Dredging challenge to ferry proposal

Carving a 7m channel through the lower Whanganui River to accommodate a large vessel would require ongoing work. Photo/ Sieska Verdonk

Accommodating large vessels at Whanganui Port such as the proposed ferry service to Motueka would require several costly upgrades including ongoing dredging.

A report commissioned by Whanganui District Council has found initial dredging of 1 million cubic metres would be required with annual maintenance dredging of 650,000cu m.

The work required for the lower Whanganui River to accommodate a proposed 180m vessel requiring a 7.1m channel includes upgrades to the north mole structure, wharves and turning basin.

Environmental engineering consultants Tonkin & Taylor who wrote the report said that came with risks.

"Any realignment of the turning basin wall to accommodate a larger vessel turning circle would create a constriction within the river potentially resulting in higher flood levels upstream."

This could be mitigated by providing another outlet or raising stopbanks but "both options are likely to be technically difficult and very high cost".

The report said previous attempts to dredge near the bar where the channel depth is deeper than the seabed had also proven problematic.

"It should be noted that a major dredging campaign on the bar in the late 1980s spent a reported $1million on dredging with the bar reportedly infilled within six weeks."

Midwest Ferries are preparing a business case for a service between Whanganui and Motueka.

An initial study commissioned by Midwest said the service was technically and financially feasible.

A peer review of that report commissioned by the council said that study contained "an unusually large number of areas 'parked' for future study and some critical assumptions" and that much more work was needed.