The port has carried out a wide range of in depth investigations to understand any effects of the proposal and is working closely with technical experts, stakeholders and interested parties to identify ways to manage or mitigate these.
An extensive optimisation exercise was undertaken to identify the most cost effective design delivering the least amount of dredging for the best operational outcome.
Alternatives for disposal have, and will continue to be considered.
Seabed sediment at the harbour entrance is clean sand and gravel that if disturbed quickly sinks to the seafloor.
Dredging activity is expected to affect water clarity much less than natural conditions such as Hutt River flows and southerly storms.
When disposed at sea, 95% of material will sink to the seafloor immediately, while the remainder would settle to the seafloor within two hours.
Thorndon Container Wharf
Material removed from Thorndon Container Wharf is finer sands and mud and will be placed in nearby deeper water in the vicinity of the wharf – where seabed sediment is similar.
Investigations show there are low levels of contamination of the sediment, typical of the urban coastal environment.
Surface sediments will be removed from the dredging site first and covered by lower, less contaminated sediment.
Modelling suggests a deeper channel will increase wave action on the western shore of the harbour entrance, while wave action will be reduced on the eastern shore of the harbour entrance.
Seatoun Beach already experiences variations in sand distribution along the beach as a result of tides and storm events. There is already a seawall in place to protect the coastal reserve at the eastern end of the beach.
Modelling suggests there may be slight changes to these natural variations in sand distribution.
Any changes will be monitored and the beach front reserve will be protected.
Any beach level changes at Worser Bay are expected to be little different from those caused by storms, wind and tides.
No beach level changes are expected at Breaker Bay, Scorching Bay, Kau Bay and Mahanga Bay.
Modelling suggests the changed wave action will slow down the natural distribution of gravel and stones along the eastern shores of Wellington Harbour/Te Whanganui a Tara.
It also suggests that surfing conditions on the Eastbourne coast would be reduced due to decreased wave energy.
CentrePort understands that these surfing locations are valued by the Eastbourne and wider surfing communities and is working with these groups to identify initiatives to sustain overall surfing opportunities across the wider region, and recreational opportunities in Eastbourne.
CentrePort worked with local ecologists to assess potential affects and identify measures to manage these.
We know Wellington Harbour/Te Whanganui-a-Tara and Cook Strait are important areas for commercial, customary and recreational fisheries.
Modelling indicates dredging and disposal would have no adverse effects on fisheries such as crayfish, paua and fin fish.
We have been working with iwi and representatives from the commercial fishing industry to provide assurance for the valuable fishery in this area.
There would be low effects on the marine species relied on for recreational fishing and seafood harvesting.
Seabirds and Marine Mammals
Wellington Harbour/Te Whanganui-a-Tara and Cook Strait are home to many important seabirds species including little blue penguin and CentrePort will put a lot of care into managing its activities with regard to these birds.
CentrePort consulted with local ecologists to assess potential effects and identify measures to manage these.
Monitoring would be put in place before, during and after dredging.
Effects on marine mammals will be low and management controls would be in place during dredging activity.
CentrePort recognises the aquifer is an important source of drinking water for the Wellington region. It has worked with Wellington Water and Greater Wellington Regional Council to understand any effects the channel deepening may have on tis resource.
These investigations show that dredging to allow a 12.5m draught vessel would havea negligible effect on the aquifer. CentrePort would gather more information, such as monitoring aquifer pressures and collecting sediment samples, before going any deeper.