NZ’S flooding future laid out
For every 10 centimetres of sealevel rise, thousands of people and billions of dollars worth of houses become exposed to potential flooding, a sweeping new assessment has found.
It also put in stark terms the potential impact of flooding away from the coast as a result of more extreme rain storms, which are an expected consequence of a warmer climate.
The new assessment, based on two reports and released yesterday by Niwa (National Institute of Water & Atmospheric Research) and the Deep South Science Challenge, examined the impact of flooding as a consequence of climate change. It considered both extreme coastal flooding events, made more damaging due to sea-level rise, and areas already deemed to be at risk of flooding during heavy rainfall.
The work built upon a stocktake in 2015, which used blunter methods to calculate the number of people and buildings exposed to flooding. Those figures informed a report by the Parliamentary Commissioner for the Environment, which alarmed decision-makers with its assessment of the threat.
The primary takeaway from the new reports is that every 10cm of sea-level rise matters.
Scientists say at least 20cm or 30cm of sea-level rise is almost certain by 2050. What happens afterwards depends on global efforts to respond to climate change, with scenarios ranging from 50cm to 150cm by 2100, and continued increases for many decades beyond.
Under the average of all sealevel rise scenarios, the modelling found about 7000 buildings nationwide become exposed to coastal flooding for every 10cm the sea rises. Those buildings have an average combined replacement value of $2.5b.
Along with people and buildings, for every 10cm of sea-level rise, 130 kilometres of road, 360km of three-waters pipes, and 45 square km of productive land become exposed to extreme coastal flooding.
The coastal flood modelling was based on extreme sea-level events, which typically occur when a storm surge arrives on a high tide, extending the reach of waves inland.
The reports’ modelling used a 1 per cent annual exceedance probability for extreme sea-level – better known as a ‘‘one-in100-year’’ coastal flooding event – as the basis for the figures. Such an event occurred in Thames in early 2018, and caused widespread damage.
These events are expected to become significantly more frequent as higher sea-levels form a higher base from which storm surges can spring. While the figures sound high, it is because they refer to people and buildings potentially exposed to such events, which occur fairly randomly when certain factors combine. It does not mean all of those people and buildings will be inundated at some point.
A handful of regions are particularly exposed to this type of flooding, the modelling found.
The worst affected area is Canterbury – specifically Christchurch city. Under a 1 metre sealevel rise scenario, the number of people exposed to coastal flooding in Christchurch triples from the present day, to more than 30,000.
The buildings exposed in that scenario are worth about $5b.
For every 10cm of sea-level rise, another 2000 buildings in Christchurch enter the hazard zone. Nationwide, under the expected 30cm of sea-level rise by 2050, an additional 27,000 people would be exposed to extreme coastal flooding, as would buildings with a replacement value of $6b. Beyond 2050, an under 0.6m of sea-level rise – a fairly modest scenario – an additional 60,000 people (compared with today) would be exposed to extreme coastal flooding, as would buildings worth $14b. Under 1.2m of sea-level rise, one of the high-end scenarios, 127,000 people and $31b worth of property not exposed to coastal flooding today would become so.
Aside from people and buildings, thousands of kilometres of roads and wastewater infrastructure become vulnerable to extreme flooding nationwide, as do major infrastructure assets such as airports and large areas of productive farmland.
The researchers emphasised it was a ‘‘first attempt’’ at quantifying possible exposure to flooding. Data limitations meant the conclusions were naturally broad in nature, and presented a ‘‘worst case scenario’’. The research also examined climate change-related flooding more broadly, including areas already exposed to flooding from storms and heavy rain. More extreme rainfall events are expected in a warming climate.
As of 2019, about 190,000 people in the Canterbury region are already exposed to flooding hazards, most in Christchurch city. Auckland has 118,000, Dunedin (35,000), and Hamilton and Lower Hutt (31,000 each).
Researcher Rob Bell said the findings emphasised the need to restrict new development in coastal areas.
Higher sea-levels form a higher base from which storm surges can spring.