Division and evaluation of dangerous areas of geological disasters

Geological hazard zoning is based on geological hazard-prone areas. The susceptibility of geological disasters represents whether geological disasters have the formation conditions and the difficulty of geological disasters. The risk of geological disasters includes the activity degree, threat range, susceptibility and inducing factors of geological disasters, which is the possibility of geological disasters. The information method is still used in the calculation.

A, geological disaster risk assessment index system

There are many geological conditions that control and influence the formation of geological disasters, but they mainly include two aspects, namely, the basic conditions and inducing conditions for the formation of geological disasters. Based on this, the index system of geological disaster risk assessment can be established (Figure 6- 12).

1. Activity degree of geological disasters

The degree of geological disaster activity mainly refers to the history of geological disaster activity. This geological hazard risk assessment mainly considers the point density and area density of geological hazard activities as the basis for geological hazard risk zoning. However, the history of geological disaster activities can only explain the past of geological disasters, and the degree and danger of future geological disaster activities mainly depend on the forming conditions and inducing factors of geological disasters.

2. Formation conditions of geological disasters

The forming conditions of geological disasters include main controlling factors and influencing conditions. This geological disaster risk zoning evaluation mainly selects slope structure type, engineering geological rock group, hydrogeological conditions, slope geometry, fault structure, human activities and so on.

3. The threat range of geological disasters

Once a geological disaster occurs, its possible influence range is the range where there is danger of geological disaster. The threat range of geological disasters determined this time mainly includes the slope belt of the prone area itself, as well as the bottom of the valley and a certain range in rivers and reservoirs.

Figure 6- 12 Evaluation Index System of Geological Disaster Risk

4. Inducing factors of geological disasters

Inducing factors refer to the internal and external dynamic geological processes that can make the geological environment system evolve towards geological disasters or lead to geological disasters. The inducing conditions in this area mainly include earthquakes, rainfall and human engineering activities. However, due to the relatively weak seismic activity in this area, there is little difference between the seismic intensity and the peak acceleration of ground motion which can represent the degree of seismic activity in this county, and the influencing factors of seismic activity are not considered in this risk assessment. The main inducing factors involved in the evaluation are rainfall and human engineering activities.

Second, the geological disaster risk zoning

1. Quantization of evaluation index

Risk assessment is calculated by information method. Extract basic geographic information from 1: 50000 topographic map, extract vegetation layer from remote sensing image, and extract rainfall index from rainfall isoline map. Human engineering activities are mainly extracted from the topographic map and development planning of Lingtai County, and various evaluation index layers are obtained. The point density and area density of geological disasters can be obtained according to the actual survey data, and these indexes are introduced into GIS operating system.

2. The superposition calculation of information analysis model based on GIS.

The information analysis model based on GIS is used for superposition calculation. By calculating the information provided by various influencing factors on slope deformation and failure, as a quantitative index of zoning, it can not only correctly reflect the basic laws of geological disasters, but also be simple, practical and easy to popularize and apply. The calculation principle and process are as follows:

The viewpoint of information prediction holds that the occurrence of geological disasters such as landslides and collapses is related to the quantity and quality of information obtained in the prediction process, which is measured by the amount of information, namely:

Study on Geological Hazards in Lingtai County, Gansu Province

Where: I(Y, x 1, x2, x3, …, xn) is the information provided by factor combination x 1, x2, x3, …, xn to geological disasters such as landslides and collapses, and P(Y, x 1, x2, x3, …, xn).

According to the conditional probability operation, Equation 6- 1 can be further written as:

Study on Geological Hazards in Lingtai County, Gansu Province

Where Ix 1(Y, x2) is the information provided by factor x2 to geological disasters such as landslides and collapses when factor x 1 exists.

The establishment process of the information method evaluation model is as follows:

1) First, calculate the information I(xi, h) provided by each factor xi to the geological disaster event (h), namely:

Study on Geological Hazards in Lingtai County, Gansu Province

Where: P(xi|H) is the occurrence probability of xi under the distribution of geological disasters, and P(xi) is the occurrence probability of xi in the study area.

Equation 6-3 is its theoretical model, but the following sampling frequencies are often used in actual calculation:

Study on Geological Hazards in Lingtai County, Gansu Province

Where: s is the total number of evaluation units in the study area, n is the total number of units with geological disasters in the study area, Si is the number of units with evaluation factor xi in the study area, and Ni is the number of geological disaster units distributed in a specific category within factor xi.

2) calculating the total information provided by n factors in an evaluation unit Ii for the occurrence of geological disasters:

Study on Geological Hazards in Lingtai County, Gansu Province

Where: Ii is the total information value of the evaluation unit, and n is the number of participation factors.

