Discussion on the Occurrence Mechanism of Debris Flow and Disaster Prevention Measures [Abstract] Debris flow refers to a special torrent that carries a large amount of sediment and stones caused by landslides caused by heavy rain, heavy snow or other natural disasters in mountainous areas or other ravines and areas with steep terrain. Debris flow has the characteristics of suddenness, fast flow rate, large flow rate, large material capacity and strong destructive power. Mudslides often wash away transportation facilities such as roads, railways, and even villages and towns, causing huge losses. Therefore, understanding the occurrence mechanism, monitoring and forecasting, and rescue of debris flows can minimize the losses caused by disasters.

【Keywords】debris flow, occurrence mechanism, danger, detection and forecasting, disaster prevention and mitigation, emergency rescue

Debris flow is a torrent formed by saturation and dilution of soft soil mountains containing sand and gravel by heavy rain and floods, and its area, volume and flow are large. A typical debris flow consists of a viscous slurry with coarse solid debris suspended and rich in silt and clay. Under appropriate topographic conditions, a large amount of water is soaked with flowing water, and the solid accumulation material in the hillside or ditch bed reduces its stability, and the solid accumulation material saturated with water moves under its own gravity, forming a debris flow. Mudslides are a catastrophic geological phenomenon. Mudslides erupt suddenly and violently, and can carry huge boulders. Because of its high speed and powerful energy, it is extremely destructive.

1 Mechanism of occurrence

Debris flow is a kind of fluid containing a large amount of sediment and stone sudden eruption, which lasts for a short time, has great destructive force and complex causes, and the amount of debris flow collapse is closely related to many factors such as rainfall, rainfall intensity, catchment range, slope slope and aspect, vegetation, rock type and weathering degree, slope soil thickness, etc., and the factors also affect and restrict each other. In this paper, the main factors such as debris flow collapse and rainfall, catchment area, slope slope, rock type and soil thickness are preliminarily analyzed.

1.1 Mudslides and rainfall

Debris flow collapse is related to the size of rainfall, and after calculation and analysis, the debris flow collapse has a logarithmic relationship with rainfall, and its formula is as follows:

y=24769Ln（x）－99408（1）

1.2 Debris flow and catchment area

The relationship between the debris flow collapse and the catchment area, the logarithmic relationship between the debris flow collapse and the catchment area, the formula is as follows:

y=5016.3Ln（x）＋16253（2）

1.3 Debris flow and hillside slope

The amount of debris flow collapse is exponentially related to the slope of the hillside, and the formula is: y=0.0005x4.8018

1.4 Debris flow and hillside soil thickness

The amount of debris flow collapse in the soil and rock mountain area is related to the thickness of the soil layer on the slope where the debris flow occurs.

The mountain is gneiss and conglomerate, and when the covering thickness is 0.3~0.7m, the debris flow collapse body is mainly dolomite and limestone; when the covering thickness is 1~2m, the collapse body is mainly dolomite, conglomerate and gneiss; when the covering thickness is greater than 2m, the shale is prone to debris flow. Under the same rainfall, with the change of slope soil thickness, the amount of debris flow collapse is increasing.

1.5 Mudslides and rock types

Depending on the lithology, the amount of debris flow collapse formed by the same rainstorm varies greatly.

2 Danger

Debris flows are often characterized by sudden, ferocious and rapid onset. It also has the dual effects of collapse, landslide and flood damage, and its harm degree is more extensive and serious than that of a single collapse, landslide and flood. Its harm to human beings is manifested in four specific aspects.

2.1 Hazards to settlements

One of the most common hazards of mudslides is to rush into villages and towns, destroying houses, factories, enterprises and institutions, and other places and facilities. Drowning people and livestock, destroying land, and even causing disasters such as the destruction of villages and the death of people. For example, in August 1969, the Nangong mudslide in Langzhang District, Liucheng District, Daying River, Yunnan Province, destroyed the villages of Xinzhangjin and Laozhangjin, killed 97 people, and caused economic losses of nearly one million yuan. On August 7 and 8, 2010, a massive mudslide erupted in Zhouqu, Gansu Province, killing 1,270 people and leaving 474 missing, leveling a 5-kilometer-long and 500-meter-wide area of Zhouqu.

2.2 Hazards to roads and railways

Debris flow can directly bury stations, railways, highways, destroy roadbeds, bridges and culverts and other facilities, resulting in traffic interruption, can also cause running trains, cars to be overturned, resulting in major personal injury accidents. Sometimes, debris flows flow into the river, causing large changes in the river, indirectly destroying roads, railways and other structures, and even forcing roads to be rerouted, causing huge economic losses. For example, in July 1978, a mudslide broke out on the other side of the Ganchuan Highway, which blocked the Bailongjiang River, and the highway was flooded for 1 km. Since 1962, this section of the route has been forced to be rerouted three times due to the impact of mudslides on the opposite bank. Since the founding of the People's Republic of China, mudslides have caused incalculable losses to China's railways and highways.

2.3 Hazards to water conservancy and hydropower projects

It is mainly to wash out hydropower stations, diversion channels and ditch buildings, bury the tail canals of hydropower stations, silt up reservoirs, and abrasion dam surfaces.

2.4 Hazards to Mines

It is mainly to destroy the mine and its facilities, bury the mine tunnel, injure the mine personnel, cause the shutdown of production, and even make the mine scrapped. [4]

3 Detection forecast

The prediction and forecasting of debris flows is very important, which is an important step and measure for disaster prevention and mitigation. At present, the following methods are often adopted in the prediction and forecasting of debris flow in China:

(1) Fixed-point observation and research are carried out in typical debris flow gullies, and strive to solve the problem of debris flow formation and motion parameters. For example, the observation and experimental study of debris flow in Jiangjiagou and Daqiaogou in the Xiaojiang River Basin of Dongchuan City, Yunnan Province, and the observation and study of Shahe debris flow in Hanyuan County, Sichuan Province.

(2) Investigate the relevant parameters and characteristics of potential debris flow gullies.

(3) Strengthen hydrological and meteorological forecasting, especially for small-scale local rainstorms. Because heavy rain is formed