Date: 11 November, 2013

Kerikeri, Whangarei latest flood maps released

Kerikeri/Waipapa and Whangarei have become the latest parts of Northland to be issued with new maps showing the likely effect a ‘one in a century’ flood would have on them.

The maps form part of the Northland Regional Council’s Priority Rivers Programme, which focusses on reducing flood risks in 26 Northland catchments at highest threat, including 17 in the Far North.

The council has already released flood maps for 22 catchments over the past year – including Kaeo and Waitangi last month (subs: October)  – to raise awareness of where flood hazard areas are and help inform public and district council decision-making.

Joseph Camuso, the council’s Rivers Programme Manager, says the Kerikeri/Waipapa and Whangarei catchments had been among the more difficult to model regionally due to factors including their shape and topography, the flood paths within them and the need for more detailed data.

Mr Camuso says the Kerikeri/Waipapa catchment had been modelled at a cost of about $200,000 and Whangarei $150,000.

The models forecast the predicted impacts of a ‘100 year flood’ and take into account the latest Central Government climate change projections on storm rainfall intensity and sea level rise.

Mr Camuso says there’s a one percent probability of a flood of this scale this occurring in any given year.  However, throughout New Zealand most authorities only have useable, scientifically valid data stretching back 30 to 50 years which can affect the accuracy of predicting extreme events.

“Our new Kerikeri/Waipapa and Whangarei maps are based on the latest available storm data from the National Institute of Water and Atmospheric Research (NIWA).

He says as with other parts of Northland, residents in both catchments are already familiar with heavy rainfall events and how these affect properties in their respective areas.

“If a 100 year event were to hit Kerikeri/Waipapa or Whangarei floods would probably be pretty much within what we already know to be flood-prone areas.”

Mr Camuso says many people living in the Kerikeri/Waipapa area still set the area’s infamous 1981 storm and floods as a benchmark for flooding.  However, in reality, that storm was so extreme that in some parts of the catchment the flood may have been as high as a one in a 1000 year event, with the rest at least a 100 year event.

He says over the next few days the regional council would be writing to the owners of about 1700 affected Whangarei properties and roughly 1100 in Kerikeri/Waipapa outlining the latest flood risk to their properties and directing them to the new maps.

The new maps will progressively replace previous hazard maps based on soil types and topographical features indicating they had suffered heavy flooding in the past.

They would also complement a range of other extensive work the regional council had planned in both areas to reduce flood risk.  These included an $8M flood detention dam in Whangarei due to start next year and a yet-to-be-finalised $2M-plus flood diversion spillway in Kerikeri, expected to start in 2015. 

Mr Camuso says as with many other technologies, the science around flood mapping is now considerably more advanced.

“In the past several years we’ve had a number of actual floods in Northland which have been very useful from a computer modelling point of view and we’ve been able to use these real-life experiences to validate some of our flood models.”

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

Mr Camuso says there are now just two catchments for which new flood maps have to be released; Whangarei Heads and Awanui.  These are expected to be released next year.

ENDS

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