Firing Techniques
Various firing techniques can be used to accomplish a burn objective. The particular fire technique chosen must be correlated closely with burning objectives as well as fuel, weather, and topographical factors to prevent damage to forest resources. The proper technique to use can change as these factors change. Atmospheric conditions should be favorable for smoke to rise into the upper air and away from smoke-sensitive areas such as highways, airports, and urban areas.
Firing techniques are often named after the direction of fire movement with respect to wind direction. Fires either move with the wind (heading fire), against the wind (backing fire), or at right angles to the wind (flanking fire).
These three fire types can be combined in various ways by other firing techniques, such as point source fires and circular fires.
Heading fire is the most intense because of its faster spread rate, wider flaming zone, and longer flames. Backing fire is the least intense, having a slow spread rate regardless of windspeed. This type of fire has a narrow flaming zone, and short flames. Flanking fire intensity is intermediate. The slope of the land has an effect on rate of spread similar to that of wind.
The following sections, from the widely used publication: A Guide For Prescribed Fire In Southern Forests (USDA Forest Service 1989), describe these firing techniques and several others that can be used to conduct prescribed burns.
- Backing Fire
- Heading Fire
- Flanking Fire
- Point Source Fire
- Aerial Ignition
- Center and Circular Fire
- Pile and Windrow Burning
If slight variations in fuel volumes or weather conditions are encountered, consider combining two or more firing techniques to achieve the desired result. A solid line of fire always spreads faster and thus builds up intensity quicker than does a series of spot ignitions spaced along the same line. Intensity increases abruptly when two fires burn together. The magnitude of this increase is greater when fires converge along a line rather than along a moving point. The line of crown scorch often seen paralleling a downwind control line delineates the zone where a heading fire and a backing fire met.
Residence time is the time it takes the flaming zone to move past a given point. The residence time of heading and backing prescribed fires is often about the same because the deeper flame depth of a heading fire compensates for its faster movement. Generally, backing fires consume more forest floor fuels than do heading fires. The total heat applied to a site may be roughly equal for both heading and backing fires, as long as additional fuels are not involved. This result can be expected even though the fireline intensity of the heading fire would be greater. In a backing fire, the released heat energy is concentrated closer to the ground.
Encyclopedia ID: p543
