Susceptibility Assessment of Rainfall-induced Shallow Landslides using a GIS-supported Deterministic Analysis – A Pilot Study

Abstract

The landscape of Guwahati city (Assam) is characterized by a number of residual hills (eighteen) scattered in and around the main urban areas. The geology of all these isolated hills is the same, comprising an overburden of soil (weathered mantle) over bedrocks of porphyritic granite, quartzo-feldspathic gneiss, migmatites and schistose rocks. Shallow landslides in the form of soil slips are regular phenomena in these hills during the monsoons which are triggered by spells of high intensity rainfall. The Narakasur hill which lies in the heart of the city was selected as a small sample area for deterministic landslide susceptibility analysis to forecast the spatial occurrence of rainfall-induced shallow landslides. A grid-based slope-stability analysis was incorporated with the GIS spatial functions adopting the “infinite-slope” geometry to balance the resisting and the driving forces acting on the sliding mass. Landslide susceptibility is expressed by Safety Factor.Input data for the analysis includes topographic slope, soil thickness, water table depth and material strength properties. The effect of Safety Factor values, arising due to variation in material properties and water table conditions, was shown over the study area. Safety factors were calculated for three different water table depth scenarios assuming that soil is completely dry, completely saturated with water and an intermediate condition. Based on the calculated values of the Safety Factor the area was divided into stable, critical and unstable zones.Comparing the results with the shallow landslide inventory map, it has been observed that more than 70% agreement between predicted shallow landslide susceptibility and the inventory.