Unusual combination of conditions helped fires spread
‘‘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.