (OCHA 2005).

There are two main components to storm surge: violent winds and low atmospheric pressure at sea level (Fig. 5.16). The first of these two components is the major one. As the winds gradually become stronger in a maturing storm, the wind stress acting on the surface of the ocean increases. This generates large swells out at sea and drives giant waves against coastlines (Fig. 5.17). This has the effect of piling the sea up against the shore. The secondary component

Tuvalu

Vanuatu

Caledonia

FUi Ji

New Zealand

Zeala

Tokelau

Tonga

Cook Islands

Tokelau

Cook Islands

Tonga

French Polynesia

South Pacific Ocean

Fig. 5.14. Path of Tropical Cyclone Percy through the atoll nation of Tokelau and the Cook Islands in late February 2006. TC Percy caused sea flooding of several atolls, including Nukunonu (9.1°S, 171.5°W) in Tokelau and Pukapuka (10.5°S, 165.5°W) in the Cooks.

Ciclone Veena Polinesia 1983

Fig. 5.15. Sea flooding of Nukunonu atoll in Tokelau, during Tropical Cyclone Percy on 26 February 2005. Photo courtesy of the Tokelau Public Works Department.

160"W

2000

Fig. 5.15. Sea flooding of Nukunonu atoll in Tokelau, during Tropical Cyclone Percy on 26 February 2005. Photo courtesy of the Tokelau Public Works Department.

Tropical Cyclone Storm Surge
Fig. 5.16. Wind and low barometric pressure components of storm surge beneath a tropical cyclone advancing right to left (not to scale). Adapted from NASA (2006).
Ciclone Veena Polinesia 1983

Fig. 5.17. Furious seas lashing the coastline of Tikopia island (12.3°S, 168.8°E) in the outer Solomon Islands, generated by Tropical Cyclone Zoe in late December 2002. Photo by Geoff Mackley.

is associated with the big drop in atmospheric pressure close to the centre of tropical cyclones. The low pressure causes a temporary but rapid rise in the local sea level. Along affected sections of coastline, this sea-level rise may take an hour to reach its peak as barometric pressure falls to its minimum value, and roughly the same time to go back down again after the cyclone has moved away. The overall effect of strong winds and low pressure is that sea level near the middle of a tropical cyclone rises in a dome of water typically 50 km across, and from 2 to 5 m higher than the predicted height of tide (Table 5.5).

Table 5.5. Examples of observed maximum surge heights produced by tropical cyclones in the Coral Sea and the South Pacific.

Tropical

Surge height above

Place recorded

cyclone

Year

normal sea level (m)

Bathurst Bay, Queensland, Australia

Mahina

1899

13

Mackay, Queensland, Australia

Unnamed

1918

6

Southern Viti Levu island, Fiji

Unnamed

1941

1.8

Funafuti atoll, Tuvalu

Bebe

1971

4

Nayau island, Fiji

Meli

1979

2-3

Tikehau atoll, Tuamotu archipelago

Veena

1983

4

Beqa island, Fiji

Oscar

1983

3-4

Southern Viti Levu island, Fiji

Hina

1985

1

Rarotonga, Cook Islands

Sally

1987

5

The extra rise in sea level produced by storm surges means that coral reefs, which normally afford protection around tropical island shores, sometimes become deeply submerged. This allows the enormous waves to attack exposed locations, leading to a variety of shoreline erosional and constructional changes (Nunn 1994). Percolation of salt water into the soils of coastal plains also kills vegetation and contaminates natural freshwater aquifers, particularly in wells (Fig. 5.18).

Tropical Cyclone Zoe
Fig. 5.18. Stagnant sea water standing on low-lying areas of the main islet on Pukapuka atoll in the northern Cook Islands. The atoll was flooded several days earlier by Tropical Cyclone Percy on 26 February 2005. Photo courtesy of Douglas Clark.

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