Peat is an accumulation of partially decomposed plant debris in a water-saturated, anaerobic environment. Peatlands cover about 5% of the earth’s surface. Their total reliance on water to maintain natural processes makes them highly significant terrestrial ecosystems. They are typically biologically hostile environments due to waterlogging, low pH and anaerobic conditions. They are under increasing pressure from human development and drought conditions.
Peatlands are potentially unstable terrains because the organic soils have very little inherent structure and are mainly bound together by roots. In contrast, mineral soils have superior structures derived from the more cohesive mineral particles in the soil.¹
There are many peat deposits in Australia and in the state of Victoria, for example: Western Port, South and East Gippsland, Western District Plains and the Eastern Highlands, with peat being found in peat swamps, grasslands and well as forested areas.
Unlike surface fire such as forest and grass fires, peat fires smoulder at a low temperature and can spread underground, making them difficult to detect, locate and extinguish. They produce little flame and much smoke, which can become a threat to public health. Peat smoke differs from normal wood smoke because of what is in the peat and how it burns. Peat consists of partially decomposed vegetation and contains varying amounts of carbon, sulphur and nitrogen compounds.
Typical peak temperatures for smouldering peat range from 450°C to 700°C with slow fire spread, burning with much lower efficiency and for much longer than typical flaming fuels. Peat fire spread can be in any direction. From the surface it can be horizontal and downwards in the shape of a shallow dish, preheating and drying the fuel in contact. Fire spread can also occur vertically within the peat layer and to other surface fuels such as trees, plants and other vegetation.
Assessment of peat fires
The rate of spread can be very slow, and in some cases peat fires may be fought over weeks and months, even years. A size-up and full appreciation of the current and potential situation will enable planners to formulate a course of action with clear objectives, strategies, tactics and tasking.
The nature of a peat fire poses in some cases some unique control options compared with surface and crown vegetation fires.
In comparison to a bushfire (wildfire), during the initial size-up, planning process and ongoing assessment for a peat fire there are a number of factors that may need to be considered:
- The size of the fire
- The fire’s behaviour
- Risk to life and property/assets
- Suppression difficulty
- What assistance or resources are needed?
For grass and forest surface fires we can use established observations, assessment tools and models such as grassland and forest fuel hazard guides, fuel moisture and weather meters, current and weather predictions together with predictive models for fire behaviour. The fire behaviour of a peat fire is very different as much of what is burning may be unseen and unknown. The initial and ongoing evaluation is important in developing a plan and monitoring its effectiveness.
Assessing fire behaviour, spread and threat potential
Assessment of areas with previous fire history can be done as well as on-site surveys of unburnt and burning areas to determine the potential for lateral and downward spread, as well as the risk to other assets.
To determine the fire activity, we may need to assess or survey the area. The factors to consider include but are not limited to:
Perimeter of the fire – In some cases, immediately following a large surface wildfire, a peat fire’s size may initially be difficult to determine and not clearly defined with other surface fuel still burning. To determine the size of the fire, observations may be a combination of visual and infrared detection ether from the ground or an aerial platform. The fire area may be an irregular shape or consist of many small to large individual fires with unburnt areas or ‘islands’ of peat. This information can then be collated and marked on a map or electronically recorded and when used for planning will identify the location of the burning edge and inactive edge.
Depth of burning, the drying of fuel and moisture layers of underlying peat material – This may have to be done in a number of locations at, or within, the fire perimeter to determine the fire’s behaviour and potential by digging with hand tools or machine. This can also be done by probing and measuring the depth and gaining an indication of temperature at different levels, depth of burning, amount of burning and levels of ash and char (the amount of energy will determine the amount of water that may be needed), or more accurately by digging or taking core samples to assess a cross section of material. Assessment can also be done a short distance away from the fire’s edge in unburnt ground for comparison and to determine the potential for fire growth. At or near the fire, consideration should be given to examining areas of different peat types and moisture levels. These areas may be influenced by near-surface fuels, drainage, roads, topography, etc. This can include the following: surface and near-surface newer organic material (root layer, duff etc.), mineral layers, type of peat (high or low mineral content), drying levels of underlying peat material, depth of undisturbed wet peat and an indication of moisture levels, as well as any other underlying inert mineral material. (Note: Excavation compliance codes for each state or territory must be followed. For example: Worksafe Victoria, Excavation Compliance code, Edition 2, December 2019.)
