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The tale of three landslides in the Western Ghats, India: lessons to be learnt

The tale of three landslides in the Western Ghats, India: lessons to be learnt In recent years, landslides have become a typical monsoon calamity in the Western Ghats region of Kerala, India. In addition to property damage, heavy rainfall (36% above normal) and multiple landslides (4728) killed 48 people in 2018. This tendency continued throughout the monsoon seasons of 2019, 2020, and 2021, resulting in the deaths of over 100 people. Anomalous precipitation is ascribed to the frequent development of low-pressure in the sur- rounding oceans. Using ground real data and satellite imagery, we evaluated the features of three large landslides in the state of Kerala, which occurred during the monsoon season of 2021. Our investigation found that the Kokkayar landslide was triggered by anthropogenic-related agricultural activities, the Plappally landslide by geomorphic and tectonic processes as well as human involvement, and the Kavali landslide by forest fragmentation with dense vegeta- tion on thin soil. The triggering mechanism for all three of these landslides, however, is the intense rainfall of 266 mm in less than 24 h. Thus, an accurate and precise forecast of rainfall can be used to define a threshold for an early warn- ing, which will be vital for saving lives. Keywords: Landslides, Human interventions, Kokkayar, Kottickal, Western Ghats, Kerala people and devastated several hutments in a tea planta- Introduction tion region (Achu et  al. 2021; Sajinkumar and Oommen Catastrophic landslides have become a common mon- 2021). Year 2021 also experienced cataclysmic landslides soonal phenomenon in India’s southwest state of Kerala, on 16th October with the most disastrous ones being which is located in the foothills of the prominent moun- at Kokkayar in Idukki district and Plappally and Kavali, tain chain, the Western Ghats. The anomalous rainfall of near Koottickal in Kottayam district. All these devastat- 2018, which was about 36% more than the normal rainfall ing landslides that occurred since 2018 showed an une- (Vishnu et al. 2019), triggered 4728 landslides (Hao et al. ven geographic distribution (Fig.  1a, b), pointing to the 2020) and killed 48 people. These landslides occurred in a possibility that many parts of the Western Ghats are sus- single storm-event i.e., 16th August 2018. The following ceptible to landslides, though these landslides are located years saw further landslides, with the monsoon season of along the same valley (Fig.  1c). In this study, we narrate 2019 witnessing disastrous landslides such as the one at the ground real data and interpretation of high-resolu- Puthumala, which killed 17 people, and the Kavalappara, tion remotely sensed images of the three landslides- Kok- which killed another 59 people (Sajinkumar and Oom- kayar, Plappally and Kavali (Fig. 2, a, b, c) that occurred in men 2020; Wadhawan et al. 2020). Both these landslides 2021. We also employed ethnographic techniques, such occurred on 8th August 2019. The Pettimudi landslide of as in-depth interviews with elderly impacted individuals, 6th August 2020 was the most tragic one that killed 70 to learn about their shared experiences. These three land - slides are amongst the tens of landslides in the vicinity *Correspondence: sajinks@keralauniversity.ac.in; skochapp@mtu.edu of the study area (Fig.  3). The reason for selecting these three landslides is because of their catastrophic nature Department of Geology, University of Kerala, Thiruvananthapuram, Kerala 695581, India resulting in many human casualties. We believe that the Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 2 of 8 Fig. 1 Location map (a) South India (Source: Google Earth) (b) Study area with elevation map draped over hill shade map showing major landslides since 2018 (Elevation data is ALOS PALSAR) (c) Google Earth image showing the spatial distribution of these three landslides along a valley Fig. 2 Field photos of (a) Kokkayar landslide (b) Plappally landslide (c) Kavali landslide A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 3 of 8 people and completely destroyed seven houses. The dimension of this landslide is 500 m (length) × 40 m (avg. width) × 1  m (avg. thickness). Rubber plantations pre- dominantly occupy the area with intermittent clusters of mixed vegetation. The area is utilized for agriculture through terrace cultivation with the cut slope protected by rubble masonry wall. Rain pits were constructed on this slope. Houses are constructed by the cut and fill method