Wellington region's stopbanks and floodgates in poor condition

Sunday, 5 November 2017

NIWA's supercomputer has helped drive some of our most important climate forecasting, including two years processing data to create ultra-long range models that predict a blazing, parched Wairarapa and Sydney-style heat for the capital by 2090.

Nearly 200 of the Wellington region's stopbanks and floodgates are in a poor or very poor condition, with some at imminent threat of collapse.

The findings, presented in a report to the Greater Wellington Regional Council (GWRC) Environment Committee on Wednesday, showed the poor-condition defences represented more than a fifth of the region's stopbanks and 16 per cent of floodgates.

The findings include seven stopbanks and four floodgates at imminent threat of collapse, but a council spokesman said no critical assets or private property was at risk.

The four floodgates deemed to be in very poor condition were given the rating as not to assume that the condition was any higher until further inspections are done.

Meanwhile a barrage gate which contributed to the protection of 40,000 hectares of Wairarapa farmland from flooding was also rated as being in poor condition, defined as having significant wear or deterioration and in need of repair.

**READ MORE:

Cars navigate the flooding on Grays Rd in Pauatahanui after two days of heavy rain which hit the Wellington region in March.

* Frequent floods during spring-summer damage defences

* Heavy rain causes surface flooding across Wellington region

Lyn Williams wades through her flooded driveway on Grays Rd, Pauatahanui in March.

* Wellington's deluge described as one-in-20-year event**

GWRC chairman Chris Laidlaw said the region's flood defences were not prepared to withstand the pressures of climate change.

'It never will be, probably, given the volume, range and numbers of stopbanks that need attention and are going to need attention and the availability of funds.

'It's a hard truth that a lot more money needs to be invested in protecting property, and protecting lives if it comes to that. We all know that, but it comes down to affordability.'

Laidlaw said anything presenting a danger to communities would be given priority, but that didn't mean there weren't communities vulnerable right now.

'There is a rudimentary stopbank system protecting Greytown and parts of Carterton, and we've been in dialogue with all the interested parties, including the councils for years, about what is best and most effective form of protection – but in the end it comes down to affordability.

'Councils are crying out to government for some form of infrastructural funding arrangment.'

Council flood protection manager Graeme Campbell said Greytown had been identified as having a 'very real flood risk' and it needed additional stopbanks to bring protections up to a modern-day acceptable standard.

'There's a long tale of smaller stop banks all around the region that need work done to them, and the work will be done in accordance with the ability to pay.'

The George Blundell Barrage Gates were used to manage water flows at lake Onoke during major flooding, and while they were unlikely to fail due to heavy rain, some concrete work was required in order to protect against earthquakes, he said.

'Once they are damaged we can't operate them. It could be 12 months or two years before they get fixed.

'It would be the productive sector that would be most effected – it's not about lives at risk in that situation.'

Flood protection team leader Colin Munn said at Wednesday's meeting the gate's electronic component had been upgraded, but structural work was required.

Ascertaining floodgate conditions in the rest of the region had been difficult, with many underwater, and some rated poorly as a stop-measure until in-depth research could be undertaken, he said.

Floodgates found to be in poor condition were located in the Lower Wairarapa Valley.

The Wellington region has come under pressure during 2017, with Metservice figures showing the region received 1283mm of rain by October's end – already above the average yearly total of 1215mm.

National Institute of Water and Atmospheric Research (Niwa) hydrologist Daniel Collins worked on the Climate Change Report for Wellington Region released in August.

He said climate projections showed rainfall would become more intense across the Wellington region over the course of the century.

'This would put additional strain on flood mitigation infrastructure and on the communities it is designed to protect, with stopbanks more readily overtopped or damaged, and homes, roads, and businesses at greater risk of inundation.'

The value of the region's flood protection assets were estimated to total $386 million, up $78m since 2012.

Munn put the increase down to some upgrades, as well as significant increases in amenities.

'Rock has become extremely expensive and earthworks, contractors and plants have increased too.'

Munn said the council were currently investigating which assets were in the worst condition, and developing a programme of works to improve them.

STOPBANKS RATED POORLY WERE LOCATED AT:

- Waikanae River at Jim Cooke Park

- Along the Lower Wairarapa Valley Development Scheme area

- At the Otaki River downstream from SH1

- Along the Waiohine River

* All identified stopbanks were included in development schemes, or had money allocated in council budgets for improvements.