Introduction

Landslide hazard is commonly shown on maps, which display the spatial distribution of hazard classes. Landslide hazard zonation refers to “the division of the land in homogeneous areas or domains and their ranking according to degrees of actual/potential hazard caused by mass movement” (Varnes 1984). Scientists are globally working on landslide studies and particularly on LSH studies. India is one among the countries highly prone to landslide hazard. The landslide hazard zonation atlas of India shows that many parts of India are prone to very high to severe landslide hazard (BMTPC 2002). Some of the densely populated hill areas fall under this category. Various methods have been deployed to prepare LSH maps by various researchers along the globe. Researchers have been using various qualitative and quantitative approaches/techniques to derive the hazard areas viz., Distribution (inventory) approach, Statistical approach, Bi-variate statistical analysis, Weights of evidence model, Weighted overlay method, Frequency ratio approach, Information Value Method (IVM), Bureau of Indian Standard (BIS) based Landslide Hazard Evaluation Factor (LHEF) method, Fuzzy logic method, Multi-variate statistical analysis, Logistic Regression (LR) analysis, Discriminant analysis method, Artificial neural network method, Multivariate analysis, Probabilistic approach, Analytic hierarchy process approach, Rainfall threshold model, Physically-based landslide susceptibility models. The qualitative approaches are very popular in late 1970’s and the quantitative approaches are become popular in last decades (Kanungo et al. 2009). The Nilgiris district in the Western Ghats of India is one of the severe to high landslide hazard areas of India (Fig. 1). Many researchers carried out landslide hazard studies in The Nilgiris district (Seshagiri et al. 1982; Ramakrishnan et al. 2002; Rajarathnam and Ganapathy 2006; Rajkumar et al. 2007; Vasantha Kumar and Bhagavanulu 2007; Thanavelu and Chandrasekaran 2008; Ganapathy and Jothimani 2009; Jaiswal and van Westen 2009; Jaiswal Jaiswal et al. 2010a,Jaiswal et al. 2010b,Jaiswal et al. 2011a,Jaiswal et al. 2011b; Naveen Raj et al. 2011; Antony and Ramanujan 2012; Ganapathy et al. 2012; Manimaran et al. 2012; Prabu and Ramakrishnan 2012; Pradeep Kishore et al. 2012; Vaani and Sekar 2012; Gurugnanam et al. 2013; Sunandana and Lakshmikanta 2013; Gomathi et al. 2013; Chandrasekaran et al. 2013; Muthukumar 2013; Bairavi et al. 2014; Nalina et al. 2014; Ganapathy and Rajawat 2015). The Nilgiris district has lengthy history of landslides and it created more damage to property and infrastructure, however the loss of life is less when compared to landslide impacts in other parts of India. Even though many of them carried out research on landslide hazard zonation, most of the studies are not properly validated with field evidences and lack of database. The present paper aims to bring out a document on the past history of landslides in The Nilgiris and revise the landslide hazard zonation impact map of The Nilgiris district, Western Ghats of India.

Fig. 1.
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Location map of the study area on the landslide hazard zonation map of India

