Wildfire assessment and control

April 29, 2013

In the world of forestry, the worst thing a forester can experience is to find that the plantation under his or her control is exposed to an uncontrollable wildfire. Such fires can start inside a plantation stand or within another vegetation type such as fynbos, grassland, savanna, wetland or agricultural crop, because plantation units are normally surrounded in the landscape by a range of vegetation and fuel types, each with their own fuel-related characteristics and fire behaviour.

by Neels de Ronde

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A wattle compartment in the KZN midlands survived this devastating fire, proving how effective it can be as a 'green' fire break.

 

Once a wildfire starts, we are faced with a range of scenarios ranging from controllable – low profile – surface fires, to turbulent and spotting crown fires, which can develop within a relative short period of time as a fire progresses. Add to this variation in the landscape (such as contrasting topography), a variety of vegetation and fuels, wind-exposed surfaces on ridges and plains, man-made barriers such as fuel-free areas (roads, railways, etc.) and natural barriers (such as indigenous forests, rivers with "green" riverine vegetation and wetlands with non-burnable vegetation margins), and a forester is clearly faced with enormous challenges when a wildfire approaches.

How will the fire behave? The forester might panic if he or she is not equipped with some vital fire behaviour assessment knowledge and experience, and there and then things can go off the rail. It is then that wrong decisions are sometimes made, which can result in unnecessary damage to property and even loss of lives. How can we equip the forest fire manager to make better decisions?

Normally the forester will be the first to sound the alarm. The next "action" while waiting for assistance will be to accept reality: some wildfires cannot be contained even with the best resources. Also, under extreme conditions, fire ignitions can become uncontainable within a few minutes and some sites are impossible to reach in time. Once such a situation has been identified, there is of course no way that such an uncontrolled fire can be contained by the forester and his resources, particularly not from the head of such a fire by means of direct attack.

No-one should ever be sent to the front of such a fire, and people in the path of such a fire should be evacuated immediately.

During the past few decades approximately 5% of all wildfires recorded have been responsible for 95% of all damage caused by wildfires. Today the forestry industry – realising this – has invested in improved fire prevention systems in many forestry regions.

These actions are indeed reducing wildfire damage significantly (according to my own estimations, at least over the past 4-5 years, and then only in the most prominent forest regions!). However, serious fires still occur, and are increasing in other (mostly non-forested) regions. For example, in the Free State 150 000ha of grassland was burnt as a result of at least 20 wildfires within a 24 hour period in August 2012.

Dynamics of vegetation and fuels

The most important factor that foresters need to note is the dynamic grassland fuel base of the summer rainfall regions, which have the potential for very fast spreading but relatively low intensity fires. In contrast, all shrublands (such as fynbos in the Cape forest regions) normally result in high intensity fires with regular flaring potential, but with a relatively low rate of fire spread.

In our plantations we are normally faced with a range of tree stand conditions. When such stands are very young, grasses persist as the main fuel in the summer rainfall regions. However, as the trees grow older, thinning and pruning slash can significantly contribute to dangerous surface fuel levels, which will mix with the mature grassland fuel component. This is the most hazardous situation in a tree stands' development! In contrast, mature stands with closed crown canopies can provide very safe lines from where even counter fires can be applied safely, to restrict lateral spread of a wildfire.

By this time I think many foresters realise that a mature and even-aged wattle stand (with slash fuel well-managed and crown canopies closed) can present a very effective firebreak. I have seen uncontrollable wildfires stop there and then.

What about the hazards of eucalyptus slash after clear felling? How many foresters know that a five-year-old pine stand is in fact an extremely hazardous five-year-old grassland?

In many instances even experienced incidence commanders and fire bosses are not properly trained in assessing vegetation and fuel dynamics, and do not know how to link this to predicted fire behaviour. I have to mention that our foresters are generally better equipped than fire managers outside of forestry regions, where the lack of fire training and experience is often shocking.

Weather and topographical parameters

No getting away from these important issues when assessing counter fire options. Here the foresters' experience comes into its own. Here are some typical questions: Is it a north-westerly wind or more typical Berg wind? How variable is the wind direction? How will the topography affect wind conditions?

I know from experience that the NW wind in the summer rainfall regions differs significantly from S. Cape Berg winds. The direction of the NW wind becomes more constant the stronger it blows, but in the Southern Cape and Tsitsikamma, Berg wind direction, strength and gusts can be extremely variable. This can change conditions within minutes, making any counter fire use impossible.

South-easterly winds in most of the Cape forest regions normally decrease by about 4 pm, but in some places it only reaches maximum strength at around 8 pm. This means you should not attempt any counter fire ignitions where such wind patterns are a significant feature, or where you have no experience of such wind patterns.

Conventional wildfire suppression methods

Our fire fighters are normally well trained to handle this scenario. However there is one aspect that I must emphasise, and that is the problem of smouldering fires when mopping up after prescribed burns or wildfires. Few fire fighters (and fire bosses/managers) understand the guarding requirements, which are linked to fuel types, and when such areas can be regarded as 'safe'.

Most foresters know the golden rule: Guard smouldering sites after a fire 24 hours a day for as long as necessary, or until adequate rain has fallen, armed with trained manpower, equipment and water supplies.

However, even foresters can be caught short by sudden changes in weather conditions, and such fires can flare up again.

The solution is to understand the relationship between fuel types and fire. Some fuels do not smoulder for long (such as many natural grasslands), while others are notoriously prone to long-term smouldering (such as slash heaps). Our bomber pilots know that one can throw water onto slash heaps until you are blue in the face, and the fires can still stand up again. Such smouldering sites can become worse where there are deep forest floor humus layers, and fires can even burn deep below the soil surface along root channels of old stumps. Many times such smouldering fires can only be brought under control by physically digging such fires out, or to surround it by deep trenches to avoid further spread.

Unconventional wildfire suppression methods

In conditions where wildfires in plantations and surrounding vegetation became uncontrollable, direct wildfire suppression methods cannot be applied, and unconventional means will have to be considered. Now we have indeed reached the most difficult decision-making stage for fire managers: how, when and where to apply fire against fire?

To recap, the first stage of fighting fire with fire is assessing vegetation and fuel, weather conditions and topography. Now the fire manager may decide that it is too dangerous to use a counter fire, for reasons such as (i) the fire is too turbulent and is spotting, (ii) wind conditions are too variable, or (iii) the topography is too contrasting for safe fire application. Additional reasons could be that there is no safe ignition line option available, or no safe escape route for the ignition crew.

Or the fire manager may decide that conditions are suitable for counter fire ignition, although this action needs to be postponed until such time as the weather conditions improve (particularly wind speed and direction), or the fire reaches less dangerous fuel conditions.

Whatever the case, there are some golden rules which apply before a counter fire is ignited:

  • Never apply a counter fire ahead of the head of a fire.
  • Apply only a fire parallel to the left and/or right flank.
  • Anchor the fire line to a flank perimeter line, nearest to the fire rear, to where the fire line is still burning, but is reachable safely with a drip torch.
  • Select a safe and as straight as possible counter fire route.
  • Always ignite the fire line from the direction where the fire started, into the direction the fire is spreading to (the head of the fire), and some distance away from the flank fire.
  • Stop ignition immediately whenever wind conditions change dramatically.
  • Avoid any conditions where the ignited fire line becomes uncontrollable, because this can be extremely dangerous.
  • If you do not know exactly what you are doing, or you are hesitating for whatever reason, do not ignite a counter fire line at all.

firrrre

Published in Feb 2013

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