Press Release on Sustainable Mangawhai Project
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Mangawhai Matters Press Release on the Sustainable Mangawhai Project
More Intensive coastal storms a threat to Mangawhai
Mangawhai Matters has released new findings from its Sustainable Mangawhai Project that warn of a growing coastal storm risk to Mangawhai Harbour, its environment, and community.
During the past 18 months researchers assessed key vulnerabilities. NIWA examined water quality risks from pollutant and sediment runoff during heavy rain. University of Auckland scientists evaluated the barrier spit’s susceptibility to storm tides and wave erosion. Coastal engineers, Tonkin & Taylor, modelled harbour inundation under increasingly intense storm scenarios. Geographer and Mangawhai resident Dr Phil McDermott assessed the impact of storms over the next fifty years using a flood-mapping tool developed by Tonkin and Taylor.
Storms shifting south and intensifying
These studies were set against the expectation from current climate modelling of larger, more powerful cyclones reaching further south from the tropics. Storm records since the 1956 shows that trend is already underway and affecting Northland. There has been a notable rise in the proportion reaching major strength (defined as Hurricane levels 4 and 5) since the late 1980s. Although they usually weaken to ex-tropical status near New Zealand, warmer seas will mean they are likely to retain more energy, with stronger onshore winds, higher storm surge than experienced to date, and more rain.
Major Storm Impacts
Storm tides could reach 2 metres above astronomical tides within 20 years, exceeding the 1.7-metre storm tide recorded during Cyclone Gabrielle in 2023. If a major storm coincides with a spring hgh tide the report warns of:
Flooding of harbourside reserves, campgrounds, and homes
Submersion and damage to roads, bridges, and causeways
Disruption of transport and communications
Failure of wastewater and stormwater infrastructure
Loss of seabird nesting habitat, cultural sites, and recreational areas, with knock-on effects for tourism, jobs, and property values
Recovery times could range from days to years, with some damage possibly irreversible and significant costs to the community.
Some defensive actions under way
The loss of the spit’s protective function through erosion or a breach (as happened in 1978) is the most immediate and serious risk. Higher storm surge and stronger ocean waves entering the harbour would lift flooding and increase erosion on its western and southern coasts.
Practical measures already in progress include targeted sand relocation on coastal low points of the spit by the Mangawhai Harbour Restoration Society, with Department of Conservation support, and more widespread dune stabilisation through fencing and planting.
Other recommended actions to reduce potential damage from storms include creating wetlands to retain floodwater, constructing sea walls and groynes to protect critical infrastructure, restricting development in high-risk zones, and raising vulnerable houses, roads, and bridges. The project calls for such measures to be phased in to spread cost and effort.
In addition, the sedimentation study identified the risk of increasing siltation and ecological damage within the harbour from heavier rain, underpinning Mangawhai Matters’ advocacy for stronger earthworks controls on development projects.
Call for a coordinated response
Mangawhai Matters convened KDC, NRC, and DoC officials and Te Uri o Hau to see if they could pool resources and plan and coordinate protection and mitigation measures.
The agencies acknowledged the project’s value and conclusions of the Sustainable Mangawhai Project but said resource constraints and legislative uncertainty are barriers to the proposed collaboration.
Mangawhai Matters Chair, Doug Lloyd responds:
“We must work together to avoid far reaching and wide-ranging damage from a more volatile climate. We are all affected, and no single agency has the capacity to provide the multiple responses needed. We urgently need a programme to implement the necessary measures, knowing that whatever we spend on protection in the short term will be far less than what we’ll lose—and what we’ll have to spend —when we’re hit by major storms.”
