Abstract
Idukki is a South Indian district in the state of Kerala, which is highly susceptible to landslides. This hilly area which is a hub of a wide variety of flora and fauna, has been suffering from slope stability issues due to heavy rainfall. A well-established landslide early warning system for the region is the need of the hour, considering the recent landslide disasters in 2018 and 2019. This study is an attempt to define a regional scale rainfall threshold for landslide occurrence in Idukki district, as the first step of establishing a landslide early warning system. Using the rainfall and landslide database from 2010 to 2018, an intensity-duration threshold was derived as I = 0.9D-0.16 for the Idukki district. The effect of antecedent rainfall conditions in triggering landslide events was explored in detail using cumulative rainfalls of 3 days, 10 days, 20 days, 30 days, and 40 days prior to failure. As the number of days prior to landslide increases, the distribution of landslide events shifts towards antecedent rainfall conditions. The biasness increased from 72.12% to 99.56% when the number of days was increased from 3 to 40. The derived equations can be used along with a rainfall forecasting system for landslide early warning in the study region.
Highlights
The state of Kerala (India) experienced the worst disaster in its history in 2018
The necessity of developing rainfall thresholds and early warning systems for the Idukki district has been emphasized in some of the specific site studies conducted by the Geological Survey of India (GSI) [23]
Considering the increased number of casualties which occurred in the study area in the recent past, rainfall thresholds using intensity-duration relationships and antecedent rainfall conditions have been developed in the current research
Summary
The state of Kerala (India) experienced the worst disaster in its history in 2018. The disaster affected around 5.4 million people and 433 lives were lost [1]. Owing to the limitations of defining process-based thresholds, this study defines the rainfall conditions that when surpassed, are likely to initiate landslide events in the Idukki district in the Western Ghats. This is an empirical approach which primarily focuses on the occurrence of rainfall and landslide events. The term rainfall intensity gives an idea about the average rate of rainfall during an event, not the peak intensities Another important factor which defines the applicability of the threshold is the area considered for the study. This research focuses on the initiation of shallow landslides which cause maximum casualties during the monsoon time in the region and thresholds based on intensity-duration plane and antecedent rainfall are defined for Idukki. The objective is to start the preliminary steps towards an effective regional scale warning system for the Idukki district
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