Slope fuels flames
Wildfires the size of the Nelson blaze are relatively rare in New Zealand, but factors have provided a perfect storm. Oliver Lewis reports.
‘‘You have a very simple hierarchy, what’s important is life and then property. You can rebuild property, you can rebuild homes, but you can’t rebuild lives that were lost.’’ Richard McNamara Fire and Emergency New Zealand
Weather, terrain and fuel. These elements combined can spread an out of control fire – and conditions in Nelson’s Pigeon Valley have been perfect.
By yesterday morning, the fire was covering an area the size of 1850 rugby fields. Homes have been destroyed, more than 170 evacuated and a state of emergency declared.
Fire and Emergency New Zealand (FENZ) rural regional manager Richard McNamara, the man who led the response to the devastating 2017 Port Hills fires, said the Nelson fire getting into the hills was especially dangerous.
‘‘I believe it started on the flat and once it got on to the slope and into the forest nothing was going to stop it.’’
A wet winter and spring created ideal growing conditions for the fine fuels, grass and shrubs, that usually start large wildfires. And after the growth: a long, hot drying period.
‘‘We’re breaking 30 degrees Celsius on a regular basis in the northern half of the South Island,’’ McNamara said.
Relative humidity was also low, meaning grass and shrubs dried out quickly.
A southerly wind also fanned the spread of the fire. ‘‘You’ve got the fuels, you’ve got the weather, and then the other thing that impacts on fire is the terrain,’’ McNamara said.
Fire runs uphill. On a slope, the embers and hot air radiating off a large fire are closer to the ground because of the angle. This preheats the fuel in front of the blaze.
‘‘The rough rule of thumb is, for every 10 degrees of slope, the fire speed and its intensity doubles,’’ McNamara said. Grass fires on a slope can outrun a person.
Hard, dirty work
McNamara said there were three main strategies for fighting a large wildfire: direct attack, indirect attack and letting it burn out. Firefighters in Nelson would be using a combination of all three, he said.
A direct attack involves the use of firefighters with hose lines on the ground, or the use of helicopters and fixed-wing planes to drop water or firefighting foam on the fire from above.
A fire like the one in Nelson could reach temperatures of 800 to 900 degrees Celsius. McNamara said the head, or main part of the fire, was too dangerous and aggressive to attack directly.
‘‘Generally you try and take out the flanks of a fire which is burning at a slower rate and work your way up towards the head, pinching it in as you go,’’ he said.
The indirect approach involved using chemical retardants and control lines to contain a fire. Bulldozers could be used to create fire breaks, or firefighters could rely on existing forest roads.
McNamara said it was common in the United States for aircraft to spray retardant on fuel ahead of a fire. The chemical released ammonia, robbing the fire of the oxygen it needed to burn.
Retardants were used in Nelson yesterday, he said. The chemical was put down on the side of a control line that was farthest from the fire.
If a fire was burning too quickly and aggressively, McNamara said the risk would be too high to commit firefighters to an area, and they would instead focus on protecting whatever was in front of it.
‘‘You have a very simple hierarchy, what’s important is life and then property. You can rebuild property, you can rebuild homes, but you can’t rebuild lives that were lost.’’
Firefighters in Nelson would be relying on aircraft to knock the wildfire back to a point where they could safely access the flanks of the fire, McNamara said.
The perimeter of the fire was about 20 kilometres yesterday morning.
Firefighters would spend time creating an area of black – where nothing was burning – around the entire perimeter, then work in.
‘‘There’ll be weeks of hot, dirty, very hard work for firefighters on the ground, backed up by heavy machinery – pushing that area of black into the fire itself.’’
An analysis of New Zealand wildfires from 1991/92 to 2007/08 found each year an average of 3033 wildfires burned 5865 hectares. The South Island accounted for 34 per cent of the fires, but made up 75 per cent of the total area burned.
In 1999, wildfires that broke out near Alexandra, in Otago, ravaged about 2500ha at Springvale and about
7500ha along the Roxburgh Gorge and Knobby Range, according to news coverage from the period.
A year later, on Boxing Day, 2000, a huge grass fire broke out in the Wither Hills above Blenheim. The fire, which burned through more than 6500ha, killed livestock and threatened houses.
In February 2017, the Port Hills fire raged above Christchurch. The fires burned through more than
2000ha and destroyed several homes. Helicopter pilot Steve Askin was killed while fighting the blaze.
The fire also exposed the dangers of wildfires to urban-fringe residents, people living on the outskirts of towns and cities near forestry plantations, grassland and other fuel sources.
McNamara said it took firefighters more than 50 days to completely put out the Port Hills fires. It would take a similar amount of time for firefighters working on the Nelson fire, he said.