The Manawatu River drops back to new course

Erosion continues under the State Highway 3 bridge at Ashhurst despite attempts to shift the Manawatu River channel.
MURRAY WILSON/STUFF

Erosion continues under the State Highway 3 bridge at Ashhurst despite attempts to shift the Manawatu River channel.

As the Manawatu River dropped, a shingle bank designed to divert its course away from an erosion-prone domain near Palmerston North emerged largely intact.

But engineers will have to take a closer look this week to see if the bank at the Ashhurst Domain has been damaged by floodwaters. 

The main flow receded to the new central channel, leaving calmer waters near the domain land as a clue to the extent of flooding a few days ago.

An engineered shingle bank emerges from the Manawatu River as floodwaters recede.
MURRAY WILSON/STUFF

An engineered shingle bank emerges from the Manawatu River as floodwaters recede.

The bank was a $100,000 temporary work put in place by Horizons Regional Council and the Palmerston North City Council to prevent further erosion upstream of the State Highway 3 bridge.

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But this month's stormy weather pushed floodwaters, which peaked over 6.5 metres at the former Manawatu Teachers College monitoring site,  over the rim of the shingle bank and carved away more domain land.

An engineered shingle bank could not stop a flooded Manawatu River from eroding more land at the Ashhurst Domain.
MURRAY WILSON/STUFF

An engineered shingle bank could not stop a flooded Manawatu River from eroding more land at the Ashhurst Domain.

There was also more damage to the shared cycle and pedestrian pathway heading downstream under the bridge.

Horizons river management investigations and design manager Jon Bell said it was still likely the flooding would have caused some damage to the temporary works.

"Until the river recedes to a safe level we are unable to get a survey team in to assess the temporary work," he said.

"We are hoping this will occur later this week, allowing us to determine the extent of any damage and what remedial works can be undertaken."

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Only then could decisions be made about whether to carry out any remedial work.

Horizons has offered to pitch in up to $300,000 for a project the city council estimates will cost up to $1.5 million to create a permanent rock lining at the edge of the domain land to protect it from further erosion.

Most of that money, $250,000, would have to be raised through extending a loan for the City Reach project for Palmerston North's flood protection.

The city council had put aside $500,000, in the hope of negotiating a three-way deal including the NZ Transport Agency, which had an interest in protecting the bridge approaches from damage.

The work was needed after bush and tracks at the domain were progressively swept downstream over more than 18 months, with the worst damage occurring in April after the remnants of Cyclones Debbie and Cook caused serious flooding.

Sorting out a solution had been complicated because the domain falls between river management plans, and is owned by the city council.

 - Stuff

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