عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Concerning the landslides, many researchers have tried to present some models of board holding for risk of this disaster and in other words, they have made effort to reach the plan for board holding of the slops in order to access to these types of plans, in addition to using various techniques, quantity and quality selection of effective factors in occurrence of the landslides is very important. Some factors such as structure of the land and type of stone, figure and direction of the scope, application of lands, rainfall, height from sea level, distance from fault, road and rivers and also landslide vibrations are some of the most important factors in occurrence of landslide that have been referred to in researches of Iranian and foreign experts, and most of them have been based on induction method, in the form that they have surveyed various effective factors on the occurrence of earthquake and then they have analyzed the manner of their effect in spread of the landslides. Except study of the effective factors on the occurrence of landslides which was referred to them, various efforts have been made in the field of assessment for these factors and the manner of collecting these factors together as well, but it shall be mentioned that all of these methods are in the field of better efficiency in applying these quantitative factors. Such methods also have become common by the method of applying numerical techniques and in the systems for analyzing geographical information with the methods of direct coverage and / or cellular numerical combinations, but what is mentioned in the research as a major problem has not been in the field of recognition and development of effective land factors or has not relied on precision in application of the mixed methods, but it mentions another problem and it is this problem that glacial system has defined the main framework for creation and occurrence of such phenomena and if the land factors have any effect, they can be surveyed only and only within this framework and they can be mentioned.
In the case under study, statistical methods and climate periodical deeds have been used with utilization of Wright Method. Recognition of the old and new landslides also has been performed through the field observations and air and satellite photos as well.
In the stages of performing research the software Google Earth, ARC GIS, SPSS have been used. In summary, the stages for performance of this research can be stated as follows :
Providing map for distribution of the landslides in the region: For providing this map on the basis of field observations ,studying air photos and software Google Earth, 360 cases of the slope movements particularly the landslide occurred on the map DEM (numerical height model with cellular size of 85 m resulting from numerical photos of radar) for the province were determined locally by software ARC GIS 9.3.
Determination of permanent snow line and water and ice equilibrium line : In order to determine permanent snow line with utilization of Wright Method, using the field observations, air photos and topographic maps, some of glacial circuses were recognized. Then with determining the location of small circuses and passing the line for 60 % of them, the permanent snow line is determined in the past, also using the obtained data the permanent snow line is specified at the present time as well.
Providing isothermal map for the past and present: with studying the statistics during a 20-year statistical period for 85 climatic and synoptic stations, the data for temperature and height were analyzed and evaluated as the main variables for determining permanent snow line and the present map was prepared with consideration of correlation relation between the elements of temperature and height. The past isothermal map was also provided using the calculated Gradient equation for the present temperature and the rate of adiabatic loss in the area and also permanent snow height. Finally, with adjustment of permanent snow line map and permanent snow line as well as the landslides distribution map in the area considerable results obtained.
With studying the air and satellite photos and also field observations of 360 cases from slope movements including new active landslides, old landslides, and the rainfalls were specified locally. Then with adjustment of the map for distribution of the landslides, and permanent snow line map and water-ice as well as water-ice equilibrium line, significant relation obtained between these limits and accumulation of the landslides
temperature limit of higher than the permanent snow line, the limit between permanent snow line and water equilibrium line and the limit lower than water-ice equilibrium line in the main landslide focal points is this that a high percent of the landslides have occurred in the limit between permanent snow line and water-ice equilibrium line.
Another considerable point is this that most of the ancient landslides have been located close to water-ice equilibrium line whereas the active landslides have more accumulation in the border of permanent snow line.
The average for the percent of occurred landslides in these 3 height ranges in the area indicate that about 75 % of the landslides occurred in the limit between permanent snow line and water-ice equilibrium line, 20 % upper the permanent snow line and 5 % within the limit lower than water-ice equilibrium line. Thus the highest occurrence of landslides occurred in the limit between permanent snow line and water-ice equilibrium line.
In general, for justifying the occurrence of these types of landslides we can state that considering the fact that the dominating process on permanent snow line and its higher heights is freezing and the dominating process on water-ice equilibrium line and the range between this line and permanent snow line is melting, dislocations of these two borders within the areas under the effect of this process usually causes to create a regular sequence of melting and freezing .
Consecutive melting and freezing of snow and depositions and also their nutrition in the cold season cause formation of special phenomena including some of the types of sloping movements. Often in the limits higher than permanent snow line these processes have less effect since persistence of snow in these areas is usually more than one year, but in the limits lower than this, permanent snow line does not exceed from one year, the changes of temperature around zero degree axis cause to melt a section of the snow layer and its penetration inside the deposits and re- freezing of penetrating waters. The rate of this process effect depends on the duration of icing period and particularly its repetition.
Key words: Landslide, Cold hole, Hot hole, Astronomic climate.