‘Disturbing’ study confirms dunes disappearing, breach threat

Mangawhai Focus, April 7, 2025

The studies confirmed the suspicions of many locals that the Mangawhai dunes have been slowly disappearing over several years.

Potential threats to Mangawhai’s distal spit, which is crucial in protecting the coastline, were outlined at a meeting hosted by the Mangawhai Harbour Restoration Society (MHRS) on March 21.

The guest speakers were marine geologist and coastal oceanographer Dr Terry Hume and Auckland University School of Environment Professor Mark Dickson.

In 2024, Hume was commissioned by Mangawhai Matters to do a study covering coastal processes, including climatic and ocean currents, wave action and storms, and their impact on the harbour’s sedimentation.

The report mapped out the spit more accurately than previous studies, providing a blueprint for future work and identified some significant threats to the environment, cultural sites and economic activity.

However, Hume said the “really big one” was a significant repeat of a breach like the one in 1978.

“As we know, storms are increasing in intensity and there will be more of them on the way. It’s better to do mitigation efforts now than clean up the mess later on,” he said.

Concerned about the potential threat of another breach occurring in the future, last year MHRS contracted Professor Dickson to study the spit’s morphology with a focus on operational issues.

Using a drone-based LiDAR system (Light Detection and Ranging) that provided a level of detail ‘considerably greater’ than previous studies, a survey of the spit’s topography was done and the results compared to the landscape changes of a 2018 aircraft-based LiDAR survey. The topographic mapping also made it possible to show possible breach locations during storm conditions.

Dickson’s conclusions were presented for the first time at the meeting and his research revealed that the spit’s large dunes have moved a considerable 10 to 25 metres westward and alarmingly, around 260,000 cubic metres – equivalent to 26,000 standard dump trucks or eight centimetres of sand from all over the entire spit – was lost between 2018 and 2024.

Dickson said Mangawhai’s spit was one of several ‘drumstick-shaped barriers’ found around the country, which were inherently susceptible to breaches, being eroded by tidal currents from the estuary and ocean sides that narrow the spit, as well as sea-flooding, especially over the thinnest point, the ‘waist’. Lack of vegetation on Mangawhai’s spit also heightened its vulnerability.

Professor Mark Dickson and Dr Terry Hume with Mangawhai Restoration Harbour Society chair Peter Wethey and Mangawhai Matters Phil McDermott at the recent presentation which emphasised the threats to Mangawhai’s iconic distal spit.

“Since about 1960, the ocean side of Mangawhai has been eroding pretty quickly, about a metre-and-a-half a year on average. But it hasn’t been eroding on the estuary side as we might expect because of the bund wall.”

Topographic simulations also presented a futuristic perspective, revealing the impact of storms with the magnitude seen in 1978, amplified by rising sea levels. The projected outcome showed that by 2080, the top part of the spit could be largely lost beneath encroaching waters.

However, MHRS’s mitigation works had had a positive impact, Dickson said.

“Although open-coast erosion and dune transition is ongoing, the effects have been mitigated due to the maintenance of the bund wall, and the fencing and planting have been highly effective at maintaining the dune’s height and width.”

Dr Hume said going forward, key actions to ensure the spit’s resilience included maintaining the height, width, sand volume and continuity of the dunes along the coast, as well as the upkeep of the bund wall. Identifying the flowpaths of possible water ingress, gaining consents and gathering funds for the work involved were also urgent.

“Vegetation is important. Mangawhai is a bit unique, it’s a bit bald compared to other New Zealand spits, but the society’s planting programme has been very successful,” Hume says. “The MHRS have been doing some really fundamental and very valuable work, and it must continue.”

MHRS chair Peter Wethey said the report presented a “disturbing picture, namely, the potential for another breach of the spit” which must be prevented from becoming a reality.

“The evidence from the study identifies the best ways of doing this, but what MHRS are authorised to do depends on how the NRC and DoC view the importance of our work,” he said.

“One very positive outcome was the validation of the sand fencing and spinifex planting that we are doing each year. The drone mapping clearly showed the benefit of this. It’s good that the key stakeholders involved in protecting the harbour and the distal spit were here so we can share the information all at one time and this will hopefully lead to good cooperation going forward.”

Among those at the presentation were Northland Regional Council Cr Rick Stolwerk, Department of Conservation senior staff, Environs Te Uri o Hau and Mangawhai Matters representatives.