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Study area

The Nilgiris district of Western Ghats parts of Tamil Nadu has an extensive history of landslides. The occurrences of landslides in the district are almost seasonal. The months of October & November are prone to landslides. The years 1902, 1978, 1979, 1993, 2001, 2006 and 2009 are notable years for landslides in the history of The Nilgiris district. In November 1891 heavy rain caused many landslips on the Coonoor Ghat, and created more damage to the Kotagiri - Metuppalayam road (Ganapathy and Rajawat 2015). The old and new Coonoor railway was blocked for a month and the ghat roads for nearly as long; and all the traffic of the eastern side of the plateau was thrown upon the Kotagiri ghat, which was itself in a perilous condition-slips having occurred throughout and being serious in six places out of its twenty one miles (34kms) length during December 1902. Boulders disrupted the railway movement between Adderley and Runneymede railway stations and subsidence were reported near Katteri railway station in the year 1926 (GSI, 1982). Arogyaswamy 1967 mentioned a large landslide was reported in a peat Landen area on the left flank of Emerald Valley. Balasundaram reported rock slides in July 1952 and November 1958 in Penstock of Nilgiri. In the year 1961 a large pre existing landslide was examined by Srinivasan in the Porthimund Dam (Seshagiri et al. 1982). During 1978, unprecedented rains triggered about a hundred landslides in the district over an area of 250sq.kms. Many people were killed in Ooty on 5 November 1978 due to the collapse of houses, landslides and drowning. During 1979, nearly 200 landslides were recorded, resulting in loss of life and serious property damage. Although it is known that the Nilgiri and other mountainous areas are susceptible to landslides, occurrences of this magnitude were previously unknown. Continuous heavy rainfall occurred between 12 and 19 November 1979, with heavy 102.2 mm rain in Coonoor and a heavy landslide in Selas where a house was completely buried in the debris together with two women and three children. Another ‘cloud burst ‘occurred in the upper reach of Coonoor Taluk’s Marappalam on 11 November 1993, damaging about 18 huts below the road and washing off Coonoor MTP Ghat Road for about 11/2 km (Ganapathy et al. 2010). For more than a fortnight, road traffic was suspended. Twelve people were killed and fifteen people were missing. Twenty-one passengers with two buses were washed away. Landslide damaged the railway station and a bridge in December 2001. A stretch over 300 m along with a railway was damaged by a major highway (Ganapathy and Hada 2012). In the early hours of 14 November 2006, numerous landslides were reported, killing one and injuring three people and disrupting traffic on National Highway 67 and blocking the mountain railway between Mettupalayam and Coonoor (Thanavelu and Chandrasekaran 2008).

Recently, in the Nilgiri Hills, casualties and damage caused by landslides have increased.. In 2009, heavy rains triggered a series of landslides in The Nilgiris regions of Ooty, Coonoor and Kotagiri.. On 10 November 2009, 42 people died within 48 h.. Seven of a family died near Ooty in the hamlet of Acchanakal.. Also the slides and uprooted trees cut off access via Mettupalayam to The Nilgiris. From Coonoor via Mettupalayam, the approach road to Ooty was severely damaged. This is the district’s major rain disaster after the 1978 landslide. Smaller landslides and fallen trees block parts of the road as well, however. Houses have been damaged, communication infrastructure has come down, and roads and railroads have disintegrated. There is unprecedented extent of damage to infrastructure (Ganapathy and Rajawat 2015; Edison et al. 2016). The approach road from Ooty through Coonoor to Mettupalayam was severely damaged.. Most of the landslides took place along the National Highways upslopes, Burliar-Ooty and few of them took place on the way to Coonoor-Kundah road. 68 fall in built-up areas out of 89 landslides. The deaths reported were 13, 23 and 9 in talks in Ooty, Coonoor and Kotagiri. Approximately 1890 houses completely or partially damaged due to the landscape and the estimated total losses are worth about USD 6 million (The Hindu 2009). The year wise details of landslides occurred in the district and their damage consequences are listed in the Table 1. From the review of literatures it has been clearly revealed that most of the landslides in The Nilgiris are triggered by the intense rainfall, human and cultural activities, the toe of the slope was removed for laying roads without any engineering measures, and unauthorized buildings gave additional weight on top of the slope.

Table 1 Historical landslides and the damage consequence in the Nilgiris District
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Methodology

Landslide hazard assessment is an important measurement for risk management and land use planning for areas prone to landslides. The landslide hazard zonation maps are not only useful for monitoring landslides and also helpful in predicting future slope failures in a region (Pardesi et al. 2013). Number of landslide hazard assessment studies carried out for The Nilgiris district from the year 1979. Tamil Nadu’s government launched detailed geological investigations of the landslides in August 1979, in collaboration with the State Geology branch, the Geological Survey of India (GSI). The landslide susceptibility map (Fig. 2) was first produced in India on a regional scale for the district of The Nilgiris (Seshagiri et al. 1982) The process of preparing a macro-scale map of the landslide hazard map is predominantly a technique of numerical superimposition. India’s Geological Survey studied the 1993 landslides in The Nilgiris. About 408 landslides / slips have been reported during this year and the serious one is Marappalam Landslide, resulting in 27 deaths. A detailed geological and geotechnical report prepared on this by GSI (Balachandran et al. 1996).

