Wildfire by Bill Mollison

Wildfire is a feature of many sites and climates. and can be created even in hot humid climates by logging. or by block plantings of eucalypts and pines. It is notoriously violent in summer-dry climates peripheral to large arid areas; “wet” savannah or chaparral scrub will burn fiercely when strong advected heat blows in from deserts.

Periods of high fire danger coincide with periods of strong ground winds from continental desert interiors, and affect many climatic types on desert borders, up to 200km from the desert edges themselves. These winds are the normal precursors of widespread and catastrophic wildfires. which in the presence of enough local fuel may develop into terrifying firestorms, which themselves generate a type of fire tornado, with fierce ground winds. In the southern hemisphere, fire winds blow anticlockwise, and in the northern hemisphere clockwise.

The critical factors for firestorm are:

  • FUEL SUPPLY; this includes the dryness of available fuels and their distribution and quantity (loose fuels of more than 6cm diameter}.
  • OXYGEN SUPPLY as winds to fan the flames, especially hot winds.
  • PREHEATING as upslope or radiant heat in front of the flames, or as advected desert winds in unprotected forests.
  • UNSTABLE AIR MASSES, so that wind shear, ground whirlwinds (dust-devils), scattered cumulus clouds and shifting winds all presage fire danger when dry fuels reach to less than 35% moisture. In unstable air, smoke does not level out at low altitude, but ascends to great heights, or is up and down in streamlines, and the air is otherwise clear (no fog or smog before the fire). In some forests, one can smell the volatile oils, terpenes, or resins, and light-blue haze develops over these forests.

Fire winds in northern and southern hemisphere

A small proportion of fires start from lightning strikes, even in remote forests. This is why many ridge forests show pyrophilous (fire-dependent) species, and on some ironstone ridges, every tree will be scarred by strikes. Such places should be noted as areas where houses need earthing for lightning strike.

However, most fires are deliberately lit, or arise from previous “controlled” burns left smouldering (often lit to reduce fire risk!) Freak fires can start from electrical shorts (power lines). backfire flashes from vehicles in grass, heat focussed by bottles or curved glass, and welding, campfire. and cigarette accidents. In all, lightning and accidents are perhaps only 4% or so of total fires; the vast majority are lit by mischievous, psychopathic, or even well-meaning people. A few pyromaniacs light and attend many fires, and even enlist in volunteer firefighting organisations; in aboriginal or tribal peoples, an angry person will sometimes burn out a camp or forest,

Factors that Increase the Fire Intensity or Spread

Once initiated, wildfire can spread with great speed; grass fires spread after 10–11am (after the dew has dried off); forest fires from midday to 3pm After an initial flareup, an hour or so suffices to develop firestorm conditions, aided by:

  • Loose fuels of smaller than 6cm diameter, grasses and sticks, at less than 20% water content (a chunk of 75 x 50mm pine, unpainted, can be weighed to judge humidity and wood dryness; where saturated, this wood is judged at 100%). Pine woods erupt at less than 30% humidity or moisture content due to high resin or oil content.
  • Winds from 10–50km/h accelerate spread, as the square of the velocity. Eg., at 20km/h, if the spread is 2km squared per hour, then at 30km/h, the rate is 4km squared per hour. At higher wind speeds, tongues of fire break up the fire front. At 80km/h, ground fires may self-extinguish.
  • Winds “backing” (shifting) late in the day may blow out a fire flank into a broad front, or even blow a fire back on itself and make it safer, However. backing winds are unpredictable, and in wildfire the best strategy is to order an early evacuation of a broad area except for teams (in safe refuges) whose job it is to put out minor house fires in the first half-hour after the fire has passed. For this reason, forested suburbs need local refuges (gravelled areas with underground shelters), as do isolated homesteads, and long stretches of roads through inflammable forests.

Wildfire will always occur on arid borders; thus we need to first be able to live with fire, and perhaps only secondarily (over a period of years) design to exclude fire from settled areas by a combination of:

  • Altering the vegetation to create more fire-immune systems.
  • Designing dams specifically to flood flow over hillsides subject to fire.
  • Mechanical or grazing removal of fuels just before fire—danger periods—this includes dead brush, long dry grasses, and the dead lower branches of trees.

In uninhabited areas, both “cool” fires (damp and weather) and “hot” fires (dry periods) are sometimes lit as a management mosaic to preserve fire-dependent flora and fauna; this is unsafe and difficult to control, and often causes fires.

