NZ's sinking city: Floods the new reality of life by the river video

DAVID WALKER/Stuff

As the water table rises along the Avon River, nearby suburbs will become more prone to flooding.

Is the Christchurch luxury of life by a river a relic of the past? CHARLIE MITCHELL investigates.

On a dry, spring afternoon, it's hard to picture the cars on this tree-lined street bobbing in a torrent, or inflatable paddle boats sailing door to door in the pounding rain.

Some of the houses still have scars – a high water mark etched on their fences, or stray weeds left when the floodwaters receded. Water settles on grass berms, even though it hasn't rained for days.

Adam Peat waiting out the floods in 2014.
IAIN MCGREGOR/STUFF

Adam Peat waiting out the floods in 2014.

Flooding is an ancient problem in Christchurch, but it has become more severe. It is the major consequence of life in a sinking city while the ocean rises.

For some in Christchurch, the fear of aftershocks has been replaced by the fear of rain.

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A new house on Riverlaw Tce is raised high due to flood risks.
STACY SQUIRES/STUFF

A new house on Riverlaw Tce is raised high due to flood risks.

 

There have been at least five major flood events around the Heathcote River since the earthquakes, three of which happened in 2014. There were six such events throughout the entire 1980s and 1990s.

The last was in July, when a dump of winter rain fuelled the usually docile river, which swelled over its banks and turned the surrounding roads into canals.

Surf lifesavers on Riverlaw Tce in July.
JOSEPH JOHNSON/STUFF

Surf lifesavers on Riverlaw Tce in July.

In historical terms, the storm was a lightweight. About 88 millimetres of rain was recorded, slightly more than half of that recorded during the major storm of 2014.

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But floodwaters rose above the floorboards in 17 houses and rescue boats patrolled the flooded roads, helping those who were trapped.

"The flooding came right up to the doorstep," recalls Sharon McDowell, who lives on Eastern Tce.

 

"If my son had his car parked down the road, it would have floated down the river. It was pretty bad."

For many living on the river terraces, flooding is a fact of life. A disaster relief worker came to McDowell's door, asking if she wanted to be rescued. She didn't –she already had a well-tested flood evacuation plan.

"All we have to do is go to the back of the property and over the fence," she says. "The neighbours don't mind us jumping the fence when it's flooding, we usually park one of the cars up there if we need to get out."

Elevation changes after the earthquakes.
TONKIN AND TAYLOR

Elevation changes after the earthquakes.

Even when the river doesn't burst its banks, flooding is a problem. It turns driveways into waterways.

The July rain was the worst Craig Henderson had seen – he could see boats travelling back and forth out his front window – but even regular rainfall was a problem.

"It comes down from the back there like a stream," he says, pointing to the area at his front door.

Houses along the lower Avon River, where the water level is regularly higher than the road.
DAVID WALKER/STUFF

Houses along the lower Avon River, where the water level is regularly higher than the road.

Somewhere between 10,000 and 15,000 houses have become more flood-prone since the earthquakes, most of them along the city's major rivers: the Avon, the Heathcote and the Styx.

In a couple of weeks, the Christchurch City Council will hold two public meetings for the residents stretched along the Heathcote, which has proved the most problematic. They will be told about options for dealing with the ongoing flooding problems.

Among them will be engineering solutions, such as stopbanks, flood basins and dredging. Another may involve raising houses. But it's understood buyouts will be on the table, too, potentially relating to around 30 houses.

A projected "one in 100 year flood" with 1m sea-level rise.
TONKIN AND TAYLOR

A projected "one in 100 year flood" with 1m sea-level rise.

"Until this report is complete it is premature to say which options will be recommended," said land drainage manager Keith Davison.

"We will be presenting all the possible options to the community … Decisions on these options and funding will then need to be made by the council."

RUFFLING THE BEDSHEET

Flooding by the Avon River in 2014.
JOHN KIRK-ANDERSON/STUFF

Flooding by the Avon River in 2014.

Christchurch's flooding problems began many millions of years ago, deep within the Earth on the Pacific-Australian plate boundary.

Tectonic forces thrust the land upwards to form the spine of the Southern Alps, and sediment pummeled by rain flowed to sea on melting glaciers and swollen rivers across what became the Canterbury Plains, leaving behind the fertile, alluvial soils near the coast that would serve as a blank canvas for English settlers looking to reconstruct their homeland in miniature, with its large cathedral and vibrant gardens and rivers that glide gently out to sea.

The first choice for the city's location was Port Cooper, now known as Lyttelton Harbour, but the necessary land reclamation proved too expensive. Instead, Christchurch was built on the flat expanse by the coast, an area once thick with forests that had been burned long before, leaving fragile and wet soils that resembled a vast, coastal swamp.

