24 October, 2013

Kaeo, Waitangi flood maps released

New maps showing the likely extent a “one in a century” flood would have on the Kaeo and Waitangi catchments will be released by the Northland Regional Council shortly.

The maps’ release is part of the council’s Priority Rivers Programme, which focusses on reducing flood risks in 26 Northland rivers at highest threat, including 17 in the Far North.

The council has already released flood maps for 20 catchments over the past year to raise awareness of where flood hazard areas are and help inform public and district council decision-making.

However, Joseph Camuso, the council’s Rivers Programme Manager, says the complexity of the issues facing the remaining six catchments meant they would be rolled out in stages over the next 12 to 18 months.

Those six – Kaeo, Waitangi, Kerikeri/Waipapa, Whangarei, Whangarei Heads and Awanui – posed particular problems due to a combination of factors including flood paths, the shape and nature of the catchments and the need for additional detailed data.

Mr Camuso says the two latest maps had been modelled at a cost of about $300,000. They model the likely impacts of what’s known as a ‘100 year flood’ (there’s a one percent probability of this occurring in any given year) and take into account the latest climate change predictions.

He says part of the issue is that nationally, most authorities (including Northland) only have useable, scientifically valid data stretching back 30 to 50 years which can affect the accuracy of predicting extreme events.

Mr Camuso says the new Kaeo and Waitangi maps are based on the latest available data from the National Institute of Water and Atmospheric Research (NIWA) and had delivered at least one surprise for the council.

“Ironically, the very high intensity storms Northland has experienced in the past decade have had quite an impact on the modelling which now indicates rainfall in a 100 year flood is likely to be up to 15 percent more than previously forecast.”

However, he says residents in both catchments are already quite familiar with high intensity rainfall events and the impacts these have on properties in their areas.

“Many residents and their properties, especially those in Kaeo, are already affected by floodwaters in much smaller events and the council is actively working with those communities to try to help reduce the impacts of these.”

This had included securing funding from central government to reduce risk to 14 of Kaeo’s most vulnerable properties.

Mr Camuso says if a 100 year event were to hit Kaeo or Waitangi, floods would probably be largely contained to known flood-prone areas, although the actual water levels involved would be higher.

He says over the next few days the regional council would be writing to the owners of about 600 affected properties in both catchments outlining the latest flood risk to their properties and directing them to the new maps.

“These maps will progressively replace previous hazard maps developed roughly a decade ago based on soil types and topographical features that indicated they had historically been subjected to heavy flooding.”

Mr Camuso says as with many other technologies, the science around flood mapping was now much more advanced with computer modelling offering more accurate predictions.

“In the past several years we’ve had a number of actual floods in Northland which have been used to validate our flood models.”

General information about flood mapping is available from council‘s website at: www.nrc.govt.nz/floodmaps .

ENDS

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