The Dhaka University Journal of Earth and Environmental Sciences, Vol. 8 (2): 2019 Geospatial Modeling of Landslide Vulnerability and Simulating Spatial Correlation with Associated Factors in Bandarban District Md. Sofi Ullah Department of Geography and Environment, University of Dhaka, Dhaka 1000, Bangladesh Manuscript received: 22 August 2020: accepted for publication: 5 January 2021 ABSTRACT: The present study aims at identifying and predicting landslide vulnerable areas in Bandarban District of Chittagong Hill Tracts (CHT) using weighted overlaying of the multiple geospatial layers to determine landslide hazard areas. The historical landslide inventory map was prepared using Google Earth image and through PRA technique. Then ten landslide triggering factors including landuse, rainfall, slope, elevation, cut-fill, soil types, geology, distance to rivers, roads and stream orders, population density, income, education of the inhabitants were chosen as effective factors on a landslide in the study area. Subsequently, the landslide vulnerability map was constructed using the weighted overlay model in Geographic Information System (GIS). Bandarban District has 348 landslides vulnerable locations. Among them, 6 are extremely vulnerable and 342 are highly vulnerable to landslides. Model results show that the Upazila Ruma and Thanchi are extremely vulnerable to landslides. About 91 percent of the landslides will occur within 10 degrees of slope, about 65 percent will occur within 50 meters elevation. The model shows that there is a strong relationship between landslides and physical, economic and social variables. Keywords: Landslides vulnerability, Simulating, Spatial correlation, Weighed overlay, ArcGIS model builder INTRODUCTION Landslides are very important hazards around the world, and they are often connected with other Landslide is the process of displacement of soil hazards such as volcanic activity or earthquakes and rocks on slopes and one of the most common (Keller and Blodgett, 2004). The occurrence of the natural hazards in many mountainous areas and most landslides are small in size and slow but some greatly affect the social sustainability of human beings landslides are faster and bigger with catastrophic (Formetta et al., 2014; Lee et al., 2017; Jeong et al., consequences. All kinds of landslides are often 2017; Cruden and Varnes, 1996). There are natural correlated with geological materials of the earth and man-made causes of landslides. The natural surface (Korup, 2004, 2005, 2006). Therefore, the causes, such as elevation, slope, soil types, geology, composition and consolidation of the materials are heavy rainfall, and earthquakes, but the man-made very important factors in the stability of the soil mass causes are deforestation, land altering, cutting, urban and rocks (Mitchell et al., 2002). encroachments, etc. Significant studies have focused rainfall-induced landslides in last few decades to There are five types of movement in landslides explore the effective measures to enhance public (Turner and Schuster, 1996); fall, topple, slide, spread safety (Safaei et al., 2011; Crosta and Frattini, 2008). and flow. As a consequence, several landslide risk modeling a) Fall is a vertical movement extremely or very applications have been developed, such as GEOtop rapid and can be as rocks or debris. This type has (Simoni et al., 2008) and SHETRAN (El-Emam and low moisture (Summerfield, 1991). Bathurst, 2007), which can calculate risk over large b) Topple is a rotation out of a mass of soil or rock areas using hydrological and mechanical elements. about a point or axis below the centre of gravity of Other applications can evaluate landslide hazards for the displaced mass. smaller areas such as catchments of a few square c) The slide is a downslope movement of soil or rock kilometers (Dietrich and Montgomery, 1988; Dhakal mass where there is a rupture of the surface. There and Sidle, 2003). are two types of slides, rotational or translational and the moisture is low or moderate Corresponding author: Md. Sofi Ullah (Summerfield, 1991). E-mail: [email protected] d) Spread is a movement of cohesive soil or rock mass over a material unstable. The moisture is DOI: https://doi.org/10.3329/dujees.v8i2.54839 moderate to high and this movement is very 52 Md. Sofi Ullah complex due to the different materials and the boulder reaches the element at risk depending on the water interaction. propagation process along the slope and mobility of e) Finally, Flow is a movement of no consolidate element at risk, (3) vulnerability which depends on materials and this movement can be fast or slow rock fall intensity and characteristics of exposed assets depends on the materials and the moisture and (4) economic value of elements