Motivation and purpose of research

Debris flow, as a common geological disaster in mountainous towns, is a special flood (high concentration liquid-solid mixed flow) containing a lot of solid substances, especially mud, sand and stone. Debris flow has the characteristics of sudden, destructive, fast movement and short duration, and has strong natural properties such as erosion and handling capacity. It destroys all kinds of urban facilities on the way by means of collision (impact), siltation and blockage, and its harm degree is often wider and more serious than that of single landslide, collapse and flood, causing serious losses to human production and living places, transportation, water conservancy and hydropower projects, mines and so on. At present, the problem of debris flow in mountainous towns in China is very prominent and the disaster situation is quite serious. Therefore, it is of great significance to analyze the debris flow disaster and its causes in mountainous cities and towns to strengthen the prevention and control of debris flow in cities and towns.

Keywords: mountain city; Debris flow; Disaster; cause

Research methods: Investigation and research were adopted, and data were sorted out through data.

research contents

I. Overview of Debris Flow

The concept of debris flow

Debris flow is a torrent formed by rainstorm and flood, which saturates and dilutes soft soil mountain with sand. It consists of thick mud suspended with coarse solid debris, rich in silt and clay. Under appropriate topographic conditions, a large number of water bodies soak the solid accumulation materials in the hillside or gully bed, which reduces its stability, and the solid accumulation materials filled with water move under their own gravity, forming mudslides.

Debris flow is a kind of natural disaster widely distributed in some special landform areas in the world. It is a mixed flow of soil, water and air between sediment-laden flow and landslide, which is excited by water sources such as heavy rain, melting of snow and ice in valleys or hillsides. Debris flows are mostly accompanied by floods in mountainous areas. It is different from the general flood in that the flood contains enough solid debris such as sediment and stones, and its volume content is at least 15% and the highest is about 80%, so it is more destructive than the flood.

(2) Types of debris flow

Namely: (1) According to the water supply, it can be divided into glacier type and rainfall type; (2) According to the valley shape, it can be divided into valley type and slope type; (3) According to the material composition, it can be divided into: debris flow, mud flow and water-rock flow; (4) According to structural rheology, it can be divided into: viscosity (capacity is 2.0 ~ 2.3t/m3), rareness (capacity is 1.5 ~ 1.8t/m3) and transition (capacity is1.8 ~ 2.0t/m3); (5) According to the scale, it is divided into: small scale (one debris flow accumulation 1 ten thousand m3).

Second, the harm of debris flow to cities and towns

Debris flow is a phenomenon that loose solid materials flow at high speed along steep slopes under the hydrodynamic action of rainfall, melting of snow and ice, dam break and so on. A large number of sediment and stones carried in the process of water flow have strong impact and destructive power, which often cause devastating damage to various urban facilities along the way. (1) Debris flow can destroy "solid" facilities in cities and towns, such as buildings, factories, bridges, water supply and power supply facilities, highways, railways, high-voltage lines, vehicles, dams, telephone poles and other fixed facilities and moving targets, and seriously endanger people's lives and property. For example, a mudslide occurred in Zhouqu, Gansu Province on August 7, 2065438, which washed away farmland 1 465438 mu, washed away 5508 houses of 307 households, flooded 20945 houses of 465438 households, washed away 2/kloc-0 rooms of office buildings of government agencies and units, and damaged 65445 vehicles. (2) Debris flow with boulders, tree trunks and other sundries will block bridges and culverts, causing debris flow to overflow the spillway, causing other secondary disasters, causing debris flow to bury, submerge and destroy urban facilities and residents in a large area, resulting in serious casualties. Such as 2011-05-11,a sudden mudslide occurred in a quarry in Caotun, Guangkeng, Luojiang Village, Salt Water Town, Quanzhou, Guangxi, and workers in the shed were buried by the mudslide before they could escape, resulting in 12 deaths and 10 missing. (3) If the scale of debris flow is large, the debris flow can cross the main river to form a barrage, and the blocked river will form a dammed lake upstream of the dam, resulting in the inundation of towns along the river; When the dam body breaks, powerful special floods will cause water damage to downstream towns and various facilities, such as washing away downstream houses, roads and farmland. (4) Because the debris flow contains solid substances, when the solid substances in the debris flow block its circulation road, the overflow will be diverted and the downstream facilities will be destroyed or submerged. (5) Squeeze the main channel. A large amount of sediment washed out by debris flow makes the accumulation fan expand continuously, forming a dangerous beach for navigable rivers, hindering navigation, and pushing the main river to the other side, causing serious erosion on the other side, causing bank slope instability and endangering various targets. Third, the reasons for the formation of debris flow disasters in mountainous towns

The formation of debris flow must have three basic conditions at the same time: rich loose solid matter, a large amount of water in a short time and a ditch-like terrain with a certain slope. Human engineering activities are also one of the factors that induce debris flow.

