8B3 The Effects of Topography
8B3.01 Topography can be described as the configuration of the Earth's surface, including its relief and position of natural and man-made features, arranged across the landscape. In the wildfire environment topography plays an integral part in determining how a fire will develop and spread across a landscape. In wildfire terms, it is the space in which a fire can move comprising the physical shape and positioning of the features within that space. These influences will have a direct and indirect impact on fire behaviour, and if the topographical influences are understood, fire officers can use this knowledge to determine the likely fire severity at different points across the landscape.
Photo B3.1 The shape of the terrain will influence fire behaviour
8B3.02 Topography should be considered to be a fixed or known factor that will influence variables such as fuel types, quantities, relative humidity, wind speed and direction, and the potential size and shape of the fire footprint. In addition, topography plays an important part in fire intensity, direction and rate of travel.
8B3.03 The shape of the landscape has a direct effect on the ability of the surface to interact with the sun, allowing some areas to become warmer and dryer while others remain cooler and damper. These variations can affect localised areas or cover larger parts of the landscape; in some situations this can cause rapid changes to fire behaviour.
8B3.04 Water drainage has also altered the shape of the landscape forming re-entrants such as valleys and gullies. These influence wind speed and direction, also providing habitats that alter the types and quantities of fuel within them.
The Profile of the Landscape
8B3.05 The shape of the landscape will influence wildfire behaviour in a number of important ways and consideration should be given to the following:
- Orientation and angle of slope in relation to the position of the sun
- Steepness of slopes
- Shape of the topographical features
- The effect of topography on the types and amounts of vegetation
- Water drainage features
- The effect of topography on wind direction and its strength
- Barriers to fire travel such as tracks, roads, streams, rivers, wetlands and other fire obstacles
Photo B3.2 Showing changes that can occur across the landscape
The Effect of Aspect
8B3.06 Aspect can be described as the direction the surface of the ground is facing in relation to its orientation with the sun. Generally, southerly-facing slopes become warmer and drier throughout the day, whilst northerly-facing slopes remain cooler and damper.
8B3.07 Although the angle of a slope is fixed the amount of solar radiation received will vary depending on the time of day and the position of the sun.
8B3.08 The amount of solar radiation the surface absorbs has a direct influence on the temperature and humidity of the air and therefore the combustibility of available fuels. As a general rule, fires that occur in fuels that have been subjected to more solar preheating will be more intense.
8B3.09 The diagram B3.1 above shows the direct relationship between temperature and relative humidity, as the temperature rises the relative humidity falls, this effect on the general landscape peaks in the afternoon but the timing of this will vary depending on the time of year.
Fig. B3.1 Diagram of how flammability of fuels can change throughout the day
The Sun's Position
8B3.10 South-eastern-facing slopes will be warmer in the morning, drying out the fuels earlier in the day. South-western-facing slopes warm later when the sun moves into alignment. Fuels on these slopes are likely to continue to increase in temperature during the day until late into the afternoon.
Photo B3.3 Showing the south-westerlyfacing slope being warmed during the afternoon
The Effect of Slope
8B3.11 Slopes can assist or hinder fire development in a number of ways, they can be in or out of aspect which will affect air and fuel temperature and the steepness of a slope also influences fire spread and intensity.
8B3.12 When a fire moves across level ground or downslope most of the heat generated by the fire
(in the absence of wind) will be lost to the atmosphere.
8B3.13 When a fire moves onto an upslope, the slope will assist the fire's development and increase its efficiency, accelerating the combustion process. The fuel is brought closer to the flame by the angle of the slope and this is then subjected to more preheating. The steeper the angle of the slope the more preheating the fuel is subjected to.
