Hauraki River Management: Scheme office Levi Renata works his way along the Hikutaia River, felling old poplars to mitigate the risk of river obstructions in the future. Photo / Waikato Regional Council
Hauraki River Management: Scheme office Levi Renata works his way along the Hikutaia River, felling old poplars to mitigate the risk of river obstructions in the future. Photo / Waikato Regional Council
Slips, river erosion protection works ripped out, blocked rivers, river channels off-course, debris and silt build-up, river flood capacities compromised - this summer’s flooding has left behind a substantial trail of damage for Waikato Regional Council to remedy for its communities.
Across the region, the estimated cost due to damage from the flooding events is nearly $18.5 million.
All areas presenting an immediate high risk of flooding to communities and properties are being done as a priority before June 2023, with further remedial work to continue in the following two years.
One job of high priority is remediation works to the Ōhinemuri River in Paeroa, beneath the Criterion Bridge, currently under way. The river, the largest tributary of the Waihou River, is part of the Waihou Valley flood protection scheme.
Works supervisor Hayden McGregor, from the council’s Paeroa office, says major silt deposits that accumulated on the inside corner (south side) of the river under the bridge during the floods significantly reduced the volume of water that could pass under the bridge.
“As a result of the river having to make room for itself, it eroded a lot of the river berm in the front of the stopbank that protects the town. So, we’re creating some rock armouring to the riverbank to prevent any further erosion and loss of bank.”
Up to a metre of silt was also removed from the surface of the berm on the downstream, south side of the bridge to improve the future flood capacity of the river within the town’s two stopbanks.
The unseasonably wet summer caused extensive damage to multiple rivers and streams in the Coromandel Peninsula and Hauraki catchments and to a lesser extent in the lower Waikato/west coast region.
Cleaning up has been the immediate focus of the council’s river management teams, so removing trees from the rivers as they cause blockages that exacerbate damage, and realigning river channels.
Rock-armouring of the riverbank of the Ōhinemuri River beneath the Criterion Bridge in Paeroa will prevent further erosion and loss of the riverbank, as witnessed after Cyclone Gabrielle. Photo / Waikato Regional Council
Coromandel catchments lead James Ferrier-Kerr says the events of the summer have culminated in the worst, most widespread damage he has experienced in his seven years working in river management on the peninsula.
“A big part of the Coromandel story is the successive heavy rain events we’ve had in May, July, September, November, December, followed by [Cyclone] Hale, Auckland Anniversary weekend in January then Gabrielle for three days in February,” says James.
“When Gabrielle made landfall, we were already in a state of catchment saturation with obstructions and bank erosion sites known but unable to be remedied between events. Since Gabrielle, we continued to have localised heavy rain in March and April.”
James says the priority has been clearing obstructions to mitigate the flood and erosion risk before starting on erosion protection works. Logistics have been difficult on the peninsula with road closures.
“We estimate a further 12 months of remedial works. We have completed 34 out of 88 clearance jobs and landowners are still making contact. We’re spending days if not weeks on the same property. As an example, we cleared multiple obstructions over 3.6 kilometres from the Whenuakite River which took four weeks and there is a further 3.4 kilometres of the Whenuakite River that requires work.”
In the Hauraki area, the flood management team have also been making their way along rivers - clearing obstacles and realigning channels.
“Basically, we’ve been working from the bottom of a river up, doing what needs to be done,” says Hayden of the Komata, Hikutaia and Pūriri rivers, also tributaries of the Waihou River, and the Kauaeranga River, which is the third-largest river system within the Waihou Valley Scheme.
Along the Hikutaia River, many of the old poplars dropped their branches into the river so, as well as clearing the blockages, staff are coppicing the trees, with the timber being sold to help cover some costs.
“The extent of the damage to our rivers is quite incredible. On some river corners, significant amounts of land have been lost.
“The Komata River basically jumped its channel and crossed the road in places.
“We get rid of all the branches in the river and get the diggers in to reinstate channel stability as soon as possible. The last thing you want is a slip remaining in the river over an extended period of time.
“Pools do naturally occur in streams and rivers, so we want the digger driver to not create big, wide, shallow channels. Pools are habitat for fish, and they also provide refuge during high flows. Sometimes I might have to say, ‘I know you have done a nice tidy job, but can you mess it up for me, please’.”
