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Floods

by  Eileen McSaveney

Surrounded by ocean, New Zealand is regularly swept by weather systems that bring heavy rain. The country’s many rivers and streams can quickly become powerful torrents, bursting their banks and causing millions of dollars of damage each year.


New Zealand’s number one hazard

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.


19th-century floods

Most major rivers in New Zealand have a history of destructive floods, and European settlers soon discovered that floods were a recurring menace. Wellington’s first settlers arrived at Petone in January 1840, intending to farm the broad Hutt Valley. Less than two months after they arrived, the Hutt River overflowed, inundating their huts and tents. After several more floods during the next few months, most of the settlers abandoned the Hutt Valley and moved to Thorndon, now part of central Wellington.

In Christchurch, settlers unwittingly chose a site for the city that was part of the constantly shifting channels of the Waimakariri River.

Blenheim – ‘Beaver Town’

Blenheim was dubbed ‘Beaver Town’ or ‘The Beaver’ by early surveyors – like a beaver colony built in midstream, it was often completely swamped by the meandering Ōpawa River. The river also acted as an overflow channel for flood water from the Wairau River. During one period in 1893, the local newspaper, the Daily Times, noted:

Again the town has been inundated with Opawa back-water, and this time to a greater height than has been the case during winter. We have now had nine floods in 11 weeks, each of them being of sufficient size to entirely suspend business operations for two days. 1

Central Otago floods, 1863

In 1863, prospectors swarmed over the mountains of Central Otago, camping along the streams and rivers where there was gold. The winter that year was severe, with thick snow blanketing the mountains. In July, warm rain deluged the region for six days, and rain and melted snow poured into the rivers.

Between 25 and 27 July, rivers swelled to disastrous levels. During a single night the tributaries of the Molyneux (now the Clutha) – the Shotover, Kawarau and Arrow rivers – rose by 6 to 10 metres. The floods overwhelmed dozens of miners asleep in tents and makeshift huts on the river banks beside their claims, or even on terraces well above normal river levels. Eight huts disappeared from a beach in the Arrow Gorge, and by morning, only one tent of many was left on a terrace opposite the Arrow township. In the upper Shotover River the torrent undermined a terrace, and a hut where 15 men were living collapsed into the river, and 12 drowned.

The sodden mountains gave way in landslides that sometimes blocked the valley floors. Weeks earlier, a landslide had dammed a stream running into the Shotover River at Sand Hills near Māori Point. Swollen by heavy rain, the stream burst through the barrier on 26 July and overwhelmed a camp, killing 12 miners.

On the Arrow River, a landslide dam broke, releasing a wall of water that swirled through the lower areas of Arrowtown, sweeping away buildings and burying everything beneath several metres of gravel. Most miners escaped to high ground, but many lost all their belongings and equipment. By the end of August, more than 100 lives had been lost because of the Central Otago floods.

Clutha floods, 1878

Although the Central Otago floods of 1863 resulted in more deaths, the Clutha River floods of September and October 1878 caused the worst damage. Heavy rain and melting snow in the headwaters of Lake Wānaka produced floods that lasted for three weeks, inundating settlements and demolishing dozens of bridges along the whole length of the river. A 1938 account described the Clutha in flood:

[i]ts angry surface [was] strewed with dead horses and cattle, houses, bridges, furniture, timber and farmstacks. Some days the spring sun shone with a ghastly pleasantry on the devastated towns, while 100 miles away more heavy rain on the mountains was preparing still greater strength for the flood. 2
Footnotes
    • Quoted in A. L. Poole, Catchment control in New Zealand. Wellington: Water and Soil Division, Ministry of Works and Development for the National Water and Soil Conservation Organisation, 1933, p. 160. › Back
    • Quoted in Catchment control in New Zealand, p. 185. › Back

The Kōpuawhara flash flood, 1938

Twenty-one people were killed in the Kōpuawhara flood of 1938 – the largest number of fatalities from a 20th-century flood in New Zealand. It is a sobering reminder of the dangers of building on low-lying land close to rivers.

In 1938, workers building the Wairoa to Gisborne railway near Māhia lived in huts and tents in public works camps along the banks of Kōpuawhara Stream. No. 4 camp was in a river valley, but appeared to be safely above the level of the tiny stream. In the early hours of 19 February, a cloudburst caused a flood that sent a wall of water nearly 5 metres high down the stream.

About 3.30 a.m., water began to pour across the lower levels of No. 4 camp. A worker there raised the alarm, banging the cookhouse gong and beating on the hut doors; his body was later found 5 kilometres downstream. Men struggled to get to higher ground through the rising water, and many scrambled onto the roofs of huts. Most huts collapsed and the people on them were swept away. Two men died after wading into the torrent to try to find the camp waitress; her hut had been one of the first carried away.

