**1. Introduction**

The state of Kerala (India) experienced the worst disaster in its history in 2018. The disaster affected around 5.4 million people and 433 lives were lost [1]. Several landslides, particularly debris flows, were associated with the event. Among the 14 districts in the state, 13 are part of the Western Ghats and are susceptible to landslide hazards. The scarps of the Western Ghats, which are the steepest parts, are more susceptible to landslides due to heavy rainfall. Attempts have been made by researchers to study the triggering factors of landslides in the Himalayas [2–8] and the Western Ghats [9,10]. However, on a regional scale, establishing rainfall thresholds for the occurrence of landslides in the Western Ghats has not yet been attempted. This paper is an endeavor to define a regional threshold for the Idukki district (Kerala) which is a severe landslide prone zone in the Western Ghats.

A rainfall threshold can be defined using process-based or empirical methods. The process-based approach considers physical and hydrological parameters which can initiate a landslide event. This requires highly sophisticated inputs, as the spatial and temporal distribution of these parameters can only be analyzed through detailed site-specific studies [11]. Owing to the limitations of defining process-based thresholds, this study defines the rainfall conditions that when surpassed, are likely to initiate landslide events in the Idukki district in the Western Ghats. This is an empirical approach which primarily focuses on the occurrence of rainfall and landslide events. Empirical thresholds can be divided into three categories: (1) thresholds which use rainfall data for specific events, (2) thresholds which consider rainfall conditions prior to failure, and (3) others which include hydrological thresholds [12]. In the current research, thresholds in the first two categories are derived for Idukki

using historical rainfall and landslide information. A rainfall event is defined by three parameters, viz., rainfall event, rainfall intensity, and rainfall duration. cumulated rainfall is the the total amount rainfall from the beginning of the rainfall event to the occurrence of failure [13]; the term duration indicates the duration of the rainfall event considered or precipitation period [14]; rainfall intensity is the amount of precipitation in a given time, i.e., the rate of precipitation over the period considered [15]. Thus the term rainfall intensity gives an idea about the average rate of rainfall during an event, not the peak intensities. Another important factor which defines the applicability of the threshold is the area considered for the study. Based on the area, thresholds are classified into local, regional, and global. The stability of the slopes depends upon the hydro-meteo-geological parameters of the region and the conditions for the triggering of landslides differ from place to place. Global thresholds give a universal minimum, below which chances of landslide occurrence is nil, without considering any physical factors. Regional thresholds deal with areas of a few to some thousands of square kilometers where climatic, physiographic, and meteorological features are similar. Local thresholds can be applied to single or a small group of landslides in regions of sizes up to the range of hundreds of square kilometers. Regional and local thresholds perform well for the area they were developed for, but they cannot be exported to other areas easily [16]. These thresholds can be used in regional/local warning systems for providing an alert level to the government and public in general.

Empirical thresholds can be classified again based on the rainfall parameters used as intensity-duration (ID) thresholds, total rainfall event-duration (ED) thresholds, and total rainfall event-intensity (EI) thresholds [12]. A general, well-accepted agreement which determines the selection of rainfall parameters is that shallow/rapid landslides are initiated by rainfalls of high intensity and short duration [16] and deep-seated landslides occur when it rains continuously over a long time [17]. This research focuses on the initiation of shallow landslides which cause maximum casualties during the monsoon time in the region and hence thresholds based on intensity-duration plane and antecedent rainfall are defined for Idukki. The objective is to start the preliminary steps towards an effective regional scale warning system for the Idukki district.

#### **2. Study Area**

In the state of Kerala, Idukki was the worst-hit district during 2018 disaster, with 143 major landslides in the state government records [1]. As shown in the slope map of Idukki, the geography of the area consists of slopes as steep as 80◦ (Figure 1) and the elevation ranges up to 2692 m (Figure 3). A significant share of the population of the district had houses in these unstable slopes, which were destroyed in the 2018 landslides irrespective of the building typology [1]. 97% of the major roads in the districts cut through the rugged mountains and hills, which are often blocked due to landslides in the monsoons [18]. Sprawling across an area of 4358 km2, Idukki supplies 66% of the electric power requirements of Kerala [19]. This district with more than half of the area covered by forests is the second largest one in terms of area in the state.

The Western Ghats can be divided into two segments, north and south, separated by the Gap of Palghat. Deep-seated landslides are reported in the northern segment and the eastern flank while the southern segment mostly experiences shallow landslides [20]. Idukki belongs to the southern part, where regolith thickness ranges from 0.25–5 m [21] and is prone to shallow landslides [1]. Geomorphic classification of the terrain divides the area into four, viz., rugged hills, ridges and valleys, fringe slope, and plateau [22]. Scarps of the Western Ghats consist of frictional soil with less cohesion, thus being stable in dry conditions and losing their strength when the moisture content increases. Plateu regions have a thick layer of top soil, rich in clay content due to their morphology and tropical climate [23]. Geologically, rocks of Wayanad, Charnockite, Khondalite, and Migmatite groups contribute the formation of a part of South Indian Precambrian metamorphic shield [22]. The primary weathering process is hydrolysis in the area, which is due to the high precipitation [24].

**Figure 1.** Location and slope map of the Idukki district.

#### *2.1. Triggering Factors*

The scarps of the Western Ghats experience an annual rainfall as high as 5000 mm as a result of the southwest monsoon, northeast monsoon, and premonsoon showers [25]. The Western flank of the Western Ghats experiences landslides during the southwest monsoon and the eastern side is affected mainly during the northeast monsoon [20]. Large amounts of high-intensity rainfalls increase the pore water pressure within the soil mass, which eventually decreases the shear strength of the soil. This is considered as the primary triggering factor of landslides in the Indian Himalayas [2,4,26] and the Western Ghats [27]. The fissures in bedrock siphons the excess rainwater to unstable zones in the slopes during the monsoon [28]. Photographs of some landslides which happened during the 2018 monsoon are shown in Figure 2. The population of this region increased rapidly after the 19th century, as the people from the midlands started migrating into the hilly region [29]. The industrially backward district was in a quest for better infrastructure due to an increase in population. As an effect, the land use has changed significantly in a short span of time, which favoured the occurrence of landslides in the region. Large scale hill-toe modifications have been done in the district in recent decades for the purpose of infrastructure development, due to which the hill slopes have become steep, without any lateral support. The terraced slopes, modified for monoculture plantations with no sufficient drainage provisions, aggravated the scenario. Due to the drain blockages, water from the intense rainfall accumulates in the top soil layers, leading to landslides.

In a detailed landslide inventory of Kerala until 2010, prepared by the Geological Survey of India (GSI), 64 major cases were reported in the Idukki district [30]. The landslide typologies vary from creep and subsidence to debris flows and avalanches. Along the major road corridors of the district, earth/debris slides have become common during monsoon period [18]. The sharp turnings and vertical cuts along the roads are highly susceptible to cut-slope failures. Incessant rainfall and the subsequent pore pressure increase adversely affects the steep slopes and results in landslides. To conclude, from the case studies conducted by GSI, a major share of the events in Idukki are of debris flow type triggered by heavy rainfall and are influenced by factors like slope, land use, overburden thickness, and disposition of streams etc. [18,23,31,32].

**Figure 2.** Damages that happened due to landslides in the Idukki district in 2018. (**a**) Debris slide at Anachal. (**b**) Debris flow at Kallimai. (**c**) Subsidence at Kallarkutty approach road. (**d**) Earth slide at Cheruthoni [22].