Houses, dense surrounds, village surrounds, and in town planting (or the forests at the base of settled slopes) should all be designed to minimise fire damage and mortality. Fire can be expected as wildfire on a more or less regular schedule in specific vegetation types at about 30 years in wet sclerophyll forest, 8-10 years in dry savannah, and even annually in unbrowsed grassland. Thus, fire provides a specific problem for designers and landowners progressing from grassland to forest operations. From 3–5 years, or until forest establishment, the system has high fire risk, and we need to programme planting mosaics to reduce district risk.

FIREBREAK is a way of decreasing fire intensity; roads act as firebreak, as do ponds, marshes, rivers, stony areas, and summer-green or sappy plant crops hedgerows. Horizontal firebreak weakens or reduces the fire front energy. Vertical firebreak, to prevent fire “crowning” in trees, relies on the removal of lower branches, dead tree material, and perhaps on planting sappy ground cover under the forest.

No firebreak (even 10km of water) is effective in firestorms, as fire tornadoes, with ascent velocities of up to 250km/h can develop on the lee ridge side of hills, travelling downhill and lifting aloft large logs and branches and pieces of houses, and creating massive aerial gaseous explosions. The ground winds near the base of this column (some tens of metres across) can reach 100km/hour and will roll people over and over. The noise is deafening.

In these tornadoes, or in more minor whirlwinds, incandescent material is carried aloft and dropped out from 1–30 km downwind to start fresh fires, and fresh firestorm sequences (Figure 5.23).

In towns, fire resistant design (for wildfire) has these features:

  • A simple roof and wall outline (no internal roof valleys or re-entrant wall corners to pile up incandescent ash).
  • No tarpaper roof lining projecting into gutters. Roof gutters should have either a leaf free profile, or can be plugged and water-filled in the event of fire (plugs are handily chained to the gutter near downpipes). Roof spaces often catch alight from leaves in the gutter.
  • No unscreened windows, underfloor, or wall cavity vent spaces; all need fine-mesh metal screens to reduce spark size. Even beds can catch alight with large embers. and cellars or underfloor spaces may have dry firewood or fuel liquids stored there.
  • No inflammable doormats, nor woodpiles or shrubs against the house walls. Large cans of petrol, or explosive materials, should be stored in a shed away from the houses, tightly lidded.

Siting of Houses and Buildings

In fire prone areas, houses are at most danger from upslope fire; few houses survive wildfire on sharp ridgetops, or in hill saddles that have diverging ridgelines creating a wind (fire) funnel effect. The same funnelling or intensification of fire is created by planting inflammable trees (eucalypts) or grasses (pampas grass) along a house driveway. I have seen funnel-shaped plantings of this type that would have the effect of a blowtorch on the house, when even concrete will powder, and steel posts behave like spaghetti influenced by Uri Geller (or an Indian snake charmer).

For every 10° increase in the angle of upslope, fire speed and intensity doubles; the effect is that of updraught plus enhanced drying out of the fuel ahead of the front due to upslope flame and wind. That is, if the fire speed is at 16km/h at 0° slope, it is 32km/h at 10°, 64km/h at 20°, and 128km/h at 30°; thus slope effect alone can wipe out hill ridge settlements. It is critically important therefore that downslope forests are not pine or eucalypts, but slow-burning deciduous trees, with low leaf oils, and are sappy or thick-leaved forests with a clean floor, or with succulent groundcovers and lily clumps, or succulent vines and crops interspersed.

To reduce the ridge effect, site houses not only off the ridge, and if possible on downslope plateaus, but also excavate the site instead of raising the downslope house wall on stilts or stumps. A house nested into a shelf on the hill is protected from radiation, has no open underfloor area, and can have a rim wall, pond, or earthbank on the edge of the plateau as further protection. Such houses can more easily develop a cave or dugout refuge behind the shelter of the house itself (these are fully earthed over and have a wet blanket door and a dogleg or curved entrance to further escape direct radiation). Each such fire or radiation refuge needs a small (270 litre) permanent water tank incorporated, a few old blankets, and a bucket of water or two. This is absolute “fire insurance” for those caught at home (often women and young children). All these fire aids also apply to barns, livestock shelters, and outbuildings.

Around house and building sites, it is essential to reduce forest and grass fuel to a distance of 30m (100 feet). This does not mean tree removal, but rather the planting of such trees as Coprosma, deciduous fruits, figs, willows, poplars (not olives, pines, eucalypts), lines and clumps of lilies (Agapanthus, spring bulbs, arum, Canna) or “summer-green” ground cover (comfrey, ice plant, Tradescantia, Impatiens, shortgrass sward) to reduce flame and radiation effects.

