INUNDATED: Anzac Parade and the Whanganui River in a photo taken about seven hours after the flood peaked in June. Photo/Bevan Conley
INUNDATED: Anzac Parade and the Whanganui River in a photo taken about seven hours after the flood peaked in June. Photo/Bevan Conley
There is worse flooding to come.
Flood waters will again top the banks of the Whanganui River and the floodplain and anything on it will be inundated with water and silt.
Once that is accepted, experts say, Wanganui can and should begin to organise for the future.
A district-wide discussion needs to be had about what flood protection is necessary and a coordinated approach to any measures needs to be put in place.
Six years ago Horizons Regional Council began working on 200-year flood protection for Wanganui.
The first stage was completed in 2011, protecting the industrial area along Heads Rd with stopbanking, flood walls and temporary barriers.
In early 2012 the community persuaded Horizons to shelve plans for further flood protection closer to the CBD.
"The primary concern that came through in these responses was the perceived cost of the proposed flood protection plans," Horizons operations manager Allan Cook said.
Stage 2 flood protection in Putiki, Anzac Parade, Moutoa frontage and Taupo Quay did not go ahead and Horizons instead proposed to assess alternative flood management methods in six years.
Horizons operations manager Allan Cook oversaw the earth stopbank at the city end of the 3.2km Balgownie flood protection project in 2010. PHOTO/ SUPPLIED
However, Wanganui District Council held concerns about the level of protection along Anzac Parade and Kowhai Park and asked Horizons to assess existing stopbanks.
Flooding in October 2013 revealed the banks' weaknesses when sandbagging was required to prevent breaches and damage to the banks. So Horizons decided moderate upgrades were needed.
"That upgrade would, in a cost-effective manner, provide the community with a level of confidence around the performance of the system up to a known performance standard," Mr Cook said.
Just over $1 million was spent on upgrades designed to provide flood protection to approximately 57 houses in a 50-year flood. But it was never going to hold the river at its highest recorded level of 9m at the town bridge as happened last month.
Dr Ian Fuller is the associate professor in physical geography at Massey University in Palmerston North.
He has expertise in river dynamics and catchment responses to land use. He knows a lot and will soon know a lot more about the history of flooding in the region.
Dr Fuller co-directs the Innovative River Solutions Group, which has done ground work in researching flood histories in the Whanganui catchment. Once funding is received the research will study flood deposits to get an idea of flood levels pre-current records.
"What we're trying to understand is how the river has behaved in the past," he said.
The research will allow scientists and engineers to make clearer predictions about the frequency of flooding, in the past at least.
"What it means is, say a 100-year event, there is a 1 per cent probability of that magnitude event happening this year. You could conceivably get that sort of magnitude event happening more than once a year and in successive years," he said.
"People do automatically think, 'well I've had the 100-year-flood, it's not going to happen again, I'm safe' which is not the case."
Horizons is also gathering more data. A recent independent review showed flood modelling for the region was dated. In response, $1.48 million has been set aside for an upgrade of seismic and flood information. Starting this year, $489,000 will be spent digitally mapping 1000sq km of flood plain in the region, including in Wanganui.
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But what are the solutions?
With a million dollars spent on strengthening stopbanks along Anzac Parade only to be breached straight away, is the answer rebuilding them higher? Dr Fuller says no.
"Ultimately stop banks can be built as high as society wants them to be built," he said.
"They're certainly effective for the smaller scale floods and they'll keep suburbs dry."
However, at some point they will be breached.
"When they work it's fine, but when they don't work clearly there are problems because water that would've drained away naturally is now ponded," Dr Fuller said.
"Also because they basically push the water up when they do fail, when you do get overspill ... that basically enhances the energy and the power and potential for destruction at that point.
"If your property is at the point of breach the damage is likely to be far more severe than if the stop bank hadn't been there at all."
But what do you do? I mean basically the reason the homes are ruined is because the homes are built on a floodplain.
Removing stopbanks allows floodwater to spread more evenly and slowly over the floodplain. "More people would've got wet, but the damage wouldn't have been as severe," Dr Fuller said.
