How to stay away from low frequency debris flow?

Water is the life resource that we rely on to survive, and it is also the craftsman who sculptures our living environment. In the past 2 million years, the strong erosion process of the global mountainous surface has shaped the types and forms of topography, and is inextricably linked with the life and production of mountain people. Laozi said in his Tao Te Ching: "Water is good for everything; the world is weaker than water. And the strong and strong, can not win." As the most important external force to shape the mountainous landscape, the water flows through the upper reaches of the valley. The function is to transport the loose solid matter on the surface to the downstream ditch and form a flat alluvial fan to provide fertile land for the mountainous area. At the same time, however, mountain rivers also use unconventional processes to destroy their own years of careful construction of fertile land under extreme conditions, and debris flow is one of the most common forms of destruction.

Debris flow is one of the common geological disasters that occur in mountainous areas. Usually, under the action of hydrodynamics such as channel runoff formed by heavy rain, dam flooding or melting of snow and ice, the loose soil in the basin is activated in a short time. A solid-liquid two-phase flow process in which water and stone are mixed. The occurrence of debris flow often has the characteristics of sudden, large scale and high speed, and carries a large number of stones to rush out, which is extremely destructive, which restricts the local economic development to a large extent, and is one of the most prominent problems in the mountainous area. .

What is the low frequency debris flow

According to the difference of the recurrence period of the debris flow outbreak, it can be divided into high frequency debris flow and low frequency debris flow. The high-frequency debris flow has a short return period, often occurring every year or even every few years; while the low-frequency debris flow has a relatively long reproduction period, which often occurs only once in a few decades. In the book "Chinese Debris Flow" published by the Chengdu Institute of Mountain Hazards and Environment of the Chinese Academy of Sciences in 2000, the mudslide with a return period of more than 30 years is called the low-frequency debris flow.

Why is the return period of the low-frequency debris flow long? On the one hand, the loose source collection speed required for the start of the low-frequency debris flow is relatively slow. The investigation shows that the low-frequency debris flow rarely develops in the weakly fractured phyllite or schist area of ​​the rock mass. However, it is mostly developed in granite , sandstone and limestone areas where the rock is relatively hard and weakly weathered. The rock mass in these areas is destroyed and accumulated under natural weathering to a large amount of loose soil required to form a low-frequency debris flow. On the other hand, the study shows that the low-frequency debris flow is mostly developed in areas with good vegetation coverage or relatively hard rock with weak weathering and weak metamorphism. The slopes in the better growing areas will be relatively stable, and the surface soil erosion intensity will be weak. It can keep the loose soils from being easily taken away by the surface runoff formed by small-intensity rainfall, and will only be washed out under heavy rain or heavy rain conditions. The probability of heavy rain and heavy rain is relatively low, so the return period of low-frequency debris flow is relatively long.

What are the characteristics of low frequency debris flow?

Low frequency debris flow has strong concealment

Because of the frequent occurrence of high-frequency debris flow, it will attract people's attention and have a strong sense of prevention. Even if a large-scale mudslide occurs, the loss is generally limited. For example, the mudflow of Jiangjiagou in Dongchuan, Yunnan Province can occur at most every year. More than 10 mudslides, local residents have a higher degree of vigilance against their potential dangers. However, for low-frequency debris flow, mudslides do not occur for decades or even hundreds of years. Vegetation coverage in the basin is good, and it is easy to cause people to make a ditch. It is a safe misjudgment. It is easy to cause major disasters when the next debris flow occurs. .

