Stopbanks are vulnerable to erosion from high velocity river water. Live river edge protection in the form of willow trees is the first line of defence to prevent this scouring – the branches and foliage slow the flood water near stopbanks, while the roots hold the soil together. A new pest called willow sawfly has defoliated and killed willows along stretches of the rivers.
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Willow sawfly (Nematus oligospilus) was first discovered in New Zealand in 1997 and was first found in Hawke’s Bay in April 2000, when the species significantly damaged willows which formed part of river edge protection schemes. Hawke's Bay Regional Council (Council) has monitored the effect of the willow sawfly on willow trees and began trialing alternative river edge protection species in the period 2000 - 2004.
In 2004 the Council commenced a programme of sawfly remediation works for the Heretaunga Plains flood control scheme and, in particular, flood control measures between the river edge and stopbanks on various parts of the Ngaruroro, Tutaekuri and Tukituki rivers that cross the Heretaunga Plains.
These works were necessary because of the significant damage caused by the willow sawfly to the live edge protection that was predominantly comprised of various species of willows which help control the active channel of all three rivers and help to safely convey a 1% Annual Exceedence Probability (AEP) flood safely to the sea. The damage caused by sawfly to the live edge protection substantially increased the risk of flooding to significant parts of the Heretaunga Plains because of the risk from lateral erosion of the river and the risk to the stopbank system from breaches which could result.
Remediation works included structural works comprised of wire rope and steel rail permeable groynes, hard engineering works in areas were extreme risk of stopbank failure from lateral erosion from the river, and extensive planting of a variety of tree and shrub species to help bind both the river edge and berm areas to reinforce the berms and protect them from lateral erosion. This work was carried out between 2004/05–2008/2009 and cost approximately $9M.
Nevertheless, while this programme of works has sought to repair the damaged caused by willow sawfly, the level of service provided by the Heretaunga Plains Flood Control Scheme for the Ngaruroro, Tutaekuri and Tukituki rivers prior to 2000 has still not been fully reinstated, and won’t be until new edge protection works are fully established i.e. 5-8 years from time of planting.
A cautious estimate of the amount of damage that could be created by stopbank breaches in key parts of the Ngaruroro, Tutaekuri and Tukituki river systems is $426M. This excludes any allowance for damage to publicly or privately owned infrastructure as well as the social and emotional damage that would be caused from such flooding. The Council maintains a scheme disaster fund to recognise this increased risk.