Tauranga Harbour sediment source confirmed

6 November 2014

New research has found that stream bank erosion may be a significant cause of sedimentation in Tauranga Harbour.

Bay of Plenty Regional Council Direction and Delivery Committee Chair Paula Thompson said that as part of its efforts to care for Tauranga Harbour, Regional Council commissioned the National Institute of Water and Atmospheric Research (NIWA) last year, to investigate how much of the sediment run-off into Tauranga Harbour is from stream bank erosion. A final report was presented to Councillors last week.

“Sediment is a concern in the harbour because it covers over sandy areas, encourages mangrove growth and has the potential to smother kaimoana (shellfish). It can also increase the amount of phosphorous in the harbour which may contribute to blooms of algae.”

“NIWA provided some good data on sediment flows into the Tauranga Harbour in 2010. It told us that pastoral areas contributed the most sediment, but we didn’t know what proportion of that was coming from land run-off such as from farm tracks or cultivated land, versus stream bank erosion,” Councillor Thompson said.

To fill the knowledge gap, NIWA undertook a case-study in the Kopurererua River catchment. Their research report concluded that at least 95 percent of the sediment that washes from that sub-catchment into the Tauranga Harbour comes from subsurface erosion off stream banks, landslides and earthworks. Stream bank erosion was identified as the single most important source.

“In the last two years we’ve helped landowners to fence and plant of more than 100 kilometres of river and stream margins on waterways that feed into Tauranga Harbour. We’ve always known that riparian restoration was important for improving water quality but we couldn’t quantify how much difference it was making to harbour sedimentation. The NIWA study has confirmed it’s a worthwhile investment that we should continue to support,” said Councillor Thompson.

Further information for landowners or Māori Land Trusts about how Regional Council can help them to care for their land and waterways is available at www.boprc.govt.nz/landmanagement or by calling a Land Management Officer, phone 0800 884 880.

ENDS

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