By Chris Dolley, Group Manager Asset Management, Hawke’s Bay Regional Council

Cyclone Gabrielle, which hit the North Island in February this year, has had a tremendous impact on people, communities and the economy in the Hawkes Bay region. Clean-up and repair of key infrastructure has been considerable, but it presents an opportunity to build in Flood resilience for the future.

Cyclone Gabrielle originated in the Coral Sea and tracked towards the north-east of the North Island. The most severe weather impacted Hawke’s Bay on Sunday 12 February and Monday 13 February, then reduced in intensity.

Prior to the cyclone, Hawke’s Bay had already recently experienced the wettest six-month period since records began, and rivers were expected to rise in response to rainfall.

Hawke’s Bay Civil Defence Emergency Management declared a State of Emergency on Tuesday 14 February 2023 at 4.31 am. A State of National Emergency was then declared by the National Emergency Management Agency at 8.43 am that same day.

Extensive rain and rapidly rising rivers led to extensive and widespread flooding across the region. Swift-flowing floodwaters in Hawke’s Bay sadly claimed human lives and resulted in many stock losses. Many other people were trapped on their rooftops or cars and needed to be rescued by helicopter or boat during the event.

Houses and buildings were flooded, and inundated with silt, and people’s possessions were lost or damaged, placing huge pressure on our communities. The loss of critical infrastructure (power, cellular coverage and roading) for many days following the cyclone caused widespread unrest for communities – even those not directly affected by floods.

Hawke’s Bay Regional Council assessed that this was the largest recorded rainfall event in the region. Actual rainfall during Gabrielle exceeded forecasts by as much as 250 millimetres. Of note was the Glengarry gauge, used to monitor the Esk Valley, which received 501 millimetres over 24 hours – equal to nearly six months of rainfall between 1 am and 7 am. Gabrielle delivered about 320 millimetres of rain to our Newstead site, in the western hills of the Ahuriri catchment near Puketapu, which is about one-third of the usual annual rainfall there – most of it falling within 24 hours.

The impact on people, communities and the region’s economy has been severe. The damage to the region’s flood protection scheme has also been considerable. The damage has been wideranging due to the scale and geographic scope of the event.

There were approximately six kilometres of breaches in our 248-kilometre stopbank network, and a further 28 kilometres of the network were damaged by erosion. There were a total of 30 breaches to our network. Stopbanks did not appear to fail through piping (erosion of the internal structure of the stopbank), but rather the design capacity of the river channel was exceeded and the stopbanks were overtopped.

The immediate focus has been on putting temporary protection in place around breaches, which includes bunding (gravel banks) and waterproof plastic wrap, moving to restoring the stopbanks to their original design standards. It is estimated that this work will take six to nine months.

Drains across the region were blocked with a combination of woody debris, leaf litter and silt. Drain clearing will need to be repeated as new rainfall continues to wash silt back into drains. Silt deposited up to three metres deep will have longer-term impacts on drainage and management of surface run-off during rainfall events that are still to be fully understood. Numerous pump stations along drainage systems were impacted during the event, and in the days and weeks following.

Another challenge has been the silt left after flooding. There is an estimated 4000 hectares of productive Hawke’s Bay land impacted by silt. Councils are waiting on the National Institute of Water and Atmospheric Research’s estimate of silt that has been deposited across the land – but assuming 300 millimetres of silt across the 4000 hectares, that would amount to roughly 12 million cubic metres.

Even if about half of it was removed, that’s five or six million cubic metres. Currently, there are around 800–900 daily truck movements carting silt, at around eight cubic metres per load. At that rate, it would take nearly 700 days to move five million cubic metres. This is a mammoth task.

With such widespread damage to our infrastructure, it has been important to get a clear picture of what has occurred and when. This has been difficult due to damage to some of our sensors. Council has a network of 53 river and 78 rainfall monitoring sites. Communication with more than 100 rain and river level monitoring sites was lost on Monday 13 February. A number of sites and instruments used for rainfall and river monitoring were damaged or lost. Restoration of these sites and building in redundancy for communications is required.

This level of destruction has meant that stopbank repair and silt disposal have been absolutely critical. It has also meant that we have had to organise our rebuild in ways that allow us to quickly respond to the aftermath.

Our stopbank repair was quickly stepped up. Within two weeks after the event, we had Rapid Repair Teams working, moving up the rivers repairing breaches and scouring. These teams include an engineer, as well as diggers and trucks, so decisions are made on site without needing to be referred back to head office. We now have 18 teams that have undertaken the equivalent of six months’ work in three months. After stopbank breach repairs, the focus will move to repairing scours.

We are also working differently to manage silt disposal across the region. Instead of each council trying to manage the disposal with multiple disposal sites and competing with each other for resources, the region’s councils have formed a Joint Silt Taskforce to manage disposal across territorial boundaries. It is likely that we will be dealing with this problem for many months to come.

Logistics and mounting costs are not the only challenges we have faced. There has also been considerable and consistent misinformation circulating on social media about the performance of the council’s flood protection assets, and the subsequent clean-up. There have also been high public expectations regarding how quickly the clean-up and rebuild will occur. This is largely due to the public’s lack of understanding around the role of recovery agencies and Territorial Local Authorities, and the time required to collect the evidence needed for sound decision-making.

We are also trying to be as upfront and transparent as possible with the scale of the problem. We have held several Facebook Live events discussing the rebuild, which have had more than 100 people viewing them live, and thousands of views afterwards.

Council has also established an internal review of our flood protection assets. This will be independently run by a panel of experts and there will be opportunities for public input. We anticipate that the review will take six months, and will provide Council with a list of recommendations.

The damage to Council’s infrastructure from Cyclone Gabrielle has been considerable. The resulting clean-up and repair of key infrastructure is a huge challenge from which it will take decades to recover. What it does give us is an opportunity to build in a level of resilience that will provide us with more confidence into the future.