(A) the objective conditions for the formation of debris flow disasters

That is: (1) landform conditions: landform can provide potential energy conditions for the occurrence of debris flow disasters and sufficient solid material source conditions. (2) The source conditions of loose materials, such as easily broken rock surface, developed fault folds and dense faults, as well as soil erosion and old debris flow accumulation caused by deforestation, quarrying and slag abandonment, provide rich sources of solid materials for the formation of debris flow. (3) Water source conditions, such as heavy rain, severe melting of snow, sudden breach of reservoir, etc. , leading to the movement of a large number of deposits on the bed and side of the ditch, which may lead to the occurrence of debris flow disasters.

(B) the lack of reasonable urban development planning, weak awareness of disaster prevention

At present, with the continuous increase of urban population and the continuous expansion of urban scale, especially with the vigorous development of economy and construction in mountainous areas, the population is growing rapidly and the scope of urban construction is expanding day by day. However, due to the lack of unified planning guidance for urban construction in some mountainous areas, such as building houses in low-lying areas, ditches, ditches and other areas with serious debris flow risks, or building houses in debris flow passages. In addition, some towns lack a certain awareness of disaster prevention, and lack of supporting awareness of disaster prevention in urban construction, which leads to major disasters for urban residents when mudslides occur.

(3) Debris flow induced by human factors

With the growth of population, the development of economy and the expansion of the scale of towns in mountainous areas, human beings compete for land in the lower reaches of debris flow ditches, and at the same time, they constantly compete for and destroy land in the upper reaches of ditches. On the one hand, improper activities such as slope cutting, deforestation, quarrying, random discharge of mining soil and slag, land reclamation on steep slopes, massive deforestation, road clearing, overgrazing and unreasonable urban construction are increasing day by day, which greatly changes the original structure of the surface, leading to the deterioration of the ecological environment, intensified soil erosion, enhanced debris flow activities and aggravated landslides and debris flow disasters. On the other hand, the diversion canal built on the mountain directly induces landslides and even mudslides due to leakage. At the same time, with the continuous growth of urban population, due to poor management and people's lack of understanding of the harm caused by indiscriminate digging and logging, man-made debris flow disasters sometimes occur or aggravate the harm of debris flow.

Research conclusion

Eliminate and reduce the harm of surface water and groundwater. Landslides are often closely related to the action of water, and water is often the main factor that causes landslides. Therefore, it is particularly important to eliminate and reduce the harm of water to slope. Its purpose is to reduce pore water pressure and hydrodynamic pressure, prevent the softening and dissolution of rock and soil, and eliminate or reduce water scouring and wave impact. The specific measures are as follows: to prevent peripheral surface water from entering the landslide area, intercepting ditches can be built at the landslide boundary; In landslide areas, drainage ditches can be built on slopes. Mortar sheet or artificial vegetation can be used to cover the covering layer to prevent surface water from infiltrating. For rock slopes, shotcrete can also be used to protect the surface or hang steel wire mesh on shotcrete. There are many measures to remove groundwater, which should be selected according to the geological structure characteristics and hydrogeological conditions of the slope. Commonly used methods are: 1, horizontal drilling drainage; 2, vertical hole drainage; 3. Shaft pumping; 4. Drainage of the tunnel; 5. Support the blind ditch. Through certain engineering and technical measures to improve the mechanical strength of slope rock and soil, improve the mechanical strength of slope rock and soil, improve its sliding resistance and reduce sliding force.

Research experience

At present, the problem of debris flow in mountainous towns in China is very serious, which has caused great damage to urban facilities, threatened the life safety of urban residents and seriously affected the economic development of mountainous towns. Therefore, attention should be paid to the prevention and control of debris flow.