Solving the city's flood problems

JOHN MCCRONE
Last updated 08:57 16/03/2014

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Take a pancake-flat city which was once a swamp. Shake it with an earthquake that tilts its main drainage outlet to the sea. Add a projected sea-level rise of one metre over the next century plus 16 per cent more heavy rain.

You can see why Christchurch City Council Mayor Lianne Dalziel wants a serious discussion - why there is a rushed public forum in Sydenham today to talk about the city's changed natural hazard risks and the response options being contemplated in a District Plan review.

Floods rather than earthquakes were always the big risk Christchurch had in mind. The Waimakariri River busting its banks and re-routing through to the Avon was going to be how the city hit the international headlines - "Hundreds drowned, whole suburbs inundated".

And now, post-quake Christchurch has its shifted geography, a new pattern of dips and hollows changing the water flows, with some places dropping half a metre closer to sea level. It is time to figure out what this means for the city. What are the priorities, and what can be afforded? Do the authorities protect or retreat? And does Christchurch even have the right general flood management philosophy? Once the waterways were ruled by a drainage board. The approach was "Ministry of Works" functional.

But in recent years there has been a return to nature. Ecological and landscape values have taken over. And some say last week the city paid for getting rather fluffy about its major natural hazard.

So, plenty of issues to chew over. Surprisingly, big-picture consideration of the problem has only just begun, three years after the earthquakes.

Stepping out of a Port Hills rockfall technical briefing in the Mayor's lounge - life has become one hazards meeting after another - Dalziel defends the time taken to bring flooding into focus. The need to fix simply overtook the chance to analyse, she says.

Just think back to the first September 2010 earthquake, she says. After that, the Government was going to swoop in and project manage fixing the land.

There was a Tonkin and Taylor report and it all seemed do-able. Avon River suburbs like Bexley, where she used to live, were going to have stopbanks and perimeter treatments - piles driven into the ground to prevent lateral spreading.

"Remember the pink maps? There were going to be large-scale earthworks around our river and wetland boundaries that were going to provide strength.

"The activities were going to be co-ordinated, so if you were rebuilding houses, it would be basically all done together, compacting the land and pouring all the foundations at the same time, like making a new subdivision."

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So the first earthquake recovery was viewed as a giant geo-engineering project. A speedy analysis followed by a speedy fix. But then came February 2011, and even the Government was overwhelmed. The need for instant action pushed the big-picture questions into the background.

Take the red-zoning of suburbs for instance, says Dalziel. The problem there was that Earthquake Commission (EQC) insurance had been set up to cover individual properties, when in an earthquake with liquefaction like Christchurch's, land damage was, of course, area-wide.

The Government's red-zone payout offer was a quick and dirty way of collectivising EQC insurance, sidestepping legalities and enabling people to move off their broken and swampy land in the shortest time possible.

It also happened to deal with much of city's changed flood risk by removing some 6000 houses closest to the Avon and the Estuary.

But as people living in other newly flood-prone areas like St Albans' Flockton basin have discovered, this still left the problem of who is in charge, who pays, when a home-owner's problems might be due to the Dudley Creek caving inwards and the surrounding land tilting.

Or where pockets of green-zoned housing like South New Brighton have been left exposed to the threat of sea-level rise. Or like Woolston, which has become riddled with re-awoken springs.

Dalziel says an insurance-based recovery was geared to deal with the immediate damage to individual homes, and not the long-term knock-on impacts in terms of earthquake-related changes in flood risk. But now there needs to be some urgent catch-up she says.

Some way of formally collectivising EQC payouts to pay for community-wide flood protection must be found to avoid money being frittered away on individual cash settlements that do not solve the problem. The council is waiting for the EQC to come to the party.

And then there has to be a global strategy for the city that includes things like sound building controls and land-use maps, as well as physical works like stopbanks, estuary dredging and stormwater retention schemes.

So this is what's happening with the District Plan review , says Dalziel - and why she has pulled out the natural hazards chapter of the plan for special "pre-draft" public discussion over the next few weeks.

