Plan now for future floods, urge scientists
The Hutt River has a long history of flooding, and a new study says it will only get worse with climate change during the next century.
Apart from the threat to the valley's 130,000 residents, $6 billion in assets would be at risk - the cost of a breach in one of Lower Hutt's stopbanks has been put at as much as $1.7b.
Focusing on the Hutt River, the Victoria University-based New Zealand Climate Change Research Institute has looked at the rift developing as the need for constantly evolving responses to climate change collides with local authorities' desire for fixed planning systems.
"Our research shows that local government agencies, which are responsible for much of the decision-making dealing with climate-related impacts, are increasingly aware of the need to take action," research team leader Professor Martin Manning says.
"However, it has also identified a number of barriers that create inertia and resistance in the communities they serve."
The team produced reports on future flood scenarios in the Hutt Valley, and studies on a sea-level rise in Auckland, water management in Wellington, and the implications climate change has for local government decision-making.
Although stopbanks and other flood-protection systems shield crucial infrastructure such as Hutt Hospital, Prof Manning says the "bottom line" is that these guardians of life and property will have to be raised again and again.
More frequent flooding in the Hutt Valley will impair the flood-protection standards of existing and planned stopbanks, and produce continuing uncertainty.
Although not dramatic, increases in extreme weather are a definite trend. The prolonged closure of the Manawatu Gorge because of slips from heavy rain last year is a prime example, he says.
"We're not trying to be alarmist. We're alerting people to the fact that they have to start the process of risk management and address the long lead time for infrastructure change."
Flood-risk management in New Zealand is primarily the domain of local government.
This creates unique issues in the context of climate change because handling that escalating risk will require ever greater localised technical and financial clout and challenge the often short-term planning horizons of local authorities.
The Hutt Valley study included a survey of residents, which highlighted some of the issues faced by local authorities and property owners.
"Homeowners who have experienced flooding first-hand tend to be better prepared," Prof Manning says. "But if they sell their house and move on a few years down the track, the new owners don't necessarily have an appreciation of the risk and often don't take precautionary measures.
"It shows that while you can build resilience to risks, that resilience can also collapse over time. There is a similarity between the responses to extreme weather events and to major earthquakes, which can start with action and then fade away by not addressing the ongoing risk."
General understanding of flood risk levels was low in the Hutt Valley and few people had seen flood-hazard maps, the survey found.
The team concluded that effective flood-risk communication was vital, but found that hazard scenarios going ahead 50 or 100 years, often used by scientists to illustrate the potential impacts of climate change, were not the most effective way of getting people's attention.
To combat the communication breakdown, the team has developed a more close-to-home flood-risk evaluation.
Instead of mapping out scenarios, the new method brings to mind wet feet or worse by showing the likelihood of above-floor-level inundation in the Hutt Valley floodplain under different flood strengths.
Greater Wellington regional council developed a 40-year blueprint for flood protection in the Hutt Valley in 2001 - the Hutt River Floodplain Management Plan. The plan is open to review every 10 years or earlier if flood hazards change.
MAJOR HUTT VALLEY FLOODS
1855 - The river rises higher than ever seen before, destroying the third Hutt bridge.
1858 - Nine people die in Taita after a massive flood bursts the river's banks.
1878 - Two big floods sweep the valley, inundating the entire floodplain.
1893 - A large flood swamps some Petone properties to more than a metre.
1898 - The largest recorded flood covers the valley floor, rising 90cm in 30 minutes. A second flood prompts building of first major stopbanks.
1931 - A flood threatens Lower Hutt city as the river rises 5.2m. Manor Park Bridge and Haywards suspension bridge swept away.
1939 - A devastating deluge covers hundreds of acres. The entire valley from Silverstream to the Upper Hutt basin is flooded wall-to-wall.
1976 - Flooding isolates Petone and leads to further reviews of the flood protection system.
1998 - Two floods within a week of each other cause extensive riverbank damage but no breaches recorded.
2000 - Like 1998, two floods strike within a week of each other - again the system copes well.
2004 - The Waiwhetu Stream floods causing an estimated $200m in damage.
2005 - Water floods 10 Lower Hutt homes and causes severe erosion on golf courses.
The Dominion Post