but without any support in the cut slope. Most of the houses have dug wells and the depth to water level is shallow (< 2  m) whereas during the landslide these were found overflowing (as per local witness), pointing to the fully saturated column of soil. Numerous springs Fig. 3 A distant view of the hills in the study area showing several spout from this area (Fig. 4a). This spouting phenomenon landslides existed before landslides because dwellings have drains to flush away storm water (Fig.  4b). These observations indicate that a seasonal first and/or second-order stream narrative of these three landslides applies to other land- flows through this area, which might have been modified slides that occurred in the immediate vicinity of this area. during the course of agriculture and/or habitation. A few fresh gullies have been formed, to which water is now confined. Site and situation of the landslides The in-depth interviews with the local people revealed Kokkayar landslide that the vegetation, mainly rubber trees were clear-felled Kokkayar landslide (9°34′21’’N; 76°53′13’’E) of Peermade after slaughter tapping a few years prior to the event. taluk in the Idukki district of Kerala has killed seven Fig. 4 a Spouting of spring at Kokkayar landslide (b) A demolished house having provisions for draining storm water (c) A highly-weathered joint in the country hornblende biotite gneiss (d) Soil profile showing dislodged material, lateritic soil, saprock and weathered bedrock Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 4 of 8 Contour bunding and rain-pits were made prior to followed by bedrock. The dislodged material was finally replanting the rubber saplings. These interventions seem dumped into the Pullakayar, a tributary of Manimala to have taken place ignoring the natural hydrological River. requirement of letting the first/second order streams to have its free flow channels. Such interventions may have Plappally landslide contributed to destabilizing of soil on the slopes. Plappally landslide (9°37′3’’N; 76°52′21’’E) in Kanjira- The area is characterized by outcrops of hornblende pally taluk of Kottayam district has killed four peo- biotite gneiss. The general trend of this foliated rock ple and demolished two buildings. This landslide of is 173°/35  W. The preponderance of feldspar in this 500  m (length)  × 20 (avg. width) × 1  m (avg. thickness) rock and its subsequent alteration through weathering was initiated in a rubber plantation whereas its runout has resulted in the formation of clay. The rock is highly stretches through areas of different land use types. In jointed, and weathering is found to be extensive along the Google Earth image (before landslide), the upslope these joints (Fig. 4c). The crown of the landslide is occu - in which the landslide occurred is confined is a trun - pied by bouldery outcrops of this rock with no soil cover. cated spur and its right boundary is marked by a straight Hence, during monsoon, all the water in the crown part lower-order river course, indicating a lineament (Fig. 5a). has surcharged the immediately downslope column of Due to the broader surface area of this spur, the run-off lateritic soil causing an increase in pore-water pressure. zone is more extensive. The storm water when crossing Near the flanks of the landslide, the soil profile shows the barren rock outcrop, situated downslope, facilitates dislodged soil followed by lateritic soil of 1  m thickness sudden surcharge to the thin veneer of soil lying imme- and another 1  m thick saprolite (Fig.  4d). This is further diately downslope. It is in this zone the recent landslide Fig. 5 a Google Earth image showing a distant view of Plappally landslide showing a suspected lineament, remnants of paleolandslide and its associated truncated spur (b) 3 m resolution FCC of Planet Lab image showing the landslide runout and its surcharge area (c) Storm water gushing through the uprooted house location (d) The ruins of the devastated house, which was constructed along the course of a lower-order stream (e) Seepage along the joints of hornblende biotite gneiss A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 5 of 8 was initiated. The surcharge zone can be well seen in the revealed thick vegetation. A Normalized Difference Veg - high-resolution (3  m) False Colour Composite (FCC) of etation Index (NDVI) map was created using the high- Planet Lab (Fig. 5b). The truncated spur together with the resolution Planet Lab image to understand the area’s bulged foothill suggests this as a paleo-landslide, within land use. NDVI revealed that the landslide occurred in which the