LESSON 3 - EFFECTS OF WEATHER, TOPOGRAPHY AND FUELS ON FIRE BEHAVIOR

3.3 Topography

Topography is the physical features of the earth’s surface. These features are assessed in terms of what will stop, resist or encourage fire spread. Topographic maps provide information which will show the location of natural or constructed boundaries such as lakes, rivers, ponds, roads and will also provide information regarding scale (measurement of distances) and the determination of the degree of slope. This can be an extremely important source of information about fire spread and potential fire behavior now and in the future.

3.3 Slope

Slope is the upward or downward slant of the earth’s surface. Because there are various degrees of slope and it`s relevance to fire fighter safety make this information vital.

An extremely important aspect of fire behaviour is the fact that it will change dramatically when slope has been introduced to the fire environment. Many injuries or deaths have occurred while fire fighters were engaged in suppression activities on the side of a slope. As a fire moves up slope, two conditions will become apparent almost right away: Both Rates of spread and Intensity levels will increase. There are two reasons why this will occur:

1. The flame tip will become closer to the fuels therefore promote pre - heating.
2. Indrafts from behind will provide an additional boost in wind and oxygen supply.
   As slope increases so will the impact on fire behaviour
   

The fire fighter needs to be extremely careful working on slope.

More pronounced the slope, GREATER THE FIRE BEHAVIOUR

While working on slope, remember the following;

20% slope — 2 times the Rate of Spread.

30% slope — 3 times the Rate of Spread.

40% slope — 4 times the Rate of Spread
(Rule of thumb)

3.3 Aspect

Aspect is the direction the slope is facing. For example, slopes facing south are said to have a southern aspect.

Slopes with southern aspect are exposed to the sun for longer periods and will be warmer and drier. These areas will ignite more readily and burn faster than those with a northern aspect. In the spring, slopes with a southern aspect will lose their snow cover and develop burning conditions earlier than northern slopes.

Slopes with a southeastern aspect will warm earlier in the day as they face the morning sun, and fires may burn more severely earlier in the burning period. Slopes with southwesterly aspect will have maximum burning conditions later in the burning period.

Example of early snow melt on a south facing aspect. The result would provide more available fuel with higher ignition temperatures than on a north facing slope.