Mangawhai Focus, January 28, 2025
Residents group Mangawhai Matters is calling on Kaipara District Council to exercise better site management and measures to protect stream banks from erosion during earthworks.
As part of the Sustainable Mangawhai Project, the group commissioned a study by NIWA into the possible contamination and sedimentation of the harbour from runoff, based on the catchment’s current and potential future runoff.
Spokesperson Phil McDermott says the results show that there is limited nutrient loading during an average year under current land cover, and that gross sediment yields – the amount of sediment entering the harbour – are not high, although they are concentrated in certain areas.
“The risks of ecologically damaging sedimentation in such areas will rise with heavy and more concentrated rainfall,” he says.
“The main concern is around the impact of urbanisation and especially at the earthworks stage.
“On the basis of this study, we are keen to see council monitor performance and enforce the maintenance of standards, particularly when extensive earthworks are left open for extended periods.”
The report was prepared by NIWA’s principal scientist catchment processes scientist Sandy Elliott, who said that if treated wastewater were to be disposed of by drip irrigation to the golf course – one option suggested to cope with Mangawhai’s growth – it would add about 1200kg of nitrogen/year to the wetland and, as a result, the Tara arm of the harbour, mainly in winter.
The report said this would add less than 2.7 per cent to the current loading, although the localised effect would depend on sustaining the flushing capacity of the Tara arm.
“The land use change most likely to increase the risk of harbour contamination was urbanisation, including large lot residential development. However, it can be expected that this would generate much the same nutrient load as the pastoral farming it displaces.”
The study identified that the key issue when considering sedimentation was the depth of deposition on the harbour floor. The threshold beyond which there was a risk of adverse ecological effects was 2mm/year.
“This means that if today’s estimated average load was spread evenly over the non-channel parts of Mangawhai Harbour, the rate would still be well under that, at around 0.59 mm/year.
”However, sediment yields vary among sub-catchments, and in different parts of the harbour. The southern arm accounts for about 44 per cent of the sediment delivered, and the northern (Tara) arm 49 per cent. With sediment from both deposited in Back Bay, the sedimentation rate of 1.2mm/year there is much higher than the harbour average, although still below the ecological impact threshold.
The study also considered the impacts of a changing climate, finding that a high temperature climate scenario (a +3oC change) could lift deposition in Back Bay to around 1.97mm/year, only slightly below the ecological damage threshold.The study examined the implications of development currently consented and proposed areas for development, at the rate of 230 dwellings a year for 10 years. The earthworks associated with this level of activity could lift sedimentation rates by an average of 2.5 per cent over the non-channel area of the harbour.
“If urbanisation accelerates, it would lift average deposition further. For example, doubling the rate of development would double earthworks sedimentation, leading to around a 5.4 per cent sedimentation increase in Back Bay.”McDermott says the study shows that the staging of development should be spread out to avoid excessive sedimentation during a major rainstorm.
“It is also important that the high-quality erosion controls assumed for the catchment study are maintained in practice, through rigorous regulation and enforcement of stormwater management and stream-bank protection during development.“Similarly, it is important that Mangawhai’s stormwater network, including council’s infrastructure (culverts, drains, detention ponds, wetlands) is maintained and managed so that it continues to operate effectively in the face of more intensive storms.
”McDermott said that while the harbour was currently in a satisfactory condition, the NIWA study justified active management to avoid deterioration in the face of more volatile weather and ongoing urban development. He said this should include:
Baseline measurements and subsequent monitoring of sediment in the mid-estuary, Back Bay, Tara, and southern compartments
Monitoring and enforcement of a high site management standards during development
Ensuring the quality of stormwater system maintenance and management under the relevant resource and subdivision consents.