Worley Parsons and Napier Port are carrying out a dredge effects study based on a hydrodynamic model of the nearby sea area. The key aspects of the dredge effects study includes the following:
The models below are early images that show the geological make-up of the sea floor where the dredging would take place. The Port has completed geotechnical investigations to understand the nature of the dredge material. This has involved drilling boreholes, vibracore sampling and laboratory analysis of the different types of material (for example, sand, silt, siltstone) that will need to be dredged. This will help determine the dredging and disposal methodologies, disposal location and quantities.
Yellow: Fine to medium sands and soft to firm silt
Orange: Firm to stiff clay-like silt
Light Green: Residual Mangaheia Group = very stiff to hard weathered Sandstone and Siltstone
Light Blue: Mangaheia Group = strong Siltstone, Limestone and Siltstone
Napier Port investigated a number of options for where it could locate its dredge material disposal site and initally considered extending its currently consented disposal sites near Westshore Beach.
However, after consulting with stakeholders and doing further investigation it decided to seek consent for a disposal site 5kms east of Napier Port.
At 20kms deep, this site is much deeper than the inshore site and investigations show that the deeper water means that the material disposed will be less susceptible to adverse weather events. Accordingly, any sediment is less likely to be disturbed.
Further investigation confirmed two coastal processes were at work that impacted on the decision to dispose of material further off-shore.
Napier Port proposes to dredge, firstly to develop the berth pocket (next to the new wharf) and the swing basin (where the ship turns). Then, as there is demand from larger ships, the channel would possibly be dredged in as many as six campaigns to a maximum depth of 14.5m.
Two types of dredges, a Backhoe (BHD) and a Trailing Suction Hopper Dredge (TSHD), could be used to carry out the programme. The BHD is suitable for dredging firmer material, while a TSHD is more suitable to sands and silt (see channel design maps to see where the different dredges are likely to be used).
The Trailing Suction Hopper Dredge (TSHD) is fitted with modern turbidity control devices which reduce the extent of any dredge plume.
Above is a typically-sized Trailing Suction Hopper Dredge, the Mahury, expected to be used for parts the proposed project. Attribution: http://www.heronconstruction.co.nz/Equipment/Machiavelli.html
Above is an illustration of a typical Backhoe Dredge.