**2. Description of Study Area**

Kerala state is located in the southernmost part of the Indian subcontinent which is characterized by all the three physiographic division: coastal plains, midlands and highlands. The geomorphic features vary from coastal plains below sea level to mountain peaks with an elevation of 2695 metres. In addition, 40% of the state's area is occupied by the Western Ghats, the most significant orographic feature of Indian Peninsula [39]. The Western Ghats is a humid forested area where debris flows, initiated by rainfall, being the primary agents of landscape evolution [40]. Torrential rains during the months of June, July and August 2018 triggered around 341 major landslides across ten districts of the state [41]. Wayanad district was one of the worst affected districts which suffered severe socioeconomic setbacks due to these landslides. About 36.74% of the net cropped area of the district was damaged in the disaster [41]. Most of the locations that were affected in the 2018 landslide disaster were reactivated during the 2019 monsoon also, making the situation critical.

The 2130 km2 area of Wayanad district lies between 11◦30' N to 12◦3' N latitudes and 75◦39' E to 76◦30' E longitudes, as shown in Figure 1. Geologically, the district can be divided into four sectors: peninsular gneissic complex, migmatite complex, charnockite group and the Wayanad group in north-central, south-central, southern and northern parts respectively [42]. Wayanad group rocks are found on the northern side as bands. Charnockite rocks form the hilly terrains of south and southeast parts of Wayanad, with narrow bands of pyroxene granulite and magnetite quartzite within charnockite. Biotite hornblende gneiss found over large parts of south-central Wayanad represents the migmatite complex.

The altitude of the district ranges up to 2084 m above sea level (Figure 1). The high altitude Western Ghats and the denuded Wayanad Plateau constitutes the physiography of the region. The plateau of Wayanadu is sloping towards the east and is bordered by isolated structural hills in the east. Most of the district is drained by the Kabani River and its tributaries. The river which flows to the east, along with its tributaries, contributed to the major carvings in the landscape of the catchment.

The topography of the region consists of features ranging from rugged high ranges to flood plains [42]. Hill ranges in the west, northwest and south-western parts of Wayanad can be classified as high ranges with rugged topography, occupied with dense forests with steep slopes. The eastern hills of the districts are high ranges with moderately rugged topography, with an elevation ranging from 1000 m to 1400 m. The valleys between high ranges are formed by the process of deposition and erosion. The flood plains of the region form productive aquifers, and the alluvial thickness of more than 10 m are typical in such plains. This topographical diversity increases the chances of landslides in the region.

**Figure 1.** Location and digital elevation model of Wayanad District (**a**) India (**b**) Kerala (**c**) Wayanad (Modified after [43]).

Four major soil types are found common in Wayanad district [42]. Reddish-brown lateritic soil, formed due to the tropical climatic conditions, has its particle size ranging from clay to silt, with minor coarse fractions. Mananthavady, Kalpetta and Sulthan Bathery areas are rich in forest soil, formed by weathering under forest cover. Deep brownish hydromorphic soil is seen in the undulating topography in the district. This type of soil is formed by transportation and sedimentation of hill slope materials. Alluvial soils are found along the riverbanks, consisting of sandy and clayey fractions. During the monsoon, landslides are frequent in the region and they can be considered as the main geomorphological process, shaping the landscape.

### *2.1. Landslides in Wayanad*

The types of landslides observed in the study region are mainly debris flows or slides of rapid to very rapid nature. A general agreement is that such landslides are triggered by high-intensity rainfalls. Hence, intensity and duration are the two parameters based on which rainfall thresholds are derived from in this study. A threshold line in the form of power-law is derived for the region using frequentist approach for the study area. Images of some of the landslide events that occurred during the 2018 monsoon in the study region are shown in Figure 2, pointing to the typology of landslides observed in the study area. In the debris flow that occurred in Pancharakkolli (Figure 2a), 10 acres of

land comprising of 4 acres forest and 6 acres agricultural land was lost. A total of nine houses were damaged in the course, out of which five were completely destroyed. In Padinjarethara (Figure 2b), four debris flows were initiated from the forest area and caused a severe loss of agricultural land. A total of 10 acres of land was lost and 24 families were affected by the disaster. Several translational and rotational earth slides also occurred during the 2018 monsoon (Figure 2c). Such events have substantially decreased the stability of existing slopes and have affected the functionality of buildings and roads. The debris flow that occurred in Kurichermala (Figure 2d) was the largest in terms of the run-out, where around 150 acres of land (130 acres tea estate and 20 acres agricultural land) was washed out along with the debris. A total of 17 families lost their homes in the disaster [44]. The high elevation zones in the district is characterized by long and large volume debris flows (Figure 2a,b,d) owing to the high regolith thickness combined with slope steepness. In zones of low elevation, debris/earth slides are observed as riverbank failures and cut slope failures.

**Figure 2.** Images of landslides that occurred in Wayanad district during the 2018 monsoon.

Concerning landslide data, a variety of methods can be combined to obtain inventories as complete and detailed as possible, including direct surveys, newspapers, internet news and official reports from technical or administration offices [27,35,45]. In this study, the details of landslide events that occurred during the study period (2010–2018) were collected from reports of the Geological Survey of India, District Soil Conservation Office Wayanad and media reports. The spatial distribution of 123 landslide events identified are plotted in Figure 3, along with the drainage map of Wayanad. Most of the landslide locations are close to the streams in the region, at higher elevations. The landslides that occurred in the low-lying regions often took place under anthropogenic influences, along cuts and slopes or along a riverbank, triggered during rainfalls. The database consists of the location, date of initiation and typology of the landslide. The precision of the location and time reported is subject to the availability of data from the report. The dates of occurrences of landslides were collected from the government reports with daily accuracy. When the dates were not mentioned clearly, as in the case of media reports, the event was assumed to occur one day before the reporting date. Locations were deduced with a spatial accuracy of the nearest mentioned place in the reports, i.e., the names of villages in most cases. A general idea about the landslide typology was available from most of the reports, but the details of the mode of failure were not available. Another constraint with the available reports is their bias towards the fatalities. As the high altitude and unstable slopes are mostly within the forest or less occupied areas, any possible slope failure in such areas is not reported by the media or government reports. As the objective of the study is also to aid in LEWS to reduce the impact of disaster on the population, ignoring such events is acceptable.

**Figure 3.** Drainage map of Wayanad overlaid by locations of landslides and rain gauges considered for analysis.
