External conditions of debris flow formation

1. Meteorological and hydrological conditions

The water flow formed by long-term precipitation, rainstorm, melting of snow and ice and water inrush from the surface outside the geological body is not only an indispensable material condition for the formation of debris flow, but also an energy provider for the initial kinetic energy and flow of debris flow. Therefore, the addition of water flow is regarded as an external condition.

For the vast debris flow active area, precipitation and its hydrological process are the most important factors in various external conditions. Among them, heavy rain and torrential rain often lead to mass mudslides, and the formation scale is also large.

Except northwest, northern Tibet and western Inner Mongolia, which are inland climate areas with scarce precipitation, the rest areas are controlled by monsoon climate, and it is rainy in summer and autumn.

The monsoon that affects the precipitation in summer and autumn in China mainly comes from two directions: first, it comes from the southwest, bringing the water vapor formed over the Indian Ocean into eastern Tibet, Yunnan, Guizhou and Sichuan through the north-south corridor of the Hengduan Mountains, and even affecting the weather in the vast areas south of the Yangtze River, causing a lot of rainfall. Affected by this airflow, the rainfall in Dongchuan, Yunnan Province reached 55 hours in September18,0963. 1977 may 14, two-hour rainfall 127mm in chengdu-kunming railway, which triggered a large-scale debris flow outbreak. The monsoon in the other direction comes from the Pacific Ocean. In summer and autumn, a tropical cyclone formed in the Pacific Ocean and containing a lot of water vapor landed from the eastern and southeastern coasts of China in the form of a typhoon, which caused a large-scale rainstorm or torrential rain every time, and its influence could reach the central and western regions of China. Affected by it, a strong tropical storm 198 1 hit the northeast on July 27th, and the rainfall in Laomaoshan area of Liaoning Province reached 395mm in six hours.

In addition to precipitation, the severe melting of snow and ice caused by rising temperature is also the main reason for mudslides in some special areas. In some high-altitude areas in northwest and southwest of China, there is snow all the year round or large-scale glaciers are distributed. In summer, the temperature rises and a large amount of ice and snow melts, which often leads to mudslides, such as Bomi area in Tibet and Tianshan area in Xinjiang.

The research shows that the frequency and scale of debris flow activities have obvious statistical relationship with the time distribution and intensity of atmospheric precipitation.

(1) annual rainfall. The greater the annual rainfall, the stronger the debris flow activity, but the annual rainfall threshold of debris flow outbreak in different regions is very different. For example, in Dongchuan area of Yunnan, there is debris flow activity when the annual rainfall is about 400mm; However, in Huaying Mountain area of Sichuan, the movement of debris flow requires more than 800 mm of annual rainfall.

In the same area, the interannual variation of rainfall also has a great influence on the activity of debris flow. For example, in Jiangjiagou, Dongchuan, Yunnan, debris flow activities have been frequent for many years, reaching about 15 times; In the dry season for many years, debris flow activity is relatively weak, only a few times.

(2) Seasonal rainfall. Debris flow activities are mainly distributed in rainy season, such as the upper reaches of the Yangtze River. Due to the advance and retreat of monsoon, the rainfall time is controlled. The monsoon comes early, the rainy season comes early, the annual precipitation is generally large, and the debris flow breaks out earlier. On the contrary, with the delay of the rainy season, the annual rainfall decreases, and the debris flow occurs later and with lower frequency.

(3) Daily rainfall. The influence of daily rainfall on the occurrence of debris flow is mainly manifested in the role of total rainfall in one day on the occurrence of debris flow. Due to the rainy season, the synchronous debris flow has the characteristics of large scale and heavy harm. The daily rainfall required by debris flow depends on the geological and geomorphological conditions of the basin.

(4) The rain is strong. Rainfall intensity is the rainfall per unit time, which is mainly manifested in two ways: 1 hour rainfall intensity and 10 minute rainfall intensity. In the triggering conditions of debris flow, rainfall intensity is a factor that cannot be ignored. According to the statistics of debris flow in Sichuan, the minimum rainfall intensity of 1 hour is about 15mm, and most of them are between 20-60 mm.

2. Human activities

(1) Manual excavation and abandonment. Rock and soil excavation and disposal in railway, highway, canal and other engineering construction. For example, in Laogangou of Dongchuan-Kunming highway in Yunnan, the mountain was destroyed due to road construction and canal repair, and the landslide and debris flow caused by the 1966 rhinoceros mountain earthquake became more serious. For example, many large-scale projects and ground buildings have been built in Hong Kong over the years, and almost every project has to be cut off from reclamation or filling in order to obtain a suitable site. 1972 a rainstorm killed 20 people at the excavation site under construction 120.

For example, in Hanluogou, Lugu Iron Mine, Mianning County, Sichuan Province, due to unreasonable waste of soil and slag, 1972 rainstorm triggered mine debris flow, which washed out loose solid materials about 1× 105m3, buried chengdu-kunming railway 300m and (Germany)-(Chang) highway 250m, stopped driving and gave transportation.

(2) indiscriminate reclamation and deforestation. Under the conditions of human activities, such as deforestation and unreasonable reclamation in mountainous areas, soil erosion, large-scale landslides and collapses all provide material sources for mudslides. An important reason why Sichuan, Yunnan and other areas have become the most serious areas of debris flow activity in China is the deforestation of forest resources in the past 1 century.

For example, the middle reaches of Bailong River in Gansu Province is now a famous debris flow-prone area in China. 1 0,000 years ago, there were dense bamboo forests and beautiful scenery here. Later, due to logging and charcoal burning, the forest was destroyed, leading to the proliferation of mudslides.

Another example is Da 'ertou Mountain in Shiaozigou of Ganchuan Highway, which was originally a forest area. Due to deforestation, a mudslide occurred in 1976, which washed away downstream villages and highways and caused serious losses to people's lives and property. The local people said: "There is a lack of acres on the mountain, and there are lights under the mountain."

The scale and types of debris flow are restricted by many factors. In addition to the above three main factors, earthquakes and volcanic eruptions may be the trigger factors of mudslides.