Characterising landslide susceptibility of an environmentally fragile region of the Western Ghats in Idukki district, Kerala, India, through statistical modelling and hotspot analysis

Landslide susceptibility of mountainous villages (locally known as Panchayats) in the Western Ghats of Kerala was critically analysed in the present research by considering numerous landslide events occurred during the mega flood event in the year 2018. A total of 11 variables such as slope, aspect, relative relief, slope length and steepness (LS) factor, curvature, landform, stream head density (SHD), normalised difference vegetation index (NDVI), topographic wetness index (TWI), distance from streams and distance from road were taken into account in the present analysis. Bivariate statistical technique InfoVal method was used to determine the contribution of each variable in increasing the terrain susceptibility by assessing the weight of individual class in the variable. InfoVal weight identified very prominent contributors of terrain susceptibility as concave slope (20–30°), facing north-east with relative relief (225–300 m/km2), having moderate slope length along with lower stream head density and toe cutting due to road development. Landslide susceptibility zonation (LSZ) map indicates more than 60% of the study area having higher landslide susceptibility (44%, 14% and 3%, respectively, for high, very high and critical zones) with more than 75% accuracy of generated landslide susceptibility map. Among the local self-government bodies, Konnathadi, Mariapuram (≥ 70% of the total area) followed by Vathikudi (69%) shows a higher level of landslide susceptibility. Though the LSZ map shows distributed nature of higher susceptibility zones in the study area, the hotspot analysis indicates very localised clustering of landslide hotspots in western side of the study area. Evaluation of the role of land use/land cover over landslide susceptibility of the study area indicates increased occurrence of landslides in areas covered by cardamom, mixed crop, dense mixed forest, open mixed forest and land with scrub. Similarly, rainfall received in the study area during MJJ (May, June and July) and August months of year 2018 indicates an excess amount of rainfall (887 mm and 366 mm, respectively) than the previous year 2017. All these together initiated widespread occurrence of landslides in the study area. Overall, landslide susceptibility zonation map at LSG level will aid in implementation of disaster informed development programmes in the study area and will ultimately help to build a better prepared and planned community in near future in the case of disasters.