recent landslide occurred. a densely vegetated area when compared to other areas This landslide is also confined to a lower-order stream consisting of a wide variety of land uses like moder- course. The two buildings, which were destroyed, were ate vegetation, grassland, barren outcrop, and built-up. constructed precisely on the river course. Water gushes Usually, landslides are less reported in densely vegetated through this during the monsoon (Fig.  5c), whereas it is areas (cf. Alcantara-Ayala et  al. 2006; Reichenbach et  al. dry during the non-monsoon season (Fig. 5d) showing its 2014). In contrast to this, a recent study by Lan et  al. seasonal nature. But seepage can be seen along the joints (2020) suggests that a densely vegetated slope decreases of the country rock, hornblende biotite gneiss (Fig.  5e). its stability. This study has been concurred with by the Here again, in the upper slope, where the houses stood recent findings of Hao et  al. (2022) wherein most of the before the landslide, plantation with young rubber trees landslides that occurred in Kerala during 2018 are spa- existed, which indicates a similar influencing factor like tially associated with forest land. However, a closer look at Kokkayar. at Fig.  6a, b reveals forest fragmentation and breaking- off of the contiguity of the forest canopy, creating scat - Kavali landslide tered and fragmented forest islands. Studies reveal that Six people died and one house was demolished by the such a process could compromise landscape integrity Kavali landslide, which is 250  m (length)  × 15 (avg. (Ramachandra and Kumar 2011; Batar et al. 2021). width) × 2  m (avg. thickness) in dimension. Hornblende biotite gneiss is the country rock, which is highly weath- Discussion ered and jointed. The attitude of this highly foliated rock The Western Ghats, especially its southern part encom - is 315°/80NE. Here too, spring water is tapped for domes- passing the entire state of Kerala, witness landslides often tic purposes. The destroyed house was constructed in a during monsoon season. Since 2018, the noteworthy fea- cut-slope, but the cut-slope is still retained after the land- ture of the monsoon has been that it triggers landslides slide. The cut-slope profile exhibits lateritic soil, sapro - during the sporadic high-intensity rainfall (cf. Vishnu lite, and weathered bedrock. The area is characterized by et  al. 2019, 2020; Yunus et  al. 2021; Sajinkumar et  al. thick vegetation when compared to the sparse vegetation 2022). Though several studies have been conducted in in the adjacent area. This thick mixed vegetation with this region, and measures suggested were not adopted, rubber plantation is the major crop, followed by nutmeg, we present here specific omnipresent reasons that facili - arecanut, and teak. Google Earth image (Fig.  6a) also tate landslides in this region. Fig. 6 a Google Earth image showing a distant view of Kavali landslide and forest fragmentation (b) NDVI of Kavali area depicting dense vegetation in landslide occurred area Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 6 of 8 Introspection of land use policy The recent landslide susceptibility map of Kerala (cf. Sajinkumar and Oommen 2021; Escobar-Wolf et al. 2021) shows an area of 3300 and 2886 k m as highly and mod- erately susceptible to landslides, respectively. It will be an arduous task to implement stringent measures such as habitation- and construction-free zones in these areas. However, some of the landslide-facilitating practices that are common, may be inadvertently so, can be averted. Kerala is predominantly an agrarian state, and the gen- eral agricultural land use seen are cash crops, with rub- ber plantations occupying the midlands and tea, coffee and cardamom in the highlands. All the three landslides occurred in the midlands, especially where rubber plan- Fig. 7 Hyetograph of Kanjirappally rain gauge, which is the nearest tation dominates the land use. The construction of rain to the landslide affected area. Note the prominent 266 mm rainfall on the landslide day pits is a common practice in almost all rubber estates. Major disturbance to the slope stability occurs when fully matured rubber trees are slaughtered after their life span Kanjirapally, approximately 10  km away from this land- of ~ 20 years, and fresh saplings are planted in a broad pit slide, which is grossly inadequate to capture the micro- of 1 m size. Rain pits are also dug here. The method of climatic conditions of the susceptible areas. Moreover, stubble mulching is not practiced here and large area of this rain gauge station is a manual one with daily