Study on Debris Flow Disaster in Gongjue County, Xizang Autonomous Region

Qi Guoqing 1 Cao Xiuding 2 Xia Baoben 3 Yun Teng 3

(1 National Professional Laboratory of Geological Disaster Prevention of Chengdu University of Technology, Chengdu, Sichuan, 610059; Institute of Hydrogeological Engineering Geological Technology and Methods, China Geological Survey, Baoding, Hebei, 071051; Xizang Autonomous Region Institute of Eco-environmental Geology, Lhasa, Tibet, 850000)

There are three rain and debris flow gullies in Gongjue County, Xizang Autonomous Region, which affect the safety of residents. Based on the strength theory of unsaturated soil, the genetic mechanism is studied, and it is suggested that the critical rainfall line model be used to establish the debris flow prediction model in Gongjue County, and the fitting analysis is made according to the rainfall data of Zhelongwa No.2 gully 20 days before the debris flow outbreak. The results show that the two critical rainfall line models proposed in this paper are both suitable for establishing the rainfall-type debris flow prediction model in Gongjue County.

Debris flow; Suction of unsaturated soil matrix; prediction model

1 Introduction

Gongjue County is located in the eastern part of Qinghai-Tibet Plateau and the northern section of Hengduan Mountains. According to the topographic zoning map of Tibet, the county belongs to the mountainous valley area with great ups and downs in eastern Tibet. The terrain slope in the county is generally 30 ~ 40, and some of them are greater than 60. Located in the southeast of the county, the alpine valley area, which accounts for about 34.4% of the county's total area, is a high-risk area for geological disasters such as mudslides.

Debris flow is an intermittent torrent carrying a lot of soil and debris. ② The distribution of solid particles below 0.0 1mm is consistent, while the distribution above 0.01mm is different. The material content of Zhelongwa Ⅱ debris flow of 3 ~ 10 mm is higher, and that of Kexilin debris flow of 0. 1 ~ 0.5 mm is higher. This is mainly because of the different sources of materials.

Fig. 2 Distribution curve of solid particles in Muxie debris flow

1. Zhelongwa Ⅱ debris flow; 2. Zhelongwa type I debris flow; 3. Debris flow in Lin Kexin

2.3 Meteorological and hydrological conditions

The inducing factor of debris flow in Gongjue County is atmospheric precipitation, which belongs to rainfall debris flow. Gongjue County and adjacent Mangkang County belong to the plateau temperate humid and semi-humid climate. The annual rainfall is 450 ~ 570 mm, with little rainfall and distinct wet and dry seasons. The annual rainfall is mainly concentrated from June to September, mostly heavy rain and heavy rain, with more disastrous weather. The average temperature in Gongjue County is 5.2℃, and that in Mangkang County is 3.5℃.

Gongjue County is located on the west bank of Jinsha River, which runs north-south along the eastern boundary of Gongjue County, with a flow of about 80km. Rivers in the territory include Hot Bend, Oblique Bend, East Bend, Guoqu, Luomai River, Brequ Gorge, Masinong and A Xiang Xi, all of which belong to the middle and upper reaches of Jinsha River. The river basin is located in the middle of Gongjue County, and it is basically distributed in the north-south direction. Among them, the Requ River basin is mainly located in the west of the basin, while the Xiequ, Dongqu, Guoqu, Hehe, Brequ Gorge and Masinonghe basins are all located in the east of the basin. The largest river in China is Requ River, which flows into Jinsha River from north to east. The flow in the area is about 1 10km, the average gradient of the whole river is 4 ‰ ~ 7 ‰, and the river width is about 50 ~ 70m. The secondary water system on both sides of the river is dendritic. Maqu, Naqu and Qu Ze are its tributaries. Most of the debris flows in Gongjue county are developed in the oblique bend valley, and Luoqu is a tributary water system in the upper reaches of the oblique bend.