8B3.14 Depending on the actual gradient of the slope, the convection plume will penetrate into more of the vegetation ahead of the fire front, drying out the air and also raising the temperature of the fuels, particularly the finer ones. This preheating of the vegetation is not limited to the surface fuels, but can also include aerial fuels, particularly those in the canopy; this can greatly increase the ease of ignition of these fuels. The longer and steeper the slope, the more influence this process will have leading to an increase in intensity. Fires on steep slopes will normally develop a narrower and more intense head fire.
Fig. B3.2 A Fire that is burning upslope is able to transfer its energy to the un-burnt fuel and is classed as being in alignment with the slope
Fire Position on a Slope
8B3.15 The position at which a fire starts on a slope is important; generally fires that start nearer the base of a slope become more intense. This is due in part to the fire remaining in alignment with the slope for a longer period; this allows the fire more time to heat up the fuel and air in the fire's path. A fire moving upslope will generally increase in speed and intensity, as it progresses up the gradient, the steeper the slope the faster the acceleration.
Fig. B3.3 Fire that is burning upslope will burn faster and with more intensity
Fires Burning Downslope
8B3.16 When a fire burns downslope the process is almost reversed with a widening of the gap between the available fuel and the fire, the movement of the fire will slow and its intensity will reduce.
Fig. B3.4 Fire that is burning downslope is not able to transfer energy to unburnt fuel as effectively as an upslope burn and will burn with less intensity
8B3.17 A significant hazard for firefighters working on a slope in un-burnt fuel is burning material may roll down hill and ignite fuels behind them, with the result that they can become trapped between the two fires.
Slope Reversal (upslope to down slope)
8B3.18 Slope reversal is a term used to describe a situation where a fire changes its alignment with slope. This can happen when a fire that has been moving upslope reaches the top of a hill and then starts to move downslope. Once burning against the slope the fire changes its behaviour, and unless supported by a strong wind will normally significantly reduce its intensity. Most of the heat generated by the fire is lost to the atmosphere and the change in fire alignment causes a significant reduction to the fire's rate of spread.
Photo B3.4 A pre-slope reversal low-intensity fire
Slope Reversal (downslope to upslope)
8B3.19 This second scenario is much more dangerous and occurs when a fire that is backing downslope reaches a point where it can start to turn and burn upslope. This change can result in a dramatic and sudden increase to the intensity and speed of the fire, this type of reversal should be considered to be particularly hazardous.
Photo B3.5 Showing a significant increase in fire intensity due to the fire now burning upslope
8B3.20 Another, and possibly more dangerous situation, is where a fire is backing downslope and moving against the wind. The appearance of the fire can be very deceptive - sometimes demonstrating very limited intensity. Should any part of the fire move into a position where it finds alignment with the slope and wind, then it may make a run back upslope through any unburnt fuel. This can result in a rapid and high intensity head fire that can develop in the opposite direction of the backing fire which may compromise the safety of firefighters that are positioned upslope.
8B3.21 Dangerous locations where these changes may occur should be proactively identified and used as trigger points (indicators) to highlight the necessity to review safety issues and operational tactics.
The Shape of Slopes
8B3.22 The shape and angle of a slope will affect the ease in which the fire is able to transfer heat to existing vegetation.
Slopes are termed to be:
8B3.23 A straight slope has a near constant angle that results in a more predictable rate of spread. Any increase in speed or intensity will be due to the effect of preheating of fuels or changes to vegetation or its arrangement.
Personnel at the top or at the base of the slope should have an unobstructed view of the slope's surface and the fire unless this is limited by other factors such as vegetation.
Fig. B3.5 The shape of slopes can affect fire spread and intensity
8B3.24 Concave slopes are ones that have an increased angle of slope nearer the summit; this can result in a significant increase in fire intensity as the fire moves upslope. A fire moving downslope will gradually increase in intensity as the angle of the slope decreases nearer the bottom where more heat can be transferred to the unburnt fuels. A concave shaped slope should not affect line of sight when standing at the bottom or top of the slope.
8B3.25 Convex slopes have a steeper angle at the base compared with the summit; therefore the fire is likely to be more intense at the base of the slope. As the fire moves upslope the angle of slope lessens and the amount of heat transferred to the unburnt fuel is reduced.
