The term fuels refers to any carbonaceous material that can be ignited and sustain fire.

This chapter examines the two broad categories of fuel:

  • Forest fuels
  • Grass fuels

It covers common fuel terminologies such as:

  • Fuel load
  • Fuel hazard
  • Fuel arrangement
  • Total fuel
  • Available fuel
  • Fuel continuity
  • Residence time

As a firefighter, you need to be able to assess the type, load and arrangement of fuel around you when required. It is extraordinarily important as a firefighter to understand the role of fuel when predicting fire behaviour. 

Common fuel terminologies


Fuel load

The fuel load is a measure of how much fuel is present and available to burn. In grass we consider the fuel on the ground. In forest we consider fuel on the ground and elevated in the understory and we often need to include bark fuel. It is a combination of these fuels together that determines the overall forest fuel load. Crown fuels are not included in calculations but may be consumed under certain conditions.

When fuel load is estimated, only items below a diameter of 6 millimetres are taken into consideration. Everything above this threshold is classified as coarse fuel and not considered as a contributor to instantaneous fire behaviour. Coarse fuel contribution is more relevant when reference is being made to a fire’s residence time as it will continue to burn for some time after a fire front has passed.

The unit of measure used for fuel load is tonnes per hectare (t/ha).

Fuel hazard

Fuel hazard is a measure used to determine the hazard of a fuel source in question. The model aims to make an appreciation of fuels across multiple forest strata and rank its hazard between low and extreme using a set of assessment rules. The final overall fuel hazard can then be used to estimate fuel load or even probability of initial fire suppression success.

Further information relating to fuel hazard can be gained from the Overall Fuel Hazard Guide, Third Edition May 1999. This is referred to as OFHAG in this manual.

The NSW RFS currently uses the Department of Sustainability and Environment’s, Overall Fuel Hazard Guide, Third Edition as its primary method of assessing fuel hazard and fuel load. The Fourth Edition has not been endorsed by the NSW RFS at March 2015 due to issues with the conversion of fuel hazard scores to fuel load

Fuel arrangement

Fuel arrangement is a vitally important concept which can have a far greater impact on fire behaviour than fuel load. It refers to the physical arrangement of the fuel and must be taken into account.

If fuel is densely packed, the surface area of the fuel directly exposed to oxygen is minimal. Inversely, a lightly packed fuel load has significant surface area exposed to oxygen allowing for much more efficient combustion.

Total fuel

Total fuel is the amount of fuel that is available to burn under the driest of seasonal conditions, that is, a day with an Extreme or Catastrophic Fire Danger Rating.

Available fuel

Available fuel is the quantity of fuel that will actually burn in a fire in the leading edge. The amount of fuel available to burn in a fire is strongly dictated by the moisture content of the fuels. This moisture content is driven by a number of factors including recent rain, long term drought effects and even topographical aspect.

Fuel continuity

Grass which grows in isolated clumps may need some wind assistance to carry a fire effectively. The fuel and its ability to carry fire is affected by its continuity.

Continuous fuel

Continuous fuel refers to a fuel structure that is connected. This allows fire to spread from fuel source to fuel source conductively without the need for any other mechanism and it is the dominant factor in the influence of fire spread in grassland.

Discontinuous fuel

Discontinuous fuel refers to a fuel structure that is broken up, making the spread of fire more difficult. Grassland made up of clumps that are generally disconnected will often rely on wind to spread from clump to clump. Similarly, eucalyptus forest, pine forest and heath are also affected by fuel continuity in their own ways. For example patchy fuel in a eucalyptus forest can become an issue during HR work. A low fire danger rating (FDR) combined with patchy fuel or obstructions will impede the spread of fire and may contribute towards self-extinguishment.

Residence time

Residence time is a term used to describe how long it takes a particular fire to burn out the fuel in a given location.

