Floods are the most frequent and costly natural disasters in New Zealand – between 1968 and 2017, the country experienced over 80 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. High sea levels at river mouths may also increase flood levels. In New Zealand, the heaviest rain commonly accompanies tropical cyclones, depressions and frontal systems. Extra-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 13 metres of rainfall per year in places.

Floods may also occur when landslides triggered by severe weather or earthquakes dam rivers and streams. Water will back up upstream of the barrier and may cause a flood downstream if the dam gives way. About 200 dams were formed by landslides in the 2016 Kaikōura earthquake.

South Island rivers

In the South Island, many major eastern rivers originate in the Southern Alps. Weather conditions in their mountain headwaters often produce floods in their lower reaches. The greatest flood ever observed on the Clutha River, New Zealand’s largest river in catchment area and volume of flow, occurred in 1878. It was the result of a succession of weather systems bringing in warm rain and warm wind, which melted the winter snow cover. At the height of this flood, more than 5,700 cubic metres of water poured down the lower reaches of the river near the coast every second.

Flash floods and debris flows

Brief but intense rainstorms may produce flash floods – brief, powerful flows that can move even large boulders. A destructive flash flood near Roxburgh in November 1992 was produced when 80 millimetres of rain fell in just 45 minutes.

If large amounts of sediment enter the flood waters, for example from landslides, the flood may turn into a ‘debris flow’. This is a fluid mix of water, rock particles and vegetation, which may be as thick as wet concrete. In 2005, over 300 mm of rain fell in 24 hours near the township of Matatā in the Bay of Plenty, triggering debris flows that heavily damaged over 100 homes. Flash floods and debris flows also regularly damage bridges, resulting in expensive road maintenance through mountain regions.