Mangonui's $5m bridge aims to fix decades of harbour damage

Work started on State Highway 10 east of Mangonui on Monday to replace a small culvert with a bridge in the hope of reversing years of environmental degradation. Photo / Myjanne Jensen

Work has started on a $5 million project to fix an engineering error of more than 50 years ago that has caused serious environmental degradation in Doubtless Bay.

Sedimentation in Mangonui Harbour's upper reaches is believed to have started when a causeway was built in the 1950s to take State Highway 10 from the western side of Paewhenua Island across a tidal estuary to the mainland near Mangonui.

Installing a single small culvert in the 1970s didn't ease the problem.

Since then what used to be part of the upper harbour, on the southern side of the causeway, has silted up and been claimed by mangroves.

Locals have been campaigning for decades to have the causeway opened to restore flow in the Aputerewa Creek.

Cost, however, has long stood in the way.

The global Covid pandemic finally provided an opportunity to fix the damage with the Government granting $5m from its ''shovel-ready'' Crown Infrastructure Partners programme.

This Google Earth image shows sedimentation of the upper Mangonui Harbour where a SH10 causeway links Paewhenua Island with the mainland. Photo / Google Earth

Work to replace the under-sized culvert with a bridge started on Monday and is expected to take about six months.

Waka Kotahi national infrastructure delivery manager Andy Thackwray said the new bridge would allow more water to pass under the road.

It would be built by installing two large concrete walls, excavating the channel between them and installing a concrete deck on the top and bottom.

"The reinstatement of the channel and installation of a bridge will re-establish more natural tidal flows and contribute to the ecological function, including fish passage, of the upper estuary and creek system," he said.

Work started on State Highway 10 east of Mangonui on Monday to replace a small culvert with a bridge in the hope of reversing years of environmental degradation. Photo / Myjanne Jensen

The first two months of the Papakawau Culvert Replacement Project would focus on relocating services and building a temporary diversion east of SH10 to take traffic while the bridge was built.

Temporary speed limits would be in place with contractors sometimes working early in the morning or late in the evening because of the tides.

Waka Kotahi would work closely with the Northland Regional Council to address sediment, which had accumulated around the causeway. That would include dredging and mangrove removal, Thackwray said.

Former Regional Economic Development Minister Shane Jones, who pushed for Covid-recovery funding for the project, was pleased to see work start at last — but sorry that many kuia and kaumatua who had fought to have the damage undone were no longer alive to see it.

They included the late Mere Lloyd who witnessed the changes in the harbour since the causeway was built. More recently Aputerewa Marae kaumātua Glen Larkin had fought the same battle.

Bob Cathcart, who headed the Northland Catchment Commission before it was replaced by the Northland Regional Council, said the causeway was constructed in the 1950s when SH10 replaced the old Kaeo-Mangonui road around the head of the harbour.

However, it removed the natural channel that used to take sediment out to sea. It also altered water flows and caused scouring of the highway and the bridge on the other side of Paewhenua Island.

Cathcart put a number of proposals to what was then the Ministry of Works to fix the problem, to no avail.

''It's good to see it's finally being done. It's tidying up something that wasn't done properly in the first place,'' he said.

Former Far North Mayor Wayne Brown, who lives at Mangonui, had also tried to get the problem fixed.

''If you block the channel on one side of the harbour, one side will silt up and the other side will erode. You don't have to be a genius to work that out.''