Grass fuels

The way and rate at which grass fuel forms is much different to that of forest fuel. An area of land can have a low grass fuel load one year and an extreme load the next. Much of this depends on the seasonal conditions leading up to any one particular fire season. Grass fuels rely on three main things:

  • The amount of fuel i.e. grass height, the growth density etc.
  • Fuel arrangement i.e. clumping, continuity, compaction etc.
  • The level of curing of the fuel i.e. the moisture content of the fuel

Grassland curing

Curing is an indication of the fuel moisture content of grass. It is part of the life and death lifecycle of grasses. Living vegetation will contain up to 35 times more moisture than dead vegetation. Most grasslands will not burn until at least 50% of the grass has cured. This is a predominant factor in determining the fire danger in grasslands. Large fast moving fires are usually not possible until the grassland is greater than 80% cured. Once grasses are 100% cured, extensive high intensity fires can occur.


Grassland curing guide

Grassland curing map

Because well cured grass is a very fine, easily combustible fuel, it reacts instantaneously to the influence of wind. The rate of spread and intensity can change very quickly. Wind gusts that may occur without warning can be life threatening and the intensity can be severe.

Grass fuel condition

As assessment of grass fuel in tonnes per hectare (t/ha) is not considered practical this factor has been simplified to:

  • Natural
  • Grazed
  • Eaten out

Natural

Natural grassland is grass that has been allowed to grow and cure without any kind of grazing activity. Grass will typically grow to its natural height and produce seed heads. It is generally more than 50 centimetres tall.


Natural grassland

Grazed

Grazed fuel, as the name suggests, is grassland fuel that has been grazed on by livestock or has been mown. Generally it is less than 10 centimetres in height. This condition is very common across the agricultural zones of Southern Australia. 


Grazed grassland

Eaten out

This pasture has been heavily grazed and is often less than 3 centimetres in height. Fuel continuity is often broken by patches of bare earth. This kind of pasture is commonly found during severe drought.


Eaten out grassland

Grass fuel load estimation

Table 2 can be used to estimate fuel load based on the observable fuel condition and assumes that the grass curing is such that it is available for combustion.


 Approximate grass fuel load

Non-native grasses

Some non-native grass species are more flammable than native Australian grasses. Examples of these grasses include African Love Grass, Molasses Grass and Setaria.

The fire behaviour exhibited by these grasses is extreme and if underestimated has the potential to cause serious injury or even death.


  African Love Grass                   Molasses Grass                         Setaria
  Examples of non-native grasses that exhibit extreme fire behaviour


It is important to be aware of the potential for non-native grasses to exhibit high intensity fire and accelerated rates of spread, which may cause a risk to firefighters and assets. 

Forest fuels

Forest fuel is made up of a number of layers which contribute various amounts of fuel to the overall fuel load estimate.

Fuel layers

Fuel in forests, woodlands and shrub lands can be divided into five layers:

  1. Surface fuel
  2. Near surface fuel
  3. Elevated fuel
  4. Bark fuel
  5. Canopy fuel

Forest fuel strata

Each of the layers contributes to fire behaviour in their own way. For example the contribution of bark is centred on spotting whereas the surface and elevated fuels primary contribution is rate of spread.

Canopy fuel is not included in the final fuel hazard calculation as its contribution can be quite variable.

Also note that the NSW RFS does not treat near surface fuel as an independent contributor to overall fuel load. Instead it is treated as a ladder fuel connecting the surface and elevated fuel strata.

Surface fuels

Surface fuels are defined to be leaves, twigs (<6 millimetres dead or <3 millimetres live), bark and other fine fuel lying on the ground. They are predominantly horizontal in their orientation. This layer will often make the greatest contribution to the overall fuel load measurement. When measuring surface fuel we include the duff layer (decomposed fuel) on the soil surface. Often the thickness of the duff layer can play a significant role in the ability to successfully black out a fireground as it can have a tendency to continue to smoulder for a number of days and become a possible source of re-ignition.

Surface fuels are measure in tonnes per hectare (t/ha). They can be more generally described as being light, moderate or heavy – see figure below.


  Light (5t/ha)                                         Moderate (10 t/ha)                             Heavy (20 T/ha)
Examples of surface fine fuel hazard

The continuity of the surface litter also plays a major role in the hazard it presents.  This feature that will often determine how effectively this layer can propagate (spread) a fire. Continuity can be interrupted by bare soil, rocks, cliffs or heavy fuel such as logs.

