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Harbour sediment rates too high for healthy habitats
A recent survey of Te Awarua-o-Porirua Harbour shows that sedimentation rates have increased in the last five years, making the harbour less suitable for healthy animal and plant communities.
The survey, which shows sedimentation has more than doubled in the last five years, is co-ordinated by Greater Wellington Regional Council and co-funded by Porirua City Council. Greater Wellington Marine and Freshwater Team Leader Dr Megan Oliver says this is the third bathymetric (or depth) survey of Porirua Harbour, and part of a long-term study of sediment deposition and erosion in the harbour.
'Results show sediment is accumulating in the deep basins of the harbour, making them increasingly muddy and unsuitable for a range of species. This is a bigger issue for the Pāuatahanui Arm and is driven by a combination of factors such as urban development, motorway construction, forestry harvest, run-off from rural pastures and erosion of stream banks,' Dr Oliver says.
Although estuaries naturally infill over time the current volume of sediment entering the harbour, and depositing there, is impacting the healthy functioning of the estuary.
“We are losing the remaining seagrass meadows, important as a habitat for fish and invertebrates and, in turn, for the rich biodiversity our estuaries are so highly valued for.”
Dr Oliver says the measurement methods used are not precise, and can have large errors, but what her team looks for are the general patterns, using a range of different tools.
'Sediment monitoring varies hugely from year to year because of the interaction between various land based activities and rainfall. As such, the results need to be viewed over the long term.'
The average rate of sedimentation over the last ten years has been 4 millimetres per year and 3.6 millimetres per year for the Pāuatahanui and Onepoto arms, respectively.
The Te Awarua-o-Porirua Whaitua Committee and Ngāti Toa have identified sediment as one of the big ticket items from the Greater Wellington whaitua process. Environmental Policy Manager Matt Hickman explains:
'The Te Awarua-o-Porirua Whaitua Implementation Programme has set objectives of less than 2 millimetres per year and less than 1 millimetre per year for the Pāuatahanui and Onepoto arms, respectively. These limits will be introduced into the regional plan next year.'
Mr Hickman says more regulation of earth disturbing activities is anticipated for the future.
'The message from our partners and community is clear – they want less sediment in the harbour. There are some big opportunities in the whaitua to demonstrate innovation with well-designed large urban development and the activities that happen on our own regional parks.'
Dr Oliver says the results of the survey will be used to paint a picture of how sediment is transported and deposited throughout the harbour, and there will be ongoing monitoring and evaluation of progress towards the objectives set out.
'Upcoming monitoring will include habitat mapping of the intertidal areas to measure changes in the area of seagrass, saltmarsh, mud and sand,' she says.
Regional Councillor, and Whaitua Committee member, Barbara Donaldson says residents have known from their own observations, over a number of years, that the harbour is silting up.
'This is particularly evident in the Pāuatahanui Inlet which is frequently brown with sediment.
'This survey confirms that the rate of sedimentation is increasing at an unsustainable rate and we must change the way we manage and develop land to prevent further damage,' Cr Donaldson says.
Sharnahea Wilson / Media Advisor
GREATER WELLINGTON REGIONAL COUNCIL
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