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Dredging

Dredging is the excavation activity to remove sediment from the bed of a body of water and is required in most ports because of sediment movement and run-off from surrounding land areas and city streets. It ensures that ships can navigate and berth safely.

The Waitematā Harbour has reasonably strong tidal movements, which reduces the build-up of sediment in the channel. Some siltation in the port basin comes from the run-off from stormwater outlets from surrounding city catchments. Auckland Council is working to upgrade the city stormwater system, which will help to reduce siltation and its impacts on water quality in the harbour.

Ports of Auckland carries out two types of dredging:

Maintenance dredging: regular dredging to maintain our channel and berths at the required depth by removing any sediments which have built up over time. Most of our dredging is for this purpose.

Capital dredging: typically, infrequent or one-off dredging carried out to deepen new or existing facilities in order to allow larger ships access.

Responsible disposal of dredged material

Until recently our dredged material has been used for reclamation at the port, specifically in most recent times the north-east corner of Fergusson Container Terminal. The dredging's have been mixed with cement to make mudcrete for the reclamation.  Because mudcreting quickly binds dredged sediment into a solid mass, there is very little impact on the harbour or the water quality. The dredged material is predominantly marine mud. Extensive monitoring has found no problems with contamination or biotoxicity.

Our reclamation project has now ended and as such we needed to find another area to dispose of dredged material. In June 2019 the Environmental Protection Authority granted Ports of Auckland a 35-year consent to dispose of dredged material at the 'Cuvier Dump Site' 150 kilometres from Auckland for disposal – see map below. This is one of five Government-approved disposal sites in New Zealand.

Further information on the marine consenting process can be obtained from the EPA website.

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Channel deepening Resource consent application

In October 2019 we applied for a resource consent to deepen the Waitematā Navigation Channel. Auckland's population is growing fast, with a million more people expected to live here by 2050. More people means more demand for the products we all buy from overseas, which means more containerised imports and – because of economies of scale – bigger ships. Most imports passing through the port are bought by Aucklanders from local shops and businesses. The ships carrying these goods have been getting bigger and will continue do so in the next few years. Ports of Auckland must be ready to handle this growth. The largest container ships we carry around 5,000 twenty-foot containers (TEU). Shipping lines want to bring 6-7,000 TEU ships here in the next 2-3 years and in future it is likely we will need to host 'New Panamax' ships that can carry around 12,000 TEU.

Just like we need roads to accommodate vehicle traffic, we need a shipping channel that is fit for purpose to allow ships to call Auckland safely. The shallowest parts of the channel are 12.5m deep at low tide, whereas 6-7,000 TEU ships have a draft of 14m and New Panamax ships have a draft of 15.2m. This is a capital dredging project that we would implement in two stages, firstly to accommodate the 6,000 – 7,000 TEU vessels and then secondly for New Panamax class vessels.

We will use a mechanical dredge – a digger on a barge – to deepen the channel. The digger will have a long arm to reach down to the seabed and scoop out sediment and rock. The bed of the channel is mostly soft material such as marine muds, mudstones and some sandstone and gritstone, which can be removed by a digger. No blasting is required.

This year we have undertaken a wide range of studies to examine the likely impacts of dredging, including work on navigation and safety, wave and current impacts and biosecurity.

For more information from Ports of Auckland, please contact:

Nigel Ironside

[email protected]

+64 27 558 8076

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