Fig. 2
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Detailed landslide hazard zonation map of Nilgiris carried out in 1980'’s

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Ramakrishnan et al. (2002) carried out a study on using aerial photos and ortho photos, thematic layers were prepared in a GIS platform. They have brought out a landslide hazard zonation map of Kotagiri, which is northwestern part of The Nilgiris district. Rajarathnam and Ganapathy (2006) brought out landslide hazard zonation map of south India using remote sensing GIS techniques in 1:6 Million scale and divided the areas in to four categories viz., low, moderate, high and very high. Also they have narrated the significance of the each zone on landuse regulations in a simple table. Thanavelu and Chandrasekaran (2008) undertaken detailed geotechnical investigations on November 2006 landslides of The Nilgiris district, they have documented numerous landslides. A study on landslide susceptibility mapping for part Ooty town carried out by Rajkumar et al. (2007) based on conventional Delphi Exercise. The effect of deforestation on landslides in the part of The Nilgiris was studied by Vasantha Kumar and Bhagavanulu (2007). A first level landslide risk mapping was carried out using socio economic data by Ganapathy and Jothimani (2009). Chandrasekaran (2010) studied the damage induced by the rainfall trigged landslides of November 2009 landslides in The Nilgiris district. He had discussed the different type of structural failures caused by landslides. Ganapathy et al. (2010) insisted the need and urgency of landslide risk planning for The Nilgiris and risk posed to urban settlements.

Naveen Raj et al. (2011) quantitatively determined the landslide hazard zonation using the relative effect method in southeastern part of The Nilgiris and compared the various thematic layers viz., geology, geomorphology, landuse, slope, lineament density, drainage density, distance to drainage, and soil with landslide inventories and classified the area in to 5 different categories viz., very low, low, moderate, high, and very high. The problem related to environmental societal issues in connection with landslide hazard mitigation practices was assessed by Ganapathy and Hada (2012). This was the first attempt in The Nilgiris. The use of soil bio engineering was strongly insisted to stabilize the slope of the The Nilgiris district. The pattern and nature of landslide occurrences, geological process involved in triggering landslides are discussed by Manimaran et al. (2012). Regional level hazard zonation and vulnerability analysis for road network was carried out by Vaani and Sekar (2012) for southwestern the part of The Nilgiris. Analytical Hierarchy Process was used in GIS to produce landslide hazard zonation by integration of thematic layers which contribute to landslide. An attempt was made by Antony and Mohd (2012) on identification of landslide prone zones in Ooty region using W-4 system of 2D electrical resistivity imaging technique. They have identified the subsurface profile for four different sites at The Nilgiris district. Pradeep Kishore et al. (2012) used the geotechnical parameters to understand the slope instability by limit equilibrium method in application of Landslide Susceptibility Analysis. The factors of safety for 22 different slopes were calculated based on the soil analysis. The combined use of socio economic, remote sensing and GIS data for landslide hazard mapping using Artificial Neural Network was carried out by Prabu and Ramakrishnan (2012) for The Nilgiris. Chandrasekaran et al. (2013) carried out finite element for three landslide affected slopes in The Nilgiris and studied the infrastructural damages caused by the November 2009 landslides. Jaiswal and van Westen (Jaiswal and Van Westen 2012 & Jaiswal and Van Westen 2013) carried out an in-depth study on hazard, vulnerability and risk assessment for rainfall induced landslides in transportation corridors from Burliar to Coonoor. This is the only detailed study so far available for risk assessment landslides in The Nilgiris. The direct and indirect risk as well as the rainfall threshold for different slopes was developed. Weighted overlay analysis was used to assess the landslide hazard zones in GIS by Muthukumar (2013). Sunandana and Lakshmikanta (2013) used image processing technique and Digital Elevation Model to identify landslide hazard prone area in GIS. An attempt was made by Gurugnanam et al. (2013) on compilation of landslide location details from various literatures using GIS. The landslide locations of The Nilgiris were collected from different literatures and geo-referenced the locations in GIS platform. The spatial locations were generated by digitization tool and the coordinates were extracted by geometry tool. The land use and land cover changes were compared for two different years and they concluded that the landuse for tea estates without considering proper drainage and slope ultimately results in loss of natural eco system and ends in massive frequent landslips. The influences of landuse and land cover on landslides were studied by Bairavi et al. (2014). Nalina et al. (2014) analyse the slope stability using geotechnical parameters for the road sector from Kallar to Coonoor sectors. They used the Artificial Neural Network (ANN) in MAT LAB to train the data set and concluded that the Backward Propagation - ANN technique give good prediction value. Ganapathy and Rajawat (2015) documented the use of hazard and vulnerability maps in particular with land use practice in landslide prone areas. Gobinath et al. (2015) studied the link between soil root and landslides in connection with slope stabilization in The Nilgiris district. Based on the review of landslide studies in the Nilgiri district it can be grouped in to three categories viz., i) landslide hazard/susceptibility zonation mapping using Remote sensing & GIS and different computational methods ii) detailed geotechnical investigation and structural analysis on landslide and its damage iii) use of landslide studies in Landuse regulations and disaster preparedness. However there is no updation of hazard zonation map after the year 1980.

