Sawfly work complete & almost paid for

Trees planted to replace willows devastated by willow sawfly are now well established and the river protection is almost back to where it was before the pest devastated the willows in 2005.

Hawke’s Bay Regional Council asset management team recently reported on progress of the strengthening of the flood protection schemes on the Tukituki, Ngaruroro and Tutaekuri Rivers in an audit report to the HBRC’s former Asset Management and Biosecurity Committee (now the Environmental Services Committee).

“In light of a potential disaster, Hawke’s Bay Regional Council took decisive action to secure river bank protection,” said Cr Kevin Rose chair of the Environmental Services Committee.

“We consulted with the community and received solid support for new mitigation measures.  HBRC staff were dedicated in their commitment to complete the work as quickly and efficiently as possible.”

Groyne structures made of specially designed concrete units were installed in the rivers, and replacement willows and native trees were planted by HBRC from 2006 to ensure that the stopbanks would be protected from flood damage.

“The groynes have performed well, and some have settled as they were designed to do. Extensive areas of new planting of both exotic and native trees and shrubs have done well in the good growing seasons,” said Mike Adye, Group Manager Asset Management.

“Willows have been replanted along the river edge as these are the only tree special able to withstand and thrive on everything the rivers throw at them.”

The $9 million cost of this work was funded by all ratepayers and people who directly benefited from the Heretaunga Plains Flood Control and Drainage Scheme.  This involved a 20% increase in the scheme targeted rates and $6.2m of loan funding.

HBRC has this week confirmed through its Long Term Plan that it will increase the level of protection provided by the Heretaunga Plains Flood Control Scheme to a 1-in-500 year level, with work commencing in 2016/17.  The rates income that had previously been used to pay for the sawfly remediation work will now be directed to pay for the new work to increase flood protection.

Willow sawfly [Nematus oligospilus] came into New Zealand in 1997 and progressively spread south from its entry point in Auckland. 

The Heretaunga Plains was first affected by willow sawfly in 1999 and severe defoliation of willows along the Tutaekuri, Ngaruroro, and the Tukituki rivers followed, three or more times each summer. Trees lost up to 90% of their root mass and died. 

This situation was potentially disastrous, putting the flood protection schemes in Hawke’s Bay at risk.

The lines of mature willows form a buffer zone between the fast flowing flood waters and the stopbank, and had been purposely planted and managed for decades.  The trees work in a flood by reducing the speed of the current along the banks, and therefore reducing the power of the water which lessens the risk of it eroding the constructed stopbanks.

Before any work began to fix the problem, HBRC investigated the situation thoroughly and looked at what other regions had done to combat the threat. 

As part of the investigation, HBRC staff constructed a 1 to 50 scale model in its Taradale depot.  Based on a section of the Tutaekuri River, the model was used as a tool to both demonstrate the risks posed by the lack of willows, as well as to experiment with various solutions.  Councillors, media and many members of the public got to see it in action.

The model enabled staff to investigate different configurations of groyne construction, rope and rail, deflection banks and plantings using water, sand and rock, to come up with the most effective solution.

By 2005 work had begun installing groynes and replanting along the river berms.

“We’re pleased that we’ve been able to get the flood protection scheme back to what it was.  And it’s good to see that ratepayers support the next step to increase that level of protection to cope with higher levels of floods,” said Cr Rose.

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