The council, Government, EQC and other agencies need to jointly deal with the city's problems in a coherent fashion. And the community has to be engaged, as the decisions will affect the very look and feel of the city.

For example, is Christchurch going to end up centred somewhere around Hornby by the end of the century? Is East Christchurch going to turn gradually into a wild bayou with homes on stilts? The future of a flat city by a rising sea does need thinking about.

It is tough to digest, as it breaks into two quite separate sources of risk.

There is the long-term problem of climate-change-driven sea-level rise. And Christchurch, of course, already has an adaptation policy for that in its creation of a tidal surge Flood Management Area (FMA).

Vulnerable coastal and river suburbs have been mapped, and building regulations brought in, so that any new buildings have floor levels to cope with a projected half-metre sea-level change.

In the natural hazards chapter being revealed today, it is being noted the FMA would now have to take in nearly a third more of Christchurch to allow for land that has sunken closer to sea level following the earthquakes.

There is also the thorny question of whether the floor level allowance ought to be doubled to a whole metre to match the latest international projections of sea level rise over the next century.

If that happens, still more of Christchurch would end up under FMA regulations. So soon it could indeed be a city getting used to homes built on earth mounds or up on stilts.

Then there are the flood issues probably closest to mind, with Christchurch having just experienced a "one-in-100-year"

rainstorm, and possibly another one on the way this Sunday.

Does the repeated flooding of the Flockton basin suggest the earthquakes have stuffed the city's natural drainage pattern? Or even exposed the very way the council deals with stormwater?

It is not hard to find critics here. Civil engineer Warren Lewis, who as chair of Cashmere Rural Landowners, has faced off against the council about its drainage policies many times over the years, says the Barrington St bridge across the Heathcote River tells a story for him.

Look at the high and wide span designed by transport engineers, he says. Then check out the narrow, vegetation choked river-way designed by the council's landscape architects.

Trees and native planting right up to water. Certainly ecological and pretty, but evidence to him that Christchurch had become muddled in its approach to flood management long before the earthquakes threw a spotlight on the matter. Therefore, his concern is whether the council even has the right mindset to fix things.

Lewis says it is easy to forget Christchurch's basic vulnerability as New Zealand's flattest city. "We're on a slope of 1 in 1000. Once something floods here, it's going to spread a heck of a long way."

So the city depends on an extensive network of good open channels to drain excess rainwater out to sea. And early in its history, Christchurch created a drainage board which took a no-nonsense approach.

"It was run by engineers, so they made sure the rivers were wide enough, the pipes big enough, to deal with the situation. When the Flockton basin started having flooding 30 years ago, they didn't muck about. They got stuck in and put in pumps and diversions to solve the problem."

Lewis says Environment Canterbury (ECan), which is responsible for the region's wider flood policy, continues to take a "belt and braces" view, at least to a threat as grave as the Waimakariri River.

If the Waimakariri were to burst its banks in a spring flash flood, as has happened in the past, it would wash right through either Kaiapoi or central Christchurch, causing up to $8 billion damage and serious loss of life.

But the Waimakariri is fenced in by a very functional looking stopbank system. And ECan has just spent another $130 million to expand that scheme with a secondary row of banks to cope with a one-in-10,000-year event.

However, Lewis says since Christchurch City Council took over control of the city's waterways when the Drainage Board was disbanded in 1989, there has been a change in philosophy. Official policy is now to manage stormwater based on the six values of culture, heritage, recreation, landscape, ecology and drainage.

Council documents explain for instance that Maori believe stormwater should be treated before being discharged into streams as "the mixing of water and waste pollutes mauri" - the life force. And people generally want natural-looking waterways, full of wildlife and attractive to walkers.

Lewis says what this means is that there has been a shift from simply digging ditches deep enough to move water fast, to creating intricate networks of rain gardens, swales, retention ponds and flood basins which are designed to hold excess water in place and let it seep away slowly.

But Lewis likens it to deliberately creating road congestion.

"You don't have to make many more lanes, and you could probably cater for your peak traffic without having jams. It's the same with rivers. You wouldn't have to widen them much, and you could take the peak flow and wouldn't need all the ponds upstream.

