Un­usual com­bi­na­tion of con­di­tions helped fires spread

The Press - - Port Hill Fires - CHARLIE MITCHELL

‘‘We didn’t get the higher-hu­mid­ity morn­ings which usu­ally help.’’ Scion Re­search ru­ral fire team re­search leader Dr Tara Strong

It looked like the worst was over. But then the wind changed, two fires be­came one, and the flames spread on the wind and spi­ralled out of con­trol.

The Port Hills fire is not just no­table for its size, but also the com­bi­na­tion of fac­tors that fu­elled it.

By 10am on Wed­nes­day, the two fires cov­ered about 1000 hectares. The largest spread from Early Val­ley Rd near Hal­swell to Cass Peak, while the smaller fire burned in a cir­cle on Mar­leys Hill. Less than 24 hours later, the fires had merged, more than dou­bled in size, and ven­tured over the hill and down gul­lies to­wards Gover­nors Bay.

It had moved rapidly to­wards Kennedys Bush, and filled into spa­ces near Kennedys Re­serve and Sign of the Bell­bird.

The fu­els, in this case, re­fer to veg­e­ta­tion.

The hills are dom­i­nated by gorse, bracken, grass and pine forests – all highly flammable, par­tic­u­larly in dry con­di­tions.

The fire’s move­ment changed as it moved through the dif­fer­ent fuel types, mak­ing it er­ratic and hard to pre­dict, Dr Tara Strong, ru­ral fire team re­search leader at Scion Re­search, said.

When day turned to night, lower tem­per­a­tures prompted an un­usual, and dan­ger­ous, fire be­hav­iour. Fire typ­i­cally moves faster up­hill, as the ris­ing warmth pre­heats the fuel ahead. Fire­fight­ers of­ten have bet­ter luck con­tain­ing a fire when it moves down­hill.

But when night fell on Tues­day and Wed­nes­day, cold air seeped down the val­ley, bring­ing the flames along with it.

‘‘In gen­eral, it’s un­usual for a fire to do that,’’ Strong said.

‘‘That was def­i­nitely a risk in this case. Then we had the wind shift which ac­tu­ally pushed the fire down­hill.

‘‘We also had drier, warmer nights than usual. We didn’t get the higher hu­mid­ity morn­ings which usu­ally help.’’

The fire is of­fi­cially classed as an ‘‘ex­treme fire’’, which are rare in New Zealand.

Over­seas, the strat­egy for large fires is to con­tain, but in a nar­row coun­try where as­sets are close to­gether, the strat­egy is to at­tack, Strong said.

‘‘In gen­eral we can’t af­ford to con­tain and let burn. It’s eco­nom­i­cally not fea­si­ble be­cause our space is nar­row. It’s ei­ther go­ing to run into peo­ple or a liveli­hood very quickly.

‘‘So in gen­eral it’s a di­rect at­tack, and at­tempts to do that on this fire were made, but it wasn’t work­ing. So it was quickly switched to pro­tec­tion of prop­erty.’’

Hotspots could be ex­pected for sev­eral weeks, she said.

Fore­cast rain would be a huge help.

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