Forest fires have broken out many times in Liangshan, endangering the safety of firefighters. Why are forest fires always difficult to extinguish?

Fires occur every year in Liangshan, Sichuan, endangering the safety of firefighters. Why are forest fires always difficult to extinguish?

The entire altitude of Xichang City is above 1,500 meters. The terrain is dominated by Zhongshan, accounting for 78.9% of the city's total area, with high mountains and low mountains accounting for 1.1% and 3.4% respectively; the valley flat area accounts for 16.4%, making it the second largest valley plain in Sichuan Province. There are many types of wild plants growing in the forest understory and the resources are rich. There are 233 families, 532 genera, and more than 2,000 species.

Currently, Xichang has a forestry land area of ??159,926 hectares, accounting for 60.22% of the total area, a forest area of ??129,427 hectares, a forest coverage rate of 48.74%, and a vast forest area.

Liangshan, where Xichang is located, belongs to the subtropical monsoon climate zone, with distinct dry and wet conditions. In the winter half of the year, there is plenty of sunshine, little rain, and dry warmth; in the summer half of the year, there are more clouds and rain, and the climate is cool. Except for the four distinct seasons in the Niger River, other places divide the seasons according to the average temperature, and the four seasons are not obvious. There is no summer in the higher altitude areas, and there is no winter in the southern areas with lower altitudes. Although the four seasons are not obvious, the dry and wet seasons are. The dry season is roughly from November to April, and the wet season is from May to October. The characteristics of dry and wet seasons are significantly different. The dry season climate is characterized by sparse precipitation, small temperature difference, abundant sunshine, and warm climate. The climate characteristics of the wet season are cool, humid, and rainy. The obvious characteristic of the wet season is that it is rainy, so it is customary to call the wet season the rainy season.

This is also the reason why Liangshan is always prone to fires in March and April.

Forest fires have occurred as long as forests have existed on Earth. On average, more than 200,000 forest fires occur around the world every year, and the burned forest area accounts for more than 1% of the world's total forest area. China now has an average of more than 10,000 forest fires every year, burning hundreds of thousands to millions of hectares of forest, accounting for about 5 to 8% of the country's forest area.

Fires not only bring ecological losses, but also pose serious threats to the lives and safety of towns and people around forest areas. But one of the biggest problems with forest fires is that they are extremely difficult to extinguish.

Take the Australian fires as an example. The Australian fires have been burning for nearly 5 months, and 2,500 houses have been burned down. About 11.2 million hectares of forest have been burned across Australia. The Amazon rainforest fires that shocked the world last year burned only about 1.8 million hectares of forest area. In addition, more than 1,400 kilometers of coastline are burning, from the northeast to Jiangsu, Zhejiang and Shanghai. According to statistics, more than 1 billion wild animals have died directly or indirectly in this forest fire.

The difficulties faced mainly include four aspects: many fire points and long fire lines; the mountainous area is small and the climate is complex; the mountain is not high but the forest is dense and the road is difficult to walk; the temperature will continue to rise and the wind direction is always changing. Moreover, the altitude of the Xichang area is over 1,500 meters, and the lack of oxygen at high altitudes is also a severe test for rescuers, which makes the rescue extremely difficult.

One of the most dangerous situations in the process of extinguishing forest fires is encountering deflagration, which will form huge fireballs, mushroom clouds and other phenomena in a short period of time. The temperature generated during deflagration is extremely high. Forest fire explosions can be said to be the biggest threat to firefighters' lives and safety.

The main reason for deflagration is the simultaneous burning of multiple combustibles in special dangerous terrains such as steep slopes, ridges, and grass pond ditches.

Forest explosive fires erupt instantly, are extremely powerful, and are sudden and accidental. According to foreign literature reports, when the fire develops to an extremely violent level, the temperature of the deflagration fire can reach more than 1000°C and the flame can reach more than 30 meters. When a fire occurs to this extent, if there are fire fighters on the ground, they will be exposed to high temperatures, smoke, and shock waves. It will be difficult for the fire fighters to escape and have no time to react, so they may often cause heavy casualties. Sichuan 2019 In the Liangshan forest fire, firefighters encountered a sudden explosion and 30 people died.

In addition, flashover will also occur in the forest. There are two main reasons for flashover. One is that combustible materials in the forest accumulate for a long time and decay, producing combustible gases mainly methane. Suddenly When encountering fire and the action of small combustibles, flashover occurs.

Another reason is that forest fires burn into special terrains such as narrow ridges, single-entry valleys, steep slopes, saddles, grass pond ditches, and rock protrusions, causing the combustibles to preheat at the same time. It burns and instantly forms a huge fireball and mushroom cloud, with a maximum impact area of ??30 acres.

