Know the bushfire’s behaviour, manage the bushfire, protect the house, protect the nation.

The principles and science of self defence

Denis O’Bryan 2010

This is an abridged version of another paper

INTRODUCTION

Two different but complementary strategies for dealing with a going bushfire can be identified, offensive and defensive suppression. In this abridged note, offensive suppression is defined as the act of attacking the moving edge of the bushfire, which is the traditional role of the fire fighter. Defensive suppression is the act of defending an area or a person or an object from bushfire attack. Typically, this applies to residents when they protect their property, but it also applies to fire fighters who defend a control line.

Sources of information and training material for offensive suppression are numerous, whereas little coverage has been given to defensive suppression, apart perhaps from training fire fighters to deal with a flame-over emergency.

The purpose of this paper is to identify the theories and principles behind defensive suppression and to show that they derive from and have similarities to the long practiced principles of dry fire fighting.

It is the author’s hope that the people who practice defensive suppression can use these principles to check the effectiveness of defence preparedness and personal safety, in the same way that fire fighters use the principles of dry fire fighting to ensure suppression effectiveness and crew safety.

THEORY OF OFFENSIVE AND DEFENSIVE SUPPRESSION

Offensive suppression

Offensive suppression is the act of attacking / stopping the spread of the moving edge of the bushfire. It usually applies to the entire fire perimeter or one or more of the flanks.

Traditional fire fighter teaching describes two key strategies of offensive suppression, direct attack and indirect attack.
– Direct attack focuses suppression activity at the fire edge. It is generally done with water from a hose or a tanker. This is known as wet fire fighting. A variation of wet fire fighting is water application from an aircraft, known as water bombing.
– Indirect attack focuses suppression activity away from the fire edge. It is also known as dry fire fighting.

This paper now focuses on the theories behind indirect attack by dry fire fighting. Luke and MacArthur (1978) confirm that indirect attack can be effective where direct attack against a severe head fire is not advisable. Cheney and Sullivan (1999) note that under extreme conditions, stopping head fire attack in grass fires by offensive suppression tactics is rarely possible. In these circumstances, they recommend the use of indirect attack.

Traditional teaching describes these requirements for successful dry fire fighting:
Firstly, the fire controller designates a control line at a distance from the moving fire front. The moving flame must be stopped at this line. The control line is a fuel free strip, normally a road or a track or firebreak.

Secondly, the fire fighters create a fuel free barrier in between the fuel free strip and the fire edge. It is usually done by burning out. In essence, the burning out extends the width of the control line. There are two variations of the burning out process – burning out and back burning (see details below).

Finally, the fire fighters systematically patrol the control line to ensure spot fires do not take hold on its downwind side.

Clarification points:
1 . Traditional teaching identifies two variations of dry fire fighting – burning out and back burning. The essential difference between back burning and burning out on a severe bushfire is the proximity of the control line to the fire edge.
– Burning out is usually a few hundred metres to a few kilometres and therefore creates a very wide burnt out strip. This means the flame at the fire edge is a long distance away from the fire fighters on the control line and the short distance spotting tends to fall into the burnt out area.

– Back burning is usually within a few tens to a few hundred metres of the control line. Its narrower burnt out strip means that the approaching flame is relatively close to the fire fighters on the control line, and while some short distance spotting falls within the burnt area, some will cross the control line.

2 . The fuel free barrier refers to the fuel bed on the ground. It needs to be wide enough to deal with the danger elements of the moving flame as follows:
– Ensure the flame does not stretch across it. Technically speaking, barrier width needs to exceed flame rollover / stretch distance. This means the flame stops at the barrier.
– Keep the moving flame well away from the defenders. This means the risk of danger due to flame contact and radiation is minimal.

3 . The role of the fire fighters is to patrol for spot fires downwind of the control line and to extinguish them when small. Traditional teaching also stresses that the success of indirect attack / dry fire fighting depends on availability of sufficient numbers fire fighters.

Defensive suppression

Defensive suppression is the act of defending an area from bushfire attack. Consider the case of fire fighters defending a control line using backburning and transpose the same principles to a resident defending their property.

Firstly, the effective defender designates a control line along which they will be defending (= defence area).
The fire fighter may be defending an asset downwind of this line, or may be aiming to stop the moving flame at this line.
The resident will be defending their designated assets (eg, house).