rainfall land will be disturbed when the trees are uprooted using recording on the succeeding day at 8.30 am (www. imd. machinery. Hence, avoiding rain pits, planting pits, and gov. in). Having automated rain gauges that report rain promoting stubble mulching practice will help reduce information near real-time will be critical for developing the probability of landslide occurrences. Avoiding rain early warning systems. pits and planting pits in susceptible landslide areas will help increase run-off rather than infiltration. In addi - tion, all agricultural techniques on the hilly slope affects Soil thickness and soil‑rock interface plane the lower-order drainage, by obstructing it with rubble- The hilly area of the entire state of Kerala is characterized masonry walls, redirecting it to a more hazardous slope, by a thin veneer of unconsolidated soil, resting above the or by constructing houses. These lower-order courses, massive Precambrian crystalline rock except for plateau except in thickly vegetated forest areas, are usually sea- regions like Munnar and Nelliyampathy (Sajinkumar and sonal, and during monsoon season, the normal flow of Anbazhagan 2015). Usually, the glide plane of the land- water is thus disturbed by these practices. Hence, a strin- slides will be the contact plane of these two litho-units gent land use policy to avoid such practices in agricul- (cf. Istiyanti et  al. 2021). u Th s, wherever the landslide tural fields is a pressing requirement. occurs, the bedrock will be exposed, which can be seen in all these three landslides. Hence, along with the under- standing of landslide susceptibility, the soil thickness of Rainfall‑ the sole triggering factor the area and the saturation capacity of that soil column As mentioned, these three landslides were also triggered have to be investigated. The contact between these two by a sporadic-high intensity rainfall of > 266 mm in a sin- litho-units is stable in a plain or gentler slope (Fig.  8a) gle day (Fig.  7) but with a 5-day antecedent rainfall of whereas it will be in a meta-stable position when in a only 109.9 mm. The comparatively higher rainfall of 48.8 steep slope (cf. Getachew and Meten 2021; Puente-Soto- (2nd October), 45.4 (8th October and 69.6  mm (11th mayor et al. 2021) (Fig.  8b). This equilibrium will be lost October) might have saturated the soil column and the when the soil column is saturated by water during the 16th October anomalous event was sufficient enough to monsoon season (Fig. 8c). trigger landslides. In order to limit the risk of rainfall- induced landslides, an accurate and exact rainfall fore- Conclusion cast that allows for the issuance of early warnings based The three landslides that occurred on 16th October on the rainfall threshold of the area is essential (Wei- 2021 are located in the same valley, and were triggered dner et  al. 2018). The sparse density of rain gauges and by a high-intensity rainfall of 266  mm in one day. These manual operation methods make things difficult. For e.g., similarities are never the same when conditioning fac- the rain gauge station nearest to these three landslides is tors are analyzed. The steep slopes of the hilly regions A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 7 of 8 Fig. 8 Sketch depicting the contact between unconsolidated soil and massive crystalline Precambrian rocks along the Western Ghats part of Kerala. (a–c) shows the different stages of stability of these two lithounits Competing interests where all three landslides occurred originally contained The authors declare that we have no financial and personal relationships with natural contiguous forests that may have held the thin other people or organizations that can inappropriately influence our work, soil and regolith layer together. The modern landscape, there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the however, is dominated by human interventions such as position presented in, or the review. the replacement of natural vegetation with plantations, highways, and human settlements. These measures facili - Author details State Emergency Operations Centre, Kerala State Disaster Management tated the triggering of the landslides by a sudden storm of Authority (KSDMA), Thiruvananthapuram, Kerala 695033, India. Sree Sank a- intense rainfall (cf. Lahai et  al. 2021). However, a closer racharya University of Sanskrit, Kalady, Kerala 683574, India. Depar tment check using ground reality and satellite photographs of Geology, University of Kerala, Thiruvananthapuram, Kerala 695581, India. Department of Geological and Mining Engineering and Sciences, Michigan revealed