KOCHI: Located on the west facing slope of the Western Ghats, landslides are a norm for Kerala, affecting 13 of the state’s 14 districts. Most of the landslides in the Western Ghats are flow of debris triggered by intense rainfall events. The landslides caused by extreme rainfall events in 2018 and 2019 devastated the hilly regions, reiterating the need for elaborate investigation on landslide mapping, monitoring and forecast. While hazard zonation considers multiple predisposing factors like slope factor, slope curvature, land use and land cover, geomorphology etc. to define regions vulnerable to landslides, ultimately it is rainfall that triggers slope failures.
Land formations with more than 15 degree slope are prone to landslides. Even areas with lesser inclination are prone to landslides, provided there is critical rainfall continuously for 48 hours. Laterite weathered soil with high clay content absorbs water and triggers debris flow. If the depth of the top soil is low, intense rain can set off landslide. Though all rainfall events do not spark slope failures, the intensity and duration of the rainfall within a particular region can cause slope failure. Hence, identifying the rainfall threshold has a major role in forecasting a slope failure.
The National Centre for Earth Science Studies(NCESS), Kerala analysed the rainfall data of Wayanad region with the help of Global Satellite Mapping of Precipitation (GSMAP) to establish the relationship between intensity/ cumulated rainfall event and debris flow. The power law-based model identified cumulated rainfall events within a rainfall period in relation to the rainfall duration of the events. Thresholds were identified comparing the landslide frequency over a given area within the time period of the estimated rainfall events, by adopting bootstrapping techniques.
The landslide inventory of Geological Survey of India (GSI) as well as data collected by NCESS were used to understand the threshold at which the slopes failed during the extreme rainfall events of August 2018. The rainfall records indicated that the majority of events had occurred on August 7, 8, 16 and 17, 2018 reiterating the prevalence of intense rainfall prior to and on the days of the failure. The results highlighted that short duration spells as well as long duration spells with an overall cumulated rainfall of 250 mm contributed to the slope failures in 2018. Eventually, high intensity short duration spells lasting up to 40 hours and low intensity long duration spells (200 hours) accounted for majority of the slope failures.
(Information sourced from authors K K Ramachandran and Vincent A Ferrer, scientists, NCESS)