Hawke’s Bay Regional Council will undertake a flood scheme review to understand how stop banks might perform to “overdesign events”, Asset Management Group Manager Chris Dolley said.
“It’s important to note that the stop banks did not burst their banks. In some locations, the enormous volume of water overtopped the stop bank, leading to erosion, which caused a breach,” he said.
The work will be peer reviewed, but the council has not yet found the appropriate person to undertake the review. Dolley said they intended to start as soon as practicable.
BayBuzz enquired about what modelling had previously been undertaken to assess both the volume of water that might come down in an event like Cyclone Gabrielle and whether that had included slash.
HBRC said it was already planning for future flood protection before the cyclone, and had begun work on the Heretaunga Plains scheme.
“We did undertake probable maximum flood analysis and this was the precursor to our decision to upgrade the stop banks from a 1-in-100 year level to a 1-in-500 year level.
“We also analysed possible breaches to understand the impact on the community, and this is how we prioritised upgrades in Taradale, Moteo and Ngatawara. Before the cyclone, we had completed a 1-in-500 year level upgrade at Taradale, and planned to upgrade Ngatarawa and Moteo.
“We had secured $19.2 million through Kānoa – Regional Economic Development and Investment Unit, for a program of upgrades across the region including for the Taradale upgrade now complete, and additional upgrades in Ngatarawa and Moteo. We are continuing to work with Government for additional funding beyond 2023.
“Our modelling didn’t include the high volume of wood debris nor did it allow for dams to form at bridges which potentially released waves of water and debris when bridges failed.”
The Tūtaekurī upgrade, which was completed last year, was instrumental in protecting much of Napier from catastrophic flooding.
The council was in dialogue with roading authorities to make sure that new bridge heights and flood control improvements were part of any new design, Dolly said.
Public interest journalism funded through New Zealand on Air
Operationally in a major storm event resources should be on bridges to remove large items of slash. Potentially the forestry industry should be charged the cost. Prevention of damage far cheaper than the consequences of damage.
Asset Management Group Manager Chris Dolley said
“It’s important to note that the stop banks did not burst their banks”. I think Chris needs to look at the before and after photos of the 70 metre section of stop bank that completely failed near Ravensdown. The failure appears to be caused by the slash being trapped in the L shaped section of stop Bank and & because there was a large discharge pipe going through the stop bank at this point, this appears to be a big weakness in the strength of the stop bank. I have a video of the failed stop bank if anyone is interested
Does the ‘Tutaekuri upgrade’ mentioned above refer to the ‘Taradale upgrade’ or was that a separate piece of work?
In a recent 1News ‘Sunday’ programme (In Harm’s Way – https://youtu.be/XCAE8W9P6Uk) Nick Rogers, an Auckland-based engineer and leading expert on natural hazard resilience and risk, stated that there is enormous risk to life from future floods in many areas of New Zealand. Hawke’s Bay is one of those areas. He pointed out the increased dangers from building back bigger.
Have a look at ‘Forest and Birds’ campaign ‘Making Room for Rivers’. Worth considering these ideas for the long-term view.
The way in which 1 in 500 is calculated needs to be upgraded in light of climate change.
Part of the way in which this risk number is calculated is based on historical records. With climate change leading to more frequent and more intense events using an assumption based on average flood levels in the past just isn’t going to cut it.
I have no idea what banks have to do with the restoration of STOPBANKS, other than possibly arranging finance for the project!
Why did the HBRC stop taking shingle out of the river beds?
The build up of shingle means there is less free board in the river to handle big rain events. The HBRC needs to get back to tasks they were created to manage.