عنوان مقاله [English]
Nowadays, scientific and comprehensive study landslides, is one of the most important issues in the world due to infliction of human and financial damages on human communities. Landslide risk in aquatic structures and dams is of great importance due to the direct and indirect impact of water presence on landslide occurrence. The construction of dams and the creation of a lake behind it causes geomorphic changes along their margins. In fact, such structures at various stages of construction and dehydration are accompanied by natural changes in the area around the dam due to the presence of water which in some cases causes a disaster because of the instability that occurs in the slopes surrounding the dam reservoir,
Materials and Methods
For this research, 1: 50000 area topographic map, 1/100000 area geological map, Global Positioning System GPS (GPS, Landsat 8 OLI satellite imagery, and DEM) with the size of 30 pixel was used.
Geographic information system (GIS) and Expert choice software have been used for zoning. In order to determine the characteristics of the factors affecting the occurrence of slope instability, especially landslides, and to determine the potential of occurrence of instability in different parts of the study area and to show the occurrence of landslide phenomenon in the form of risk zoning by Analytical Hierarchy Process (AHP) method was used.
In this method, the elements of each level are compared in pairs to their respective higher level elements, and their weight is calculated.
AHP is based on the following three principles:
1)The principle of hierarchical tree drawing
According to this principle, since understanding a problem in general is very difficult for a human to do, it may not address the various important aspects of a problem. Therefore, the breakdown of a larger problem into smaller elements, the relationships and concepts present in the decision-making problem and the relationship between each element with the other elements, will be accurately understood and easily perceived. By doing this, the tree creates a hierarchical structure that helps in understanding and solving the problem.
2) The principle of formulating and setting priorities
Since the concepts are relative, which means they are they are compared respectively, and we have no absolute analogy to compare them with, we make pairwise comparisons on the concepts and measure the two by two criteria. We compare and set priorities for indicators and options.
3) The principle of rational consistency of judgments
This shows the degree of correlation between different criteria at one level of hierarchical structure, which means we examine pairwise comparisons in terms of compatibility.
In order to achieve this goal, information layers including slope maps, slope direction, elevation, distance from road, lithology, land use, distance from waterways and vegetation were prepared in GIS environment. After preparing and weighing all the above mentioned layers of information, all of these layers were overlaid using Arc GIS software. Finally, the study area was divided into zones with different landslide potentials and the final model was presented.
Results and discussion
This study is aimed to investigate and map the risk of instability in the Alavian Dam area. To achieve the objective, 8 effective landslide maps were prepared by Arc GIS and all maps were classified and standardized to quantify the factors including slope maps, slope directions, Altitude levels, lithology, land use, vegetation, road distance, waterway distance. The role of each factor in the study area was independently assessed and their relationship with the amount of landslide occurring in the study area was studied. AHP model was used to achieve the objective, determine the preference of factors and convert them into quantitative, expert opinions, by filling out questionnaires and analyzing their opinions. Finally, instability susceptibility zoning was performed in the Alavian Dam study area and the result was mapped into five sensitivity classes (very low risk, low risk, medium risk, high risk, very high risk). Based on the maps produced and the percentages of classes in each map, it was found that the lowest slope class is 0-12 degrees slope, which corresponds to the western and central parts of the study area, which comprises 60% of the area. The highest slope class is 51- 26 ° slope, which corresponds to the eastern part of the study area which covers 4.5% of the area and the highest altitude class is 1948 to 2178 m. Percentage of area covered is located in the eastern and northeastern part of the region and the lowest altitude class is 1543 to 1653 meters, which covers 17.8 percent of the area is located in the central area. This shows that the eastern part of the region has the highest slope and elevation compared to the other parts of the region. These conditions are susceptible to instability in the eastern part of the study area due to the young and weak lithology of the study area.
Based on field studies and identifying effective criteria, pairwise comparisons, weighting between them and determining their weight using expert opinion, it was found that all the criteria presented in this study are at risk of landslides in the area. The study area was effective and considering the landslide risk zoning map, the area with average landslide risk occupied the largest area in the area. The results also showed that slope and lithology factors with the highest weight are the most influential factors in the occurrence of instability in the study area and high landslide risk, considering the zoning map, consists 9% of the study case area which is located on the eastern slopes of the area. The eastern highlands of the region are therefore prone to landslides due to the loose and young topography and lithology conditions and reduced land use and vegetation. Therefore, in the implementation of development and development plans (road construction, etc.) in the east of the region, attention to the potential of high slope instability is essential.