Low frequency mudslides are seriously devastating

For the mountainous areas with poor land resources, the old stacked fan formed after the mudslides is fertile and open, it is often an ideal place to live, and a large number of houses, villages and even towns are built, causing a sharp increase in the risk of debris flow. The risk increases, and once a mudslide occurs, it often causes serious disasters. According to statistics, in the many mudslide disasters, the most serious one-time disaster is the low-frequency debris flow. The serious disasters caused by low-frequency mudslides at home and abroad are not uncommon. For example, the 400-year low-frequency mudslide occurred in Zhouqu County, Gannan Tibetan Autonomous Prefecture, on August 7, 2010, causing almost all of Zhouqu County to be destroyed, resulting in 1463 deaths, 302 missing, and direct economic losses of several hundred million yuan; 1999 In mid-December, a group of low-frequency mudslides occurred in the Vagas region of Venezuela in South America for about 300 years, resulting in a rare and major disaster stream with direct economic losses of 20 billion U.S. dollars and more than 30,000 deaths, resulting in direct economic losses of 20 billion. dollar, 30,000 people died of the world's rare major disasters; July 9, 1981, China's Sichuan Province Ganluo Toshiko Wajda ditch landslides due to heavy rainfall in 50 years of the occurrence of the Chengdu-Kunming railway bridge washed away, resulting in three passengers The carriage fell to the bridge, killing more than 360 people; from June 27 to 28, 2012, a 50-year low-frequency mudslide occurred in Dianzigou, Baihetan Town, Ningnan County, Sichuan Province, Sichuan. A three-story hotel was destroyed by mudslides, causing 41 people. A major disaster that has been killed or missing.

Formation conditions of low frequency debris flow

As with high-frequency debris flows, the formation of low-frequency debris flows must have three essential conditions: abundant loose soil, steep topography, and adequate hydrodynamic conditions. The abundant loose soil is an important condition for determining the size and destructive force of the debris flow. If there is not enough loose soil in the basin, the heavy rain can only form a flood, and its destructive power and erosion ability to the river are far more than the debris flow. a lot of. The vertical height difference formed by the steep terrain conditions makes the debris flow fluid have sufficient potential energy, and transforms into kinetic energy to maintain its movement during the movement, which plays a decisive role in the characteristics of the debris flow rushing distance and flow velocity. Sufficient hydrodynamic conditions are the main cause of debris flow. Only under the action of hydrodynamics, the loose soil will start and form a debris flow. The hydrodynamics of most debris flows are from the heavy rainfall process. The hydrodynamic conditions directly affect whether the debris flow starts and rushes. Characteristics such as scale and fluid gravity.

High-frequency season of low-frequency debris flow

Studies have shown that almost all low-frequency debris flows are induced by heavy rain, while heavy rains occur mostly during the rainy season. The rainy season is the most concentrated month of precipitation in a year, characterized by wet and rainy weather. The rainy season in southern China is from April to September, and in the north is from June to September. These are high-risk seasons in which low-frequency debris flows occur. The statistics of typical low-frequency debris flow disasters that have caused heavy casualties and property losses in 26 mountainous areas in southwest China show that 21 low-frequency debris flows occurred in June-August, accounting for about 85% of the total. The results also show that during the 26 low-frequency debris flow disasters, 25 low-frequency debris flows occurred at night, accounting for 96% of the total, and only one occurred during the day.

How to prevent low frequency debris flow

For the government departments: When selecting the flat fan-shaped area of ​​the ditch as the land for urban construction or infrastructure construction, it is necessary to be cautious, and in the form of laws and regulations, it is required to carry out the geological disaster risk assessment of the construction land, and the danger of low-frequency debris flow. Construction areas or engineering monitoring or early warning measures will be carried out to minimize the risk of potential disasters.

For the local residents: the construction of the house should be as far away as possible from the river. Do not choose the area with relatively low ditch. At the same time, when encountering heavy rain, try to avoid activities near the river. You should always observe the rise of water flow in the river and prepare for evacuation to a safe area in advance. If the water level of the river is found to rise rapidly or suddenly, it should be quickly evacuated to a relatively high-level safety zone.

For tourists: In recent years, the quality of urban air has declined, and the heat island effect of high-rise buildings and concrete floors has made the city's summers hotter and hotter. The special geographical environment of the mountain area makes it have unique scenery and cool climate, so the mountain area has become a summer resort favored by many tourists in summer. Summer in the mountains is an extra season, and the risk of low-frequency debris flows is higher. Therefore, tourists need to know the weather forecast of the destination in advance when they choose to play in the rainy season, and try to avoid entering the mountain when there is rain.

In recent years, the Yiyou family has been very popular among young people. They often go to some sparsely populated mountainous areas to enjoy the scenery. However, they lack professional safety training before departure, and reports of frequent mudslides and people causing casualties are not uncommon. Therefore, it is extremely important for the best friends to choose a safe zone for camping. When choosing a camp, try not to choose the ditch at the mouth because of the convenience of water intake, which will largely avoid the hazard of low-frequency debris flow.

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