that the Kokkayar landslide was completely Technological University, Houghton, MI 49931, USA. Indian Institute of Sci- caused by humans, whereas the Plappally landslide was ence Education and Research, Mohali, Punjab 140306, India. also affected by geomorphic and tectonic causes. The Received: 13 February 2022 Accepted: 2 July 2022 third site, the Kavali landslide, was caused by forest frag- mentation on the forest island. Consequently, regardless of the contributing components, the common and vital feature to be researched is the rainfall dynamics, which References can be converted into early warning systems, thereby sav- Achu AL, Joseph S, Aju CD, Mathai J (2021) Preliminary analysis of a cata- ing countless lives. strophic landslide event at Pettimudi, Kerala state. India Landslides 18:1459–1463 Acknowledgements Alcántara-Ayala I, Esteban-Chávez O, Parrot JF (2006) Landsliding related to The authors thank Kerala State Disaster Management Authority (KSDMA) for land-cover change: a diachronic analysis of hillslope instability distribu- facilitating fieldwork in these areas. Jobin Sebastian, a freelance photographer tion in the Sierra Norte, Puebla, Mexico. CATENA 65:152–165 and paraglide trainer, is highly thanked for providing photos (Figs. 1d and 2). Batar AK, Shibata H, Watanabe T (2021) A novel approach for forest fragmenta- The lab work was carried out at the Laboratory for Earth Resources Informa- tion susceptibility mapping and assessment: a case study from the Indian tion System (LERIS) housed at the Department of Geology, University of Kerala. Himalayan region. Remote Sens 13(20):4090 LERIS is a collaborative initiative of Indian Space Research Organization and Escobar-Wolf RV, Sanders JD, Oommen T, Sajinkumar KS, Vishnu CL (2021) University of Kerala. A GIS tool for infinite slope stability analysis (GIS-TISSA). Geosci Front 12(2):756–768 Author contributions Getachew N, Meten M (2021) Weights of evidence modeling for landslide RSA-Field visit, manuscript writingDN-Field visit, manuscript writingAR-Data susceptibility mapping of Kabi-Gebro locality, Gundomeskel area central analysis, figure preparationTO-Data analysis, figure preparation, manuscript Ethiopia. Geoenviron Disasters 8(1):1–22 writingYPA-Data analysis, figure preparation, manuscript writingKSS-Field visit, Hao L, Rajaneesh A, van Westen C, Sajinkumar KS, Martha TR, Jaiswal P, McA- Data analysis, figure preparation, manuscript writing, supervision doo BG (2020) Constructing a complete landslide inventory dataset for the 2018 Monsoon disaster in Kerala, India, for land use change analysis. Funding Earth Syst Sci Data 12(4):2899–2918 The author declare that there is no funding in the manuscript. Hao L, van Westen C, Rajaneesh A, Sajinkumar KS, Martha TR, Jaiswal P (2022) Evaluating the relation between land use changes and the 2018 landslide disaster in Kerala, India, for land use change analysis. CATENA 216:106363 Declarations Istiyanti ML, Goto S, Ochiai H (2021) Characteristics of tuff breccia-andesite in diverse mechanisms of landslides in Oita Prefecture, Kyushu Japan. Ethical approval Geoenviron Disasters 8(1):1–14 The data employed in this work is accessible to the public upon request and Lahai YA, Anderson KF, Jalloh Y, Rogers I, Kamara M (2021) A comparative therefore ethical approval is not required. This paper is not being submitted to geological, tectonic and geomorphological assessment of the Charlotte, any other journal at the same time. Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 8 of 8 Regent and Madina landslides, Western area Sierra Leone. Geoenviron Disasters 8(1):1–17 Lan H, Wang D, He S, Fang Y, Chen W, Zhao P, Qi Y (2020) Experimental study on the effects of tree planting on slope stability. Landslides 17:1021–1035 Puente-Sotomayor F, Mustafa A, Teller J (2021) Landslide susceptibility map- ping of urban areas: logistic regression and sensitivity analysis applied to quito Ecuador. Geoenviron Disasters 8(1):1–26 Ramachandra T, Kumar U (2011) Characterisation of landscape with forest fragmentation dynamics. J Geogr Inf Syst 3(3):242–253 Reichenbach P, Busca C, Mondini AC, Rossi M (2014) The influence of land use change on landslide susceptibility zonation: the Briga catchment test site (Messina, Italy). Environ Manage 54:1372–1384 Sajinkumar KS, Anbazhagan S (2015) Geomorphic appraisal of landslides on the windward slope of Western Ghats, southern India. Nat Hazards 75(1):953–973 Sajinkumar KS, Oommen T (2020) Rajamala landslide: continuation of a never- ending landslides series. J Geol Soc India 6:310 Sajinkumar KS, Arya A, Rajaneesh A, Oommen T, Ali P, Yunus RVR, Avatar