Unit 3: Topography & Fire Behavior

Topography   ::   Canyons & Drainages   ::   Retarders   ::   Slopes   ::   Exercises

Box Canyons & Intersecting Drainages

Fires in a box canyon can have an upward draft like a fire in a stove. This dangerous condition occurs when:

1. Unstable air conditions at surface cause a convection current through the canyon.
2. Air is drawn in at the base of the canyon to support the convection currents.
3. Fuels are available to support a rapid burnout in the head of the canyon.

The images below illustrate a topographic situation that has claimed the lives of several firefighters.

Fires Burning in the Bottom of Narrow Canyons

Fires burning in the bottom of narrow canyons are dangerous to firefighters when these events occur:

1. The fire is burning under an inversion or stable air conditions.
2. Smoldering fire is slowly drying out aerial fuels.
3. The inversion breaks and winds increase into the canyon.
4. Increased fire activity often produces crowning and spotting.
5. Fire crosses to the opposite slope by radiation and convection (spotting).

The figures below illustrate more canyon bottom problems for firefighters. Fuels on the opposite slope from a fire in a narrow canyon are subjected to intense heat and flying embers. This fire situation is especially dangerous to firefighters when the fire is burning under an inversion or stable air conditions, and the smoldering fire is slowly drying out aerial fuels. When the inversion breaks, winds will increase into the canyon.

Increased fire activity often produces crowning and spotting, and fire crosses to the opposite slope by radiation and convection. This change can happen in a matter of a few minutes, giving little warning to firefighters working in the canyon. Obviously, firefighters need to recognize when these situations can occur, and later units of this course will describe methods of monitoring weather and predicting fire behavior in mountainous terrain.

Drainage Intersections

Where drainages intersect, fire might follow one or both drainages depending on:

1. The direction of canyon winds as determined by aspect and time of day.
2. The dominant winds in the canyon.
3. Wind eddies at the forks of the canyon.
4. The availability of fuels in the forks area.

This complexity of factors often makes predictions on fire spread very difficult at this point.
The figures below illustrate a fire at the forks of two drainages. Which way will the fire spread?