Dredging was another often mentioned flood mitigation measure but is expensive and environmentally dubious.
"The problem with that is there's a continuous supply of sediment coming down from the catchment," Dr Fuller said. "You've just got to keep dredging and that isn't the best thing for the environment, you're disturbing the bed, you're disturbing the habitat of fish ... that live in the river."
His view on stopbanks is shared by associate professor Asaad Shamseldin, from the Department of Civil and Environmental Engineering at Auckland University who says: "Whatever you decide, there is always something that will exceed it. This is where economics will come in."
There will be a political and philosophical, and as Dr Shamseldin said, an economic element to the flood protection debate.
"These extreme floods are not new," Dr Shamseldin said.
"They have happened in the past. I think what has changed is the media and also people's expectations. In the past it would happen and people would get on with it. People would take floods as part of nature. Now people don't want to accept it."
"We need as a society to change our mindset and to change our attitudes towards floods," he said. "I think we need to learn to live with floods (and) accept that it's going to happen.
Which is all very well for me to say because I haven't just been flooded and I'm conscious obviously that there's people whose homes have been ruined.
"But what do you do? I mean basically the reason the homes are ruined is because the homes are built on a floodplain."
People in Wanganui's flood-prone areas and others such as Whangaehu and Waitotara will have to adapt and learn to live with occasional flooding.
On such land, development could be restricted, land could be re-zoned, homes could be flood-proofed by building living areas high and in the worst spots assisted re-location.
"It would be cheaper, otherwise you're building stop banks higher and higher and higher.
"It's a different way of viewing floods," Dr Fuller said.
"Rather than shoving the river off by these stopbanks that we are very keen on using in New Zealand, actually recognising that it's just part of the natural landscape. We're going to have floods, we're going to have to learn to live with them."
The aftermath of such a big flood is the perfect time to discuss the future as a society and decide what protection we want and how much we are prepared to pay. "That's the challenge," Dr Fuller said.
"The engineers will do whatever you want them to do. If you want the stopbanks built higher, they'll build the stopbanks higher (but) in my opinion we need to allow controlled flooding. There are certain areas, obviously because of development, that will need to be protected and there are certain key essentials in urban areas that need protection. But I think there are other ways of trying to manage it ... taking stopbanks away where they're not so necessary and those in flood-prone areas, living with it and recognising the risk."
That means that floods could become bigger than they are at present ... and or they become more frequent.
Mr Cook said relocation was often raised. "While it is a credible option, it becomes very difficult to implement when you are dealing with people and their properties and it requires a coordinated approach," he said.
"Were you to start with a completely clean slate ideally you wouldn't have stopbanks at all, but when you have established communities living on the edge of a river the reality is that engineered solutions are necessary."
He said Horizons was considerate of flood risk in future planning and said there were consent measures in place for new development.
"Living alongside a river does bring a risk of flooding and there is a level of personal responsibility that must come into play," he said.
"Many residents have a strong desire to continue living in these areas and do not want to relocate from their homes. Horizons will continue to refine and provide hazard information to help people and authorities make informed decisions."
Horizons councillor Rod Pearce thinks council has it about right. He said the flood protection along industrial Heads Rd "came through with flying colours". He said he backed 50-year protection along Anzac Parade at the time and stands by it.
Mr Pearce would like to see it rebuilt to the same level. "It's cost us a bit and it's cost the people that live there (but) I actually think Wanganui came out of it extremely well for the strength of the flood."
But what happens if the risk changes? The indicators used to predict floods now may not hold much value in the coming years.
"We're in this age of warming climate ... clearly we've seen warming in the last 150 years and that is projected to continue," Dr Fuller said.
"Obviously the warmer the atmosphere becomes, the more moisture the atmosphere can hold and the more moisture the atmosphere can hold, the greater the intensity of rain storms.
"That means that floods could become bigger than they are at present ... and or they become more frequent."
Also, because the 20th century did not have dramatic changes in climate or relatively big floods, we have a false sense of what to expect.
"That's an issue because our estimation for flood magnitude and frequency ... are based on gauge records in a relatively quiet period of time.
