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.