$50m Wairarapa flood feared

A major flood in the Waiohine and Mangaterere Rivers could cause damage costing more than $50 million to private property alone - and the chances of such a catastrophic event happening are becoming more likely.
Now Greater Wellington Regional Council is asking people living on the floodplains of the two rivers, mostly in Greytown but also some rural Carterton people, for ideas on how best to protect the community from hazard flooding.
Two drop-in sessions are being organised for June 22 and July 2 in Greytown Library for people to bring their ideas along.
The regional council's flood protection manager, Graeme Campbell, said it was vital for people to understand flood hazards and what was involved in managing them.
Research has shown a one in 100-year flood would cause more than $54 million in damage to private property and there would be damage to roads, bridges, railways and other infrastructure that would cost tens of millions of dollars more to repair.
Mr Campbell said the council had plenty of information to share with people about the flood hazard "and there are people out there who know the river well and we want to hear their ideas on how to manage it".
Many of the stopbanks constricting the Waiohine River would be overtopped in any flood larger than a one in 20-year flood.
"Even a small overtopping will release a lot of water on to the floodplain."
Mr Campbell said the Waiohine River catchment could get 60-70mm of rain an hour, enough to cause intense and damaging flooding.
A report from the National Institute of Water and Atmospheric Research (Niwa) predicts more intense storms will hit the Tararua Ranges more often in the future, as a result of climate change.
"Based on that we can expect worse flooding, more often."
Property values on the floodplain have greatly increased, especially with the relatively new-found popularity of Greytown.
This, along with the predictions of climate scientists, made looking at better flood management a sensible approach
Mr Campbell said there were "obviously" some areas at high risks from floods and they should be avoided when it came to new development. The regional council description of the Waiohine River shows it to have a relatively long and narrow catchment area of 378sq km on the eastern side of the Tararua Range.
From its headwaters, the Waiohine flows out on to the alluvial gravel deposits of the Wairarapa plains, being joined by its major tributary, the Mangaterere River, as it makes its way to a confluence with the Ruamahanga River.