at risk, in terms of (Summerfield, 1991). damaging of road and vehicles, interruption of traffic and loss of life (Corominas et al., 2005; Agliardi et al., Studies show that there are five factors in the 2009; Mavrouli and Corominas, 2010; Ferlisi et al., landslides (Keller and Blodgett, 2004); materials, slope, 2012; Wang et al., 2014). climate, weather, vegetation, and water. These factors may act together or separately and they have different In Bangladesh, level of precipitation is in consequences in the landslides. The inclination of the increasing trend, while temperature has increased 1C slope is important as well because in general, when the or less in the last fifty years (Sofiullah, 2018). slope is higher the driving force is also higher. If there Increase of pre-monsoon (7-8 percent) and monsoon is a high slope, fall is the more usual type and when the rains (5-7 percent) is also remarkable (Sofiullah et al. slope is low, flows are the most frequent. Climate is 2017), which could have led to a significant rise in different around the world and mainly it has an landslide incidence in the country in recent past important role in superficial landslides. In arid climates (Figure 1). Furthermore, landslide risk period might where vegetation is sparse, landslides are frequent like be prolonged due to increasing pre-monsoon fall, debris flow, and superficial landslides because soil precipitation followed by heavy monsoon downpour mass and rocks are exposed to high erosion. However, and eventually would lead to a significant incidence of humid and sub-humid areas have abundant vegetation landslides. and soil mass cover most of the slopes and the Landslides occur almost every year; however, the landslides are more complex (Hildenbrand et al., 2003). scope and scale of 2007, 2010, and 2017 were some of Weather can be a complex problem in different types of the worst. The landslides of June 2007 killed 135 landslides. Extreme weather events such as the high people and affected 1.5 million people when heavy intensity of rainfall or long periods of rain may produce monsoon rainfall intensified by a strong storm from dangerous landslides. the Bay of Bengal caused abnormal precipitation in Assessing and mapping landslide susceptibility is the landslide area (BBS, 2011). During the landslides an established practice, often aimed at comparing of June 2017, it was reported that 80,000 people were different predictive methods, which offers the affected across all five districts of Chattogram possibility for a dynamic, integrated and on-going Division. However, among these, 42,000 were management of the territory and its sudden considered severely impacted because their homes had modifications (Devkota et al., 2013; Kavzoglu et al., been destroyed. The most affected districts were 2014; Pellicani et al., 2014; Shahabi et al., 2014; Chattogram, Rangamati and Bandarban. Though the Pham et al., 2016). On the contrary, quantifying, in number of people severely affected is quite large, mathematical terms, the landslide risk can be very there is a clear decrease in the affected population complicated, due to several aspects related to the since 2007 (Table 1). This can be attributed to the complexity in assessing the temporal probability of a resilience planning being done to address landslide specific landslide event with given intensity (hazard) risks. and the probability of damaging a given element at Table 1: Landslide Affected Population risk, i.e. vulnerability (Glade 2003; Uzielli et al., 2008; Pellicani et al., 2014; Abdulwahid and Pradhan, Disaster and its June, June, June, 2016). Associate 2017 2010 2007 Severely affected 42,000 13,900 1,500,000 The risk assessment in probabilistic terms requires population the analysis of (1) the probability of occurrence of a Source: BBS, 2011 rock fall event with a given magnitude, depending on Numerous studies were conducted for modeling the rock fall frequency– magnitude relationship and and mapping landslide susceptibility based on the triggering event frequencies, (2) the probability that a analysis of the relationships among the existing Geospatial Modeling of Landslide Vulnerability and Simulating Spatial Correlation 53 historical landslides data and various factors predisposing instability as well as weighted according to their influence on the instability process (Soeters and Van Westen, 1996; Van Westen, 2000; Ayalew et al. 2005; Fell et al. 2008). To perform a landslide susceptibility prediction model, GIS and Remote Sensing are the tools of paramount importance since it allows, because of their (1) computational power, to manage data with a high degree of spatial variability (Van Westen, 2000). Therefore, GIS and Remote Sensing offer the possibility for a dynamic,