Results and discussion

Predicting future landslide in an area needs understanding of the condition of the geological process and the nature of landmass. There are varieties of factors associated with landslide activity in any area such as past landslides and their distribution, bedrock, slope steepness or inclination, hydrologic factor, human effects. Among these the inventory of past landslides plays a major role (OAS/DRDE 2007). A map of existing landslides serves as the data source for understanding conditions controlling to landslide occurrences. Interpreting the likely hood of future landslides occurrences requires an understanding of condition and process controlling past landslides in the area of interest (OAS/DRDE 2007). Landslide inventory analysis can be done by various methods such as i) landslide distribution maps based on image interpretation (event based inventories) ii) landslide activity maps based on multi temporal image interpretation iii) generating inventories based on historical records iv) inventory based on radar interferometry, v) representation of landslide inventory as density information, landslide isopleth maps (Wieczorek 1984; Crozier 2005; Keefer 2002; Reid and Page 2003; Guzetti et al. 2000 & 2005; Jaiswal and van Westen 2009; Squarzoni et al. 2003; Colesanti and Wasowski 2006; Coe et al. 2000; Bulut et al. 2000; Valadao et al. 2002).

Even though many researchers carried out research on landslide in Nilgiri district there is no fully or fairly documented landslide inventory for the district. To update the existing data, a detailed inventory on past and recent landslides was collected and a GIS based database was created. Historical landslide information were collected from various agencies viz., Geological Survey of India (1824–1993), Railways records (1994–2006), Media and News paper (2007–2008; 2010–2014), Geotechnical Cell, Coonoor, limited field checks (2009). Totally 3453 landslide details were collected, out of this only 414 landslides were spatially documented (Fig. 3) and the year wise landslide incidences are presented in (Fig. 4). However most of the landslide inventories don’t have detailed information for further analysis. Out of this 89 landslides occurred in the year 2009 have more details segment wise such as co-ordinates, dimensions, type of slide, history, material involved, area of landslide occurrence, structures affected by the landslide, causalities caused by the landslides, cause, existing remedial measures etc (Fig. 5).

Fig. 3
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Spatial location of landslides from Mettupalayam to Ooty

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Fig. 4
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Year wise landslide occurrence in Nilgiris District

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Fig. 5
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Segment wise location of landslides considered for analysis

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Landslide triggering factors are varying from region to region based on the different terrain condition and different parameters. Various knowledge driven methods recommended by researchers landslide susceptibility assessment viz., geomorphological mapping, direct mapping methods, multiclass weigh methods, spatial multi-criteria analysis, Analytical Hierarchy Process and Fuzzy logic approach (Lee 2005; Yin and Yan 1988; Van Westen 1993; Suzen and Doyuran 2004; Chung and Fabbri 1993; Luzi 1995; Carrara 1983; Gorsevski et al. 2000; Lee et al. 2004; Ermini et al. 2005; Kanungo et al. 2006). Landslide Hazard Assessment needed a multi hazard approach as different type of landslide may occur, each with different characters & causative factors and spatial, temporal and size probability (Guzzetti et al. 2005).