"Yet the problem is you have the environmentalists screaming any time they see a digger near the Avon or Heathcote rivers."

Lewis argues Christchurch has been taking chances that as a flat city, a former swamp, it cannot afford. And this has been compounded by the rise of a managerial culture where engineering experience is short and maintenance work is contracted out.

Lewis says he jumped in his car at the height of last week's rains to see just how the various ponding systems were coping.

At a Bryndwr subdivision, where he had been involved in the design, he found the valve of the retention basin had been left open. It wasn't holding water back upstream as it was meant to.

He found other, newer, ponds in Wigram only a quarter full. "So the modelling must have been wrong there."

Lewis says what he saw did not inspire confidence in the council's approach to stormwater management, because it demanded too much care to insure it actually worked.

But others defend the council.

Canterbury University hydrological engineer Dr Brian Caruso says some think the council's stormwater retention policy is merely a cheapskate measure to avoid spending on better drains.

However, there is a matching cost in giving up developable land to swales and ponds within the city limits. So it is more about the modern view that working with nature is, in fact, the smarter option.

Caruso says, with the Estuary having lifted at the Heathcote end, and with sea level rise on its way, simply digging deeper channels to the sea may not help muchy. "All the dredging in the world might not solve the problem."

Soaking water away locally becomes the best policy.

Caruso says psychologically, when it comes to flooding, the first instinct of people is to fight to defend what they have - to expect the engineers to come to the rescue with pumps, drains and stopbanks.

But if there are forces at play that will overwhelm such defences eventually, then the decisions have to become about how to best work with nature. And this is why Christchurch needs to be thinking broadly about its waterlogged future, when even groundwater will come closer to the surface.

He, too, can see a future in which the city and its suburbs inch steadily westward over the coming century, while the east has to adapt to an ever-wetter footprint.

It is all a matter of time scales.

Dalziel says it is clear the post-February 2011 recovery effort had to be launched ahead of any proper long-term analysis, and so now there is a need to update the city's strategic planning framework in a way that reflects all its natural hazards - its flood risks, its Port Hills landslip risks and its future earthquake risks.

Yet, while climate change is going to be a big problem, it is a threat to housing that will unfold over decades. So it is about developing sensible policies to match that timeframe.

And council drainage manager Mike Gillooly - although he fears he will be shot for saying so - says at least last week's extreme rainfall shows the earthquake damage to the city's stormwater system is not as drastic as some may fear.

Much depends on whether the storm was in fact a one-in-100-year event.

Gillooly says the figure is being disputed because the weight of the rain varied across the city. Some rain gauges saw it as a one in 20, but the Botanic Gardens recorded a one-in-140, and Bowenvale Valley a once in several hundred years' downpour.

He expects the original estimate will be proved right when the final calculations are done. So he can say the network was stress-tested and it mostly performed.

Gillooly says the Flockton basin was hit hard, as everyone knows, with water getting into at least 60 homes. The earthquakes have changed the local geography there, and the council is fast-tracking a solution - a no-nonsense option like a new pump station and bypass, or a general area-wide drain upgrade.

New Brighton and the lower Heathcote also saw quake-related flooding. Gillooly says the lifting of the Estuary has reduced the Heathcote River's flow, so stopbanking may need to be considered through the affected suburbs.

Then there were some surprises. Gillooly admits the council was caught out by the flooding of five houses in Redwood, another few homes down Colombo St towards Beckenham, and some spots in Fendalton.

He says these are most likely to be "network issues" - blocked culverts or other maintenance failures - although landscape changes might have played a part.

But Gillooly says the council has been busy ever since the earthquakes, sucking silt out of drains and checking pipes. The primary stormwater network looks intact on the whole.

And the secondary network - the ponds and streams which are relied on to deal with the excess - is naturally resilient to earthquakes anyway. So again, Christchurch has problems but not a crisis.

Gillooly agrees it would have been nice if the big picture discussion of how the earthquake repairs fit into a general land use strategy for Christchurch had started sooner. Flood risk is a major part of the city's equation.

But - the latest rainstorm permitting - it is going to be cards on the table time for the council at its public forum today.

- Canterbury

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