Serious storms and floods becoming more common

Devastation from flooding at Matata this month. Picture / Rotorua Daily Post

The floods which devastated the Bay of Plenty this month are part of a trend for more serious storms and floods which has experts warning New Zealand has to be better prepared.

Over the past 35 years, the country has been hit by more than 84 serious storms and floods. Only four occurred between 1968 and 1980, jumping to 24 in the 1980s, and 34 in the 1990s. In the past five years, there have already been another 25.

Insurance losses alone have totalled more than $1 billion, and the situation is expected to get worse with global warming.

Regional councils are battling to bring flood mitigation systems up to standard as well as recover from earlier floods.

But predicting exactly where the next big one will hit is not so easy, they say, as there are few places in the country that are not vulnerable to extreme storms.

Northland is prone to high-intensity storms that dump huge amounts of water in a few hours, says Bob Cathcart of the Northland Regional Council. More than 20 storms have hit the area in recent years. Although major flood spot Kaeo has had engineering works, flooding would still occur as the work was only what the area could afford.

Auckland is fortunate in that it is one New Zealand's few urban centres with no major rivers flowing through it, says Greg Patterson of Auckland City Council. However, an ageing stormwater system makes it vulnerable to sudden intense downpours, which cause flooding in local catchment areas, most recently Cox's Bay.

The council has allocated $256 million over the next 20 years to flood alleviation.

The Coromandel cops the same high-intensity storms as Northland, while low-lying areas like Thames are hit by on-shore winds and high tides. Environment Waikato is looking at the impact of soil erosion and pests, says the council's natural hazards officer, Adam Munro.

"However, we have certainly seen an increase in cloudbursts, and no catchment can handle that."

The Hawke's Bay Regional Council is praying no heavy rainfall hits the rivers crossing the Heretaunga Plains, says group manager Mike Adye.

The willow sawfly bug has decimated the willows holding the riverbanks together and the council is spending $8.8 million along 105km of rivers to bolster the stopbanks and plant new tree species.

In other areas it is a case of living with floods, Adye says. When Wairoa floods, the town goes under as it is too expensive to put in flood protection. Instead it is monitored to warn people in time.

Rivers on the southern slopes of Mt Taranaki are hotspots after 13 floods last year raised the river beds, says Dex Knowles of the Taranaki Regional Council. Meanwhile, Waitotara residents are waiting to hear if they will be moved to higher ground after three floods in 15 years.

Canterbury has suffered numerous floods over the last decade because of its braided river systems. Kaikoura has been flooded 16 times and is undergoing major flood mitigation works.

The 1984 flood in Invercargill still stands as one of the most costly, with more than $100 million worth of damage. Flood alleviation work has been done since 1984 but the standard varies throughout the region, says Dallas Bradley of Environment Southland.

It is usually a community decision as to what they can afford and the level of risk they are prepared to live with. However, Mr Bradley says officials are concerned that people moving to the area are often not aware of the flood risk.

"Some are super-cautious when they buy, but others don't bother to find out, and discover it later when their properties flood," he said.

- HERALD ON SUNDAY