Work to upgrade stopbanks along the Waipaoa River to protect the region from flooding continues, but might not offer a higher level of protection with the possible arrival of Cyclone Cody on Sunday.
Gisborne District Council senior project engineer Joss Ruifrok said while the upgraded sections of stopbank would provide a level of increased protection, it was difficult to quantify since the structure still needed to be completed in full to provide the required protection as per the design.
The MetService has confirmed the possibility of Cyclone Cody intensifying to Category 2 tropical cyclone with severe rainfall and strong winds expected.
A total of 64km of stopbanks on the Waipaoa River are being upgraded over 10 years to protect 10,000 hectares of fertile flood plain, with the upgraded design to cater for a 100-year rain event, accounting for climate change.
The ongoing upgrade is an expansion of the original stopbanks that were built from 1953 to 1967 following a severe flood in 1948.
Mr Ruifrok said engineering reports promoting an upgrade of the Waipaoa Flood Control Scheme had been regularly discussed since Cyclone Bola hit Tairawhiti in 1988.
The project was first included in the Gisborne District Council's budget in the 2015-25 Long Term Plan.
Although construction started in February 2019, there were three years of a significant package of work related to planning, design and consenting challenges to overcome before work could begin.
“The project has been progressing very well and is on track to be fully completed prior to 2031 providing construction works continue at current rates,” Mr Ruifrok said.
Over 16km of the Waipaoa Stopbanks have been successfully upgraded to date all on the eastern (city side), a total length of 25km of stopbank upgrades, expected to be completed by the end of May.
Mr Ruifrok said the current upgrade works were primarily bulk earthworks, which involve transporting massive volumes of silt material from the grass berm areas next to the Waipaoa River, then heavily compacting it to form the higher and wider stopbank structure.
The company contracted to do the work is Earthwork Solutions. Director Matt Mead said they were moving a volume of about 25,000 cubic metres per kilometre on average — and increasing the width and height of the structure.
With the top crest being widened to 4m and bottom base by 12m wide, the height of the triangular structure averages 5 metres.
“This will all make it beefier and wider to help protect homes and high-value horticulture on the other side of the stopbank,” he said.
Mr Ruifrok said the existing pipes that drain stormwater through the stopbank were also being fully replaced.
Mr Mead said by the end of the current contract about 20 stormwater pipes (culverts) will be installed within the stopbank to replace existing old pipes.
“The culverts ensure that any water runoff coming from the protected side of the stopbank can drain into the Waipaoa River. The newly installed culverts have flood gates installed that shut and stop water flowing back in a flood event,” he said.
In August last year the council was awarded $7.5 million from Kanoa (previously the Provincial Development Unit) to accelerate delivery of the project.
Mr Ruifrok said based on the “excellent” progress over the last three years, the total cost is expected to land under the $32-$35m estimate.
“However, remaining construction costs will continue to be heavily influenced by inflationary pressures and competitive market prices,” he said.
Mr Mead said the company was using bigger, better, and more efficient machines this year. Technology such as 3D for the semi-automated diggers, tractors and scrappers minimised overbuild and time surveying, keeping the costs down.
“Part of our contract is to use local people and resources and this helps spread the money locally which is a part of the Kanoa (PGF) funding that this job is set up for,” he said.
Mr Ruifrok said the cost of the upgrade, the ability of the community to pay (affordability), and to what level of protection the scheme should be upgraded (which is directly related to the cost of the scheme upgrade) have been debated over the last 30 years.
“It also focused on cost and what level of protection should be adopted. This, and an awareness of climate change helped to bring focus,” he said.