6.4. 1 Introduction to general methods of debris flow observation
6.4. 1. 1 Line Selection and Station Construction
In the survey and site selection of debris flow observation station, firstly, we should choose the river basin where enough debris flow occurs every year to meet the observation needs, and have good facilities such as transportation, electricity, communication and living conditions. The station is generally selected near the circulation accumulation section in the lower reaches of the basin to control the whole output information. At the same time, we should fully consider the layout of observation sections and observation points necessary to control the whole basin in order to achieve synchronization and continuity of observation.
6.4. 1.2 Analysis of Basin Background Data
Collect background data of river basin.
The change of natural environment in the basin includes the change of nature itself and the environmental change caused by human factors.
(1) Hydrometeorology of the basin;
(2) Geological and geomorphological background information of the basin;
(3) Social, cultural and economic activities in the basin;
(4) Natural resources in the basin.
2) Investigation of various natural and man-made disasters in the history of the basin and its adjacent areas.
(1) earthquake;
(2) disastrous weather such as rainstorm, hail and high temperature;
(3) Landslides, landslides, avalanches, ice collapses and other events;
(4) mountain torrents;
(5) Forest fires;
(6) Main economic activities and their impacts on the basin environment.
6.4. 1.3 Debris Flow Observation Contents
1) controlled measurement of solid-liquid two-phase material source in formation area
(1) Hydrometeorology
(2) Landslide and its deformation observation: It is very necessary to detect the important landslides that supply the debris flow source in the basin, and its instruments and equipment include: landslide displacement meter, inclinometer, pore water pressure gauge, positioning pile, etc.
(3) Groundwater observation: Groundwater activity in the source area of the basin plays an important role in the formation of debris flow, and logging and spring outcrop are usually used for observation.
(4) Observation of soil water content: Especially, saturated water content plays an important role in soil, and it is tested by profile sampling analysis or soil moisture meter.
2) Observe the moving elements
The observation of debris flow movement process and various movement elements is the focus of debris flow observation test. The observation section is selected in a relatively straight circulation area, and 2 ~ 3 measurement sections are arranged.
(1) Select section: the section spacing is 20 ~ 20~200m, depending on the specific situation. After selecting sections, measure the geometric characteristics of each section. Its transverse characteristics indicate the relationship between water depth and cross-sectional area, and its longitudinal characteristics indicate the total slope of the gully bed. After each debris flow, if the cross section changes greatly, it must be measured repeatedly.
(2) Measurement of faucet flow rate: Usually, a stopwatch is used to record the time when the faucet passes through two sections with a known distance, and the average flow rate of the faucet can be obtained.
(3) Mud level and mud depth: use ultrasonic water level gauge to measure the elevation of debris flow surface. Before the debris flow, the elevation difference between the mud surface of the faucet and the bottom of the ditch is the height of the faucet. The height of the faucet does not represent the mud depth in the whole process of debris flow, because the mud depth changes too much along the array, the mud depth of the faucet is the largest, and the mud depth of the dragon tail is zero.
(4) Surface slope: Debris flow is unstable and uneven, so it is necessary to measure the slope of mud surface.
(5) Whole process measurement: observe the whole process of debris flow from its formation to the dispersion of accumulation fans or the inflow into the main river channel through accumulation fans.
3) sample analysis
(1) density and water content;
(2) particle analysis;
(3) Analysis of chemical composition and colloidal composition;
(4) Rheological test.
4) Measurement of trench bed deformation
Debris flow gully bed has the characteristics of large amount of erosion and deposition, and often after a mudslide, the gully bed is unrecognizable, which is of great significance to engineering design. Usually, many measurements are made before and after the debris flow process to compare the erosion and deposition changes of different parts of the gully bed.
5) Measurement of physical and mechanical characteristic values
There are mainly debris flow impact test and acoustic characteristics measurement.
6.4.2 Main methods of debris flow observation
Generally speaking, there are two basic conditions for the occurrence of debris flow: rich solid matter and a large amount of rainfall with a certain intensity. Abundant solid matter means that there are a large number of loose solid matter in the channel, including the debris flow driven by solid matter provided by collapse and landslide during rainfall; A large number of solid materials on the slope move to the channel (landslide or slope debris flow) and form debris flow in the channel; Landslides form earth dams, which are washed away by the current to form dam-break debris flows. According to the basic conditions of debris flow formation, combined with rainfall observation, rainfall intensity and rainfall duration analysis, the critical conditions of debris flow outbreak are given, so as to predict the occurrence of debris flow.
The critical rainfall index of debris flow outbreak will be different because of the different geographical, geomorphological and geological characteristics in different areas. Therefore, it is two main contents of pipeline debris flow monitoring to investigate the environmental conditions and background of debris flow in the small watershed where the oil and gas pipeline is located and determine the critical rainfall index of debris flow outbreak.
Rainfall observation
Set up a rainfall station in the debris flow formation area. Collect the rainfall duration data of a rainfall process as much as possible to provide basic conditions for finally determining the rainfall threshold of debris flow.
Study and Determination of Critical Rainfall Index for Debris Flow Outbreak in 6.4.2.2
Generally speaking, the rainfall observation of debris flow emphasizes the observation of 24-hour rainfall, while the 24-hour rainfall observation emphasizes the observation of 3-hour rainfall. 0 ~ 3 hours rainfall observation, especially 10 minutes and 1 hour, is the focus of debris flow rainfall observation. Through the study of debris flow under different rainfall conditions, the critical rainfall index of debris flow outbreak is determined.
6.4.3 Debris Flow Early Warning Observation
The research shows that infrasound signal will be generated during the start and movement of debris flow, which is a unique signal in the process of debris flow. Therefore, it is very effective to monitor and capture this signal with infrasound alarm, and then make an early warning of debris flow.
Debris flow infrasound alarm was developed by Chengdu Institute of Mountain Disaster and Environment, Ministry of Water Resources, Chinese Academy of Sciences after 10 years. Since 1995, there have been dozens of applications for missing or misreporting debris flow events, and patents have been approved (patent number ZL0 1256480X). At present, the equipment has been used for debris flow observation in Venezuelan, Spanish, Sichuan, Yunnan and Taiwan Province provinces.