Quake left us with a slip haz­ard

Marlborough Express - - INSIGHT -

the shape of the Earth and its sur­face char­ac­ter­is­tics – to study the de­tailed to­pog­ra­phy af­ter Gita.

That would be com­pared with LiDAR from be­fore and im­me­di­ately af­ter the earth­quake.

‘‘That will tell us where the sed­i­ment has come from, where it’s ended up, how thick it is,’’ Del­low said. ‘‘We will be able to trace that sed­i­ment through the land­scape.’’

In Oc­to­ber 2017, GNS started a five-year, $8.2 mil­lion project un­der the Min­istry of Busi­ness, In­no­va­tion and Em­ploy­ment’s En­deav­our pro­gramme to de­velop pre­dic­tive tools for man­ag­ing the risks from land­slide and sed­i­ment haz­ards gen­er­ated by earth­quakes and post-earth­quake rain and af­ter­shocks.

‘‘Ev­i­dence from pre­vi­ous earth­quakes in New Zealand and over­seas sug­gests that the fre­quency of land­s­lid­ing af­ter a large earth­quake is sig­nif­i­cantly higher than be­fore the event. This is be­cause strong earth­quakes cause slope crack­ing and gen­er­ate a lot of land­slide de­bris,’’ the project out­line said.

‘‘The de­bris gen­er­ated by the Kaiko¯ura Earth­quake, when mo­bilised, will create new haz­ards, in­clud­ing fur­ther land­slides, land­slide dams and dam fail­ures, rapid aggra­da­tion, and in­creased river chan­nel in­sta­bil­ity, as the de­bris cas­cades from hill slope to sea.

‘‘These haz­ards may per­sist for decades and there­fore rep­re­sent a pro­longed risk that must be con­sid­ered by the im­pacted com­mu­ni­ties and stake­hold­ers.’’

NZ Trans­port Agency earth­quake re­cov­ery man­ager Tim Crow said about 300,000 cu­bic me­tres of ma­te­rial needed to be cleared from 60 high­way sites on ei­ther side of Kaiko¯ura af­ter Gita. The largest at Okiwi Bay, north of Kaiko¯ura, had around 200,000cu m of ma­te­rial.

South of Kaiko¯ura at Rosy

Morn – where two houses filled with de­bris – about 5000 cu mof ma­te­rial had washed down the hill.

The amount of dam­age caused by Gita to the high­way through Kaiko¯ura was the most by a storm or cy­clone since 1975, when for­mer cy­clone Ali­son brought down 83 slips north and south of the town.

Del­low said re­searchers would be look­ing closely at the dam­age caused by Ali­son, com­pared to that of Gita.

Last Thurs­day, sci­en­tists also flew over the largest of the land­slide dams re­sult­ing from the Kaiko¯ura Earth­quake – on the Ha­puku River – to see what im­pact Gita had on it.

The dam was among about only 10 re­main­ing of the orig­i­nal 196 land­slide dams cre­ated by the earth­quake, Del­low said. The sur­viv­ing dams were slowly get­ting smaller. Ha­puku hadn’t failed cat­a­stroph­i­cally and was episod­i­cally de­clin­ing in size.

The Kaiko¯ura Earth­quake gen­er­ated tens of thou­sands of land­slides in a 10,000-squarek­ilo­me­tre area of North Canterbury and Marl­bor­ough. The most in­tense land­slide dam­age was con­cen­trated in a 3500sq km area around the ar­eas of fault rup­ture.

PHOTO: NZTA

More than 200,000 cu­bic me­tres of de­bris came down across State High­way 1 at a sin­gle site near Okiwi Bay, north of Kaiko¯ura.

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