Study on Landslide Early Warning by Using Rainfall Indices in Sri Lanka

Japan International Cooperation Agency (JICA) and National Building Research Organisation (NBRO) have been conducting projects to prevent and minimize landslide damages. Landslide early warning is one of the targets of the JICA-NBRO projects based on the analysis of rainfall indices. The analysis of these indices, such as working rainfall and Soil Water Index (SWI), was carried out. The Soil Water Index is an output of conceptual hydrological model representing soil water content. Time series of observed rainfall at 25 gauging stations from 2014 to 2020 were utilized to calculate rainfall indices. Moreover, past landslide records were utilized to analyze the correlation between landslide occurrences and the calculated rainfall indices. Results showed that most of the past landslides were caused by severe rainfall events in which Soil Water Index exceeded 112. Results also showed that small-scale slope failures were caused by relatively minor rainfall events. The critical values of rainfall indices causing landslides depend on the regional characteristics. The critical value in the south-western region tends to be higher than in the northern and southern regions. It seems that the higher rainfall in the south-western region increases the critical rainfall value of landslide occurrence. There is a possibility to improve the accuracy of landslide early warning by using the rainfall indices considering regional characteristics.