My own family survived in a dense eucalypt forest only by prior removal of all lower limbs, loose bark, twigs, fallen leaves, dry brush, dry grass, and dead stumps (every year); most of this material was chopped down and stone weighted into hollows and swales, or burnt as cooking fuels. The tall tree stems not only saved the house from fire wind, but regenerated after the fire. A downslope line of willows and fern-leaved (not hard leaved) Acacia was fire-killed but rejuvenated from the roots when cut down. This sacrificial hedge, or fire barrier, dampened out the ground fire, and even the leaves of the willows did not combust, but shrivelled and gave out steamy ash as the flames reached the trees. The house was blistered, and the zinc roof coating flaked off, but the close fitted boards were unburnt. All walls were white-painted, and screens fitted; we had ample bucket water stored both indoors and out for the many small spot fires that were left after the front had passed. Apart from sore eyes and some short beards and hair, little damage resulted (Hobart 1967, a firestorm condition). In my street alone, 70 houses were burnt to the ground, with 1,100 houses burnt in the area, and 90 people killed.

The safest house sites are in damp valley mouths, in well-tended built-up areas, on farms with flood-flow or Keyline irrigation fitted, in irrigated areas, on peninsulas in dams and lakes, and in any plateau site where the design and maintenance criteria are rigorously applied.

It follows that all designers should take fire into account over many climatic regions, and especially where we are developing forest from grazed areas, as long grass or tall stands of straw are the worst fuels for fast fire spread and for ground survival.

Important to human and plant survival are RADIATION SHIELDS; these are solid or reflective (or both) objects that reflect or harmlessly absorb the radiant heat from the fire front. It is radiant heat which quickly kills plants and animals. Most human casualties of fire are not at first burnt, but either smothered by toxic smoke or fumes from furniture or plastics, or killed (unburnt) by radiation.

House sites in order of safety

Thus, radiation shields can be houses, stone walls, thick tree trunks, hollows or caves, hedgerows, and car bodies; a white-painted brick wall is ideal. White paint on houses reduces radiation absorption, as white roof areas reduce sun heat. Fireproof or slow-to-burn insulation in houses (mineral wool, seagrass, sawdust, feathers, wool) all keep the interior cool and assist fire control. Wooden panelling transmits little heat, while stone, brick, and mud may convey heat indoors, unless of sufficient thickness to absorb and disseminate it. Thatch and shingle areas must be replaced by tile or metal roof cladding in fire prone areas (by law in some districts).

Note that a fire shadow is tapered to about 4–5 times the height and width of the solid radiation shield, so make shields of trees or walls extend past the house (Figure 12.32).

A Note on Fuel Reduction

Wildfire will always happen, often every 8–30 years, on many sites. It will not be severe if normal annual fuel reduction is practised; the most unsafe way to do this is to “cool burn”. The safe ways are to graze off, slash, compost in swales, use as firewood, or to replace tinder with sappy green plants. Part of bioregional planning must be to keep monocultures of inflammable trees to uninhabited ridgetops, or better to scatter such stands throughout grazed or wet forested areas, or to tend them very well indeed in the matter of fuel reduction.

Ida and Jean Pain (Another Kind of Garden, 1982) have clearly laid out a broadscale, beneficial, fire–reduction system of chipping all dry forest fuels, composting or using them for biogas, which in turn fuels the chipping and carting operations, and lastly using compost and sludge to grow gardens, improve soils, and further reduce litter. Every bioregion should, perhaps must, adopt these methods if forests are to be preserved and eventually made fireproof.

Likewise, in the case of scattered suburbs, it should compulsory for houses to build to fire specifications, have large roof tanks and ponds, and for developers to build fire-damping dams able to operate by radio-control to sheet water over slopes on a Keyline principle. Fire will then be restricted to remote dry-ridge forests, and lightning strikes (as it should be).

Combined design for fire safety

Fire Effects in Forests

Fire sharply reduces litter, and leaves nutrient-rich light ash that can wash or blow away, or wash into lakes and streams as a clear or cloudy nutrient load. Hot fire will remove from the soil in low-intensity burns:

  • Nitrogen: 54–75% (109kg/ha}; replaced in 11 years by legumes, rain.
  • Phosphorus; 37–50% (3.0kg/ha); replaced in 20+ years by rain.
  • Potassium 43–66%
  • Calcium: 31–34%
  • Magnesium: 25–43%
  • Boron: 35–54% (Figures from Ecos, 42 Summer ’84/85)

Additional losses come from those shrubs and trees where foliage is burnt, and particularly so if the fire occurs early in the growth season. As fires “glow” at 650°C and burn fiercely at 1,100–1,400°C when strong winds blow, sulphur and nitrogen are volatilised, as is carbon. Phosphorus and potassium are volatilised at 774°C and calcium at 1,484°C (cement structures powder at this temperature). However, organic compounds containing these elements may volatilise more easily than soil elements.

