New Zealand flood

New Zealand number 1 hazard

Floods are the most frequent and costly natural disasters in New Zealand – between 1920 and 1983, the country experienced 935 damaging floods. The Insurance Council of New Zealand calculated that industry payments for flood damage between 1976 and 2004 averaged $17 million per year in 2004 dollars. But this covers just part of the actual cost – for example, government expenditure on civil defence responses during flood emergencies alone averages about $15 million per year over the same period.



Early floods

Floods have cost an uncounted number of lives. Māori history tells of a pre-European flood in the Tūtaekurī area of Hawke’s Bay in which a party of 50 men, women and children were drowned by the rising of two streams. The early European settlers failed to realise the intensity of rainfall in New Zealand and how rapidly rivers could rise. The broad gravel-bed rivers were particularly deceptive: although usually shallow enough to wade across, in flood their currents become powerful. By 1870, just a few decades after European settlers first arrived, rivers had been responsible for 1,115 recorded drownings. Drowning became known as ‘the New Zealand death’.



A god’s revenge

Māori legend includes a story of a great flood. Tāwhaki, god of thunder and lightning, was almost murdered by his brothers-in-law. When he had recovered, Tāwhaki took his warriors and their families, and built a fortified village on top of a mountain. Then he called to his ancestors – the gods – for revenge, and they let the floods of heaven descend. The earth was overwhelmed by the waters and the entire population perished. This was known as Te hurihanga i Mataaho (the overwhelming of Mataaho – one of the places that were destroyed).

Causes

Most floods occur when water from intense or persistent rain, and sometimes from melting snow, enters rivers, streams and lakes, causing them to overflow. In New Zealand, the heaviest rain commonly accompanies tropical cyclones, depressions and frontal systems. Tropical cyclones, such as the storm of 28 January 1936 (considered New Zealand’s worst storm in the 20th century) and Cyclone Bola in 1988, have caused major floods, but such storms reach New Zealand only infrequently.



Much more common is flooding caused by the heavy rain that may accompany depressions and frontal systems moving over the country. New Zealand’s rugged topography often enhances the effects of these weather systems, as moist air forced up and over mountainous terrain condenses to produce additional precipitation. As a result, the western side of the Southern Alps is one of the wettest places in the world, with over 10 metres of rainfall per year in places.