Note that the above describes how to measure fuel for scientific purposes and for hazard reductions where there is enough time to make more precise calculations. Later in this chapter methods for making more rapid calculations in the field under the pressure of a fire are outlined.

Elevated fuels

Elevated fuels are defined as the fuel layer between the canopy of the forest and the surface litter on the ground. These fuels are mainly upright in their orientation with most plant material generally to the top of the layer.

Typical species found in the elevated fuel layer would include but not be limited to banksia, leptospermum (tea tree), acacia and so on. It is also possible for dead fuels from the canopy and forest above to be suspended on top of this layer.

The elevated fuel hazard is typically at its highest when foliage and twigs are very fine (1–2 millimetres maximum) and the proportion of dead fuel is very high. A high hazard is also present if there is significant horizontal density and continuity. The following table provides a simple description of how vegetation density roughly translates to a fuel load.


Elevated fuel contribution

Near surface fuels

In the NSW RFS, near surface fuel is not treated independently as a contributor to fuel load; however it is assessed as a ladder fuel, connecting the surface and elevated fuel layers.

Bark fuels

Bark fuel is the fuel on tree trunks and branches. Bark that has fallen from the tree and is lying on the ground is considered surface fuel. Bark that has fallen and is suspended in the branches is elevated fuel.

Spotting activity induced by bark from various species of tree can make or break fire suppression efforts through short and long range spotting.


Bark hazard in play

The aerodynamic properties of some barks actually assist spotting processes by allowing embers to remain aloft for extended periods of time. Under extreme fire conditions, some embers have been known to remain aloft and burning for up to 25 minutes.

Levels of bark hazard

Barks can present different levels of hazard:


  Low                                           Moderate                                 High

  Very high                               Extreme
  Levels of bark hazards

Bark fuels take many forms, depending on the species in question. In order to simplify this, bark is broken down into 3 main subgroups:

  • Stringy bark
  • Candle bark
  • Other bark

The key attributes for assessing the effect of bark on suppression difficulty are shown in the following table.


Bark key attributes


Failure to properly assess the potential impact of bark (underestimating or overlooking spotting behaviour) on a fireground can have very serious consequences.

Stringy bark

At an FFDI of 25, stringy bark types can produce massive amounts of embers and short distance spotting. Stringy bark is fibrous with easily visible fibres less than 1 millimetres thick but may form long string like strands when peeled off. New bark is held tightly to the trunk with older long unburnt bark held very loosely. This kind of bark is very flammable with fire readily climbing the trunk and branches of the tree.

Very fine fibres of this bark will typically burn out within one minute with small wads taking up to 2–3 minutes to burn out. Large wads of bark may have a burnout time of up to 10 minutes.

The hazard of this bark type can reach extreme and increases over time as the thickness and looseness of the old bark increases.

With a high bark hazard, this bark will cause infrequent spotting and fire will climb some of these trees. When the hazard increases to extreme, in other words there is a significant amount of fibrous bark available to ignite and break away from the tree, the quantity of spotting generally makes fire control very difficult or impossible with fire climbing virtually every tree.


Example of stringy type bark

Candle bark

At an FFDI of 25, candle bark types can produce substantial quantities of spotting at distances greater than 2 kilometres. Short distance spotting is also possible with this bark type. Trees are characterised by the annual shedding of old bark layers exposing a smooth bark underneath.

As this bark sheds it often forms long ribbons which, when long enough, tend to fall and drape over the limbs of the tree and surrounding shrubs. This particular bark type is moderately flammable with fire typically climbing up the ribbons of bark under the right conditions.

Large quantities of this bark may tend to drape over upper parts of the tree trunk forming ladder fuels from the elevated fuel layers into the trees crown.

Bark pieces are relatively light and have been known to travel like a kite up to 30 kilometres downwind under the right atmospheric conditions. Airborne bark can burn and smoulder for up to 10 minutes or longer.


