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Tens of thousands at risk

With the region still sodden after months of high rainfall, just-acquired information shows that tens of thousands of residents across Wairarapa could be affected in a severe flood.

The Greater Wellington Regional Flood Exposure Assessment – obtained from Greater Wellington Regional Council [GWRC] under official information legislation – has revealed more than 43,000 people, and almost 18,000 properties, in Masterton, Carterton, and South Wairarapa could be affected in a one-in-a-hundred-year flooding event.

The risk assessed – a ‘100-year flood’ – means that there is a one per cent chance this type of flood could occur in a given year, possibly even multiple times.

Wairarapa has been hit by major flooding in the past, notably in 1947 and 1976. More recently, Cyclone Gabrielle tore through the small settlement of Tinui earlier this year, causing widespread damage.

The report indicates 700 commercial properties are currently at risk; 100 in Carterton, 400 in Masterton, and 200 in South Wairarapa. Additionally, 17,100 residential properties are at risk; 3000 in Carterton, 9000 in Masterton, and 5100 in South Wairarapa. The risk is expected to increase, with numbers rising by 2110.

GWRC group manager, environment Lian Butcher said modelling shows all except one of the main towns across the region are likely to be significantly impacted. “We anticipate the major towns of Masterton, Carterton, Greytown, and Featherston to be most at risk from major river flooding. We also anticipate widespread flooding of rural land from the Ruamāhanga, particularly in the lower valley,” Butcher said.

While the town of Martinborough itself is not specifically mentioned, rural South Wairarapa is.

The report applies population projections from Statistics New Zealand to compare the number of people exposed to flooding now and by 2110 – in an “undefended scenario”.

Although the report does not explicitly define “undefended” risk within a flood context, other sources say the term refers to the flood hazard existing in the absence of structural flood defences [like stop banks].

The report assesses flood risk across the wider Wellington region, with territorial authorities analysed separately.

“In all instances, the undefended scenario has been considered. This is considered a true flood exposure as continued investment is required to maintain defences and schemes,” it said.

“When applying the population projections provided by Stats NZ, the number of people exposed to flooding across the Wellington Region by 2110 in the undefended scenario could increase further to approximately 260,000.”

The Waiohine catchment, primarily located in Carterton District, is assessed with the greatest proportion of people currently exposed to flooding at 64 per cent.

The number of people in Wairarapa who could be currently impacted is 7800 in Carterton, 23,000 in Masterton, and 12,600 in South Wairarapa. By 2110 this is estimated to rise to 10,700 in Carterton, 28,400 in Masterton, and 13,400 in South Wairarapa.

“By 2110, both the South Wairarapa and Masterton Districts have nearly 50 per cent of all buildings exposed to flooding, [approximately 6100 and 10,900 respectively]. This represents approximately 60 per cent of the commercial buildings in South Wairarapa and 70 per cent of commercial buildings for Masterton.

“Both councils have nearly 50 per cent of all residential buildings exposed to flooding by 2110,” the report said.

Butcher explained GWRC manages risk from major rivers across the region, and communities are also likely to be at risk from surface water/stormwater flooding that’s managed by the district councils. Coastal flooding is a further hazard.

Climate change is expected to worsen the current risk.

“The impacts of climate change do not result in any new catchments having significant increases in flood exposure,” the report said.

“Climate change is therefore expected to exacerbate issues that are already being experienced across the region,” the report said. –NZLDR

    Public Interest Journalism funded through NZ On Air


  1. Alarmist dramatic bullshit every dwelling business is possibly affected by 100 year flooding in NZ

  2. It is a matter of when not if, The big floods of 1976 and 1994 will be nothing when climate change is added to the mix. The most vulnerable will be Greytown which will be totally inundated and to think they are putting new houses in the path of future floodwaters. Bad planning and not looking at what future may hold.

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