Motorway causes water worries
Christchurch residents are concerned a new motorway has affected the level of the water table in their area, causing their yards to be more susceptible to flooding.
But the NZ Transport Agency (NZTA) says the new road is designed to mitigate effects on the nearby land and environment.
Mark Hookham has lived in his Te One Place home in Redwood since 1980. He said the property had been getting more waterlogged than usual since fill for the Christchurch Northern Corridor motorway was built up over his back fence.
It was now like ‘‘walking on a sponge’’ when he crossed his yard after rain and damp spots were appearing in the cracks in the concrete floor of his shed.
He first noticed the issue in the middle of last year.
There had been water at his property before, but ‘‘we never had so much’’.
He thought the new highway being built behind his property had affected the water table.
‘‘You pressurise one area, well, water’s going to go somewhere.’’
The flooding ‘‘seems a bit strange’’ because workers installed a ‘‘great big ‘V’ gutter’’ on the other side of his fence, he said.
The new road was closer to his back fence than he expected – and higher.
From his yard, he looked up at the trucks working on the road.
Next door, Dave Coulter remembered trying to plant a rose in November, but the hole quickly filled up with water, despite there being no rain at the time.
Coulter said the water table was ‘‘definitely’’ a lot higher, but that could be caused by ‘‘a combination of quite a few things’’, including a wet winter and the earthquakes.
He thought the motorway was likely contributing to it.
Coulter said he knew about the motorway when he bought his house 20 years ago, but he ‘‘thought I’d be looking over the top of it’’.
‘‘Now our complete outlook is gone. Well you can’t put a value on that.’’
NZTA portfolio manager Colin Mackay said the motorway was being built to consent conditions designed to reduce construction effects on the area.
The high water table was considered in the design and ‘‘significant vertical drainage’’ was installed before embankment construction started, allowing subsurface water to discharge as they were built.
‘‘The wet conditions through the
2017 winter and the saturated soil conditions may have contributed to the surface ponding being experienced in the area.’’
Mackay said the motorway had been designated for ‘‘nearly 50 years’’, pre-dating the development of the nearby properties.
The project’s design included the creation of a parkland environment, which would soften the edges of the motorway, he said. More than 600,000 plants and native trees and more than 400 specimen trees would be planted along the motorway and adjacent shared walking and cycling path.
‘‘Residents in the Grimseys Rd north area between QEII Drive and Willowview Drive will mostly view the landscaped bund which is specifically designed to reduce noise.’’
University of Waikato geotechnical engineering lecturer Dr Ali Shokri said without knowing more about the site he could not say what caused the water issues, but it was possible it was linked to the highway.
The foundations could be working like an underwater dam, blocking groundwater from flowing like it used to, he said.
There was ‘‘not an easy answer’’ though.
An Environment Canterbury spokeswoman said they were not aware of any issues with the motorway affecting the water table.
It was ‘‘conceivable’’ the motorway could have an impact, but she could not say more without more detail.
The $240 million Christchurch Northern Corridor upgrade is a joint project between the NZTA and the Christchurch City Council, due to be finished in 2020.
When finished, a new four-lane motorway will run from just south of the Waimakariri River to Cranford St. QEII Drive will be four-laned between Main North Rd and Innes Rd.
NZTA estimates 42,000 vehicles a day will use the new motorway by 2026.
Last week, 18 months of roadworks started on Cranford St as part of the project, causing safety concerns for residents.