Canterbury flooded during ‘code red’ storm – Expert Reaction

The Canterbury region is being hit hard by heavy rain and rising floodwaters.

Canterbury Civil Defence has warned of further flooding, dangerous river conditions and disruptions to travel. Hundreds of residents have been evacuated and MetService has extended its code red heavy rain warning.

The SMC asked experts to comment on this situation.

Professor James Renwick, climate scientist, Victoria University of Wellington, comments:

“As the climate warms, there is more moisture in the air on average, so when it rains it is likely to rain harder than it used to. That’s why we expect the occurrence of heavy rainfalls and floods to increase over time as it continues to warm up. At the same time, the very heaviest rains are getting heavier, so records will be broken and the once-rare events will become more commonplace. Unfortunately, the terrible damage we’ve seen done in Canterbury over the past couple of days is something we are likely to see more often in future.

“Increased flooding leads to increased ponding of water and the need for bigger stormwater drains. If water is unable to drain away safely, there is the danger of stormwater mixing with sewage or other pollutants, which can lead to the contamination of water supplies and the risk of disease, on top of the direct damage that flooding does. One important way to adapt to climate change is to build stormwater and drainage infrastructure that is more able to cope with extreme flows.

“Rivers flood naturally and may change course unpredictably over their floodplains. For instance, the Waimakariri River has in the past flowed through the area where Christchurch now is. Building stop banks reduces the chances of such events and are a good protection against most floods. The downside is what’s called “maladaptation” where communities develop a false sense of security through the presence of the stop banks. That can lead to increased urban development close to the river, and even greater damage and misery when the really big flood comes.

“A notable feature of the floods in Canterbury is that they have come on top of a prolonged period of low rainfall and near-drought conditions. This is unfortunately what we expect to see with climate change. When the weather is fine, soils dry out faster in a warmer climate and drought can develop more quickly. When it’s stormy, the greater amount of water vapour in warmer air leads to heavier rainfalls, so both ends of the rainfall spectrum become more extreme in a warmer climate.

“Canterbury is expected to dry out a little on average as the climate continues to warm. But as we have seen over these past few days, that doesn’t rule out extreme heavy rainfalls when a major storm comes through.”

No conflict of interest.

Professor Caroline Orchiston, Research Associate and Acting Director, Centre for Sustainability, University of Otago; Programme Co-leader, Rural Theme, Resilience to Nature’s Challenges National Science Challenge, comments:

“The severe flooding in Canterbury has impacted several districts over the last three days, which is particularly challenging for many areas that were experiencing drought conditions. The floods have affected both rural and urban places, and communities are rallying to support each other. The MetService Code Red warning was delivered with enough time for preparations to be made, which has undoubtedly saved lives and prevented major stock losses.

“Farmers in particular will have a long road to recovery due to damaged farm infrastructure and pastures. They will be relying on the support of their community network to get them through the hard times ahead. In Ashburton, Hakatere Marae has opened its doors to provide welfare and manaakitanga to the local community, again reinforcing the important role of iwi during disaster response across Aotearoa New Zealand.”

No conflict of interest.

Dr Daniel Kingston, Senior Lecturer in Geography, University of Otago, comments:

“The current flooding situation in Canterbury has been caused by a relatively slow moving low pressure system. This slow movement is partly responsible for the very high rainfall accumulation over the past days. In some places, more rain has fallen over the weekend than in the entire year so far.

“Another reason for the very high rain totals is because this weather system is connected to warm moist tropical air via an ‘atmospheric river’. Atmospheric rivers are a long thin filaments of atmospheric moisture transport that can carry more water than the Amazon River. Atmospheric rivers more commonly occur on the west coast of the South Island (e.g. the cause of the SH6 Waiho bridge washout in 2019), but this one made landfall on the east coast instead.

“Although it is too soon to say exactly what role climate change has had in the severity of this event, it is likely to have made it more severe. Rising temperatures as a result of climate change mean that the atmosphere can hold more moisture, typically making heavy rain events such as this just that bit heavier.”

No conflict of interest.

Dr Judy Lawrence, Senior Research Fellow, Climate Change Research Institute, Victoria University of Wellington, comments:

“The floods we are witnessing at the moment serve to remind us that heavy rainfall events can happen at any time and anywhere, and that we must plan for them to become more frequent. Warming of the atmosphere means more moisture and that means more heavy rainfall events.

“The impact of such events is exacerbated following prolonged drought conditions because runoff becomes overland flow more quickly until soils rewet. This means maintenance of flow paths, stop banks, culverts and stormwater infrastructure become super important. It means that freeboard, which is the flexibility built into design that gives a factor of safety in floods, will need constant recalculation for assets and infrastructure as climate change increases flows from heavy rainfall events; and that new developments adequately consider the implications of  heavy rainfall events. These are in addition to disaster preparedness. We have to adapt to a changing climate that will surprise us if we don’t take a risk-based approach that deals with uncertainty over the next 100 years.”

Conflict of interest statement: Judy is a Climate Change Commissioner and Chair of the Peer Review Panel of the Christchurch City Council Multi-hazards and Liquefaction Studies. 

Dr Kendon Bell, Economist, Manaaki Whenua Landcare Research, comments:

“The flooding in Waitaha Canterbury highlights the extent to which communities in Aotearoa are vulnerable to extreme events and our thoughts are with the residents at this time. This is an area that routinely experiences dry conditions, and this event adds to the burden on farmers and residents.

“While this flooding event is devastating for those affected, it should also remind all of us that preparation and investment can help to lessen the impact when it’s our turn. For whānau, this means understanding how big the risk is in your community and at your house. If you’re at risk of flooding (or any natural disaster; most of us are) then prepare using the Get Ready advice online.

“For councils, this means working with flood modellers and engineers to understand the risk and propose flood protections that benefit the community enough to justify the costs. In some cases, these flood defences can be a modest investment now in exchange for substantial savings when the flood hits.”

No conflict of interest.

Associate Professor Asaad Shamseldin, Faculty of Engineering, University of Auckland, comments:

“The recent extreme rainfall and flooding in the Canterbury region appear to be linked to an ‘atmospheric river’. The atmospheric rivers are large, flowing amounts of vapour water, and they are a major contributor to the water cycle, severe floods, droughts breaking, and strong winds worldwide.

“There is very limited understanding about the impacts of atmospheric rivers in New Zealand. However, our recent published research showed that these atmospheric rivers are linked to extreme rainfall in New Zealand.

“The current floods have caused significant disruptions to the daily life to the communities in the Canterbury region and pose health risks in terms of water contamination with wastewater. The key question in the aftermath of these recent flood events is whether or not these extreme events will become more frequent – taking into consideration the envisaged climate change effects. Addressing this key question would be enable decision makers and communities to determine the appropriate level of additional mitigation and adaptation required to deal with these extreme events.”

No conflict of interest.