Mangawhai Focus | December 2, 2024
A recent survey of the Mangawhai spit has found that the barrier has lost just over 260,000 cubic metres of sand in the past six years, equivalent to 26,000 standard dump trucks or about 1.8 per cent of the total spit sand volume.
The information is contained in a mapping report, which compares data from a survey
done this year with a similar survey in 2018. The report is part of a larger project which is endeavouring to compile an accurate scientific picture of Mangawhai Harbour’s terrestrial and marine environments. It was prepared by Professor Mark Dickson, of Auckland University’s science faculty, on behalf of residents’ groups, Mangawhai Matters and Mangawhai Harbour Restoration Society.
The survey found that erosion on the estuary side of the spit has occurred while the ocean- facing dunes in the northern part of the spit have increased in elevation. It also identified low points in the dunes, which could be vulnerable to breaching.
Mangawhai Matters chair Doug Lloyd says this latest report identifies precise areas where the spit is vulnerable to flooding from the sea and harbour. “More severe and longer lasting storms associated with a warmer climate could open the spit up to a major breach, with all the attendant risks to the natural environment and the community,” he says. “While the frequency and severity of storms cannot be predicted, the evidence of the spit’s weaknesses identified in the Auckland University report justifies increasing defensive measures now.”
So called “bathtub modelling” indicates that a typical annual storm coinciding with spring high tide would be unlikely to inundate the spit, but there were at least two areas toward the southern end which have low points in the dunes that could potentially overtop.
“Relatively modest increases in sea level by 2040 are unlikely to materially change inundation or the risk of breaching in a typical annual storm,” the report said. “But by 2080, if the morphology of the spit does not naturally adjust upward in response to sea level rise, the annual storm is likely to inundate the spit both through the distal northern portion of the spit and at several discrete locations on the oceanside of the spit where the dunes are low-lying and discontinuous.”
However, Dickson warned against jumping to any conclusions.
“We are unclear whether the considerable (10 to 25 metres) westward movement of dunes on the southern end of the spit over the last six years is transitory or part of a longer-term pattern. More monitoring data are required,” he said. Commenting on the benefits of revegetating the spit, he said that there was real value in adding vegetation.
“Plants help build a reserve of sand in the dunes, which can prevent erosion during a natural weather event, but if it is a significant storm, where you can get coastal erosion of up to 50 metres, then no amount of vegetation will help.”
On a positive note, Dickson said the spit was notably wider than it was in 1978 so it was less vulnerable to breaching today than it was then. He said a storm with similar properties to the extreme event of 1978 would likely produce total water levels of around 3.2 metres, which would still inundate large portions of the spit and could potentially cause breaching.
The report also drew attention to the estuary channel and how it was a factor in breaching.
The spit neck was unusually narrow prior to breaching in 1978 and the harbour shoal, adjacent to the spit neck, was unusually wide, the report said. This combination had made the spit particularly vulnerable to breaching.
The report recommended on-going monitoring of the spit, with particular focus on the position of the estuary channel, the growth of the estuary shoal adjacent to the spit neck, the width of the spit neck, and the ongoing east-to-west movement of the beach shoreline and spit dunes.
The survey was undertaken using funds from a targeted rate. Kaipara District Council collects the rate on behalf of the Mangawhai Harbour Restoration Society and expects to hand over just over $400,000 (including GST) this financial year.
Auckland University used a drone based LiDAR system to conduct the survey, which allowed researchers to compare topographic changes with data from the earlier aircraft- based LiDAR survey done in 2018.
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