A Riverlaw Tce resident floats the essentials on a bodyboard during the 2014 floods.
JOSEPH JOHNSON/STUFF

A Riverlaw Tce resident floats the essentials on a bodyboard during the 2014 floods.

The city's geological history was largely forgotten, buried beneath concrete and asphalt, until the earthquakes. 

Liquefaction was the ultimate re-emergence of the swamp: soft, wet soils turned to mush beneath hardened urban surfaces, cracked on top but wobbly underneath, like a creme brulee.

The network of hidden pipes many of us take for granted were broken, making drainage a major problem – those repairing the pipes rediscovered the underground swamp forests beneath the broken, century-old pipes.

The recent flooding problems in Christchurch have attracted international interest, in part because it provides a glimpse of what many coastal cities can expect in the decades to come.

During the earthquakes, the city experienced many decades or even centuries of relative sea-level rise overnight, explains Dr Mark Quigley, an associate professor of earthquake science at the University of Melbourne.

It was largely because most of the city sunk, particularly in the riverside parts of the eastern suburbs. Land along the tidal stretches of the Avon and the Heathcote rivers sunk between 0.5 metres to 1m, according to research Quigley co-authored in 2015

Lateral spreading – the process which causes liquefied soils to stretch and expand towards waterways – damaged many buildings, but it also narrowed the river channels. When liquefaction on the riverbeds solidified, the rivers became more shallow.

On the Heathcote, there was an additional problem. Near its bottom end, around Woolston, the land rose about 30 centimetres; further up, around Opawa and St Martins, it dropped a similar amount.

Not only is it thinner and shallower, it's also flatter, meaning it can no longer efficiently handle floodwaters.

"Since the rivers got shallower and narrower, and the flood plains subsided, this amounted to a perfect recipe for increasing flood hazard," Quigley says.

"Liquefaction and subsidence throughout eastern Christchurch was a bit like ruffling up a bedsheet. There were some areas that were higher relative to their surrounds prior to the earthquake that are now lower."

While it has experienced the equivalent of many decades of sea-level rise already, actual sea-level rise is expected to continue, along with the other effects of climate change.

Modelling by NIWA determined that a "one in 100 year" rainfall event – today about 138mm of rain in a 24-hour period – could be as high as 193mm by 2090, depending on the level of warming.

With 1m of sea-level rise, what would today be a historic high water level will happen every tide, according to a 2015 report by the Parliamentary Commissioner for the Environment.

Whatever decision the council makes regarding the Heathcote River, it will be a short-term one – the question of its future will remain.

"Over the next 100 years, I suspect we will see progressively larger and thicker band-aids being applied to flood problems that increase in severity and frequency," Quigley says.

"Christchurch is well poised for an almost globally unprecedented landward migration [but it's] unlikely to happen for a variety of reasons, including social and economic factors. It is not so easy to abandon parts of a city.

"I'm hopeful that phased retreat will continue, but this is likely to occur after big stimuli cause major loss, like the earthquakes."

FEAR OF FLOODING

Along Riverlaw Tce, a meandering street that trails the Heathcote, new houses are going up, perched high above their neighbours.

After the earthquakes, thousands of houses were included in the city's already sizeable flood management zone. Some of the flood-vulnerable areas aren't even near rivers. In July, parts of Hoon Hay flooded severely, likely due to the sinking following the earthquakes.

A long time ago, houses along the Heathcote were built high up, but for a while they weren't – one of the many poor development decisions exposed by the earthquakes.

"There's a frustration out there – it's not even a frustration, it's a fear," city councillor Tim Scandrett said at a council meeting last month.

"[The decision] is going to be a number of things that are inter-connected that's not going to solve the problem, in a sense, but it's going to reduce the effects. We've got to be very honest and clear about that."

While the pressure is on to address the issue around the Heathcote, there is a backlog behind it.

There are 900 houses within 50cm of the spring high tide water mark, most of which are along the lower stretches of the Avon River.

Nearly 10,000 houses and 30,000 people throughout Christchurch are at risk of sea-level rise, along with assets that would cost more than $3 billion to replace. 

While the efforts around the flood-prone Flockton Basin have been successful – during the July floods, it was notable that few issues were reported there – it was a localised example of a city-wide problem.

"Yes, priorities have been applied because of the level of exposure that has been identified," Dalziel said last month.

"It's really hard to talk about priorities when peoples' floor levels have been breached, and in some areas it will only get worse, so that's why we have to consider all of the options."

With the summer months approaching, it is an ideal time to take stock of the options to address the Heathcote, before the next round of floods.

It will be a delicate balance, and may not address the long-term problem: whether the Christchurch luxury of life by the river is a relic of the past.

"There is no magic wand, no silver bullet. It's really important that we take people with us through it," Dalziel says.

"It is frustrating, it is challenging, and it is difficult."

 - Stuff

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