To understand the regional hazard initially the Building Materials and Technology Promotion Council (BMTPC) methodology for large scale mapping is used. The landslide hazard zonation Map of India presented in the Landslide Atlas 2003 is based on a systematic study of the literature, information available on intensity and spatial distribution of landslides, preparation and processing of thematic maps at small scale of 1:6 million on a GIS platform (BMTPC 2002). To understand the district level severity of landslide hazard for The Nilgiris district, a study has been carried out using various thematic maps viz. Geology (3) – 12%, Slope (10) – 40%, Landuse (9) – 36%, Rainfall (3) – 12% using Analytical Hierarchy process. Using this landslide hazard zonation map was prepared and the hazard wise landslide severity is calculated and presented in Fig. 6. This map will be useful for only regional studies and for quick socio economic risk assessment for The Nilgiris district.

Fig. 6
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Landslide hazard severity and population exposure in the Nilgiris District

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The landslide hazard map prepared by GSI in the year 1982 at small scale based on numerical superimposition technique using thematic layers such as geology, slope, land use, geomorphology, rainfall etc. and the resultant map shows different severity of landslide hazards and none of them revised this map. Since most of the thematic factors are same for the region it was planned to revise the hazard area based on the impact of the landslides coupled with existing hazard maps directly by using additional information’s viz., the road, railway network and the latest landslide locations were used in GIS platform (Fig. 7). However the resultant areas are very vast and a site specific slopes were identified as major hot spots by incorporating size, volume of debris, reactivation of landslides, built up area, fatalities etc.,. Based on the Analysis totally 28 landslide hot spot locations (Fig. 8 and Table 2) were considered as most vulnerable locations in The Nilgiris district. By using documented landslides the updated landslide inventory map of the district is presented in the Fig. 9.

Fig. 7
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Landslide Hazard Zonation map of the Nilgiris District

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Fig. 8
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Major Landslide hotspot identified as vulnerable locations

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Table 2 Major Landslide Hotspots selected
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Fig. 9
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Updated landslide inventory map of the Nilgiris District

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Conclusion

Landslide hazard information provide as one of the many components in an integrated development planning study (OAS/DRDE 2007). The landslides are differing from place to place and based on the nature of study area and its environment. The hazard zonation studies should keep in mind that depending upon the need the scale and map unit will vary in place to place (Corominas et al. 2014). Even though the history of landslides recorded in early 1800’s the detailed landslide studies in The Nilgiris started in late 1970’s only. Later many researchers carried out work on landslide hazard zonation studies. However the work carried out by Geological Survey of India (GSI) in association with State Geology of Mines in the year 1982 is the more authenticated in detailed study at 1:12500 Scale and full-fledged work for The Nilgiris district. Even though many landslides occurred in The Nilgiris district there is no much loss on life compare to the other parts of India. The 2009 landslides of the district have created more damage to infrastructure and this can be considered as major landslide disaster after 1978 & 1979. The present study on landslide inventory and damage consequences are the detailed catalogue one for landslide inventory concern. Only one study carried out by the authors towards early warning system for the landslides in the district and that too not published. The study for The Nilgiris district clearly reveals that even though many studies carried out on hazard zonation level however there is no detailed study on prediction of landslides. The landslide hazard zonation maps should be used properly and there should be a detailed investigation on individual slopes which is prone to landslides. It should include detail maps in large scale at planning level which can show development, existing landslide scars, and predicted new slopes where we can deploy the early warning system. The future researchers can focus on landslide initiation susceptibility maps which focus on whether the geological, topographical, geotechnical and climatic conditions really contributing for slope failure based on past landslide locations. Also an investigation of slope stability in connection with the developmental activity should be investigated for future probable slope failure areas in The Nilgiris district. There is no early warning system followed in The Nilgiris district as per the study and one of the landslide related focus area for this district is development of landslide early warning systems. Guidelines can be prepared for assessing and planning policy and consent requirement for landslide prone lands in The Nilgiris district.