Sediment study to help improve harbour

A three-year study of sedimentation in Tauranga Harbour will help Environment Bay of Plenty when it comes to improving the health of Tauranga Harbour and ensuring it remains a taonga, a treasure for all.

The Tauranga Harbour Sediment Study, carried out by the National Institute of Water and Atmospheric Research (NIWA), looked at where the sediment in Tauranga Harbour is coming from; how much is deposited in different areas of the harbour; and what effect both future urban development and climate change may have over the next 50 years.

The study has shown pastoral land is the biggest contributor of sediment into the harbour, while the impact of urban development has not had such a major effect as first thought, although still remains a significant contributor on a site by site basis.

Pastoral land use generated the most sediment (64 percent) followed by bush, scrub and native forest (28 percent). Urban areas generated 11 percent of sediment, urban earthworks 0.5 percent and orchard and crop land just 0.3 percent. The study found that over time, urban development will in fact result in a small decrease in sediment into the harbour.

Increased sedimentation affects the harbour’s natural ecosystem and can increase the levels of nutrients, such as phosphorus, into the harbour as well as affecting water flow. Sedimentation isn’t always a bad thing. It can often be beneficial to the local ecology but the danger areas are when mud settles on sandy beds.

Environment Bay of Plenty Planning Frameworks Manager David Phizacklea said more than 100,000 cubic metres of sediment goes into the Tauranga Harbour annually.

“It’s not always obvious where the sediment comes from – the analysis in the NIWA study will help us to understand what the priority sub-estuaries and sub-catchments are for reducing sediment into the harbour and what the best management strategy for the future is,” Mr Phizacklea said.

He said this could include a more targeted and strategic approach to mangrove removal, land retirement or conservation planting in erosion-prone areas, more targeted riparian planting, maintaining best practice for earthworks, forestry, cropping and quarries through the consents process and other catchment interventions to reduce sediment.

Regulation Monitoring and Operations Committee Chair Malcolm Whitaker said there is increasing pressure from the community, regional councillors and local iwi to work together to protect and restore Tauranga Harbour.

“With threats to the health of Tauranga Harbour, high quality research, like the NIWA study and other science-based programmes focussed on the harbour, are fundamental to enable the regional council to make sound management decisions when it comes to Tauranga Harbour,” Councillor Whitaker said.

Sub-estuaries that are likely to become progressively muddier and where sedimentation rates will increase as a result of land use and predicted climate change include Rangataua Bay, Welcome Bay, Waimapu, Waikareao, Waikaraka and Te Puna.

Environment Bay of Plenty will now look to develop a Tauranga Harbour Sediment Action Plan.

Environment Bay of Plenty has committed around $1.7 million per year to Tauranga Harbour on a range of measures, including mangrove removal, sea lettuce clean-up, Estuary Care programmes and catchment works. In addition, the regional council has recently announced it is to fund a Chair in Coastal Science located in Tauranga as part of the Intercoast partnership between the University of Waikato and Germany’s Bremen University.

Environment Bay of Plenty is also supporting the Manaaki Taha Moana research programme, a partnership between Tauranga iwi and the Foundation for Research Science and Technology (FRST), looking at the Tauranga Harbour from a Maori perspective.

ENDS

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