Eleven men at No. 4 camp had climbed onto a truck to escape the flood. The truck, used for carrying shingle, was tossed over in the stream and the men were washed away. The bonnet was later found 10 kilometres downstream, but the rest of the truck was never found.

Fourteen people survived by climbing onto the cookhouse roof, then leaping to the adjoining caterer’s quarters as the cookhouse collapsed. An elderly man tied himself to a hut with electric cable, and held a five-year-old girl above the water for an hour. Eventually a rope was carried from higher ground to the rooftop survivors, and they were hauled to safety.

At No. 2 camp, about 5 kilometres downstream, 47 people were sleeping. Men woke to find waves dashing against their tents, but the alarm was raised in time for everyone to struggle through the rising water to high ground.

In all, 20 men and one woman at No. 4 camp were drowned. One man was also drowned at Boyd's Camp at the Gisborne end of the railway line. He was swept away by the flooded Maraetaha Stream, as a result of the same cloudburst. In 1942 a memorial, which named all 22 victims, was set up to mark the site of the disaster.


Southland floods and Cyclone Bola

Southland floods, 1984

In late January 1984, devastating floods struck communities in Invercargill and nearby Southland. The cause was a combination of north-westerly rain in the mountain headwaters of Southland rivers, and a slow-moving southerly front that dumped heavy rain over the lowlands. Steady rain on 26 January – a record one-day total of 84.8 millimetres – had by 9 p.m. caused extensive surface flooding in the streets of Invercargill, Riverton, Ōtautau and Bluff. Local waterways soon overflowed, and by 4 a.m. a state of emergency was declared. By morning many streets, houses, shops and factories were under water, and local streams sent torrents of water through Invercargill. Levels rose further still as high tide prevented floodwater from draining into the Invercargill estuary. By mid-morning of 27 January, the state of emergency included all of Southland. The rain stopped by noon, but the rivers continued to rise.

Rescue work

Army and air force personnel were dispatched south to aid rescue efforts. An estimated 5,000 people abandoned their homes, and helicopters lifted dozens of people from rooftops. Roads and railway lines were submerged, cutting off the region from the rest of New Zealand. Water up to 3 metres deep flooded Invercargill airport, partly submerging 10 light aeroplanes.

Damage

It would be a week before most people could return to their homes, and several weeks before the state of emergency was lifted. When the flood receded, extensive damage was revealed: at least 1,200 homes were uninhabitable, and residents eventually discarded more than 5,000 tonnes of ruined personal possessions. Hundreds of flooded cars had been abandoned. No human lives were lost, but livestock losses were heavy – more than 12,000 sheep, 330 pigs, 100 cattle and 75 deer were drowned. A relief appeal raised more than $3 million, and tens of millions of dollars were paid out in insurance claims.

Cyclone Bola, March 1988

One of the most damaging cyclones to hit New Zealand was Cyclone Bola, which struck the Hawke’s Bay and Gisborne–East Cape region in March 1988. The cyclone slowed as it moved over the area, resulting in over three days of torrential rain. Worst affected was the hill country inland from Gisborne, where winds forced warm moist air up and over the hills, augmenting the storm rainfall. In places, over 900 millimetres of rain fell in 72 hours, and one area had 514 millimetres in a single day.

A ruined harvest

Flooding from Cyclone Bola had a devastating effect on horticulture in the East Coast region. The damaged crops included 3,000 tonnes of grapes, 1,300 tonnes of squash, 7,000 tonnes of sweetcorn, 13,500 tonnes of tomatoes, and several million dollars in vegetables for the local market. Large amounts of horticultural produce were swept into the sea – fishing boats were dredging fruit from the sea floor in their nets for several months after the storm.

The ensuing floods overwhelmed river stopbanks, damaged houses, swept away bridges and sections of roads and railway lines, and destroyed parts of Gisborne’s main water-supply pipeline. Three people died in a car that was swept away by flood waters, and thousands were evacuated from their homes – 3,000 in Gisborne, 300 in Wairoa, and 400 at Te Karaka.

The downpours triggered innumerable landslides on the region’s hillside pastures. Some farmers lost 30% of their grazing area, with landslide scars taking decades to heal. Huge quantities of sediment were dumped into the flood waters. One 11,000-hectare catchment deposited a million tonnes of sediment into the Waipāoa River. Thick sediment from the ebbing floods smothered pastures, orchards, and crops ready for harvest. Cyclone Bola was estimated to have cost $90 million in losses to horticulture and farming, and the cost to the government was around $112 million.


