Omahu was among the 30 sites where stopbanks breached during Cyclone Gabrielle. Photo / Paul Taylor
Omahu was among the 30 sites where stopbanks breached during Cyclone Gabrielle. Photo / Paul Taylor
Hawke’s Bay Regional Council (HBRC) is adopting a one-in-500-year flood strategy for the repair and rebuild of the region’s stopbanks.
The stopbank network suffered 30 breaches during the peak of Cyclone Gabrielle flooding, with several stopbanks also “overtopped” by oncoming water.
An estimated one million cubic metres of fill will be required to bolster the stopbank network, which had been designed to withstand a one-in-100-year flood.
Quite what the new stopbank system will look like - or where the fill will come from - remains to be seen. For the time being, the main task is to identify and repair the estimated five kilometres of breaches to the existing 248km stopbank network.
“This is a major logistical exercise to shift these volumes of dirt, while maintaining quality control,” HBRC asset management group manager Chris Dolley said.
“We have a lot of work ahead of us, but the people of Hawke’s Bay should be confident we are getting on with it as quickly as we can.”
Dolley described the “unprecedented flooding event” as simply being more than the stopbank system could handle. That wasn’t helped by overtopping, which was partly caused by the build-up of material at structures such as bridges.
“The capacity of the scheme is to provide one-in-100-year level protection, or a 1 per cent chance [of breaches] in any one year,” said Dolley.
“Significant lengths of stopbanks were overtopped leading to erosion of the banks, which led to full breaches in a number of locations.”
‘Bunding’ is the temporary measure being employed at the moment by the more than 160 staff undertaking repairs. Put simply, bunding amounts to using mounds of gravel to plug the gaps.
That should see the stopbanks reinstated to pre-cyclone levels, before the real work of upgrading them for the future begins.
The Hawke's Bay Regional Council say upgrades to stopbanks near Taradale prevented "catastrophic" flooding in Napier. Photo / Paul Taylor
Dolley said the HBRC will lift the flood protection levels to one-in-500 on the Heretaunga Plains, with one-in-100 having been easily “exceeded.”
“The Taradale stopbank upgrade, completed late last year, was instrumental in protecting Napier from catastrophic flooding, so we know these upgrades are vital.”
The HBRC have received $19.2 million of funding for some of the work, but there’s no timeframe for when that might be completed.
In the meantime, the priority remains repairs and investigations.
“We have commenced a full analysis of the event and the flood protection network, including stopbanks, pumping stations, river-edge tree protection and more,” Dolley said.
