16 December, 2015

Awanui flood maps to be released

The Awanui-Kaitaia area is poised to become the latest part of Northland to be issued with new maps showing the impacts of a ‘one in a 100 year’ flood.

The new maps – developed over the past several years – 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’s Te Hiku constituency representative, Monty Knight, says staff will be writing to the owners of about 2600 affected properties shortly detailing the updated flood risk to their properties and directing them to the new maps.

He says the council has already released flood maps for 24 catchments over the past three years to raise awareness of where flood hazard areas are and to help inform both public and district council decision-making.

“It will probably come as no surprise to locals to hear the Awanui catchment has been one of the most challenging for us to map for a number of reasons.”

These include the way floodwaters flow over the State Highway and into a different catchment; the Tarawhataroa Stream. “Flow through the Kaitaia itself is also heavily influenced by the Whangatane Spillway on the north side of town, which typically diverts roughly two-thirds of flood flow from the Awanui River.”

Councillor Knight says the new models cost about $250,000 to develop and forecast not only the predicted impacts of a '100 year flood', but also incorporate Central Government climate change projections on storm rainfall intensity and sea level rise.

“There's a one percent probability of a flood of this scale occurring in any given year.”

He says most people living in the Kaitaia area are already all-too-familiar with heavy rainfall events and how these can affect local properties.

"Most of the areas covered by our new flood maps have previously been identified as ‘flood-susceptible’ and are already shown as such in the Far North District Plan.”

However, the new Awanui flood maps do incorporate a variety of new information and modelling, including the latest available storm data from the National Institute of Water and Atmospheric Research (NIWA).

They show that should a 100 year event hit Kaitaia, the worst affected areas would be within known flood-prone areas.

The largest flood in recent times occurred in July 2007 and resulted in over-topping of the Tarawhataroa Stream stop bank, flooding adjacent low-lying areas."

Councillor Knight says the new maps will progressively replace previous hazard maps which were based on soil type 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 has planned to reduce flood risk in the catchment. This includes a $600,000 project to modify the Whangatane spillway intake due to start next year.

Councillor Knight says the models assess flood depths above ground level, not building floor heights.

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

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