(1) What is a debris flow?
A mudslide is a temporary turbulent flow that occurs in mountainous areas and carries a large amount of sediment and rocks. It tends to erupt suddenly, when it comes with ferocity, moves quickly, lasts for a short time, and has a powerful destructive power. For example, there are about 239 debris flow ditches along the Chengdu-Kunming Railway, and there were 86 debris flows along the line in the six years from 1980 to 1985 alone. In the early morning of July 9, 1981, the Lizi Dayi Ditch debris flow, with a flow of 3,200 m3/s and a total amount of solid matter of 70×104 m3, washed two locomotives and the first two carriages of a passenger train out of the tunnel into the Dadu River along with the bridge, killing 275 people (Figure 15-7).
(2) Mudflow formation conditions
There are three basic conditions that must be met for the formation of debris flows, namely topographical, geological and meteorological and hydrological conditions.
1. Topographic conditions
Debris flows always occur in steep mountainous areas, usually along narrow valleys with large slopes, and each debris flow has its own watershed. A typical debris flow basin can be divided into three sections: formation area, circulation area, and accumulation area (Fig. 15-8). Topographic conditions are the premise and activity site for the formation of debris flows.
2. Geological conditions
Geological conditions determine the source of loose solid material, in the complex geological structure, weak rock formations, weathering
In areas with strong action and undeveloped vegetation, it is easy to form a large number of loose detritic materials in the slopes and valleys, forming the recharge source area (formation area) of debris flow, which is the material condition for the formation of debris flow.
3. Meteorological and hydrological conditions
Debris flows must have strong surface runoff as a dynamic condition for the formation of debris flows. The surface runoff of debris flows comes from heavy rainfall, strong melting of alpine ice and snow, and water outburst, and meteorological and hydrological conditions are the decisive factors that trigger the occurrence of debris flows.
(3) Prevention and control measures for debris flows
The prevention and control of debris flow should implement the principles of comprehensive management, people-oriented, adapting measures to local conditions, and stressing practical results. Specific control measures are divided into two categories: biological measures and engineering measures.
1. Biological measures to prevent and control debris flows
Biological measures include the restoration or cultivation of vegetation, and the absolute prohibition of farming in collapsed areas. This prevents slope erosion, regulates runoff and reduces flash flood power, and controls and reduces the material sources of debris flows.
2. Engineering measures to prevent and control debris flows
The engineering measures to prevent and control debris flow mainly include blocking projects, water storage and diversion projects.
The damming project is to build a dam body to block the debris flow in the circulation area to block the debris flow and protect the bed and stabilize the slope, and the dam body is left with a drainage hole to discharge the water flow. The retaining dam can be constructed in multiple stages to reduce the total amount of solid matter discharged and the peak flood flow. The spacing between dams can be calculated according to the formula
(Figure 15-9).
Water storage and diversion projects include flood diversion reservoirs, interception ditches and diversion canals, etc., which are built in the formation area, and their role is to intercept most of the floodwater, reduce the flood peak, and control the hydrodynamic conditions of the outbreak of debris flows. Drainage works can also be built in circulation and accumulation areas, the role of which is to adjust the flow direction and prevent overflow to protect nearby settlements, industrial and mining sites and transportation lines. The debris flow prevention and control project can be shown in Figure 15-10.
The above-mentioned engineering and biological measures are often used in combination in the whole basin of a debris flow ditch.