Basic Forest Fire Suppression Course - Online Lessons


3.4 Fuel Arrangement

Fuel arrangement is the distribution of all combustible materials in the horizontal or vertical planes. Fuel arrangement is described as being compact or loose, and continuous or separated.

Loosely arranged fuels allow free movement of oxygen that is required for combustion. This free movement of air also promotes drying of the fuels. Compacted fuels do not allow free air movement and consequently dry out and burn slowly.

Continuous fuels on the horizontal plane (earth`s surface) refer to an unbroken supply for fire consumption. The fuel variety (softwood vs hardwood ...) may change, but the supply is constant. Separated means that the supply is not constant, it has been broken by exposed mineral soil between clumps of trees or vegetation, or rock, or possibly water.

Vertical fuel arrangement is divided into four levels

Ground fuels - A mat of partially decomposed organic material between the surface fuels and the mineral soil, consisting of roots, duff, and other partially decayed vegetation.
Surface fuels - Leaves, grasses, young trees, shrubs, slash and other litter on the forest floor.
Ladder fuels - Fuel layer that allows fire to climb from the surface to the crown.
Crown fuels - The fuel layer consisting of the tops of trees.

Continuous vertical arrangement creates a ladder of fuels for surface fires to climb, contributing to torching or crown fire development.

Torching occurs when sufficient heat from the surface fire is produced to burn vertically to the tops of individual or clumps of trees.

The lateral movement of fire through the tops of trees is called a crown fire. This type of fire can occur only if the crown fuel layer has a continuous horizontal fuel arrangement.

In the horizontal plane, a continuous fuel arrangement will promote fire spread. A separated horizontal fuel arrangement will inhibit fire spread.

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