Wildfires

What are Wildfires?

Wildfires are unprecedented fires that burn in highly combustible, natural spaces such as forests or prairies. Possible causes include human activity and natural phenomenon, like lightning, however in many cases, the cause remains unknown as their occurrences are completely random. Uncontrolled wildfires can cause significant amounts of damage to surrounding areas and pose a threat to nearby cities. These fires appear more commonly in places that experience dry seasons and have occurred more frequently due to climate change. When forests are burnt down, an abundance of carbon dioxide is released into the air which adds to other harmful aerosols emitted as smoke. The smoke emitted from such devastating fires affects air quality downwind and contributes further towards climate change [2, 3].

Characteristics of Wildfires

Types of Wildfires

• • Ground fires are common in soils rich with organic matter that fuel the fire. The lifespan of ground fires can last up to an entire season before growing into a surface or crown fire.

  • Surface fires burn above ground, on the surface, where they feed off dead or dry vegetation.

  • Crown fires occur in trees or shrubs where leaves and canopies act as fuel [4].

Causes

In order for a wildfire to start, there needs to be a sufficient combination of fuel, oxygen, and heat present in an area. Fuel comes in the form of live or dead trees, vegetation, and other organic matter. Oxygen comes from the air in the atmosphere, and heat comes from lightning or human sources. The cause of a wildfire also depends on the source of the heat that ignited a fuel source [5].

Natural Wildfires

The majority of natural wildfires are caused by lightning. According to the BC Wildfire Service, lightning strikes cause approximately 60% of wildfires in the province of British Columbia in an average year [5]. Other natural events such as rock falls, volcanos, and meteorites can also generate enough heat to ignite a wildfire. 

Human Wildfires

The percentage of wildfires caused by person-activity varies by location. According to the U.S. National Park Service, 85% of wildfires in the United States come from human activity [6], while in British Columbia, 40% of wildfires come from human activity. Most human-made wildfires result from campfires left unattended, the burning of debris, equipment use and malfunctions, improperly discarded cigarettes, and intentional acts of arson [6].

Spread

The primary factors that influence the spread of a wildfire are fuel, weather, and topography. 

Fuel

Light fuels such as dry grass, dead leaves, tree needles, and small trees ignite quickly and cause rapid spread of fire. They act as kindling for heavier fuels such as logs, stumps, branch wood, and dead plant matter. Heavy fuels take longer to ignite, burn longer, and spread slower. Spacing of fuel also matters, as when fuels are scattered or separated by natural barriers such as rock, outcropping, streams, or areas of bare ground, the fire will be irregular and spread more slowly [7]. A common fire suppression tactic is to break the continuity between patches of fuel by separating ignited fuels from unignited fuels. Quantity of fuel also plays a factor, as without adequate fuel a wildfire cannot spread effectively.

Weather

The stronger the wind the faster a fire spreads, as wind brings additional oxygen to sustain flames. Wind also flattens flames which pre-heats the fuel ahead and causes spot fires by blowing sparks and embers away from the main fire into new fuel sources. Temperature of the ground also affects the movement of air currents [7].

Topography

The steepness of the land, or slope, affects both fire rate and direction of fire spread. Fires usually move faster uphill than downhill, and the steeper the slope the faster a fire moves. These tendencies occur because on the uphill side the flames are closer to the fuel and wind currents normally move uphill and can push heat. Burning embers and chunks of fuel may also roll downhill onto unburned fuel sources, igniting more fires [7].

Physical Properties

There are three elements to a fire triangle which must be present for a wildfire to occur. The first is the ignition of combustible materials, the second is high temperature, and and the final element is adequate amounts of oxygen in the air. Natural materials that are less dense, like grass, are easier to ignite as their water contents are significantly less than that of denser materials, such as a branch. Naturally, plants lose water through evapotranspiration, however, this loss is replenished through rainfall and absorption of water in surrounding soil. Plants that grow in unfavorable conditions that may throw off this balance are subject to dryness and become more susceptible to burning [8]. As for the air in which fires burn, it must contain at least 16% oxygen, which is easily accessible as ambient air contains 21% oxygen [9].

