Analysis and zonation of geomorphologic hazards (landslide and flood) in the Alborz Province by VIKOR-AHP and Fr models

Introduction Natural hazards with their varieties and their extent of influence as repetitive and destructive phenomena have always been present through the life in the planet; they have always been a serious threat to human beings since the advent of mankind. Accordingly, it can be said that no community claims immunity from natural hazards, and humans always suffer from objective and subjective harmful effects. Natural hazards by destroying income sources, biological resources, and people activity centers (houses, workshops, farms, etc.), increase their economic and physical harms. Related to the history of the hazards studies, the following notes could be mentioned. Materials and Methods In this research, in order to zone the flood and landslide hazards in the Alborz Province based on the used models, the indices of each model were extracted to be utilized in providing the risk map of these hazards as follows: for the landslide hazard zonation of the Alborz Province, one of the outranking methods, entitled VIKOR consensus optimization method, based on calculation of maximum utility and minimum losses was utilized and the susceptibility map of the sub-basins of the Alborz Province was prepared according to the occurrence of the landslide phenomenon. IN order to study the flood hazard of the Alborz Province, the Frequency Ratio Model (FR) was used. In order to perform these two models, it is necessary to extract the most important indices affecting the occurrence of the hazards; for this purpose, based on a deep investigation on previous studies in this field as well as the features of the study area, 9 indices were determined for landslide zoning using VIKOR-AHP compound model as follow: lithology, drainage density, soil type, precipitation, altitude, distance from the fault, land use, dip gradient, vegetation As well, for the flood zoning, using the frequency ratio model, 11 indices were applied which are: lithological factors, land use, distance from the river, soil type, dip gradient, dip direction, surface curvature map, Topographic Wetness Index (TWI), drainage density, altitude and amount of precipitation. Discussion and results In order to zone the risk of the landslide in the Alborz Province, initially nine criteria were considered regarding the basin tendency to land-sliding in sub-basins. After selecting the main criteria, in the next stage, the maps for each one of the selected criteria for weighting and evaluating sub-basins in the GIS environment were prepared and mapped. Considering the effect of the nine criteria in the occurrence of landslide (distance from fault, precipitation, vegetation, land use, soil type, lithology, height, drainage density and dip) distance from fault and vegetation criteria have a decreasing effect and the other seven criteria have an increasing effect. After preparing the selected criteria map, the significance coefficient of the mentioned criteria was determined by the AHP hierarchical algorithm, based on the importance level of the landslide occurrence and basins susceptibility to this phenomenon. In this investigation, in order to weight options based on the role of each criterion in the considered option, the definitive weighting range of one to ten is used; so that weight 1 has the least effect and weight 10 indicates the most effect in the landslide risk. After determining the importance coefficient of the criteria, weights decision matrix was prepared for the VIKOR algorithm. As it is known, nine criteria are efficacious in evaluation of the watershed basins. Table 2 shows the decision matrix based on the effective parameters in the sub-basins. After weighting and preparing the weight matrix, the matrix values were normalized. Conclusion The results of the landslide study in the Alborz Province indicates that in Taleghan and Karaj counties, the occurrence possibility of landslide is high due to the natural conditions of the region in terms of considered indices; since in these counties, the precipitation conditions are relatively good and vegetation is mainly of poor pasture type, there are no proper drainage conditions. The presence of less developed soils and almost impermeable bedrock, high altitude and relatively steep slope in these areas and the presence of frequent faults, provided the occurrence condition of landslide phenomenon. Therefore, as the output of the landslide zoning map shows, more than 60% of the area of these two counties are located in high-risk areas. Also, the natural conditions in relation to the flood risk is persistent and some indices such as susceptibility to erosion, land use, rivers distance, amount of precipitation, altitude, precipitation, slope angle and slope direction have led to the presence of possibility of flooding in the Alborz Province; consequently, due to the dominance of these indices in Taleghan, Karaj and Savojbolagh counties, the highest flood risk could be observed respectively. The situation of Taleghan and Karaj counties are more susceptible and vulnerable to geomorphic hazards; because the area percentage of the high and high risk classes in these two counties is considerable (high). On the other hand, most of the habitat areas of the Province are situated in high risk places in terms of flood and landslide hazards; it shows the necessities of more detailed planning to prevent the hazards and the related damages. In the following, in order to investigate the status of residential centers of the Alborz Province in relation to geomorphic hazards and the degree of adaptation and correct location-defining of these centers (urban and rural), a study was conducted; it indicates the establishment of most urban and rural centers in high-risk areas. For example, 52% of the Karaj County with 28% of the Province’s area and about 2 million populations are located in very high flood and landslide risk zones; in order to reduce their probable losses, people should be aware of probable risks and necessary plans should be considered.