Topography's effect on Fire Behavior

The term “Topography” refers to the earth’s surface, in particular to the character or physical features of a place or region.  Topography is:  The configuration of the earth’s surface including its relief and the position of its natural and man-made features.  It is the third of the “Big Three” influences of fire behavior. Topography is generally static (Except changes made by man, hurricanes or tornados).  It is the opposite of weather, which is always changing. 

Some of the effects are indirect.  Remember that they are interrelated and react with each other. In the coastal plains, most factors of topography are not very important in the suppression of wildfires, except as barriers. However, you may be sent to assist another state or federal agency and find yourself fighting fires in the mountains.  In the mountains, all factors of topography that affect fire behavior are important.  

In using prescribed fire, slope becomes very important because of the potential smoke problem.

We will examine the following topographic factors:

SLOPE 

Slope affects the spread of fire in two ways:

  1. Preheating (convection & radiation)

  2. Draft 

On slopes, the less dense air next to the surface (warmed by the surface) forms a pathway for this lighter air to rise along the slope causing a draft. Cooler air to replace the warmer, less dense air comes from below.  Consequently local winds usually blow up-slope during the day.  Because of the local, up-slope winds, wildfires usually burn up-slope.  The steeper the slope, the more rapidly the fire will burn up-slope (and more intensely).  The reason is because of both greater radiant heat and greater convective heat.  A fire will spread uphill because of the preheating of the fuel and the up-slope draft unless the general wind is strong enough to overcome these two forces.  The flames are closer to the fuel on the uphill side and they receive more radiant heat.  This results in more preheating and faster igniting of the fuel.  The heated air rises along the slope increasing the draft that further increases the rate of spread.  As a result of winds blowing up-slope, more convective heat also reaches the fuel in front of the fire and it is pre-heated more quickly to the ignition temperature. 

The opposite is  true at night.  When the slope becomes shaded, the surface loses heat rapidly and becomes cool.  The air adjacent to the surface also cools and becomes more dense thus heavier and it will begin to flow down-slope.

Down-slope winds usually are no stronger than 2 to 3 mph, whereas upslope winds can be as high as 5 to 10 mph.  The steeper the slope and the longer it is, the stronger the wind. The change of air from upslope to downslope is usually gradual so the air may become calm for an hour or more during the change.  Downslope winds begin as soon as slopes go into shadow (Refer to "local winds").

If the fire is at the bottom of the slope, the entire slope to the ridge top lies in the path of the fire.  Such fires will usually be larger because most are not controlled until they reach the top of the ridge. 

Slope is not as important in the coastal areas when suppressing wildfires.  Even a slight slope, however, can be an important factor in the use of prescribed fires.  It compounds the smoke problem due to the down-slope winds at night.  Even in areas that are considered "flat", residual smoke will drift toward and then down drainage. Neither does it spread out. Instead, it will concentrate in the lowest areas. (Refer to "Smoke management”).

Slopes Increase Radiant Heat Transfer and Updrafts

ASPECT 

Aspect is the direction that a slope faces.  The direction a slope faces determines how much radiated heat it will receive from the sun.  Slopes facing south to southwest will receive the most solar radiation.  As a result, this slope is warmer than slopes facing a northerly direction.  The warmer slope results in lower relative humidity, higher temperatures and rapid loss of moisture.  The fuel will tend to be dryer “flashy” type that ignites and burns readily.  The period that fires will ignite and burn will also be longer on south-facing slopes.

SHAPE OF COUNTRY 

The direction and speed of the wind can be greatly affected by topography.  Ridges and mountains are barriers to the horizontal movement of air.  The wind is deflected over them adding to the local up-slope convective winds from the surface heating by the sun.  When the ridge tops are reached, updrafts from the other side may bend the flames back.  Ridge tops are good places to place control lines.  Slightly down on the lee side may be even better if spotting is likely.  Eddies will form on the lee side of ridges and whirlwinds may form on unstable days.

Ravines and gullies will form paths for the flow of air and may change direction of the fire.  In narrow ravines, heat will dry out fuels on the opposite side and they will readily ignite.  Intersecting drainages and sharp turns will cause turbulence. 

Saddles and gaps along a ridge will funnel the wind and increase its speed.  Winds will also be gusty and spotting is more likely.  Fires will tend to burn toward them and increase in intensity and rate of spread.  This change can be abrupt. 

Box canyons are ravines that end at or near the ridge top.  They have no gaps or prominent saddles for the preheated air to escape.  They provide avenues for intense updrafts into the ravine and heated air is trapped as the fuel is preheated.  Heated gases trapped in a ravine or cove can all ignite at one time as ignition temperature is reached trapping anyone that may be in its path.  Such occurrence is known as a “flashover.”

BARRIERS 

Barriers are anything (natural or man-made) that can stop or slow down the spread of fire.  Examples are:  fields, roads, streams, lakes, swamps (if wet) rocky outcrops, and old burns.  They should be considered in planning a prescribed burn or in control of a wildfire.  They can also be barriers to equipment.

A thick stand of trees will also act as a barrier to the wind.  It will be forced over the top causing gusty conditions.  An opening in the stand will channel the wind much the same way as saddles.