چكيده لاتين :
Introduction Landslides are among the most hazardous natural disasters. In the North parts of Iran (Alborz Mountain belt), landslides occur frequently due to geologic and climatologic conditions with high tectonic activities. In recent years, growing population and expansion of settlements and life-line projects over hazardous area increased the impact of natural disasters in Mazandaran Province. These events caused widespread damage to roads, farmland and other infrastructures in the area. That results, annually,millions of dollars financial defects excluding casualties and unrecoverable resources. Landslide Hazard Zonation (LHZ) maps were produced to evaluate the potential of slope instability in the region. The aspect factor is one of the useful and applicable layers in many landslide hazard zonation methods. According to previous studies and existing definintions, The highest number of landslides event occurs in the north (and west) aspect and The lowest number of landslides in the south (and east) aspect. This phenomenon is said to be due to the low sunshine and also the accumulation and existence of water and moisture in north and west aspect in the northern hemisphere. Nevertheless, it is observed that this condition is not true in many regions and the aspect itself is influenced by other factors in this study. In addition to examining whether this layer has a positive role in involved landslide events or not, we calculated landslide density in each class of aspect and then we investigated the most determining factors in the performance of the aspect factor on landslide events in Babolrood Watershed. Study area Alborz Mountainous in the north of Iran constitutes a narrow belt with 100 km wide only. These mountains are a Part of Alpine-Himalayan system in western part of Asia which wraps around the South Caspian Sea from the northwest to northeast of Iran (more than 1500 km). The study area (Babolrood watershed )is located in Mazandaran province in Central Alborz. It covers an area of about 1492/97 km2 and is located between Latitudes 36 N to 36°33 '00 N and Longitudes 52° 25' 00 E to 52° 55' 00E. The elevation of the study area decreases from south to north along the Babolrood River. The geological setting of the area consists of a series of sedimentary rocks mainly limestone, dolomite, marlstone, siltstone and sandstone. These rock units belong to marine geological formations of Jurassic to late Tertiary age. Quaternary deposits of alluvial have a widespread distribution over the land surface especially to the northern part of the area adjacent to the Caspian Sea. The area is part of Alborz Mountain and is known for its active and strong tectonic activity with several destructive earthquakes in the past. The highly folded and faulted rocks are associated with active tectonics in the region. The morphology of the area is directly related to the rock types and geological features of the predominant formation. Materials and Methods Data preparation is the first fundamental and essential step for this study. In this study GIS database was mainly composed of two parts: landslide location map and landslide predisposing factor maps. Although the region faces high potential of landslide occurrence, because of the imposing geomorphological characteristics and high density of forest cover, the available information is very limited. Therefore Landslide locations of the area (1:20,000 scale), were detected by image interpretation (aerial Photos 1:20,000) and verified by fieldwork .an essential spatial database of landslides was established using GIS techniques. digital geology map and fault map of the area were prepared based on combination of two analog geological sheet maps at 1:100,000 scale, namely Ghaem Shahr and Amol, (Geology Survey of Iran). Constructions of Aspect thematic map was from Digital Elevation Model of the area with 10 m resolution. Results and Discussion To find positive role of aspect factor on landslide occurrence in babolrood watershed, Landslide density was calculated in each class of aspect layear. The result shows , Aspect can not has positive role on landslide occurrence in this area. In the next step, we investigated the factors that might change its positive impact. According to studies was conducted, lithology and faults in the study area have the highest impact on landslide occurrence. We therefore evaluated the impact of aspect with regard to these factors on the landslie occurrence in study area. In Central Alborz, geology is a major controlling factor for landsliding. We observed that the performance of aspect is difrent on Various lithologies. it can be seen that the lowest Aspect impact belong to areas with some geological layers outcrops, such as, K2 , PEm,l,s , M2,3 m,s,l , Q , OM1m,c,s These groups are mainly including, Shale, Marl, Marly limestone and old landslide, which mostly are soluble , fissile and easily weathered materials. The highest Aspect impact belong to some geological groups such as Jl , PlQ cms , TR3Js ,TRe2. it shows very low correlation with landslide occurrences and Aspect factor where lithology is mainly including Marl, Marly limestone, Shale and where is the nearest distance to the fault for all formations. Also result shows very highe correlation with landslide occurrences and Aspect factor where lithology is mainly including resistance and strong stones like sandstone, dolomite, cherty dolomite and limestone and also in farthest distance from fault for all formations. Conclusion Landslides are natural phenomena which often have detrimental consequences. Landslide hazards can be systematically assessed by using different factors and methods. The aspect factor is one of the useful and applicable layers in many landslide hazard zonation methods. in some regions the aspect itself is influenced by another factors . In this study the relationship between landslide locations and Aspect factor was investigated and it was found that the relationship between the Aspect and the occurrence of landslides is low. Also the importance factors that cause negative impact of aspect on landslide occurrence in study area was identified. We therefore evaluated the impact of aspect with regard to these factors (lithology and fault) on the landslide occurrence in Babolrood Watershed in the north of Iran. We observed that the performance of aspect is difrent on Various lithologies. The results of calculation landslide density in each aspect class and for each formation seperatly shows that the aspect factor in some formation mainly including Shale, Marl, Marly limestone has negative impact and in formation inclouding strength rocks like sandstone, dolomite, limestone shows positive impact on landslides occurrence. Also aspect impact in nearest distance from fault and Furthest distance from fault was studied. and results shows that in nearest distance from fault , aspect factor has negative impact and in Furthest distance from fault , it has positive impact on landslides occurrence . Finally, it was found the Aspect effect is minimized in the formation which mostly are soluble and easily weathered and where is nearest distance from fault
