The river of rain that pummelled the top of the South Island last month expectedly caused devastating floods. Less expected were the landslides.
Wellingtonians have been experiencing a winter where 100s of landslides have occurred over several weeks. Media reports suggest this is surprising and unusual.
Landslide risks are nothing new. One of the most memorable ones occurred in Dunedin in 1979 when 69 homes in Abbotsford were lost. This event gave birth to Toku Tū Ake EQC covering land damage within 8m of the residential home and up to 60m of the main access way to it. As insurers do not cover land, homeowners are fortunate.
Even before that time though, researchers had identified the conditions that are required to cause landslides. Recently, Dr Jim Salinger, an internationally renowned climate scientist, sent me a 50-year-old piece of research into landslips in Wellington in 1974.
The winter that year was exceptionally wet in the capital. The city experienced over 1,100 landslips which drew attention to slope stability more widely.
The research found a high number of slips in 1941, 1943, 1955 and 1956. It identified the majority of slips were linked to where hillsides had been cut into and backfilled for housing development. At the time of their creation, the shear strength of the slope, a term used to measure the susceptibility to slippage, appeared sufficient to withstand the initial stress of cutting and filling.
However, the research also found conditions that made those slopes more vulnerable to slip. It found that slips tended to be triggered where there was more than 80cm of rain over a four-month, or a deluge of at least 12cm over a 24-hour, period; particularly following a long dry period.
There is a lot more to landslip causes. More recently, research by GNS and NIWA has seen tools developed that take into account climate change impacts to inform future planning decisions in Wellington.
Going back to last month, in Nelson the ‘Tahunanui Slump,’ an area of a well-known very slow-moving landslide, has a number of houses on it where it will take some time to assess the implications for the land on which they stand.
This is an area where damage has been recorded to houses as far back as 1929 and again in 1962 following heavy rain and earthquake shaking. Last month, it was a deluge of biblical proportions.
Over in the Marlborough Sounds, homes were cut-off as roads were washed away by landslides and some houses were destroyed. Even undamaged homes and farms still have no road access to the rest of the world.
New Zealand is well-known as a high-risk country largely due to big earthquakes and the potential for other seismic events. The mistake of having built on flood plains is coming home to roost with climate change.
But the legacy of the past may prove to have a wider and longer tail. Those living on hillsides are far from immune too more extreme and frequent rainfall events, especially in earthquake prone areas.
