A Study on Risk Assessment of Geological Disasters in Pingba District of Guizhou Province Based on GIS

: To conduct regional geological disaster risk evaluation zoning in a scientific and reasonable manner, and provide a decision basis for local territorial spatial planning, geological disaster risk prevention and control, and disaster prevention and mitigation work. This study, on the basis of collecting relevant data sources and field investigation, comprehensively analyzed and studied the geological environmental conditions of disaster pregnancy and the development and distribution pattern of current disasters in Pingba District, built an information volume model for susceptibility evaluation based on the influence factors of known disaster areas, and also reasonably determined the hazard and vulnerability evaluation model by combining the characteristics within Pingba District, thus making the risk evaluation of the county highly accurate, operable and scientific, and the results of its risk evaluation zone zoning are more consistent with the reality within the county. The risk evaluation methods and processes described above are synthesized with a view to providing reference and guidance for their application in similar work.


INTRODUCTION
As a country where geological disasters occur very frequently and the damage is extremely serious, China is especially prominent in the southwestern mountainous provinces. The geological disasters constitute a great threat to people's lives and social properties, causing significant losses and becoming a major factor affecting the public safety of our society. Hence, to safeguard people's lives and properties, to improve disaster prevention, mitigation, resistance and relief capabilities has become an important issue that needs to be addressed at home and abroad. Whereas, in this study, based on the detailed survey of geological hazards, supported by GIS spatial analysis technology [4], and using the information quantity method, the integrated geological hazard and vulnerability evaluation results, the division of geological hazard risk zoning has become a common method adopted by scholars at home and abroad, and then propose countermeasures for risk control, providing a basic geological basis for local disaster prevention and mitigation, territorial spatial planning and government decision-making.

Overview of the Research Area
Located in the central part of Guizhou, Pingba District is part of Anshun City, Guizhou Province, and is a major transportation route connecting Guiyang with western Guizhou and even Yunnan-Guizhou. The study area includes Gaofeng Town and Machang Town under the jurisdiction of Gui'an New Area, with a total land area of 998.9km², 7 towns, 2 townships and 2 streets. Located in the western section of the Miaoling Mountain Range, the terrain has the characteristics of being high in the northwest, low in the southeast and flat in the middle, with most of the elevation in the area between 1100m and 1500m, the highest point is on the county boundary of Qinjiawanzi in Cross Township, with an elevation of 1675m; the lowest point is where the Bodu River comes out of the area, with an elevation of 960m. The largest height difference is 715m, and the general height difference is less than 250m. The geological structure development in the area, the diverse lithology of the strata and their combinations, the complex geological environment conditions, as well as the variable climate and heavy rainfall in recent years, have led to the occurrence and frequency of geological disasters in the area [5].

Geological Disaster Samples
According to the results of the previous work and field survey, the samples of this analysis and evaluation are composed of three types of disaster sites that have already occurred in the existing ledger (as of March 2021), disaster sites in the historical ledger with credible data from the survey and disaster sites found during the field survey, with a total of 112 samples, including 44 landslides, 48 landslides, 4 unstable slopes, 7 ground cracks and 9 ground subsidence sites. Currently there are 75 ledgers, 28 historical hazards and 9 disaster spots found during the on-site investigation.

Rainfall
The region is a northern subtropical monsoon humid climate, mild climate, abundant rainfall, inter-annual average multi-year precipitation of 1305.6mm, maximum precipitation 1656.7mm, and minimum precipitation 720.4mm. The theoretical values of 24-hour maximum rainfall for each station for 20-year and 50-year recurrence periods were calculated by the Person-III probability distribution based on the daily rainfall at a total of 16 meteorological stations in and around Pingba District since records were available, as shown in Table 1.

INDEX SYSTEM ESTABLISHMENT
It can be seen from the sample data that the types of geological hazards that have occurred in Pingba District are mainly landslides and landslides, followed by ground subsidence, and the analysis and study of the disastercausing factors , the geological environmental conditions of disaster-pregnancy and the development and distribution patterns of the current disasters [1] are used to establish the information volume model of the vulnerability of each disaster type respectively. To ensure that the factors are independent of each other and meet the accuracy of the model input parameters, the correlation test must be done for the selected factors, and the correlation matrix is calculated by using the multivariate analysis tool in ArcGIS software, so that the correlation analysis of the primary factors can be performed and the correlation coefficient of each factor with a significance of 0.01 can be obtained. Regarding the factors with 0.2 <| correlation coefficient|, they were excluded in order to avoid the mutual interference and superposition of information, which would affect the accuracy of the evaluation. A total of 11 control factors based on the above principles were selected: slope, cover thickness, slope type, engineered rock group, distance from tectonics, slope structure, land use type, slope length factor, terrain undulation, slope height, and human engineering activities as the evaluation factors for this time.

4.1Evaluation of Regional Geological Disaster Susceptibility
Whereas each evaluation unit is influenced by the combination of many factors, and there are several states of each factor, the total amount of information about the occurrence of geological hazards under the combination of factors in each state is The distribution map of the information volume of the whole area was calculated based on the total information volume of the occurrence of unit geological hazards respectively, and the evaluation map of the vulnerability of the whole area was obtained by the natural breakpoint method, and then the results of the vulnerability evaluation were generalized and the boundaries were modified with the actual situation of the region to divide the area into four grades of very high, high, medium and low with a total of 34 subzones, among which the medium and low vulnerability zones were mainly, accounting for 28.73% and 38.29% of the total area of the region respectively, and the extremely high and high susceptibility areas account for 13.91% and 19.07% respectively. The map of susceptibility zoning is shown in Figure 1.

