As flooding worsens, new ideas needed
Damaging floods like those in Dunedin are expected to become more common and New Zealanders are being urged to find innovative ways to deal with them.
It is even being suggested that in some cases it might be better to move people out of areas, rather than spend large sums to improve infrastructure.
Climate change, as well as the growth and densification of some cities, were causes of the rise in surface flooding, Waikato University environmental planning professor Iain White said.
The good news for New Zealand was that with its low population density it was in a better position than many countries to cope with surface flooding, although much more needed to be known about the problem.
Rainfall distribution patterns were changing, much in the way predicted by scientists as a result of climate change.
That meant more rain falling in intense bursts, and events like that in Dunedin on Wednesday could be expected to become more common, White said.
Dunedin City Council said the flood was a more than one in 100-year event, with about 175mm of rain falling in the 24 hours to 4am Thursday.
In Lower Hutt, the 48mm of rain that fell in an hour in May was greater than the one in 100-year rainfall event figure. Flooding in Christchurch in March last year was similarly described as a one in 100-year event. Christchurch also had some surface flooding on Thursday.
White said huge problems with surface water flooding had caused billions of dollars in damage in Europe.
"The lesson on the horizon for New Zealand ... we should expect to get more of these and we should start to think differently about water."
In some parts of Europe houses were required to store most of their runoff onsite. That could involve keeping some water to use for such things as flushing toilets, as well as having grassed areas where water could soak into the ground, White said.
Instead of having drains to take water away from communities as quickly as possible, verges could be built with dips in them where water could go, or water could be directed to playing fields.
"You are redesigning parts of cities to slow water down to give drains a chance to cope.
"At the moment we try to push water as quickly as possible from across a catchment into confined infrastructure," White said.
Auckland was perfectly placed to become a more water-sensitive city Its comparatively low density had probably saved it from some surface flooding already, although low density living did lead to other problems.
"There's an awful lot of green space around roads and houses that you just don't have in cities like London. There are lots of opportunities to have solutions in the city which can change things," he said.
"We have a lot of opportunity because historically we've had this low density model, so we have a lot of scope to store water in urban areas ... It means we can be a bit smarter. It's not just a matter of infrastructure and bigger pipes."
Putting in bigger pipes was known to work but it was the most expensive approach and the cost was high.
New developments could even be built that did not need drainage infrastructure. The drawback was that some space would be needed for water which would mean less of the site could be developed.
Green spaces could be landscaped in many innovative ways to slow down or reduce the flow of water to pipes. As a general principle that approach would be cheaper than installing bigger pipes, White said.
In some already built-up areas prone to surface flooding there could be a point where questions arose about whether it was worthwhile spending more money on infrastructure. Some flood prone places had been built on swamps which were the places water tended to flow to.
A better approach than spending on infrastructure in some cases might be to work out some way to compensate property owners so they could move from such areas.
"That could cost a very small amount of what new infrastructure would," White said.
Deciding which areas those might be, and working out ways to do it that were acceptable to communities, was the job of politicians.
Insurers were also factors in driving change, as insurance costs rose in areas increasingly prone to flooding.
Dunedin Mayor Dave Cull was another to discuss the possibility of depopulating some areas, rather than spending money on infrastructure.
South Dunedin - hardest hit by Wednesday's flooding - was probably one of the lowest built-up areas in the country, Cull said. It used to be a swamp, had the oldest infrastructure in the city and was next to the coast.
"So the groundwater levels already pose a problem in some instances, even when we don't have heavy rain."
The council had been looking at the surface flooding issue, which would be a big challenge in coming decades as climate change and sea level rise kicked in, Cull said.
"We're talking tens of millions of dollars to replace that system, in a situation where there may be some areas, with sea level rise, that we end up retreating from and not putting any more infrastructure in, and actually taking the buildings out of it."
Much more analysis needed to be done before it was known which areas those were.