Dredging and Disposal

A staged dredging programme

To build 6 Wharf and welcome more and bigger ships, Napier Port needed to create a new vessel turning area and deepen parts of the harbour using a dredge so ships could safely use the wharf. 

Dredging is an underwater excavation process that uses a specialised ship to remove sediment in the harbour and deposit it to a disposal area.

For 6 Wharf, we secured resource consents to undertake capital (new) dredging in the wharf area and maintenance dredging.

We also gained resource consent to deposit sediment in a new disposal site 5 kilometres to the east of Napier Port, which will help to protect Pania Reef during dredging and disposal.

Heron Construction and Dredging were the sub-contractor for the stage one dredging work.

Dredging Stage 1

1. Stage one dredging planned for mid-2020 to the second half of 2021.

For ships to use 6 Wharf, we needed to create a berth pocket that was 13.0 metres deep and a slope underneath the wharf to place the shore protection.

We also needed to deepen and create a larger swinging basin (where ships turn), dredge parts of the inner harbour and deepen the shipping channel closest to the port to 12.5 metres. That stage involved dredging and disposing approximately 1.3 million cubic metres of material.

Dredging Stage 2-5

2. Stages two to five dredging will only take place as required.

This part of the project would allow the port to handle ships with deeper drafts and is not required at this stage. The resource consents we have to undertake this work are valid for 35 years. We will only undertake this work if ships with deeper drafts indicate they are going to call at Napier Port.

It would involve extending the shipping channel and increasing its depth in four stages. Each stage would take up to nine weeks, with the channel being deepened 0.5 metre at a time, taking it to a maximum depth of 14.5 metres. Each stage would involve dredging and disposing a similar volume of material; the overall total being around 3.2 million cubic metres.

Disposal site: Finding the right place

Following detailed investigations and community consultation, we have secured resource consents to deposit the dredged material from stage one of the project at a newly consented disposal site.

The site is 20 metres deep and around 5 kilometres east of the port. It is a more suitable site as it is most likely to have minimal effects on Pānia Reef.

In agreement with Napier City Council and Hawke’s Bay Regional Council, we will be depositing material suitable for renourishing Westshore Beach at the inshore disposal area (R EXT) for the maintenance dredging we undertake every two to three years and when the shipping channel needs deepening at the later stages of the 6 Wharf project. We will monitor the environmental impacts of disposing at the site during dredging via our environmental dashboard and immediately cease work if there are adverse effects on the wider marine ecology.

Read the announcement abour our agreement with Hawke’s Bay Regional Council and Napier City Council here.

Managing and monitoring the impacts of dredging

We’ve undertaken in-depth studies to find out how we can minimise the impact of dredging on the environment around us. 

Working with mana whenua and the fisheries community, we’ve produced management plans and will implement a robust monitoring programme to manage the impacts of dredging and disposal on the environment and adapt our operations as required.

This includes a Biosecurity Management Plan, which will protect against unwanted pests from entering the marine environment. Biosecurity measures include thoroughly inspecting the dredges and support vessels that will be used during 6 Wharf for pests prior to arrival, the introduction of settlement plates in the inner harbour, visual searches and inspecting navigation aids.

You can read the management plans here.

Find out more about how we are managing the impacts of construction and dredging in our Building Sustainably section.

We’ll use a backhoe dredger for the majority of stage one dredging

The bulk of the material to be removed from the sea floor in stage one is consolidated stiff silt and mudstone. This requires a type of dredge called a backhoe dredge.

The backhoe dredge is a stabilised floating pontoon with a long-reach hydraulic excavator, which excavates the seabed and places the material onto a barge.  The barge is towed by a tug to the disposal area. A continuous process of barges moving back and forth will ensure the planned dredging and disposal maximises the use of the dredging equipment.

A trailing suction hopper dredge will be used for a small portion of stage one. These types of dredges are used for finer material, such as sand.  A trailing suction hopper dredge would also be used in stages two to five of the project, if they go ahead.