Predicting a landslide
In the wake of the devastation caused by ex-Cyclone Gita, scientists are working on a warning system to identify slips and landslides before they happen. reports.
People like those in the path of a landslide in Nelson on Tuesday, stand to receive better warnings about the threat of being hit by landslides, under research being carried out by GNS Science.
Residents on Kaiteriteri-Sandy Bay Rd near Marahau fled their homes as ex-cyclone Gita struck on Tuesday afternoon, just before water and debris came crashing down around their houses.
They waded out through the floodwaters, when emergency services couldn’t reach them.
It happened in an area known as ‘the Separation Point Granite’, a geological unit identified by landslide experts as being vulnerable to ‘debris flows’.
While any warning system for residents in such areas was potentially years away, hazard experts have begun researching how to identify which infrastructure is at risk from landslides during heavy rain, and how to communicate that information.
Currently, the MetService can only warn about the possibility of slips in general, when they issue a severe weather warning for heavy rain.
GNS scientists aim to create models to more accurately predict where slips could occur.
‘‘The underpinning science we’ve got to do first is to understand how the landscape would respond to different rainfall amounts,’’ GNS landslide researcher, Sally Dellow said.
‘‘We can get that from looking at historical events.’’
The plan was to then produce a series of models the MetService could run against rainfall forecasts, as severe weather approached, he said.
‘‘As the rain started falling ... the MetService could say well actually, the amount of rain that we’re forecasting would have this impact on this landscape.’’
48 hours out from heavy rain, he envisaged the warning system might still be quite general.
‘‘So [for example] we know there’s going to be rainfall in Tasman district ... this forecast tells us there could be landslides in this area.
‘‘Three hours out, we might be saying the rain cloud band is trending towards an area around Kaiteriteri and Marahau, and that we are going to tell people in those areas that there’s a possibility of landslides affecting the roads and affecting houses in the hills.’’
Just how differently landscapes could respond to rainfall, was highlighted during a series of thunderstorms over Roxburgh in Otago last November.
Dellow referred to one thunderstorm that caused flash flooding.
‘‘We’ve got the rain radar data from MetService ... that the rainfall was enough to cause debris flows. But there was another thunderstorm on the other side of the river that produced the same amount of rainfall, but there was only flooding.’’
The envisaged warning system was unlikely to get down to the level of risk to individual properties from slips, or exactly where on a road would be blocked by a landslide.
But it might break down areas as being at a high, medium of low probability of being hit, Dellow said.
There was then the complex matter of how to advise people in at-risk areas, on what action to take.
‘‘We’d have to think very carefully about how we communicated, and what we communicated,’’ he cautioned.
Researchers were looking into how to avoid ‘‘hazard fatigue’; whereby people ignored warnings after being regularly alerted to hazards that didn’t happen.
There was also a fine line
‘‘Warning systems needed to be supported by good education programmes so that in times of crisis, people can immediately remember what they are meant to do.’’ Sally Potter
between getting people to act, and frightening them.
The challenge was communicating warnings during fast-moving events, Dellow said.
‘‘We could tell people to stay in their homes with the risk their home might be hit, or we could tell them to get in the car and get out of the area with the risk they might be hit by a landslide on the road.’’
Context would be everything, he said.
Any landslide early warning system would be coordinated with civil defence in order to include the right guidance messaging, such as for evacuations, GNS Hazard and risk management researcher Sally Potter said.
‘‘Civil defence are the experts in knowing what actions to order, where, and when,’’ she said.
The tricky thing with communicating warnings was that people responded to them in different ways, based on a multitude of factors, she explained.
‘‘Their response depends on things like how threatened they feel about the hazard, and their understanding of the potential impacts; what they can feel, hear, or see out the window, or on TV or the internet; their gender and age. Women are more likely to respond to warnings than men.’’
The made it hard to make a ‘one size fits all’ warning that worked for everyone, Potter said.
‘‘Warning systems needed to be supported by good education programmes so that in times of crisis, people can immediately remember what they are meant to do.’’
Dellow said whatever the context, it would be important to give people very clear instructions about what they should do.
‘‘If you’re in your house, move to the side of your house that has the good views, and don’t be in the back part of the house where the landslide might hit. It’s going to get down to that level of detail.’’
It wasn’t easy to please everyone though, he said.
‘‘There will always be people who will say warnings were unnecessary and an overreaction because they personally were not affected. And conversely there will be those who say they were not given enough warning and that what was given was inadequate,’’ Dellow said.
‘‘Our goal would be something like to be good enough to meet the needs of most of the people most of the time, and that is probably the best we can do.’’
Tim Wraight’s home on Kaiteriteri Sandy Bay Road, was hit by a landslide on Tuesday, during ex-cyclone Gita.
Brunna Lee and her three children were forced to flee their home on the Kaiteriteri Sandy Bay Road, when a landslide came down during heavy rain from former tropical cyclone Gita.