3) Taking the total information amount Ii as the comprehensive index of the unit's influence on geological disasters, the greater the value, the more favorable it is for the occurrence of geological disasters, and the higher the risk of geological disasters in the unit.

4) Classification of information result value: In general, the result value Ii obtained by this model is an absolute value with continuous distribution, so it is necessary to classify the final information values of all units and divide the whole area into different grades of prone areas.

According to the size of Ii, determine the stability grade of the unit. Ii < 0 means that the possibility of unit deformation and failure is less than the possibility of regional average deformation and failure; Ii = 0 means that the possibility of unit deformation and failure is equal to the possibility of regional average deformation and failure; Ii > 0 indicates that the possibility of unit deformation and failure is greater than the possibility of regional average deformation and failure. In other words, the greater the information content of the unit, the better the deformation and failure of the slope.

5) Statistical analysis (subjective judgment or cluster analysis) finds out the mutation point as the demarcation point, and divides the area into different grades.

The basic data of evaluation indicators are all quantitative description data, which must be standardized, normalized, homogenized or unified by logarithmic, square root and other numerical transformation methods, or become dimensionless data after normalization before being substituted into the evaluation model.

3. Steps and methods of modeling

Preparation and reclassification of (1) factor layer

First, prepare the factor layer for effective analysis. According to the method of selecting factors in the information model, the factors involved in the evaluation are determined, and then they enter the GIS system in the form of 1: 50000 layer, and the obtained factor map is reclassified.

(2) Superimposed analysis of each factor layer and geological hazard distribution map.

After each factor layer is prepared, it can be analyzed in combination with the geological disaster distribution map in GIS. By transforming Equation 6-5, we can get:

Study on Geological Hazards in Lingtai County, Gansu Province

Among them, N/S is the total distribution density of geological disasters in the region, which is a constant value under certain conditions in the study area; N/Si is the distribution density of geological disasters in a specific category of a certain layer. It can be seen that a key point in the model is to find out the distribution density of various types of geological disasters in each factor layer. In order to calculate Ni/Si, it is necessary to obtain the distribution of geological disasters in each category of each factor and realize it in GIS by using its spatial analysis function.

(3) calculating the information value of each layer and category

After spatial analysis of the attribute table of each layer element and the corresponding geological hazard distribution map, the information values of each layer and category are calculated according to the following formula:

Study on Geological Hazards in Lingtai County, Gansu Province

From this, we can get all kinds of information in various factors. According to the relationship between each category and the corresponding amount of information, each category can be given the corresponding amount of information, and the information map of single factor layer can be obtained. After calculating the information value of each category in each layer factor, the corresponding grids between different layers can be accumulated, and the final result is the comprehensive information map of the whole study area, that is, the geological disaster-prone zoning map.

(4) Selection of grid size

Whether the unit grid division is appropriate or not affects the rationality of the results of geological disaster susceptibility division and the difficulty of obtaining various parameters in the division process. The selection of grid cell size in the study area depends on the data accuracy required for analysis, which is small enough to obtain the required details and large enough for computer storage and ensure the efficiency of analysis. When determining the unit size, the following factors should be considered:

1) Accuracy of input data;

2) The size and disk capacity of the result database;

3) Expected corresponding time;

4) Application and analysis to be carried out.

A unit that is more accurate than the input cannot obtain more accurate data than the input. The resulting raster dataset should be as accurate or coarser than the input dataset. The size of the unit varies according to different studies. The smaller the unit, the higher the precision and accuracy, but the cost of coding, database storage and analysis processing speed increases accordingly. For most analysis, the efficiency based on element analysis is more important than the loss of accuracy.

The selection of suitable grid cell size is influenced by many factors, but the sensitivity of each factor to cell selection is different (the most sensitive factor is terrain), and the influence process of these factors on cell size selection is still worth further study. In many cases, choosing the appropriate grid cell size depends entirely on the knowledge and experience of experts. No matter how much other factors affect the selection of grid cell size, the resolution of original data, especially terrain data, is the primary factor to consider when selecting grid cells. The risk zoning evaluation of geological disasters in Lingtai County still uses 500m×500m grid units, and there are 8084 evaluation units in Lingtai County.

According to the above information amount calculation method, the total information amount of each unit is calculated. The greater the contribution of various factors to geological disasters such as landslides and collapses, the greater the possibility of geological disasters. Using the natural breakpoint method commonly used in statistics, the risk zoning map is reclassified, and the risk of geological disasters in Lingtai County is divided into three levels: high risk area, medium risk area and low risk area.

