عنوان مقاله [English]
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 can be safe 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 make a zonation of the flood and landslide hazards in the Alborz Province based on the used models, the indices of each model were extracted to provide the risk map of these hazards. For the landslide hazard zonation of the Alborz Province, one of the outranking methods entitled VIKOR consensus optimization method is based on calculation of maximum utility and minimum losses. 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, we used the Frequency Ratio Model (FR). 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 by VIKOR-AHP compound model. The model is conducted via the layers of lithology, drainage density, soil type, precipitation, altitude, and distance from the fault, land use, dip gradient, and vegetation. For the flood zonnation using the frequency ratio model, we applied 11 indices including lithological factors, land use, distance from the river, soil type, the dip gradient, the dip direction, surface curvature map, Topographic Wetness Index (TWI), drainage density, altitude and amount of precipitation.
Results and discussion
In order to make a zonation of the risk of the landslide in the Alborz Province, we initially considered nine criteria regarding the basin tendency to land-sliding in sub-basins. After selecting the main criteria, in the next stage, the maps for each selected criterion were prepared for weighting and evaluating sub-basins in the GIS environment. Given the effects of the nine criteria in the occurrence of landslide, the criterion of distance from fault and vegetation cover have a decreasing effect and the other seven criteria have an increasing effect. After preparing the selected criteria, the significance coefficient of the mentioned criteria were determined by the AHP hierarchical algorithm based on the importance level of the landslide occurrence and basin susceptibility to this phenomenon. In this investigation, in order to make a weighting of the options based on the role of each criterion in the considered option, the definitive weighting was ranged from one to ten. Thus, weight 1 has the least effect and weight 10 the highest in the landslide risk evaluation. After determining the importance coefficient of the criteria, the weights decision matrix was prepared for the VIKOR algorithm. As it is known, the nine criteria are effective 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.
The results of the landslide study in the Alborz Province indicates that in Taleghan and Karaj counties, the possibility of the occurrence of landslide is high due to the natural conditions of the region in terms of these indices. Since in these counties the precipitation conditions are relatively poor and vegetation is mainly of 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 increased the occurrence possibility of landslide. Therefore, according to the output of the landslide zoning map, more than 60% of the area in these two counties is located in high-risk zones. 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 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 in these areas in order.
The Taleghan and Karaj counties are more susceptible and vulnerable to the geomorphic hazards; because the percentage of the high and high risk classes in these two counties is considerablly 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.
The results also indicated that most urban and rural centers are established in high-risk zones. In order to reduce the probable losses, people should be aware of possible risks.