4.2Risk Assessment of Regional Geological Hazards
The geological hazard risk evaluation is carried out on the basis of susceptibility evaluation, considering three types of factors: historical factors of disasters, basic environmental factors and predisposing factors. The hazard history factor is the density, scale and frequency of geological hazards that have occurred. The basic environmental factors are the set of susceptibility evaluation factors; the predisposing factors are various exogenous dynamic factors such as rainfall, earthquake, human engineering activities, etc. that induce the geological environment to evolve in an unfavorable direction or even cause the occurrence of geological disasters.
The evaluation model of geohazard hazard is the same as that of susceptibility evaluation [2], and factors such as the density of existing geohazard distribution are considered in the evaluation, and the rainfall is considered for the 50-year working condition. The regional basic seismic intensity is VI degree, the rare seismic intensity is VII degree, and the seismic working condition under consideration is VII degree.

Fig 3 Risk Grade Distribution Under Rare Earthquake
Conditions (VII Degree) The geological hazard evaluation results are divided into four grades of the low, medium, high and very high according to the natural breakpoint method. The hazard evaluation for the 50-year rainfall recurrence period is shown in Figure 2, The hazard evaluation for the rare earthquake conditions is shown in Figure 3.

Vulnerability Assessment of Regional Geological Disasters
The evaluation of vulnerability to geological hazards is based on the collection and use of the third land survey data, remote sensing interpretation and field survey, and the calculation of vulnerability of people and property. Considering that the type of hazard-bearing body is complex throughout the region, the distance from the hazard source varies, and the destabilization time, scale and intensity, and transport space of the geological hazard potential or hazard source vary, these factors determine the vulnerability or damage rate of the hazard-bearing body, therefore, it is more difficult to determine the vulnerability of the hazard-bearing body, so the vulnerability coefficients in this study area are all taken as the most dangerous consideration.
The quantity and value of different property types are calculated by overlaying the "three surveys" into the working area, and then the population quantity and property value are divided by the area of each unit to find out the population density [3] and property density value. The population and property distribution levels are assigned and the population density distribution and property density distribution maps are made.
The number of threatened people and threatened properties are calculated by the aforementioned method using the population distribution density map and property distribution density map within the hazard area with different risk probabilities after creating appropriately sized raster cells, using the following formula.
Threatened Population = Σ Population Damage Rate × Grid Calculation Unit Area × Grid Calculation Unit Population Density.
Threatened Property = Σ Property Damage Rate × Grid Calculation Unit Area × Grid Calculation Unit Property Density Value.
The full domain susceptibility was classified into four categories: very high, high, medium and low by the natural breakpoint method. As shown in Figure 4.

Risk Assessment of Regional Geological Hazards
The regional geohazard risk evaluation mainly adopts a qualitative to semi-quantitative evaluation method. This time, the superposition analysis method of geohazard risk and vulnerability evaluation results is used to obtain the risk value of each evaluation unit in the area, and the matrix analysis method is used according to the superposition operation of risk and vulnerability evaluation results in the area. R=H×V R:Geological Hazard Risk Value; H:The Risk Value of Regional Geological Disasters; V:The Vulnerability Value of Regional Geological Disasters Then, the risk level distribution of geological hazards in the area was obtained by considering the seismic conditions, 50-year rainfall recurrence period rainfall conditions and generalizing the evaluation results; finally, the risk level of Pingba District was divided into 31 subregions of very high, high, medium and low levels after reviewing and revising the zoning boundary and risk level through field verification. Of these, the very high risk area accounted for 0.75% of the total area, and the remaining high, medium and low risk areas accounted for 11.15%, 52.32% and 35.78% respectively. The risk evaluation zoning is shown in Figure 5.

CONCLUSIONS AND SUGGESTIONS
(1) This study, based on the collection of relevant data sources and field investigation, comprehensively analyzes and studies the geological environmental conditions of the Pingba area and the development and distribution pattern of the current disaster, and deduces the amount of information that marks the susceptibility based on the influencing factors of the known disaster area, and uses GIS technology to extract 11 factors to build the susceptibility evaluation model in a more objective and scientific way, which can be used as a reference for similar projects. The resulting susceptibility zoning results are very high susceptibility (13.91%), high susceptibility (19.07%), medium susceptibility (28.73%), and low susceptibility (38.29%) overall with the same spatial trend of geological hazard distribution.
(2) Taking into account the characteristics of Pingba District, the evaluation of geological hazard is based on the evaluation of susceptibility, using the same evaluation model as the evaluation of susceptibility, taking into account the historical factors (density, scale and frequency of geological hazards that have occurred), the density of distribution of existing geological hazards, the 50-year working condition and the VII seismic working condition, etc. The method of evaluating the hazard in the district is reasonable, and the results of the hazard evaluation are more consistent with the actual situation in the district.
(3) It is difficult to determine the vulnerability of the disaster-bearing body in view of the complexity of the type of the disaster-bearing body and the influence of the distance of the hazard source, the destabilization time, scale and intensity of the geological hazard potential or hazard source, and the transport space. Therefore, this study chose population susceptibility, material susceptibility and comprehensive susceptibility as the evaluation factors of susceptibility, and the susceptibility coefficients are all taken as 1 according to the most dangerous consideration, which can achieve the purpose of evaluating the susceptibility of regional disasterbearing bodies, and the evaluation model has the characteristics of high accuracy and operability [3], which is suitable to be promoted and applied in the future practical production work.
(4) This study adopts the superposition analysis method of geological hazard and vulnerability evaluation results to obtain the riskiness value and risk level subareas of each evaluation unit in the area, and finally, through field verification, the risk level in the territory is divided into four levels of very high (0.75%), high (11.15%), medium (52.32%) and low (52.32%) after reviewing and revising the zoning boundary and risk level A total of 31 subareas can provide a basic geological basis for disaster prevention and mitigation, territorial spatial planning and governmental decision-making in Pingba District.