Effect on Climate

Wildfires burn over forests, grasslands, prairies, and other such landscapes which are considered carbon sinks as they store significant amounts of carbon dioxide. Upon burning, these landscapes release carbon dioxide and other greenhouse gases, as well as soot and other aerosols, contributing to climate change. From 1998 to 2018, it is estimated that approximately 8 billion tons of CO₂ were emitted per year from wildfires alone. This amounts to 5 to 10 percent of CO₂ emitted annually around the globe [10].

As wildfire emissions contribute towards climate change, climate change lengthens fire seasons which in turn, promotes more wildfires. This pattern continues in a vicious cycle and worsens with every year that the Earth continues to rise in temperature.

Effect on Water Quality

Similar to how wildfires can impact climate, water quality can also be affected by the presence of wildfires. The effects of wildfires on water quality include [12]:

• Water levels due to changes in storm runoff and snowmelt timing
• Water quality from build-up of ash, soil erosion, and fire debris
• Changes in taste, colour, and smell of drinking water
• A rise in chemical levels in soil and water due to fire retardant 

In the aftermath of a wildfire, rainstorms flush vast quantities of ash, sediment, nutrients, and contaminants into streams, rivers, and downstream reservoirs. The absence of vegetation in a region after a wildfire can create conditions that lead to erosion and flooding. Naturally occurring substances and fire retardant can also impact drinking water quality, discolour recreational waters, and may potentially contribute to harmful algal blooms [13]. 

Designing to Resist Fires

In 2019, there was a significant amount of destruction to homes in the United States due to wildfires. That year approximately 14,000 homes burned down due to wildfires in California alone [14]. After witnessing the destructive nature of these wildfires, constructing homes to be fire-resistant is becoming increasingly important, especially in areas that experience longer dry seasons and potential droughts. Besides direct contact with fires, the main cause of houses catching fire are embers, which can be blown up to a mile from the source. Once these embers latch onto a flammable part of the house, it will catch fire and spread quickly, causing greater destruction. Many components in houses can easily be made fire-resistant by simply swapping out the combustible materials for non-combustible materials such as metal, tile, stucco, fiber cement, and treated wood [15].

Roofs

Wood or shingle roofs are highly combustible, meaning they can be an extremely vulnerable part of your home [15]. Protecting the roof from fires is achieved by selecting non-combustible materials such as composition, metal, or tile [15]. Eaves and soffits are openings under the roof in which embers can travel into and start a fire. These openings should be protected with an ignition-resistant mesh or cleared of flammable debris to avoid ignition [17].

Rain Gutters

Vinyl rain gutters should be replaced by metal gutters to avoid melting and dropping to the ground where burning debris caught in the gutter will cause surroundings to light on fire [17].

Vents

Flying embers can easily travel to the inside of homes through vents. If a flying ember enters a vent leading to the attic, it can cause the house to burn down from the inside. To combat this issue, a metal mesh can be used to cover these openings and block the entrance of any embers [19].

Windows

Radiant heat from fires will break single-pane windows due to the temperature difference on the surface of the glazing [20]. It is highly recommended to swap out single glazed windows for a double glazing as this will provide extra protection from fires. In general, all newer homes have double-paned windows, however, older homes are likely built with a single pane. A tempered glass exterior is also a good alternative as it is more durable than regular sheets of glass [21]. Since glass is not indestructible against fires and cracking is likely to occur even with double glazing and a tempered glass exterior, the safest method is to have "roll-down metal fire doors" which will protect the glass surfaces [20].

Cladding

Non-combustible materials like stucco, metal and fiber cement are better alternatives to typical wood siding. Another option is to use fire-retardant treated wood that protects the wood from weather damage, fungi, and even fires [22].

Fence

Similar to cladding, fences made from wood should be swapped with fire-retardant treated wood. Though it may not provide enough privacy, steel is also a good alternative for fencing as it is a non-combustible material [23]. 

Protection Systems

Sprinkler systems are recommended to keep the surrounding area moist and less susceptible to burning if kept dry [24]. 

Landscape

Being conscious of the landscape surrounding a home helps to reduce risk of catching fire. Within the immediate surroundings, up to 30 feet away, the land should be cleared of any debris and other combustible materials. If there's a deck within 10 feet of the house, fire-retardant-treated wood should be used as the primary building material [25]. Past the 30 feet mark, any and all vegetation should be adequately spaced out and thinned to avoid the spread of larger fires [18].

References