People, roads and billions in assets vulnerable to climate change-related floods

Thursday, 22 August 2019

For every 10 centimetres of sea-level 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 on Thursday by Niwa 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 a flood hazard 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 formed the basis of a report by the Parliamentary Commissioner for the Environment, which alarmed decision-makers with its assessment of the scale of the threat of rising sea-levels.

* Climate change: Rising sea level could hit 43,000 Aucklanders' homes

* Insurers reveal $226 million extreme weather bill for 2018

* Climate change costs are hitting households through insurance premiums

Extreme river flooding in Christchurch after heavy rain in 2014.

* Rising seas 'a slowly unfolding red zone'**

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 sea-level rise scenarios, the modelling found around 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, 130km of road, 360km of three-waters pipes, and 45km² of productive land becomes 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 – what is better known as a 'one in 100 year' coastal flooding event – as the basis for calculating its figures.

Damage to the State Highway north of Thames as a consequence of flooding in 2018.

Such an event occurred in Thames in early 2018, and caused widespread damage. They 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 was Canterbury – specifically Christchurch city, which has more people and buildings exposed to extreme coastal flooding than any other area, by a reasonable margin.

Under a 1m sea-level 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 around $5b.

For every 10cm of sea-level rise, another 2000 buildings in Christchurch enter the hazard zone.

For New Zealand as a whole, 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.

What happens beyond 2050 would have stark implications for New Zealand's coastal communities.

Emergency responders use boats to rescue residents trapped in flooded parts of Christchurch.

Under 0.6m of sea-level rise – a fairly modest scenario – an additional 60,000 people (compared to 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 across all parts of the country, as do major infrastructure assets such as airports and large areas of productive farmland.

'In terms of that first metre of sea-level rise, we're looking at doubling or tripling the current elements at risk or exposed to coastal flooding,' co-author Ryan Paulik from NIWA told researchers at a seminar earlier this month.

'Currently, we have in New Zealand a lot of elements potentially at risk with present day [flooding]. In the case of coastal flooding, we see quite a dramatic increase in terms of the exposure elements at risk with not very much sea-level rise.'

The researchers cautioned some of the findings, emphasising that it was a 'first attempt' at quantifying possible exposure to flooding.

Data limitations meant the conclusions were naturally broad in nature, and present a 'worst case scenario'. In particular, the modelling could not account for any barriers to coastal flooding, such as sea-walls, and used only one hypothetical event (the 'one in 100 year extreme sea-level') as an input.

'There's obviously a lot more work that needs to be done to fine tune this information,' Paulik said earlier this month.

Alongside coastal flooding, the research also examined climate change-related flooding more broadly, including areas already exposed to flooding from storms and heavy rain.

There is no national map of flood hazard areas, so the researchers pieced one together from publicly available data, mostly taken from local authorities.

The modelling estimated around 675,000 people and 411,000 buildings worth $135 billion nationwide were currently exposed to flooding following storms and heavy rain, either as a result of the rainfall itself, or from associated river flooding.

More than 20,000 sq km of New Zealand potentially lies inside the hazard area, as does more than 19,000km of roads, more than 1500km of railway lines, 20 airports, 3400km of electricity transmission lines and more than 21,000km of drinking water, stormwater and wastewater pipes.

This was significant because more extreme rainfall events are an expected consequence of a warming climate. This raises the frequency of events such as the storm earlier this year which brought record rainfall to the West Coast, prompting a state of emergency and considerable infrastructure damage, including destruction of the Waiho bridge.

Again, Canterbury has the most to lose in flood-prone areas. As of 2019, around 190,000 people in the region are already exposed to flooding hazards, most of whom live in Christchurch city.

It is followed by Auckland (118,000), Dunedin (35,000) and Hamilton and Lower Hutt (31,000 each).

Most of the buildings exposed to flooding nationally are residential: They total more than 300,000 buildings, worth nearly $100b. Around one-third of these buildings are in Canterbury.

Again, the findings had limitations, and were intended as a high level view of exposure, the researchers said.

One of the researchers, Rob Bell, said the findings emphasised the need to restrict new development in coastal areas.

'We need to put the brakes on development in coastal areas even if areas may not be impacted for a few decades - given sea level will continue rising,' he said in a statement.

Christchurch City Council head of three waters and waste Helen Beaumont said the reports 'confirm the vulnerability of Christchurch to both fluvial and coastal flooding' and would provide further guidance for floodplain management strategies, city land-use and infrastructure planning.

'The Christchurch City Council has excellent models of flooding from the rivers across the city and the coastal inundation modelling carried out in 2017 has informed our approach to floodplain management.

'The district plan already incorporates one-metre sea-level rise into the mapping of flood management areas across the city,' Beaumont said.