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Mass movement in Sri Lanka is mainly triggered by heavy rainfall. International literature is rich of works defining rainfall intensity-duration models to
identify the rainfall threshold for various types of Mass movement. However,
studies have not focused to establish a relationship between intensity and duration of rainfall in Sri Lanka. Therefore, this study focused to establish rainfall intensity-duration models to identify the rainfall threshold for mass
movements in Badulla district in Sri Lanka, where forty four (44) rainfall
events that resulted in same number of landslides during the last three decades were considered. Results indicate the rainfall threshold relationship fits
to the log linear model of the exponential function, I = α·D−β . The constructed
I-D curve revealed that short duration (<2 h) and high-intensity (>54 mm/h)
in rainfall events can potentially trigger the landslide. However, long-duration
(>8 h) and low-intensity (<25 mm/h) in rainfall events may also trigger mass
movements in Badulla. As per the results, most mass movements occur during
northeast monsoons and inter-monsoons. In general, higher mean rainfall intensities trigger the debris flows, while long-duration rainfall events can trigger both landslides and debris flow. When compared to Sri Lankan mass
movements triggering threshold intensities are fairly higher than the global
threshold values. It confirms that within Badulla, mass movements are triggered by very high intense and/or long duration rainfalls events only. Further,
time series analysis of the rainfall events shows an upward trend of extreme
rainfall events, which increased landslide occurring frequency in last six (6)
years.