For example, narrow ridge lines are affected by thermal radiation and thermal convection, resulting in extremely high temperatures; and they are prone to forming erratic air currents, making the direction of wildfires difficult to predict.

It can be said that in forest fires, terrain and wind direction are the biggest problems hindering fire fighting. Because important factors closely related to the speed of forest fire spread are wind and topography. Under the same conditions of wind, combustibles and oxygen, fires on hillsides will spread faster than on flat ground.

This is because when a fire burns from the bottom of the mountain to the top of the mountain, when the terrain blocks the wind, an updraft will form. This upward airflow can accelerate the spread of forest fires to the hillside. So after a fire breaks out in the forest, the fire will spread quickly and burn bigger and bigger.

If you encounter narrow ditches, ridge lines, depressions on sunny slopes, rock cliffs and vegetation, places with lush mixed trees, shrubs and grasses, young forests, and dense shrubs, especially if the density is above 0.8 Forest fires in turbulent areas where young and medium-sized coniferous forests, shrubs, shrubs and grasses are connected, and where the wind direction of the small environment of the fire site is inconsistent with the main wind direction of the fire site, will cause the fire to explode instantly, which will seriously endanger the safety of firefighters.

If you encounter strong windy weather and special fire behavior occurs, the fire field will expand rapidly and the fire will evolve into a crown fire. Firefighters and helicopters will not be able to get close to the fire scene, and large firefighting equipment will not be able to enter the mountains to participate in firefighting. This kind of The situation is extremely dangerous and is basically inaccessible to manpower. It is difficult to put out the fire. The only way to extinguish the fire is to open an isolation zone from the outside, attack the fire with fire, or wait for weather conditions to change (relying on natural forces rather than manpower).

At present, the main equipment used by firefighters is not only remote water supply systems, large water tanks and foam fire trucks, and carrying sufficient hand-held motor pumps, fire hoses and other materials, but also wind-powered fire extinguishers, because This will not be limited by the water source problem. The wind fire extinguisher uses high-speed airflow to impact the flame, causing the ambient temperature around the burning combustibles to drop sharply below the ignition point, and blowing the flame away from the combustibles, thereby blocking combustion. The instantaneous wind speed of a wind fire extinguisher can be as high as 90 meters per second, while the wind speed of a Category 12 typhoon is only about 32 meters per second.

If you encounter a forest fire, avoid dangerous fire environments. It is not suitable to approach the fire line directly, or when the approaching route has major safety hazards, you should actively avoid dangerous terrain, dangerous time periods, and dangerous vegetation types, and choose a safe area to rest or a safe approach route.

When forest fires threaten personal safety, you should choose rivers, streams, areas with no or sparse vegetation, and low-lying areas. You can use water-soaked clothes to cover your head, mouth and nose, or wipe your head. Open the soil layer, put your hands on your chest and lie down to avoid danger. When lying down, try to stay away from flammable items such as oil dispensers, igniters, and oil drums, or throw out flammable items.

In addition, you can also use an igniter to light a head-on fire in a relatively open flat area, relying on rivers, streams, and roads, so that the new fire head will spread against the wind in the direction of the big fire head, blocking the fire. Frontal rescue.

If conditions do not allow, you can also take the following four methods to save yourself:

Light a fire to rescue? When there are no rivers, streams, or roads to rely on, and time permits, Light a fire with the wind, follow the fire into the burned area to avoid the fire, dig out the wet soil from the ground with your hands, breathe close to the wet soil or cover your mouth and nose with a wet hand towel to prevent carbon monoxide poisoning.

Forced to cross the fire line against the wind? When ignition or other conditions are not met, do not run with the wind. Choose an area that has been burned or has sparse weeds and has flat terrain. Cover your head with clothes and rush against the wind quickly. Cross the fire line and enter the burned area to escape safely.

Lie down to avoid fireworks? When it is too late to light a fire and there is a nearby river (river ditch) or a windward flat area with no vegetation or sparse vegetation, wet your clothes with water and cover your head, put your hands on your chest, and lie down Stay away from fireworks. When lying down to avoid fireworks, in order to prevent suffocation from the smoke, cover your mouth and nose with a wet towel, make a pit, and breathe close to the wet soil to avoid smoke damage.

Quick transfer? When you find that a fire is coming and cannot be controlled by manpower, as long as time allows, quickly transfer to a safe area to avoid casualties.

No matter what, I hope there will be no casualties in the Xichang fire and that all fire fighters can return safely.