The Role of Oak Forest on Landslide Prevention (Case study: Ardal, Chahar Mahal and Bakhtiari Province)

Many factors like topography، climate، geology، tectonic، vegetation، soil properties،… are effective for occurring mass movements، that some of them are natural and some are created due to human activities. Deforestation and degradation of forests is one of the factors that aggravate the landslide occurrence and create mass movements directly. To investigate this important problem، in this research، two hillslopes with the same conditions (from view points of geology، pedology، topography and climate) in the field (Ardal township- Chahar Mahal & Bakhtiari province) were selected as one of them includes deforestation (unstable) and another one has been covered by oak forest and is stable. Based on the studies، both hillslopes have been located in alluvial formation with marl layers and soil texture in both of them is also clay-loam that dominant mineral clay is esmectite. From point of view of soil mechanical properties (internal friction angle and soil cohesion)، both hillslopes are the same and they classified in ML-CL groups. From thehydrology view point، water resources in upstream of both hillslopes are common and the rate of infiltrated water is 152600 cubic meters per year. The most difference between these two hillslopes is the vegetation percent as one of them has been covered by oak forest with 40 percent canopy while other one (unstable hillslope) has less than 5 percent canopy. After determining the all effective parameters in landslide، itwas specified that in landslidy hillslope، the factor of safety is equal 1 (Bishop method) and hillslope has been slided. On the other hand، in hillslope with oak trees، the rate of safety factor (without considering the effect of tree roots) has been obtained equal to 0. 9 and hillslope is stable yet. Then، it can be concluded that oak trees with 3-4 root depth، have increased the soil strength and its consolidation. Furthermore، oak forest has been able to extract the further water and decrease the mass weight through increasing the evapotranspiration. Finally، it can be concluded that in Zagros region، deforestation and cutting the oaks is one of the important factors for occurring landslides.