R, Thrivikramji KP (2022) Migrating rivers, consequent paleochannels: the unlikely partners and hotspots of flooding. Sci Total Environ 807:150842 Sajinkumar KS, Oommen T (2021) Landslide atlas of Kerala. Geol Soc India, p Vishnu CL, Sajinkumar KS, Oommen T, Coffman RA, Thrivikramji K, Rani VR, Keerthy S (2019) Satellite-based assessment of the August 2018 flood in parts of Kerala, India. Geomat Nat Hazards Risk 10(1):758–767 Vishnu CL, Rani VR, Sajinkumar KS, Oommen T, Bonali FL, Pareeth S, Thrivikramji K, McAdoo BG, Anilkumar Y (2020) Catastrophic flood of August 2018, Kerala, India: partitioning role of geologic factors in modulating flood level using remote sensing data. Remote Sens Appl Soc Environ 2:100426 Wadhawan SK, Singh B, Ramesh MV (2020) Causative factors of landslides 2019: case study in Malappuram and Wayanad districts of Kerala. India Landslides 17:2689–2697 Weidner L, Oommen T, Escobar-Wolf RV, Sajinkumar KS, Rinu S (2018) Regional scale back-analysis using TRIGRS: An approach to advance landslide hazard modeling and prediction in sparse data regions. Landslides 15(12):2343–2356 Yunus AP, Fan X, Subramanian SS, Jie D, Xu Q (2021) Unraveling the drivers of intensified landslide regimes in Western Ghats, India. Sci Total Environ 770:145357 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in pub- lished maps and institutional affiliations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

The tale of three landslides in the Western Ghats, India: lessons to be learnt

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Abstract

In recent years, landslides have become a typical monsoon calamity in the Western Ghats region of Kerala, India. In addition to property damage, heavy rainfall (36% above normal) and multiple landslides (4728) killed 48 people in 2018. This tendency continued throughout the monsoon seasons of 2019, 2020, and 2021, resulting in the deaths of over 100 people. Anomalous precipitation is ascribed to the frequent development of low-pressure in the sur- rounding oceans. Using ground real data and satellite imagery, we evaluated the features of three large landslides in the state of Kerala, which occurred during the monsoon season of 2021. Our investigation found that the Kokkayar landslide was triggered by anthropogenic-related agricultural activities, the Plappally landslide by geomorphic and tectonic processes as well as human involvement, and the Kavali landslide by forest fragmentation with dense vegeta- tion on thin soil. The triggering mechanism for all three of these landslides, however, is the intense rainfall of 266 mm in less than 24 h. Thus, an accurate and precise forecast of rainfall can be used to define a threshold for an early warn- ing, which will be vital for saving lives. Keywords: Landslides, Human interventions, Kokkayar, Kottickal, Western Ghats, Kerala people and devastated several hutments in a tea planta- Introduction tion region (Achu et  al. 2021; Sajinkumar and Oommen Catastrophic landslides have become a common mon- 2021). Year 2021 also experienced cataclysmic landslides soonal phenomenon in India’s southwest state of Kerala, on 16th October with the most disastrous ones being which is located in the foothills of the prominent moun- at Kokkayar in Idukki district and Plappally and Kavali, tain chain, the Western Ghats. The anomalous rainfall of near Koottickal in Kottayam district. All these devastat- 2018, which was about 36% more than the normal rainfall ing landslides that occurred since 2018 showed an une- (Vishnu et al. 2019), triggered 4728 landslides (Hao et al. ven geographic distribution (Fig.  1a, b), pointing to the 2020) and killed 48 people. These landslides occurred in a possibility that many parts of the Western Ghats are sus- single storm-event i.e., 16th August 2018. The following ceptible to landslides, though these landslides are located years saw further landslides, with the monsoon season of along the same valley (Fig.  1c). In this study, we narrate 2019 witnessing disastrous landslides such as the one at the ground real data and interpretation of high-resolu- Puthumala, which killed 17 people, and the Kavalappara, tion remotely sensed images of the three landslides- Kok- which killed another 59 people (Sajinkumar and Oom- kayar, Plappally and Kavali (Fig. 2, a, b, c) that occurred in men 2020; Wadhawan et al. 2020). Both these landslides 2021. We also employed ethnographic techniques, such occurred on 8th August 2019. The Pettimudi landslide of as in-depth interviews with elderly impacted individuals, 6th August 2020 was the most tragic one that killed 70 to learn about their