Costly remedies for Ashhurst Domain
The cost of protecting the Ashhurst Domain from further erosion by the Manawatu River has been estimated at as much as $800,000, and possibly rising.
Hundreds of metres of the lower domain were swept away in flooding caused by the remnants of Cyclone Debbie in the first week of April.
It compounded damage that had occurred in two previous winters, to the disgust of local man Gary Tanner, who said the delays in acting to protect the banks were ‘‘incompetence’’.
On Wednesday, Palmerston North City council City Networks general manager Ray Swadel will outline the proposed solutions and costs to councillors.
The options proposed by Horizons Regional Council range from $260,000 to $800,000.
Horizons is only prepared to contribute up to $80,000 of the costs, as the area is outside its flood-management scheme boundaries.
Swadel said there were still questions about why the 700-metre section of the Manawatu River, from where it joined the Pohangina River to the State Highway bridge, were outside the scheme.
And there were also issues about river management decisions that had allowed gravel deposits to build up in the middle of the river, forcing the main flow toward the domain.
But he said those issues would have to be set aside or worked around in order to get a result.
The cheaper of the two options is a series of willow-tree structures anchored with railway irons and wire-rope ties, to prevent erosion.
Horizons advice was that the structures, known as groynes, would need a high level of monitoring and maintenance, and carried some risks of failure.
The more expensive option was rock lining of the exposed bank.
Swadel said continued erosion since the price of the works was estimated meant that the size and cost of the works could have increased.
Given the limited budget available from Horizons, the costs to city council ratepayers could range from $182,000 to $720,000, or more.
The other potential source of money could be the New Zealand Transport Agency.
Late last year, the agency had considered advice that there was no immediate risk to the State Highway bridge approaches that would warrant it contributing to the cost of bank protection.
However, the further erosion extending much closer to the western bridge abutment this month has prompted the agency to reconsider its decision.
Swadel said the erosion was continuing even during modest river flows and there was clearly a need to do some bank protection work as soon as possible.
He said solutions would be discussed at a three-way meeting between the two councils and the agency. ‘‘The local community is expecting action regardless of which authority has jurisdiction over this matter.’’
But Swadel said whatever solution was agreed, nothing could be done before September at the earliest. ‘‘The opportunity to work on the site has now been lost until the lower river flows in spring.
‘‘Horizons do not believe there are any interim works that can be undertaken to arrest the current erosion.’’
The works would take four to six weeks to complete.
There is a lot of significant damage to land around the Manawatu river near Ashhurst Domain after heavy rain.