Effects of Topography on Fire Intensity and Rate of Spread

Slope steepness, elevation, aspect, and configuration of land are all variations in topography that influence fire intensity and rate of spread.

Slope steepness

Slope steepness affects fire behavior in a similar way as wind by affecting the flame angle. As slopes increase, flames are closer to fuels upslope from the flaming front. This preheats fuel, increasing its combustibility and rate of spread (Rothermel 1985). The effects of slope on rate of spread and flame length for a typical open pine stand with a grass understory are illustrated in the following figures. Both parameters, as modeled by Behave Plus 2.0, increase at a faster rate as slopes increase from zero to 50%.

Slope effects on rate of spread

Slope effects on flame length

Elevation

Elevation influences temperature, precipitation, and relative humidity, all of which affect fuel moisture and in turn, fuel availability. As elevations increase, temperatures tend to decrease while precipitation and relative humidity increase; all these changes reduce rate of spread. One exception to this general pattern is the occurrence of thermal belts at midelevations where warm air at night is trapped below cooler air at both higher and lower elevations.

Aspect

Aspect is the direction a slope is facing, which affects fire behavior due to differences in wind and solar radiation. Solar radiation, in turn, affects relative humidity and fuel types. Ignition and spread are most favorable on south and southwest aspects because they receive more sunlight and have higher fuel temperatures and lower relative humidity than on north and east-facing slopes. Daytime upslope windsare also stronger on south and west facing slopes (see Slope Winds).

Land configuration

Configuration of land influences the way a fire spreads through slope winds or coastal breezes, eddies at the top of ridges or other sharp topographic breaks, steep canyons, and barriers. As the sun heats land surfaces during the day, air begins to rise on slopes, or convectionally above flat land. Air from cooler locations (lower slopes or the ocean) pushes in behind the rising air, creating upslope winds in the hills or mountains or coastal breezes that flow onshore during the day. The upslope winds and coastal breezes may be major influences on the direction and rate of fire spread during the day. At night, the general air flow reverses to downslope or offshore directions, which can reverse the direction of fire spread.

In mountainous topography, the upslope winds on opposite slopes can slow the spread of a fire as it approaches ridge tops. However, air current eddies over ridges, through passes, and around other prominent land surface features can also spread fire brands and create erratic fire behavior (see Effects of mountains on winds).

In ravines or narrow canyons, fire burning on one side can radiate significant heat towards the other side, drying, preheating and increasing the availability of fuel on the other side (Pyne et al. 1996). Steep canyons may also produce a chimney effect in which a strong convection current is created from unstable air conditions at the surface, drawing air in at the base of a canyon and pushing it out the top. The rapid preheating of fuels and strong convectional winds in narrow canyons may lead to very rapid fire spread from the bottom to the top of the canyon. At times, the entire slope can ignite within minutes (see Valley Winds).

Fires can be controlled or slowed by land barriers. Rocks, bare soil, lakes, streams, roads and trails are all possible barriers.