GIS-based spatial analysis of landslides in Rathnapura District, Sri Lanka

Abstract

The levels of landslide hazards are dependent on land physical attributes, agriculture practices and other human activities. Landslides are a serious concern in steep land areas where intensive storms are common. Southwest monsoon brought heavy rains and strong winds to Sri Lanka. Rathnapura District is a landslide prone area in Sabaragamuwa Province, Sri Lanka due to southwest monsoonal rain. GIS technologies could provide a powerful tool to model the landslide hazards for their spatial analysis and prediction. This is because the collection, manipulation and analysis of the environmental data on landslide hazard can be accomplished much more efficiently and cost effectively. The objective of this study was to identify the spatial distribution of landslides in Rathnapura district. Four variables, slope, elevation, rainfall, soil type and land use, were evaluated as landslide inducing factors in the area. Eighteen Divisional Secretariat Divisions in Rathnapura district was the sampling area for the study. Four Landslide locations and risk areas in the area were identified by GPS coordinates and soil samples were collected in identified areas. Grain size distribution that was used to classify the soil was done by sieve analysis. Stream ordering was calculated by Shreve and Strahler method and the water volume, number of streams and slope gradient were analyzed to create classified layer of slope, rainfall intensity, land use, drainage and slope. Classified layers were weighted according to their contribution percentage for the landslides. By applying weighted overlaying method, study area was categorized into five groups as high-risk area, risk area, moderate risk area, low risk area and no risk area. Natural factors like slope, soil type and drainage density as well as man-made factors are caused for the landslides in the area. Landslide events in Rathnapura district are strongly correlated to many factors. As slope increased, the percentage of land affected by landslides, when the soils were saturated, increased sharply on land used for crop production, indicating that agricultural activity and the associated removal of deep-rooted permanent vegetation increased the landslide hazard on steep sites.