shared experiences. These three land - slides are amongst the tens of landslides in the vicinity *Correspondence: sajinks@keralauniversity.ac.in; skochapp@mtu.edu of the study area (Fig.  3). The reason for selecting these three landslides is because of their catastrophic nature Department of Geology, University of Kerala, Thiruvananthapuram, Kerala 695581, India resulting in many human casualties. We believe that the Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 2 of 8 Fig. 1 Location map (a) South India (Source: Google Earth) (b) Study area with elevation map draped over hill shade map showing major landslides since 2018 (Elevation data is ALOS PALSAR) (c) Google Earth image showing the spatial distribution of these three landslides along a valley Fig. 2 Field photos of (a) Kokkayar landslide (b) Plappally landslide (c) Kavali landslide A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 3 of 8 people and completely destroyed seven houses. The dimension of this landslide is 500 m (length) × 40 m (avg. width) × 1  m (avg. thickness). Rubber plantations pre- dominantly occupy the area with intermittent clusters of mixed vegetation. The area is utilized for agriculture through terrace cultivation with the cut slope protected by rubble masonry wall. Rain pits were constructed on this slope. Houses are constructed by the cut and fill method but without any support in the cut slope. Most of the houses have dug wells and the depth to water level is shallow (< 2  m) whereas during the landslide these were found overflowing (as per local witness), pointing to the fully saturated column of soil. Numerous springs Fig. 3 A distant view of the hills in the study area showing several spout from this area (Fig. 4a). This spouting phenomenon landslides existed before landslides because dwellings have drains to flush away storm water (Fig.  4b). These observations indicate that a seasonal first and/or second-order stream narrative of these three landslides applies to other land- flows through this area, which might have been modified slides that occurred in the immediate vicinity of this area. during the course of agriculture and/or habitation. A few fresh gullies have been formed, to which water is now confined. Site and situation of the landslides The in-depth interviews with the local people revealed Kokkayar landslide that the vegetation, mainly rubber trees were clear-felled Kokkayar landslide (9°34′21’’N; 76°53′13’’E) of Peermade after slaughter tapping a few years prior to the event. taluk in the Idukki district of Kerala has killed seven Fig. 4 a Spouting of spring at Kokkayar landslide (b) A demolished house having provisions for draining storm water (c) A highly-weathered joint in the country hornblende biotite gneiss (d) Soil profile showing dislodged material, lateritic soil, saprock and weathered bedrock Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 4 of 8 Contour bunding and rain-pits were made prior to followed by bedrock. The dislodged material was finally replanting the rubber saplings. These interventions seem dumped into the Pullakayar, a tributary of Manimala to have taken place ignoring the natural hydrological River. requirement of letting the first/second order streams to have its free flow channels. Such interventions may have Plappally landslide contributed to destabilizing of soil on the slopes. Plappally landslide (9°37′3’’N; 76°52′21’’E) in Kanjira- The area is characterized by outcrops of hornblende pally taluk of Kottayam district has killed four peo- biotite gneiss. The general trend of this foliated rock ple and demolished two buildings. This landslide of is 173°/35  W. The preponderance of feldspar in this 500  m (length)  × 20 (avg. width) × 1  m (avg. thickness) rock and its subsequent alteration through weathering was initiated in a rubber plantation whereas its runout has resulted in the formation of clay. The rock is highly stretches through areas of different land use types. In jointed, and weathering is found to be extensive along the Google Earth image (before landslide), the upslope these joints (Fig. 4c). The crown of the landslide is occu - in which the landslide occurred is confined is a trun - pied by bouldery outcrops of this rock with no soil cover. cated spur and its right boundary is marked by a straight Hence, during monsoon, all the water in the crown part lower-order river course, indicating a lineament (Fig. 5a). has surcharged the immediately downslope column of Due to the broader surface area of this spur, the run-off lateritic soil causing an increase in pore-water pressure. zone is more extensive. The storm water