21st century floods

Lower North Island and Marlborough, 2004

Between 14 and 17 February 2004, intense rain (up to 300 millimetres in two days) fell on land already saturated after previous severe weather. Worst affected were rivers in the South Taranaki, Manawatū–Whanganui, Wellington and Marlborough areas. The Manawatū River peaked at its second-highest level on record. Many rivers breached their banks, spilling silt-laden flood water through towns and across farmland. A number of rural communities had to be evacuated, and at the height of the emergency about 2,300 people had to leave their homes and farms.

About half the roads in the Manawatū–Whanganui region were closed and more than 20 bridges were damaged. The civil defence operation was the largest in 20 years, with air force helicopters rescuing stranded people and dropping supplies to communities cut off by flood waters.

In Marlborough, in the upper South Island, the Waitohi River flooded parts of Picton, and 500 people were evacuated for fear an overflowing water supply dam might collapse.

The February floods cost over $112 million in insurance payouts, and the government granted $135 million in aid to farmers. Around 2,600 farmers were affected by the flooding, with some having to abandon farming their properties. The total economic impact was estimated to be about $400 million.

Eastern Bay of Plenty, 2004

In July 2004, the eastern Bay of Plenty was extensively flooded when a frontal system stalled over the area, causing prolonged and intense rain. The Whakatāne River spilled into Whakatāne’s central business district and the Awatapu area. Water had to be released into the Rangitāiki River from the Matahina Dam to prevent the dam from bursting. The Rangitāiki overflowed above Edgecumbe, creating a 100-metre-wide breach in its stopbanks. Flood waters entered Te Teko and Edgecumbe, and swamped some 17,000 hectares of farmland.

At the height of the floods, about 3,200 people had to leave their homes, many sheltering in evacuation centres and on local marae. A swarm of shallow earthquakes north-east of Rotorua added to the misery, triggering many landslides on saturated hillsides. In all, more than 450 farms were affected by the floods, with over 200 homes made uninhabitable.

Cyclone Debbie and Cyclone Cook, 2017

In April 2017, within a week of each other, remnants of two tropical cyclones, Debbie and Cook, passed over New Zealand. Cyclone Debbie dumped up to three times the normal April rainfall in three days on some areas. The Rangitāiki River rose due to the heavy rain, and on 6 April a section of river stopbank at Edgecumbe gave way. A torrent of water abruptly flooded into the town and inhabitants had just a few minutes to flee their homes. Many were marooned and had to be rescued by boats, tractors and trailers. In all about 2,000 people were evacuated from the town. In the South Island, Kaikōura was cut off as State Highway 1 south of the town was blocked by mudslides washed down from loose landslide debris from the November 2016 Kaikōura earthquake.

Rain from Cyclone Cook remnants caused flooding, slips and power outages along the North Island east coast. Hard hit areas included the Thames-Coromandel District, Matamata, Tauranga, Whakatane, and parts of Hawke’s Bay.


Flood control

A flood-prone land

Settlers in New Zealand commonly chose to live next to rivers and lakes, as these were a source of fresh water, and the adjacent plains usually had fertile soil. Consequently, about two-thirds of New Zealanders now live in areas that are naturally prone to flooding. Nearly 70% of towns and cities with populations of over 20,000 have river flood problems.

Human activity has also increased the likelihood of flooding. Large areas of native forest were cleared by both Māori and European settlers, leading to a more rapid run-off of rain into stream networks, and to erosion that raised the levels of river beds. In urban areas, buildings, footpaths and roads have replaced ground that would normally soak up falling rain. This leads to surface flooding during heavy rain, and increases the run-off into storm drains, causing higher water levels in local streams.

Wild water

At Avonhead lived one Mister Bray,
Who every morning used to say
‘I should not be much surprised today
If Christchurch city were swept away
By the rushing, crushing, flushing, gushing Waimakariri River.’

This 1860 poem by Canterbury politician and poet Crosbie Ward refers to the engineer William Bray, who predicted that the Waimakariri River would break its banks and run through Christchurch. His prediction came true in February 1868, when the river overflowed into the Avon River and other old stream channels, flooding parts of the city.

Legislating flood control

Early attempts at controlling rivers were piecemeal – to protect a homestead, individuals would build stopbanks that sometimes deflected river flow onto neighbouring properties. The River Boards Act was passed in 1884, setting up a national network of river management. But a single river was often controlled by several different boards.

