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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 at random. Uncontrolled wildfires can deal 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 global warming. 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 the air quality downwind and contributes to a vicious cycle in global warming [2, 3].

Characteristics of Wildfires

Types of Wildfires

• • Ground fires are common in soils rich with organic matter that fuels 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 (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, negligently 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 and 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 movements 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 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 the rainfall and the 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 component, the air in which fires burn 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 global warming. From 1998 to 2018, it is estimated that approximately 8 billion tons of CO2 was 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 to global warming, global warming 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 our 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 soil and water chemical levels 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 even flooding. Naturally occurring substances and fire retardant can also impact drinking water quality, discolor recreational waters, and may potentially contribute to harmful algal blooms [13]. 

Designing to Resist Fires

In 2019, there was major destruction dealt to homes in the United States due to the wildfires. That year, approximately 14,000 homes burned down in the California wildfires, 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 the fires, the root cause of house fires are the embers that 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 be easily made fire-resistant by simply swapping out the combustible materials for non-combustible materials like metal, tile, stucco, fiber cement, treated wood, etc [15].

Roof

Wood or shingle roofs are highly combustible, meaning that this is "the most vulnerable part of your home" [15]. Protecting the roof from fires is done by simply selecting non-combustible materials like "composition, metal or tile" [15]. Eaves, soffits and rain gutters are all openings in which embers can travel into to settle and start a fire. These openings should be protected with an ignition-resistant mesh or cleared of flammable debris to avoid ignition [17]. Vinyl rain gutters should be replaced by metal gutters to avoid melting and dropping to the ground where burning debris will cause surroundings to light on fire [17].

Vents

Vents are an easy access to the inside of houses for traveling embers. These vents leading to the attic will 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 [18].