We have been granted consent to deepen Auckland's shipping channel and undergo maintenance dredging.
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.
We have to be ready to handle this growth??. The largest container ships we handle at the moment carry up to 5,000 twenty-foot containers (TEU). Shipping lines want to bring 6-8,000 TEU ships here in the next 2-3 years and in future we will need to host 'New Panamax' ships that can carry around 12,000 TEU.?
The shallowest parts of the channel are 12.5m deep at low tide, whereas 6-8,000 TEU ships have a draft of 14m and New Panamax ships have a draft of 15.2m.
Keeping dredging to a minimum
To keep dredging to a minimum, we will use tidal windows. Tidal windows are in common use in other New Zealand ports, and simply mean that deeper draft ships enter or leave port when the tide is high enough. To create a suitable tidal window for New Panamax ships to access port safely we will need a channel which is 14m deep on the straights and 14.2m deep on the bends. As it takes around 15 hours to unload and load a container ship, our berth will be dredged to 15.5 metres so ships can stay through a full tide cycle.
Maintenance dredging
Sediment from the city and wider catchment area builds up in and around the wharves and the shipping channel and needs to be removed every couple of years. We currently has two permits, issued by Auckland Council, which allow the removal of this natural sediment so that our berths and the channel is safe for ships to use. We are authorised to remove up to 50,000m3 of maintenance dredging of natural sediment material a year for this purpose.?
Channel deepening
Capital dredging in the Rangitoto channel and Fergusson North berth is required to accommodate larger vessels. We have carried out capital dredging several times over the past 70 years and will do some more over the over the next 15 years.
Managing environmental impact
The current channel is a highly modified environment which has been dredged many times. As a result we already know a great deal about how dredging impacts the environment. Regardless, we have undertaken a wide range of studies to examine the likely impact of this new dredging campaign. This includes work on navigation & safety, wave & current impacts, biosecurity and more.
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 to scoop out the bottom material. The bed of the channel is mostly soft material like marine muds, mudstones and some sandstone and gritstone, which can be removed by a digger. No blasting is required.
Disposal of dredged materials
We were granted a marine dumping consent EEZ400011 on 27 June 2019 for a period of 35 years. The consent will expire on 27 June 2054.? The permit (EEZ400011), granted by the Environmental Protection Authority (EPA) authorises the disposal of dredged material at the designated Cuvier Disposal Site (CDS), 27 nautical miles (50 km) east of Cuvier Island (Repanga). The CDS disposal site, also known as the Auckland Explosives Disposal Site, has been used periodically since the end of WWII for the disposal of unwanted munitions, vessels and dredged material.
Where possible, we will reuse dredged materials to reduce the amount of dredged material being disposed at sea. Before any dredged material can be disposed of at the CDS site, the consent requires that comprehensive sediment sampling, testing and assessment is undertaken. On an ongoing basis EPA review and must approve each step in this process as well as provide final approval to confirm that the dredged material is clean and safe to be disposed of at the site.
The CDS disposal site is one of five official disposal sites around New Zealand. The CDS is outside the Hauraki Gulf Marine Park and inside New Zealand's exclusive Economic Zone.