In April 1938, a flood in Hawke’s Bay left the lower Esk valley buried under metres of silt. The disaster spurred an effort to control floods, resulting in the Soil Conservation and Rivers Control Act 1941. This pioneering legislation, as well as the later Water and Soil Conservation Act of 1967, authorised catchment boards to deal with complete river systems – both carrying out river control works and controlling erosion in their catchments.

In 1991, the Resource Management Act turned over flood control to regional and territorial authorities as part of their obligation to avoid or mitigate natural hazards.

Controlling the rivers

A number of techniques are used in New Zealand to control rivers. Thousands of kilometres of stopbanks have been built to keep high river levels within the channels; this has allowed dense settlement of areas such as Christchurch, the Hutt Valley and the Heretaunga Plains. Christchurch, for example, is situated mainly on a flood plain. It occupies part of the huge alluvial fan of the Waimakariri River, which has frequently changed course, sometimes shifting as far south as Lake Ellesmere. Stopbanks now keep the Waimakariri in one place along the northern edge of the fan.

Stopbanks, however, may also cause problems. Near Franz Josef, stopbanks built to protect the township narrowed the bed of the Waiho River. The river bed built up so much that in 2011 the SH6 bridge had to be raised to stay clear of the river. In 2016, the river breached the stopbank, flooding two hotels and a campground.

Solution – or problem?

In 1877, Hawke’s Bay County began to plant willows along river margins to help prevent bank erosion. Less than 20 years later, in 1894, the county set up a sub-committee to deal with ‘the willow nuisance’. The trees, which impeded the flow of rivers during floods, continued to be a worry for the next half-century.

One method of preventing rivers overflowing their stopbanks is to lower the river bed by removing gravel. Speeding the passage of flood waters through an area can involve straightening river channels and removing obstructions such as vegetation. Another method of flood control is damming or diverting flood water until river levels drop. To protect communities downstream, some river stopbanks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland. The Moutoa Sluice Gates, for example, divert excess water from the Manawatu River into a floodway that rejoins the river 10 kilometres downstream.

New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.


River monitoring and flood warnings

Regional and District councils have the primary responsibility for managing flood hazards. They monitor rainfall, river flows and lake levels, and maintain flood protection works.

The earliest indications of potential flooding are the heavy rain warnings issued by weather forecasters. In addition, the councils independently operate networks of automated instruments that measure rainfall and river levels. Data from these instruments, and high rainfall rates or rising river levels, may trigger automatic warnings to staff.

Council staff also use computer models of rainfall and river flow to determine likely rises in river and lake levels downstream, and supply warning information to communities. Among other operations, they organise evacuations, build sandbag barriers, and close roads. Most councils maintain websites and telephone services that inform the public about rainfall and river levels.

Information on lake and river water levels, river flows and sediment loads also goes into a national Water Resources Database, managed by the National Institute of Water and Atmospheric Research (NIWA). In collaboration with regional councils, NIWA has developed new techniques for forecasting floods using computer models of atmospheric conditions and river catchments.

From a drop to a deluge

Frederick W. Furkert, a New Zealand public works engineer in the 1920s, was experienced in river problems. He recalled a statement that the first engineer he worked under used to make: ‘You have never seen it rain so hard that you could not imagine it raining a little harder or a little longer; only one of those conditions is necessary to make a bigger flood than you have ever seen.’ 1

Researching climate change

NIWA also carries out research on climate change and long-term climate cycles, such as the El Niño–Southern Oscillation that affects New Zealand’s weather patterns. In El Niño years, stronger westerly winds tend to bring more rain to southern and western regions. By contrast the La Niña pattern brings rainy conditions to the north-east of the North Island.

Managing flood-prone areas

As even major river works may not prevent flooding, there are now measures to alter the way flood-prone areas are developed. Under the Resource Management Act 1991, territorial and regional authorities can regulate land use and construction on high-risk flood plains. Local authorities use instrument records and historical accounts of rainfall, river levels and floods to determine hazard zones, based on the probability of land being inundated. The public can obtain a Land Information Memorandum (LIM) from Councils which includes information on the potential for any property to be flooded.

In many regions, riverside areas are reserved as parks, sports fields and parking areas, so flooding will cause minimal damage.

Footnotes
    • Quoted in A. L. Poole, Catchment control in New Zealand. Wellington: Water and Soil Division, Ministry of Works and Development for the National Water and Soil Conservation Organisation, 1933, p. 10. › Back

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How to cite this page: Eileen McSaveney, 'Floods', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/floods/print (accessed 19 November 2018)

Story by Eileen McSaveney, published 12 Jun 2006, reviewed & revised 1 Aug 2017