Obviously, there is a very slow recovery of soil nutrient after fire, and this depends on trace elements brought in by rain or birds, and minerals recycled to topsoil by deep-rooted vegetation. Clearly, fires never improve soil status. Humus loss of 10–12cm occurs in forest soils. and peats often combust to greater depths. Clays lose structure, and mud flows can result.

Stock Losses in Fire

Good stock managers can, by preplanning, reduce losses in fire. Some ideal situation would be to make sure that some paddocks (small areas) have been close-cropped from late winter (or late summer in monsoon areas) to early summer; so that these small fields carry no inflammable fuel and can be used as general refuges in fire. Even more effective is to place a water trough in deliberately bare soil area, even one where topsoil and shrubs have been bulldozed into a surrounding earthbank (for protection against radiant heat), and to use such areas as always open refuges off 4-6 prepared range paddocks; stock can be confined there early fire danger days. Stock enclosed by a temporary electric fence will clean off rocky knolls of grasses before the fire period; these too can be used as refuges.

Tethered goats or sheep reduce patches of fuel near houses, as do close grazers such as geese, wallaby, and rabbits. Wild wallaby or geese can be “fed in” with pollard on such areas, and thus encouraged to create marsupial lawns (or rabbit swards). A forest of young deciduous trees, fig, mulberry, or oak will make a stock refuge if close browsed before fires are expected (dry grass period). Short swards are fairly quickly developed by close grazers if first slashed, then limed and fertilised (phosphate). Eventually, such fire refuges become popular places with stock.

With respect to a house or village, close-grazed or mown areas form part of the (upwind or downslope) firebreak system, and in fire danger periods stock can be fed in or mustered, and secured in these areas or in the settlement itself. Chickens survive well in solid housing, as they often produce bare areas around such houses, or chicken sheds can be part-buried in banks to give complete protection.


Personal Survival in and After Fire

For people, the main survival factor is to cover the body, wearing wool or cotton, and to shelter from fire front radiation behind a tree, car, house, or in a trench; all the better if the whole body can be caped in a wet blanket (wet blankets are no longer considered a good idea, best is to cover yourself with dry wool blankets – Adam). Wait until flames have passed, then move cautiously onto burnt ground.

A fire shields shadow

For civil authorities, radio stations must be commanded to keep constant reports going on fire direction, open escape routes, family location centres and refuge areas, and to give constant instructions to householders and travellers.

Just as the fire passes, well-equipped ground teams preceded by a bulldozer should clear roads, and put out spot fires in unoccupied houses. Police need to roster guards, or well-disciplined volunteers, to prevent looting until the area is re-occupied and services reconnected. Such services must be on standby at the fire periphery as it is suicidal to put forward teams in the path of the wildfire.

In firestorms, oxygen is periodically exhausted. and the fire goes out briefly. People cannot breathe and they faint. so they should never take refuge in small water tanks, dams, or rivers, as they will drown while unconscious. The sea, and large rivers of 100 m or so across, are safe to run to. but beware of fainting from lack of oxygen in the air and water (the skin needs oxygen too). Drink a lot of water to prevent dehydration, and make sure children and stock get water to replace lost body fluids.

We survive better if we have planned ahead, built fish ponds or bought metal buckets, prepared blankets and important papers to be picked up by the rear door, filled roof gutters, removed doormats, hosed down the garden, tied up or penned the stock, prepared woollens to wear, taken in the clothes, filled baths and sinks with water, and so on.

We recover faster if we have expected the noise, confusion. sense of isolation, and are prepared for looters and for “survivor guilt” (that is, “why should I have survived when people around me have died?”); for weeks, months, perhaps years, we can feel desolated by the losses we know of. It greatly helps if we have assisted others before, during, and after the fire, as we know we did our best. This applies to all catastrophes, not just fire. Even so, in every large wildfire in settled areas, people will be lost or very badly injured, much property and stock will be destroyed, and the psychological and social effects will persist for months or years.

And if your house burns down, do not build another “just the same” as most people do; build one to survive the next disaster. Thus, have realistic expectations, act on them, have some planned moves, and prepare better for next time!

Additional Resources

archive posted on 11 Jan 2020 in #designing, #healing & #teaching