8B3.26 If a fire is moving downslope the angle between the fire and the fuel is increased resulting in a decrease in potential fire development. Due to the shape of this type of slope, personnel may have their view of the fire obstructed. In particular personnel at the base or at the top of the slope may not be able to observe the fire at all.
Photo B3.6 The feature shown in the photograph has all three types of slope in evidence
8B3.27 The shape of the landscape can influence fire behaviour in a number of significant ways. One of the most important topographical influences is on wind which can be channelled and funnelled through and along features of terrain; this may alter its direction and strength.
Photo B3.7 Showing a hill
8B3.28 The slopes forming the sides of a hill will either support or hinder fire development depending on whether a fire is burning up or down slope.
On the ground definition:
"A hill is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric circles. The inside of the smallest closed circle is the hilltop."
Photo B3.8 A saddle is shown in the foreground
8B3.29 Saddles are identified as a curvature in the landscape formed between two areas of higher ground. The wind passing within saddles tends to concentrate and can fluctuate or alter direction.
On the ground definition:
"When in a saddle and facing forward there will be an uphill slope in front of you, an uphill slope behind and downhill slopes to your left and right. On a map the contour lines might look something like an hourglass."
Fig. B3.6 Showing how hills and saddles are represented on an OS map, the shape of this terrain could easily alter wind direction and strength
8B3.30 Spurs are often steeper than the slopes that surround them and this may alter a fires speed, direction and intensity.
On the ground definition:
"When heading up a valley spurs will often look like steeper ground sticking out of the surrounding slope, at the end of which will be a steep slope dropping into the valley. From above, spurs will not seem as steep as the surrounding slope and have increasingly larger slopes running downhill to the left and right of it, with a steep slope at its end."
Fig. B3.7 The contours on the map show a number of spurs on the landscape formed between re-entrants
Photo B3.9 Showing several ridge lines
8B3.31 Ridges are features formed between two opposing slopes. They are sometimes the most suitable locations to construct a fire break because when the fire reaches the ridge line it loses alignment with the supporting slope and its intensity and rate of spread will reduce.
On the ground description:
"When standing on the centre-line of a ridge, you will normally have low ground in three directions and high ground in one direction with varying degrees of slope. If you cross a ridge at right angles, you will climb steeply to the crest and then descend steeply to the base. When you move along the path of the ridge, depending on the geographic location, there may be either an almost unnoticeable slope or a very obvious incline."
Water Drainage Features
Photo B3.10 A number of re-entrants formed by drainage features
8B3.32 Re-entrants are normally drainage features with slopes on either side. They can have a significant effect on fire behaviour as they can act as a chimney channelling the wind and concentrating its strength. They are often critical points on the landscape allowing fire to alter its direction and speed of travel.
On the ground description:
"When facing up a valley there will be smaller valleys running uphill to your left and right. These will become smaller and may look like folds in the landscape the higher up the valley you go."
Photo B3.11 Showing a steep-sided gully
8B3.33 A Gully is a re-entrant feature that has been formed by running water which can cut sharply into the ground, often the sides can be very steep. This type of feature can cause what is termed to be the chimney effect acting as a funnel for winds to follow and in certain situations concentrate its strength. This can increase fire intensity and rate of spread along these features substantially.
8B3.34 In narrow gullies, as a fire moves down one side the heat generated can preheat the fuel on the opposite side resulting in a much more intense fire if slope reversal occurs.
On the ground description:
"Having already gone up a slope there will be a steeper, narrowing in front of you, and steeper uphill slopes on either side. There may be water running down the gully and some scrambling or climbing might be needed to get up it."
Photo B3.12 Steep-sided gully
Photo B3.13 A narrow valley
8B3.35 The main effect of valleys is in the way they can alter the direction and strength of the general wind, normally the wider the valley the less concentration of wind strength although this might increase as the wind moves upwards where the valley is narrower.