On the basis of quantitative calculation of zoning by information model and comprehensive consideration of various factors, the risk zoning map of geological disasters in Lingtai County is finally drawn by hand (Figure 6- 13).

Figure 6- 13 Geological Hazard Zoning Map of Lingtai County

According to the results of geological disaster risk zoning, fully considering the development of geological disaster prevention and control planning, combined with the characteristics of Lingtai County, such as landform, geotechnical characteristics, geological structure, annual rainfall distribution law and human activities, the risk degree of landslides, collapses and mudslides in Lingtai County is divided into high-risk, medium-risk, low-risk and extremely low-risk areas, and further divided into 16 subregion according to the distribution and combination characteristics of geological disasters (Table 6

4. Risk zoning assessment

(1) Geological hazard high-risk area (Ⅰ)

The high-risk area of geological disasters in Lingtai County covers an area of 232.49km2, accounting for 1 1.35% of the total area. Include six high-risk sub-zones, namely, Hengqu-Fujiagou-Guancun-Zhujiawan-Dujiagou-Jingjia Zhuangzi high-risk sub-zone (I1), Zhang Jiayuan-Wenjiazhuang-Dongmen-Zhujiawan high-risk sub-zone (I 2), north bank of Daxi River (I 3) and south-central bank of Daxi River. The geomorphic units in this area are mainly loess hilly and gully region and loess hilly region. The lithology is mainly Quaternary loess and Cretaceous purplish red mudstone, sandstone and glutenite. The surface loess is thick, and most of the loess tends to gully. The local terrain slope is large, the engineering geological conditions in the area are poor, and the surface of rock and soil layer is seriously weathered. Human activities in this area are relatively strong, and the valley slope is densely populated. Human activities have a strong environmental transformation, mainly including building slopes, digging caves, building roads and so on. Human activities are likely to induce landslides and collapses. The vegetation in this area is sparse, mainly crops, which is not conducive to soil and water conservation. Gullies in hilly areas are mostly in the prime of life or infancy, and the erosion is relatively strong. In the flood season, landslides and collapses are easy to form when encountering heavy rain and continuous rainy weather. Special geotechnical and meteorological conditions provide the possibility for the formation of geological disasters. This area is also the most serious landslide and collapse area in the county.

Table 6-4 Description Table of Geological Hazard Zoning

(2) Geological disaster danger zone (Ⅱ)

The dangerous area in Lingtai County is 604.03km2, accounting for 44. 12% of the total area. Geological disasters include landslides, collapses, mudslides and unstable slopes. It includes four geological hazard sub-regions, namely, the loess hilly region in Liangyuan Township on the north bank of Heihe River (Ⅱ1), the loess hilly region from the south bank of Heihe River to the north of Lion Plateau (Ⅱ 2) and the loess hilly region from the south of Lion Plateau to the north of Daxi River (Ⅱ 3). The Quaternary Middle-Upper Pleistocene loess in this area is covered with Cretaceous glutenite, sandstone and mudstone bedrock with different thicknesses, and it is easy to form landslides along the contact surface between loess and bedrock under the action of precipitation. The human engineering activities in this area are relatively strong and the population is relatively large. Intense human engineering activities are mainly manifested in slope cutting for various purposes, which increases the free surface of collapse and landslide. The loess layer is a disaster-prone area with weathered rocks and developed joints and cracks. Gullies in hilly areas are mostly in the prime of life or infancy, and the erosion is relatively strong, and the gullies and slopes are mostly stepped steep slopes. Landslides and collapses are prone to occur in flood season. The disaster spots are distributed around the village, along the highway and on the slope of the valley. Once geological disasters occur, they are very harmful.

(3) Geological hazard low risk area (Ⅲ)

The low-risk area of geological disasters in Lingtai County covers an area of 9 12.48km2, accounting for 44.53% of the total area. Geological disasters are less developed and less dangerous. It includes six low-risk sub-regions, namely, Wang Jiagou Loess Plateau in Liangyuan Township (85438+0), Heihe Broad Valley Low-risk sub-region (Ⅳ 2), Lion Plateau Low-risk sub-region (Ⅳ 3), Daxihe River Basin Low-risk sub-region (Ⅳ 4), Shaozhai Town Loess Plateau Low-risk sub-region (Ⅳ 5) and Baili. The loess tableland, loess tableland and wide river valley in this area have good engineering geological conditions and flat terrain. Although human engineering activities are frequent, geological disasters rarely occur. Baili Township Forest Farm has dense vegetation, sparsely populated, less human engineering activities, weak human engineering activities and relatively superior geological environment, which is a low-incidence area of geological disasters.