*2.3. Landslide Characterization*

Landslide inventory is a vital dataset required for modelling landslide susceptibility. Scientific records of landslide incidences, including their spatial and temporal attributes, are rare in the region. The inventory has been constructed for the period between 1992

and 2018 based on available records from Government departments, literature, and field survey of known locations. Landslide data collected from the records of the Geological Society of India (GSI) shows that nearly 367 landslides have occurred in the period of study (1992–2018) and includes landslides of small, medium, and large volumes. The information on the length, width, and depth of the recorded landslides is available for only 270 landslides. The volume of these landslides ranges from 3.925 m<sup>3</sup> to 2,512,000 m3. Nearly 38% of the landslides have a volume less than 100 m<sup>3</sup> and 31% above 1000 m3. Landslides have primarily occurred due to cut slopes' failure, toe erosion in road cuts or natural slopes, removal of material from the toe, and failure of steep cuts. Approximately 74% of the landslides have occurred due to road cutting activity. The slope failures are shallow in nature, and rainfall is observed to be the triggering factor most often. Hence the landslides triggered by rains alone are considered for this study. Nearly 57% of the landslides recorded are earth slides, slumps, and soil slides, and about 40% of the landslides fall in the category of debris and debris slides. Slope failures like subsidence, boulder fall, and rock toppling are also observed in the region but rare. Figure 2 shows the location of the landslides used for the training of the logistic regression model.

(**a**) **Aspect**

(**b**) **Slope**

**Figure 2.** *Cont.*

**Figure 2.** *Cont.*

(**i**) **Average Annual Precipitation**

**Figure 2.** Spatial Databases of Factors Causing Landslides in Coonoor, Tamil Nadu. (**a**) Aspect (**b**) Slope (**c**) Relative Relief (**d**) Topographic Wetness Index-TWI (**e**) Soil (**f**) Land Use (**g**) Settlement-2006 (**h**) Settlement-2016 (**i**) Average Annual Precipitation.

### *2.4. Spatial Database of Causative Factors*

Numerous factors, including topographical, geotechnical, geological, environmental, hydrologic, and climatic factors, contribute to landslides occurrences. The factors are often a function of the local geo-environmental set-up. Therefore, it is necessary to assess each factor's influence with respect to its local geo-environment based on the landslide occurrences. Table 1 presents the various factors commonly used to map landslide susceptibility in literature. The factors that influence landslides in this area were selected based on their correlation to causing landslides. Factors such as aspect, slope gradient, curvature, relative relief, land use, soil, topographic wetness index, distance from lineaments, distance from streams, and average annual rainfall were considered for assessing landslide susceptibility in this region [13,15,19,31,32,34,39]. A Pearson's correlation analysis was carried out for each of the factors listed above to assess their relationship to landslides in this specific geo-environment. The factors that showed a correlation greater than 0.4 were selected to model landslide susceptibility using logistic regression. The factors selected for the study based on Pearson's correlation were aspect, slope, relative relief, TWI, soil, land use and annual precipitation.

**Table 1.** Common Physical and Environmental Factors causing Landslides used in Literature.


Thematic layers aspect, slope gradient, relative relief, and topographic wetness index were extracted from ASTER GDEM of 30 m × 30 m resolution using the spatial analyst tool of ArcMap. Aspect represents slope direction and was divided into eight cardinal

directions and a category flat (Figure 2a). Slope gradient was reclassified into five classes as 0◦–5◦, 5◦–15◦, 15◦–25◦, 25◦–35◦, 35◦–45◦, and greater than 45◦ (Figure 2b). Relative relief represents the difference in maximum and minimum elevation within a pixel. Relative relief was classified into three categories low (92–250), moderate (250–495), and high (495–922) and is presented in Figure 2c. The spatial distribution of topographic wetness index (TWI) was generated using flow accumulation and slope datasets. Flow accumulation was also extracted from ASTERDEM of 30 m resolution using flow direction raster. It was reclassified as low (5.77–9.44), moderate (9.44–12.64), and high (12.64–22.78) and is shown in Figure 2d.

The soil was classified based on the region of occurrence as Kallivalasu, Attakatti, Attavalai, Chinnakupam, Karumpalam, Kuchimuchi, Masinagudi, Milithenu, Murugali, Puvattihalli, and Reedinguvayalu soil series (Figure 2e). The soil's textural classification in the region indicates that loam, loamy sand, sandy clay loam, sandy clay, and clay were present as stratified layers. Rock outcrops were also noticed in the north-western part of the study area. They were converted into plantations taking advantage of the terrain. The thickness of the soil varies between 51 cm in Murugali and 7 m in Attavalai. Land use was extracted from BHUVAN data provided by the National Remote Sensing Agency, India. The land use map is of scale 1: 50,000. Figure 2f shows the land use map for the year 2016. The major land use categories are agriculture, forests, land with scrub, settlements, and water bodies. Tea cultivation and agriculture occupy nearly 78.4% of the area in the region. Settlements are dense in the northern part of the study area. An analysis of the spatial spread of settlements in a decade between 2006 and 2016 indicates an increase of nearly 28% (Figure 2g,h).

Daily rainfall data were used to calculate the average annual rainfall. Rainfall data from six rainfall stations—Coonoor, Ketti, Kothagiri, Runnymede, Kundah, and Uthagamandalam, was used to map the spatial variations of average annual rainfall in the study area using spatial kriging (Figure 2i). It was classified into four classes based on natural breaks— 1275 mm–1436 mm, 1436 mm–1545 mm, 1545 mm–1603 mm, and 1603 mm–1890 mm.