Rate of lateral spread – Peat fire may take weeks or months to control. Wind may influence a peat fire but not to the extent of a surface fire; lateral spread may be influenced by surface and near-surface fuel types and dryness. Identifying the rate of spread may be an important factor in determining control and suppression strategies. This could be difficult to establish at first and may have to be done over time and in different locations. It can be done by initial and ongoing mapping of the perimeter, visually or by thermal imaging from the ground or by crewed aircraft or drone. Additional information can be gained by placing items such as steel fence pickets at strategic locations, numbering them and recording the location with reference to the fire’s edge at the date and time and using them as a guide for measurements over time. While a surface wildfire rate of spread may be measured in kilometres per hour a peat fire may be able to be measured in millimetres per hour. We need to consider if and when it may move into areas that will have more complex control issues, such as burning into forest area or towards other assets.
Fire activity within the perimeter – While the majority of a peat fire’s activity will be on the perimeter, it is important to identify the amount of fire activity within. This material can be isolated unburnt islands of fuel that are still burning or peat material that is still available to burn until it is ether extinguished or reaches a moisture level or mineral material that will not sustain combustion and self-extinguishes. This assessment may be an observation of the amount of energy using visual and infrared observations ether from the ground or by aircraft. These observations can be noted as a percentage or mapped for planning or comparison at a later stage.
Underground fire spread – Identify if there a risk to underground or in-ground assets or infrastructure such as roads, bridges, fencing, building and other foundations as well as utilities including pipes, wires, poles etc. In some cases, peat fires can spread considerable distances underground with very little sign of fire activity on the surface. This could be as a result of cracks, fissures or other mechanisms for underground fire spread. Consider and examine if there are potential consequences for underground fire spread beyond the visible surface perimeter.
Topography, natural and constructed drainage – Evaluate how this may impact retention or drainage of water currently on the site and the potential for water run-off or retention if water is applied or rainfall occurs.
Surface fuels – Examine and assess the impact burning peat may have on surface fuels. Identify changes in surface fuels that may indicate a change in organic fuel under the surface and the impact it may have on fire behaviour. Surface fuels such as grass and other small green vegetation can also indicate signs of pyrolysis as a result of peat burning nearby that may not be otherwise visible.
Smoke, dust and air monitoring – Current and forecast weather conditions including smoke-plume modelling. Observations and forecasts may allow planning so that smoke and dust may be avoided and community warnings given. Peat-fire-generated smoke may contain high concentrations of carbon monoxide, organic gasses and particulate matter, as well as windborne dust and ash particulates impacting on the health and wellbeing of firefighters and the community. Other factors such as forecasted temperature inversion during morning and evening hours, which may result in dense smoke that settles in low-lying areas trapping smoke close to the ground, can also be considered. Smoke may also obscure visibility of nearby road users creating a risk of traffic accidents.
Other assets at risk – Peat fires can impact environmental as well as cultural and heritage assets. This can include fauna and flora, the impact of acid sulphate soils to ground water and the environment. Peat fires can impact assets both above and below ground, including buildings, utilities (power poles, water and gas pipes, underground wires), roads, bridges, drains, fencing, farmland, stock and cattle, etc.
Optional assessment to consider:
Soil testing – This can provide information on pH levels of soils as a benchmark for Acid Sulphate Soils for long-term planning including the possible need for emergency soil stabilization and post-fire rehabilitation. Representatives from state and territory agricultural agencies such as Agriculture Victoria or other similar organizations may be able to conduct pH and other more general soil assessment and offer specialist advice.
Assessing suppression difficulty and resource requirements
We need to establish the chances of initial first-attack success and the potential need for additional resources. A number of factors need to be considered:
- Is access to the fire available? This will include vegetation and potentially ‘boggy’ ground conditions that may restrict the movement of personnel, vehicles and machinery.
- Is sufficient water available ether on site or which can be relay pumped or transported? In comparison to surface fires, peat fires may require large volumes of water. Supplementary water supplies, water tankers, portable/collar tanks, large-diameter hose, pumps, etc. may be required. In some cases kilometres of hose or pipes may have to be laid. Are the required water application rates achievable and sustainable?
Are specialist resources needed? These may include but are not limited to:
- Specialist advice
- Additional personnel
- Air and health monitoring
- Machinery and specialist equipment
- Sprinklers, ground monitors, hoses, class-A foam and other equipment
- Portable toilets, hand-washing and other facilities.
Just as stated in the famous military quote ‘Time spent in reconnaissance is seldom wasted’, a detailed size-up and appreciation is important in establishing an understanding of a peat fire’s behaviour and potential for planning.
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- M Pemberton, Journal of the Royal Society of Western Australia, 88:81-89, 2005.