On the ground description:
"Valleys drop from higher ground and generally become broader as they lose height. When standing in one they can have steep, even slopes, narrower near the end of a valley, though they can also be wide, with a gentle curve on either side getting slowly steeper."
Fig. B3.8 An example of how valleys are shown on a map
8B3.36 Contour lines forming a valley are either U-shaped or V-shaped. To determine the direction water is flowing, look at the contour lines. The apex of the contour line (U or V) always points upstream or toward high ground, and contour height numbers are always orientated up the slope.
The Effect of Topography on Vegetation
Photo B3.14 Changes in the direction a slope faces can influence fuel arrangement and continuity
8B3.37 The orientation of a slope affects the amount of sunlight it will receive. In the short term this can affect the temperature and moisture content present within available fuels. In the longer term it can influence even the type and condition of fuels growing on different slopes. Southerly facing slopes are normally warmer and dryer while northerly facing slopes are cooler and damper.
8B3.38 These variations create microclimates that can influence the species that will colonise different parts of the landscape. The soil found in re-entrants is normally damper and this encourages more vegetation growth which may lead to heavier fuel loading.
Wetter areas of the landscape can often be identified by the vegetation types that grow on it.
The Effect of Topography on Wind
Fig. B3.9 This shows the way in which the shape of the terrain can influence wind direction and strength
8B3.39 The illustration above demonstrates how the wind can be influenced by the lay of the land. The terrain can channel wind along valley's and re-entrants sometimes causing it to significantly change its direction. Narrow valleys and gully's can funnel and accelerate the wind increasing its influence on fire behaviour.
Photo B3.15 Terrain will affect the direction and strength of wind
Barriers to Fire Travel
8B3.40 A landscape may contain areas that will restrict fire travel. A barrier is any area where there is a break in the horizontal fuel arrangement that prevents fire spread. Natural or man-made features can cause this break in continuity. Barriers to fire spread should, wherever possible, be taken advantage of within any suppression plan.
8B3.41 Common barriers to fire spread include:
- Rock outcrops or cliffs and bare patches of land.
- Lakes, rivers, streams and wet areas of land.
- Changes to the fuel type, its condition or moisture content. An example of this might simply be an area of wet, green grass.
- Roads, tracks or other man-made barriers that prevent fire spread.
- Tactical control lines either dug into the soil or burnt into the vegetation.
Photo B3.16 An outcrop of rock that has broken fuel continuity and could prevent fire spread
8B3 Key Considerations
- The shape of the topography has a direct impact on the drying environment; fuel that is in direct sunlight will be warmer and dryer than fuels that are not.
- Warmed air will move upslope during the day and cool air will fall downslope during the night.
- Slope has a major impact on fire behaviour and an upslope will cause a fire to increase its intensity and speed.
- The steepness of a slope has an incremental effect on fire behaviour and its speed.
- Fire intensity and rate of spread normally reduces when fires are burning downslope.
- When constructing fire breaks look to take advantage of reverse side of ridges whenever possible.
- Gullies and Re-entrants can be dangerous - having personnel working in them should be avoided due to the danger of the 'chimney effect'.
- Barriers such as rivers, rock outcrops and roads can often prevent fire spread and, whenever possible, should be incorporated into the tactical fire plan.
- Parts of a fire burning downslope may find an alignment that allows it to make a run back up the slope presenting a danger to personnel working above the fire in unburnt fuels.
- The shape of the terrain will have a significant impact on fire behaviour; plan to take advantage of areas where intensities will be low and avoid areas where fire intensities will be high.
- Topography will influence wind speed and direction, be aware of local conditions and remain alert to possible change.
- Southerly-facing slopes will generally be warmer and dryer than those that face towards
- Eastern-facing slopes warm early in the day; western-facing slopes are warmer in the afternoon.
Email: Dean Cowper