Influenced by geographical location, topography and meteorological factors, the hydrological situation in the basin is very complicated and varies greatly from place to place. Runoff in this basin is mainly supplied by precipitation, and groundwater and snowmelt water also account for a considerable proportion. The annual variation of runoff is about 65438 0.5 times. In a year, with the change of drought and rainy season, there are dry season and rainy season. The flood is mainly caused by precipitation, and the peak flow is not large. Generally speaking, flood and low water flow changes about 10 times.

3 Analysis of the formation mechanism of rainfall debris flow

The formation of rainfall-type debris flow can be divided into two stages [2]: in the first stage, unsaturated solid loose matter reaches saturation due to the increasing water content, and loses its shear strength due to the decrease of matrix suction; In the second stage, due to the increasing water content, the water pressure of saturated solid loose objects increases, the effective stress decreases, and mudslides occur.

3. 1 Shear strength loss stage caused by matrix suction

According to (Fredlund et al., 1978) unsaturated soil shear strength formula [5], the shear strength of unsaturated solid loose materials can be expressed as:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: c' is the effective cohesion; σf is the total normal stress on the failure surface during failure; U is the pore gas pressure on the failure surface when the failure occurs; Uw is the pore water pressure on the failure surface during the failure process; (σf-ua)f is the state of net normal stress on the failure surface; (ua-uw)f is the matrix suction on the failure surface; φ′ is the internal friction angle of solid loose material; φb is the rate at which shear strength increases with matrix suction; Is the shear strength caused by matrix suction. The change rule with water content is [2]:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: (ua-uw)r is the matrix suction corresponding to the residual water content θr; (ua-uw)b is the air intake value of the soil; θ is volume water content; θs is the saturated volume water content.

Formula (2) shows that in the first stage of the formation of rainfall-type debris flow, due to rainfall infiltration, the water content θ of unsaturated solid loose material increases continuously, and the matrix suction (ua-uw) decreases continuously, resulting in the shear strength loss caused by matrix suction.

3.2 The increase of pore water pressure leads to the decrease of effective stress and the emergence of flow stage.

After a certain duration of rainfall, the water content of unsaturated solid loose materials increases, and after reaching saturation, the water content continues to increase, which will produce pore water pressure uw in solid loose materials. The more water in solid loose materials, the greater the pore water pressure uw and the lower its shear strength. The relationship between the shear strength of saturated solid loose materials and pore water pressure is as follows:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: c' is the effective cohesion of solid loose matter; φ′ is the effective internal friction angle of solid loose matter.

When the solid loose material reaches saturation, it enters the second stage of rain-type debris flow formation. At this point, the discriminant [6] of whether the saturated solid loose matter is started is:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: a is the contact area between solid loose material and trench bed; G is the weight of solid loose matter; T is the flow thrust, which is a secondary factor; β is the slope of the bottom slope of the trench bed; K is the stability coefficient of solid loose matter. When K= 1, the saturated solid loose matter is in the limit state. When k > 1, the saturated solid loose matter is in a stable state and debris flow will not occur; When k < 1, the saturated solid loose material is in an unstable state and debris flow will occur.

Formula (4) reflects the mechanical mechanism of whether rainfall-type debris flow starts. At this stage, the short-term rainfall with a certain intensity makes it too late for the water infiltrated into the solid loose objects to be discharged, and the surrounding rainfall confluence will start the solid loose objects and form mudslides.

4 Discussion on debris flow forecast model in Gongjue County

The occurrence of rain-type debris flow is the result of the interaction between effective rainfall in the early stage and rainfall with a certain intensity in a short time. In the first stage of its formation, the increase of water content of solid loose matter is closely related to the effective rainfall in the previous period. In the second stage, rainfall with short duration and certain intensity plays a leading role. From the mechanism analysis of the formation of rainfall debris flow, it can be known that the greater (smaller) the effective rainfall in the early stage, the smaller (larger) the short-duration rainfall index required for the formation of debris flow.

