storm surges

storm surges ‘The deadliest tropical cyclone in history’ on 12 November 1970 in the Bay of Bengal resulted in a storm which elevated water levels by 9 m and caused extensive flooding in the Ganges delta. An estimated 300 000 people lost their lives and 4.7 million people were affected. The effects were disastrous, but such storm surges are not uncommon, and their frequencies and intensities have changed through time.

Storm surges are set up by changes in atmospheric pressure and associated wind stress. They are particularly severe in shallow seas, and occur both in the tropics and in middle latitudes in both hemispheres. Tropical storm surges are generated over the oceans between 7° and 25–30° latitude where the sea-surface temperature is at least 26 °C. They are known as typhoons in the western North Pacific, as hurricanes in the eastern North Pacific, Caribbean, and Atlantic (for example, Hurricane Gloria, 1985), and as tropical revolving storms in the Indian Ocean, including the Bay of Bengal and Arabian Sea, and around Australia.

Strong onshore winds associated with the passage of a deep depression cause sea water to pile up along the coast, generating a positive surge, the height of which is increased by low pressure; whereas offshore winds and rising pressure produce a fall in water levels and a negative surge. A fall of atmospheric pressure of 1.005 millibars (mb) will result in a rise of sea level of 1 cm. The disastrous Moroto Typhoon of 9 September 1934 that affected the Osaka area of Japan was set up by an intense low pressure of 954.3 mb and maximum mean wind speeds onshore of 42 m s⁻¹ which produced a surge of almost 3 m.

A tropical revolving storm will have an ‘eye’ of 20–50 km diameter where pressure is at its lowest, and here sea level may rise as much as 4 m. Most (70 per cent) of all tropical revolving storms occur in the northern hemisphere, and of these 97 per cent are recorded in July to November. An increase in the area where the sea-surface temperature is 26 °C or more will result in an increase in the incidence of tropical revolving storms, which will penetrate middle latitudes more frequently.

In middle latitudes, an increase in temperature gradient between high and middle latitude surface water masses will also lead to an increase in storm surge frequency. Such increases were characteristic of the Little Ice Age from the sixteenth to the eighteenth centuries, for which there are many historical records. The ships' logs of the Spanish Armada graphically illustrate the great storms of 14–18 August 1588 that scattered the fleet; surface wind speeds of 20–35 m s⁻¹ can be estimated from these records. Storm surges were particularly frequent in the seas around the British Isles between ad 1570 and 1720, beginning with the ‘All Saints Flood’ of 11–12 November 1570, which was one of the greatest North Sea storms recorded, and during which up to 400 000 people are estimated to have drowned, particularly in The Netherlands.

Improvements in sea defences in north-western Europe have meant that loss of life has been minimized, but during the infamous 1953 storm surge in the North Sea on the night of 31 January to 1 February, 1800 people were drowned in the The Netherlands and 307 in England, particularly on Canvey Island in the Thames estuary. Gusts of wind exceeded 100 knots (31 m s⁻¹) and atmospheric pressures were as low as 968 mb. The surge added 2.2–3.0 m to the predicted astronomical tide levels. At the time, the recurrence interval of this storm surge was calculated as 1 in every 200 years, but the extreme water level recorded then has been exceeded in eastern England no less than 12 times in the past 40 years. Hubert Lamb has recorded the great storms and associated storm surges in the North Sea and adjacent areas for the period ad 1509–1990, and summarized the meteorological conditions inferred from the historic records. A similar compilation and analysis has been undertaken by Yoshito Tsuchiya in Japan; for the Osaka area he has established the frequency of storm surge disasters, and has noted that since ad 700 the average interval between major storm surges is about 150 years, which is close to the recurrence interval for the disastrous Moroto Typhoon of 1934.

Neither the instrumental record nor the documentary record of storm surges is long enough to make it possible to calculate, with any accuracy, the recurrence interval of extreme water levels and the extent of flooding associated with storm surges. The record can be extended by identifying and dating storm surge signatures in the sediments of coastal lowlands or staircases of gravel beaches in formerly glaciated areas. By dating shells deposited during storm surges in the Dutch sand dunes, Saskia Jelgersma has demonstrated a progressive increase in the height of extreme water levels associated with storm surges since 2450 bc. In coastal freshwater lagoons, isolated peaks of marine planktonic diatoms bear witness to storm surges that have taken place during the past 10 000 years, since the beginning of the present interglacial (the Holocene).

M. J. Tooley

Bibliography

Lamb, H. H. (1991) Historic storms of the North Sea, British Isles and northwest Europe. Cambridge University Press.
Tooley, M. J. and Jelgersma, S. (ed.) (1992) Impacts of sea-level rise on European coastal lowlands. Blackwell, Oxford.
Tsuchiya, Y. and and Kanata, Y. (1986) Historical study of changes in storm surge disasters in the Osaka area. Natural Disaster Science, 8 (2), 1–18.