Example of extreme candle bark fuel

Other barks

Other bark types cover species such as iron barks, papery barks, coarsely fibrous barks and slab bark. At a FFDI of 25 these barks only contribute up to a high bark hazard with infrequent spotting being the maximum effect.

Bark contribution to fuel load

Bark does have a measurable contribution to total fuel load; however it isn’t the dominant one. In some cases, bark fuel can contribute up to 7 t/ha. The impracticalities of calculating the actual bark fuel load on the fireground mean that it isn’t used in fuel load calculations at the WFB level.

Instead, we need to focus on how it contributes to spotting behaviour and the complexities it presents as the FFDI increases.

Forest fuel load assessment in the field

A fast field based assessment can be made using the Knee / Waist / Hip estimation Guidelines for Fuel Reduction Burning (Guidelines for Fuel Reduction Burning Under Dry Forests, Forestry Commission of Tasmania, 1984). 

Step 1 – Estimate Surface Fuel

  • Find a representative spot and visualise a 2m radius circle around you.
  • Estimate the depth of surface fine fuel less than 6mm in diameter.
  • Estimate the % coverage surface of fuel within the circle.
  • Every 10% of cover at 20mm depth approximates to 1 t/ha.
  • If the coverage is 100% with 20mm of depth, this approximates to 10 t/ha.
  • If the coverage is less than 100% reduce the fuel load e.g. 80% coverage = 8 t/ha.

The table below provides some examples


 Calculating surface fuel loads in the field

The following figure may assist in estimating % coverage.


  Assessing surface fuels

Step 2 – Estimate near surface and elevated fuel.

After estimating surface fuel, at each site estimate the percentage coverage in 0.5m increments within your 2m radius circle. Every 20% = 1 t/ha for each layer.


 Knee + waist + shoulder method

Step 3 – Estimate total fine fuel


Calculating total fine fuel loads in the field

Do not worry too much over exact measurements as using an average will smooth out results. Taking an average of two people’s samples will also reduce any bias in estimating.

Keep in mind that this method is an estimate and results will vary from person to person. However, results will still be useful in identifying general trends and highlighting areas of higher or lower fuel across a fireground. 

Bark fuel

The knee + waist + shoulder estimation does not include bark fuel. Reference could be made to the Overall Fuel Hazard Guide bark section for this fuel element. This will provide an indication of the likely contribution of bark to fire behaviour.  Do not attempt to combine a t/ha score as the two systems were developed separately.

Fuel accumulation curves

A fuel accumulation curve is a graph that shows estimates of how much fuel will accumulate over years – see following pages for fuel accumulation curves for different types of vegetation.

Fuel curves show an average value, actual fuel load could vary based on site conditions such as climate, topography and past fire intensity. Fuel curves can be used to validate field estimates. If there is a significant variation between the field measurement and the fuel curve then it is worth considering if any site features such as rock outcrops or fire severity could vary the actual fuel load.

The University of Wollongong has completed a report titled Fuel Load Dynamics in NSW Vegetation, Part 1: forests and grassy woodlands. This project combines several fuel accumulation studies to develop fuel accumulation curves for key vegetation types. Additional vegetation types are being included as fuel information is collected.

The data from this report has been used to produce fuel accumulation curves in an excel workbook titled Forest and Woodlands Fuel Dynamics Calculator. This is available from the RFS Community Planning Section.

The accumulation of fuel load is not an infinite one and fuel loads will be bounded by natural limits depending on the locale.

The important information to obtain from these diagrams is the levelling off that occurs after a number of years, indicating that a theoretical maximum for fuel load does exist. Note that the fuel load reaches 90% of its theoretical maximum after only 10 years.

A maximum value is reached as the fuel at the soil interface will gradually decompose and break down. As more fuel drops to the surface, the breakdown beneath continues until equilibrium is reached where the amount of fuel added is essentially equal to the decomposition rate of the fuel beneath.


Dry sclerophyll fuel accumulation

Dry sclerophyll example

Wet sclerophyll forest fuel accumulation

Wet sclerophyll example

Grassy woodland fuel accumulation

Wet sclerophyll example

Grassy woodland fuel accumulation

Grassy woodland example