Fire by Andrew C. Scott

Author:Andrew C. Scott [Scott, Andrew C.]
Language: eng
Format: epub
ISBN: 9780192565617
Publisher: OUP Oxford
Published: 2020-05-06T00:00:00+00:00

Landscape fires

As the cost of suppressing landscape fire has spiralled over the past decade there are a number of issues we need to consider in regard to whether or not all wildfires are bad and should be suppressed. For example the fire suppression costs in the United States of America have been steadily increasing so that they now represent 50 per cent of the annual budget of the federal government’s Forest Service. The Thomas Fire north of Los Angeles in California that threatened and destroyed many homes, including those of several Hollywood stars, in 2017 alone cost 177 million dollars to suppress.

To make decisions about fire suppression, we need a greater understanding of how fires start and spread; we can then make predictions of both these aspects as well as developing appropriate strategies. Fire behaviour is intimately tied up with the nature of a biome, but human influences may also have changed our approach to fire intervention or suppression.

In the early 1950s Jack Barrows, who worked on the northern Rocky Mountain forests, wrote on the behaviour of fire—the manner in which a fuel ignites, flame develops, and fire spreads and exhibits other related phenomena. He outlined five steps involved in the practice of predicting or forecasting wildland fire behaviour: basic knowledge; forest knowledge; aids and guides; estimation of the situation; and decision.

As part of the first step, basic knowledge, we need to consider the fundamentals of combustion. We have already seen, using the fire triangle, that three elements are involved in fire: a fuel to burn, a source of heat, and availability of oxygen. We can develop this concept by considering combustion as occurring in four phases. The first may be called the pre-heating or pre-ignition combustion phase, in which an unburned fuel may be altered by the heat from an advancing fire front. The heat may drive off moisture from the fuel and raise the temperature to ignition point. During this phase, the cellulose in the plant tissues begins to break down, releasing flammable gases.

The second phase is the flaming combustion phase. Here the flammable gases that have been released mix with oxygen in the atmosphere and an oxidation reaction takes place that is essentially an exothermic chemical chain reaction producing heat and light.

The third phase is the smouldering combustion phase, and it is more important than often recognized. This is where the combustion reaction is sustained by a low heat, and the oxygen directly attacks the solid fuel. There are no flames. During this phase, the combustible gases and other volatile vapours continue to escape into the atmosphere, often as visible smoke.

The final stage is the glowing combustion phase. By this time most gases have been driven off, but the carbon-rich fuel continues to oxidize, producing a significant amount of heat with only embers visible and no flame. We are familiar with this form of combustion in charcoal on a barbecue.

Although it is clear from first principles how a fire may start, we need to consider how a

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