Unit 3: Topography & Fire Behavior

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

Objectives:

Upon completion of this unit you will be expected to:

• Explain how the factors of topography affect fuels and their availability for combustion.
• Explain how topography can affect the direction and rate of spread of fires.
• Describe how changes in fuels and topography can provide barriers or partial barriers to the spread of fires.
• List five mechanical effects topography can have on weather.
• Describe how topography induces several local wind conditions including slope and valley winds.
• Explain the "chimney effect" in canyon topography.
• Determine the slope percent from topograpnhic maps, and describe how slope percent is determined or estimated in the field.

Topography & the Fire Environment

Predicting fire behavior is a difficult job because of the many variables in nature. Burning conditions, relating to weather and fuels, are constantly changing as a fire spreads over time and space. Although the terrain usually does not change over time, it can change considerably over space. All of the topographic features illustrated here are important in predicting the behavior of fire in mountainous terrain. A common method used to depict these various land features is the topographic map. We will be using topographic maps in the unit exercises. If you are not familiar with these maps and the interpretation of features from contours, you should seek help in map reading.

Topography affects the fire environment by:

1. Altering the normal heat transfer processes,
2. Modifying general weather patterns, thus
3. Producing localized weather conditions, that
4. Influence the type of vegetation (fuels). These, in turn,
5. Result in microclimates with localized moisture conditions.

When we consider all of these ways together, we can state that topography directly or indirectly affects fire intensity and the direction and rate of spread of a fire.

In rare cases of mass ignition or fire storms over which large areas of moderate to heavy fuels are consumed in short periods of time, topography will have the least influence on the fire. The objectives of this unit are intended to give you a better understanding of these direct and indirect affects on fire behavior.

Mechanical Effects of Topography

Topography modifies general weather by:

1. Friction layer modifies general winds. more
2. Induces slope and valley winds. more
3. Creates thermal belt conditions. more
4. Produces orographic thunderstorms. more
5. Contributes to foehn or Chinook winds. more

Effects of Elevation

Let's move on to some other effects of topography. Below, we'll consider how elevation above sea level influences general climate and the effects thereof.

1. The amount of precipitation received.
2. The snow melt dates.
3. The fuel types and loadings.
4. The dates of curing of vegetation.
5. The length of the fire season.
6. The general fire danger.

Effects of Slope and Elevation on Fuel Types, Loadings, and Availability

more

The position on a slope or the relative elevation is also important because this influences the types and loadings of fuels and their availability.

Effects of Microclimate Conditions on Fuel Availability and Type

more

We've discussed fire danger in general areas or zones. It's important to recognize that fire danger changes substantially due to the micro-climate conditions at all elevations.