Investigation of the hillslope stability of Javanrood using with the process-based model (sinmap)

1- Landslide is considered as one of the natural disasters for human beings which makes a lot of deaths and sever property damages worldwide. Therefore, it is necessary to investigate the effective factors in order to make urgent planning and to present management solutions for the sensitive regions. In Iran, more studies on mass movements and landslide hazard zonation, is based on descriptive statistical methods, while in the world today, most studies of mass movements is based on physically-based models and mathematics and numerical methods. Therefore, in this study to analyze the stability of the hillslopes and landslide azard zonation in the study area was used physically-based (process-based) model.
Keywords: Landslide, Stability of hillslope, Process- based Model, Sinmap, Javanrood
2. Research This research was done by both field and experimental methods. Research steps are summarized as following.
2.1. Data Bank Preparation: Geomorphologic, hydrologic and soil mechanic characteristics of slopes in the considered zone were the required information in this study. To obtain this information, at first, it was necessary to recognize sample slopes to measure the above- mentioned variables. Therefore, first of all, landslides distribution map was prepared in the considered area then, on this basin, sample slopes were selected to measure essential variables. Sample hillslopes were recognized as 12 hillslopes, 5 stable (lack of land sliding) and 7 unstable hillslopes (having land sliding mass). They were 1- 12 numbers. 1-5 hillslopes are stable and 6-12 unstable. After selecting sample hillslopes, necessary parameters were assessed as following:
2.1.1. Mechanical features of soil: soil sampling from each hillslope was done the mechanical features of soil, so 50 kg soil was removed from each slope, from 75 cm to 1cm depth. In order to sampling soft and coarse soils, a core cuter devise and shovel were used, respectively. Sampled soils were transferred to Kermanshah soil mechanic laboratory, Kermanshah provincial transport office and necessary parameters, including dry soil specific weight(γ d), wet soil specific weight (γt), hydraulic conductivity , soil internal friction angle(φ) ,soil cohesion , and soil porosity were determined by using direct shear test.
2.1.2. Determining the geometric parameters of slopes: except for using topography map, altitude numerical model (Dem) with a pixel size of 20 by 20 m and satellite images were used to determine morphology parameters and to identify various hillslope types. Applying GIS software, manual and laser tape measure, clinometer, slope geometry characteristics such as mean slope (beta), slope width (W), slope length (L) area, were extracted.
2.2. Model implementation: The model used in this study was Talebi (2008) model which was, in fact, an extended model of process-oriented (physically based) model, being a combination of geometry model, hydrology model (permanent condition) and infinite slope stability theory. After obtaining necessary parameters to get slope factor of security (F S) including laboratory, topography and hydrology parameters, Fs values for each slope were measured by Matlab software
3. The findings of slope stability analytical model introduced hillslopes as unstable with FS less than 1. This hillslopes are classified as extremely high unstable. Main reason for instability is low Internal friction angle (often 25- ) and higher slope (more than 35%) and hillslopes concave longitudinal profile. Concave hillslopes drain more slowly than other slopes; consequently, soil saturation storage would increase, while stability would decrease. The results of the landslide hazard zonation mapping indicate that the majority of the study area located in protection class. So that constitutes 26% of the study area. The lowest percentage of study area belonging to the middle stability class and quasi-stable, which is) 9.2%( and( 6.2( % of the study area respectively.
4. According to the results of the study and analysis of model zoning map sinmap, it can be included that percentage of stability is less than percentage of instability in Javarood region. So that the highest percentage of area is located in the protective area (26%) and high threshold of instability (15.9%) and a lower percentage of the region is located as stable and quasi-stable (20 percent).This results indicates that the susceptibility of the scope of the study area in terms of inherent instability. So that natural factors (concave longitudinal profile and low friction, high angle of slope and high saturation coefficient) in these areas will certainly cause the landslide phenomenon. Therefore, this hillslopes will unstable under the influence of human activities such as leveling hillslope for the construction of residential or cutting their road.