The effective rainfall in the early stage [7] P is composed of the rainfall H24 of that day and the rainfall Pt (existing in solid matter) of the other days. Where: r is the decreasing coefficient; N is the influence period of previous rainfall. The attenuation coefficient and the influence period of the previous rainfall should be determined according to the local climatic conditions and the composition lithology, water content, porosity, permeability coefficient and matrix suction of solid loose materials.

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

The rainfall indicators with short duration and certain intensity generally adopt 10 minute rainfall, 60 minute rainfall, 24-hour rainfall, etc.

The critical rainfall line model is mainly used to predict rainfall-type debris flow. However, the differences in the characteristics of debris flow gullies and the mechanical properties of unsaturated soil with solid loose materials will lead to the differences in the prediction model frames. Jiangjiagou model [8] is a critical rainfall line model, and its model framework can be written as follows:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

In which: short duration rainfall index R 1. Rainfall10min (mm); The effective rainfall p is the effective rainfall within 20 days, and the decreasing coefficient R = 0.8a, b and m are fitting parameters. In Jiangjiagou model, the critical rainfall lines of debris flow are a = 5.5, b = 0.098 and m = 0.5mm；; Debris flow rainfall line, a = 6.9, b = 0. 123, m =1.0mm ... Jiangjiagou model forecast advance time17 ~ 20min, accuracy rate 86%, false alarm 3%, false alarm1/kloc.

Another key rainfall line model framework is:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Where: a and b are fitting parameters. Debris flow was observed in the dump [9], and the rainfall line was composed of EF section and FG section. For EF segment, a = 293.33, b =-5.93；; For FG section: A = 76.46, B =-0.48；; At the connection point f of the two curves, r 10 = 1.44 mm, p. = 39.79 mm.

Taking Zhelongwa Ⅱ debris flow in Gongjue County as an example, the above two prediction models (Formula (6) and Formula (7)) are discussed. Zhelongwa Ⅱ debris flow broke out on June 20th, 2003. See table 1 for the rainfall of 20 days before the occurrence of the debris flow in Zhelongwa.

Table 1 June 20031Daily Rainfall from June 20th.

According to table 1, the effective rainfall p in the previous period is obtained by formula (5). 30.57mm, the critical rainfall of debris flow 10 minute calculated by formula (6) is 2.33mm, and the critical rainfall of debris flow 10 minute calculated by formula (7) is 3.67 mm. However, the annual average maximum rainfall of 10 minute in this area is 6.0mm[ 10], which is greater than the critical rainfall of 10 minute obtained by formulas (6) and (7), and it is in a critical state of debris flow. This is in line with the actual situation. The above discussion shows that these two critical rainfall line model frameworks can be used to establish the prediction model of rainfall-type debris flow outbreak in Gongjue County.

5 conclusion

Judging from the inducing factors, the debris flow in Gongjue County, Xizang Autonomous Region is mainly rainfall debris flow. The two critical rainfall line models proposed in this paper are both suitable for establishing the prediction model of rainfall-type debris flow outbreak in Gongjue County.

For rain-type debris flow, when the material conditions (solid loose materials accumulated in a certain slope, a certain catchment area and other conditions) are available, the occurrence of debris flow is the result of the joint action of effective rainfall in the early stage and short-term heavy rainfall. According to the theory of unsaturated soil strength, the formation of rainfall debris flow can be divided into two stages: the first stage, the shear strength loss stage caused by matrix suction, which is related to the effective rainfall in the early stage. The second stage, the stage of debris flow, is related to short-term heavy rainfall.

The advantage of applying unsaturated soil mechanics principle to study the formation mechanism of rainfall debris flow is that by studying the unsaturated physical and mechanical properties of solid loose materials that may form debris flow, it is possible to predict in advance whether debris flow will occur under rainfall conditions, as well as the required rainfall conditions and rain patterns, thus providing a stronger theoretical basis for accurate prediction of debris flow. The author will further strengthen the application of this theory in the field of debris flow in the future research.

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