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.

Marlborough Express - - NEWS -

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.

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