For the fire fighter, the control line is the track or fire break.
For the resident, the control line is a fuel free area that they have prepared around their house. In the self defence system, this area is called the NO FLAME ZONE.

Secondly, the effective defender widens the fuel free barrier. The fuel free barrier needs to be wide enough to:
– Ensure the flame does not stretch across it. This means the flame stops at the barrier.
– Keep the moving flame well away from the defenders. This means the risk of danger due to flame contact and radiation is negligible.

For the fire fighter, this is done by removing fuel on the ground by planned burning.
For the resident, this has already been done by making the fuel free area wide enough between the defence area and the approaching flame.

Finally, the effective defender systematically patrols the defence area to ensure spot fires do not take hold.
For the fire fighter, this is done by actively extinguishing spot fires in the downwind section beyond the track.
For the resident, this is done by actively extinguishing spot fires within the defence area surrounding the house.

APPLICATION OF PRINCIPLES TO SELF DEFENCE ON A PROPERTY

This paper proposes that just as the principles of dry fire fighting have proven successful over a long period, applying the same principles to defensive suppression whether on the control line or at a property will also be successful.
Like dry fire fighting (offensive suppression), defensive suppression has three requirements – a fuel free barrier, management of flame in upwind zone (ie, direction of attack) and a team of defenders. They are both required because they have to deal with the moving flame and with ember attack.

Core requirements for successful defensive suppression and self defence

Like dry fire fighting (offensive suppression), defensive suppression or self defence has three requirements – a fuel free barrier, management of flame in upwind zone (ie, direction of attack) and a team of defenders. They are both required because they have to deal with the moving flame and with ember attack.

The following list summarises the similarities of the core requirements for successful defensive suppression by fire fighters and self defence by residents.

Defence area must be a fuel free barrier
The barrier that protects the defence area where the defenders are, must be fuel free (refers to the on-ground fuel bed)

This applies to both fire fighter and resident.

Fuel free barrier must be wide enough or be protected by an upwind low flame zone

The fire fighter’s chance of stopping the moving flame on a given control line by dry fire fighting is improved by pre planning. If the fuel bed has been fuel reduced upwind of the control line before the fire season, its flame height will be low and its residence time will be short, which means radiation level and its duration and risk of flame contact by rollover across the control line will not be a threat to the fire fighter. If this has not happened, the back burn strip must be made wider.
Similarly, the resident is able to protect their defence area by nominating an area whose maximum flame height in the worst weather will be low enough to protect the defender.
In the self defence system, this area is called the LOW FLAME ZONE.

It well known that at a given distance from a flame, incident radiation is related to flame height. It is also known that flame rollover distance, or flame stretch, varies with flame height upwind of a fuel free barrier. It is also well known that flame height is manageable because it is proportional to fine fuel load. Therefore, incident radiation on an object can be managed by managing flame height on the upwind side of the fuel free gap (all other things being equal).

What is flame height? It is proposed that predictions be made for the most severe fire danger weather. This means the flame height predicted will be the maximum expected.

Managing Ember attack
The principles of fuel free gap width and flame size and duration in the adjacent upwind zone will stop an approaching flame but will not stop an ember attack. However, having stopped and weakened the flame, the defender is free to focus on ember attack. Ember attack is also called spotting. Ember attack is the main cause of ignition and loss of buildings during a bushfire. There is ample recent and historical evidence that houses burn down in residential areas by ember-generated stationary flames

The defender uses these two observations to help his defence efforts:
If a live ember lands in a flammable fuel bed, it ignites and burns.
If a live ember lands in a non-flammable fuel bed, it cannot ignite.

The fire fighter works within the fuel free barrier, which is non-flammable, and extinguishes spot fires as soon as they develop, ie, while small, in the downwind area.
The resident, who has already minimised the surface area of flammable sites within the defence area, works within this area to extinguish spot fires as soon as they develop.

Physical capacity of defence team
Just as the length of control line that a team of fire fighters can defend is limited, so is the area a team of self defending residents can defend is limited
It usually applies to a specific part of the control line or to a specific property or neighbourhood.

CONCLUSION

This paper shows that the principles and core requirements of dry fire fighting are relevant and transferable to defensive suppression and suggests they are essential to the success of self defence. Therefore, if people wish to use self defence, they should be aware of the principles and requirements of defensive suppression. Furthermore, if advice is given to people to self defend without including the principles and requirements, the advice is seriously deficient.

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