when crossing Near the flanks of the landslide, the soil profile shows the barren rock outcrop, situated downslope, facilitates dislodged soil followed by lateritic soil of 1  m thickness sudden surcharge to the thin veneer of soil lying imme- and another 1  m thick saprolite (Fig.  4d). This is further diately downslope. It is in this zone the recent landslide Fig. 5 a Google Earth image showing a distant view of Plappally landslide showing a suspected lineament, remnants of paleolandslide and its associated truncated spur (b) 3 m resolution FCC of Planet Lab image showing the landslide runout and its surcharge area (c) Storm water gushing through the uprooted house location (d) The ruins of the devastated house, which was constructed along the course of a lower-order stream (e) Seepage along the joints of hornblende biotite gneiss A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 5 of 8 was initiated. The surcharge zone can be well seen in the revealed thick vegetation. A Normalized Difference Veg - high-resolution (3  m) False Colour Composite (FCC) of etation Index (NDVI) map was created using the high- Planet Lab (Fig. 5b). The truncated spur together with the resolution Planet Lab image to understand the area’s bulged foothill suggests this as a paleo-landslide, within land use. NDVI revealed that the landslide occurred in which the recent landslide occurred. a densely vegetated area when compared to other areas This landslide is also confined to a lower-order stream consisting of a wide variety of land uses like moder- course. The two buildings, which were destroyed, were ate vegetation, grassland, barren outcrop, and built-up. constructed precisely on the river course. Water gushes Usually, landslides are less reported in densely vegetated through this during the monsoon (Fig.  5c), whereas it is areas (cf. Alcantara-Ayala et  al. 2006; Reichenbach et  al. dry during the non-monsoon season (Fig. 5d) showing its 2014). In contrast to this, a recent study by Lan et  al. seasonal nature. But seepage can be seen along the joints (2020) suggests that a densely vegetated slope decreases of the country rock, hornblende biotite gneiss (Fig.  5e). its stability. This study has been concurred with by the Here again, in the upper slope, where the houses stood recent findings of Hao et  al. (2022) wherein most of the before the landslide, plantation with young rubber trees landslides that occurred in Kerala during 2018 are spa- existed, which indicates a similar influencing factor like tially associated with forest land. However, a closer look at Kokkayar. at Fig.  6a, b reveals forest fragmentation and breaking- off of the contiguity of the forest canopy, creating scat - Kavali landslide tered and fragmented forest islands. Studies reveal that Six people died and one house was demolished by the such a process could compromise landscape integrity Kavali landslide, which is 250  m (length)  × 15 (avg. (Ramachandra and Kumar 2011; Batar et al. 2021). width) × 2  m (avg. thickness) in dimension. Hornblende biotite gneiss is the country rock, which is highly weath- Discussion ered and jointed. The attitude of this highly foliated rock The Western Ghats, especially its southern part encom - is 315°/80NE. Here too, spring water is tapped for domes- passing the entire state of Kerala, witness landslides often tic purposes. The destroyed house was constructed in a during monsoon season. Since 2018, the noteworthy fea- cut-slope, but the cut-slope is still retained after the land- ture of the monsoon has been that it triggers landslides slide. The cut-slope profile exhibits lateritic soil, sapro - during the sporadic high-intensity rainfall (cf. Vishnu lite, and weathered bedrock. The area is characterized by et  al. 2019, 2020; Yunus et  al. 2021; Sajinkumar et  al. thick vegetation when compared to the sparse vegetation 2022). Though several studies have been conducted in in the adjacent area. This thick mixed vegetation with this region, and measures suggested were not adopted, rubber plantation is the major crop, followed by nutmeg, we present here specific omnipresent reasons that facili - arecanut, and teak. Google Earth image (Fig.  6a) also tate landslides in this region. Fig. 6 a Google Earth image showing a distant view of Kavali landslide and forest fragmentation (b) NDVI of Kavali area depicting dense vegetation in landslide occurred area Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 6 of 8 Introspection of land use policy The recent landslide susceptibility map of Kerala (cf. Sajinkumar and Oommen 2021; Escobar-Wolf et al. 2021) shows an area of 3300 and 2886 k m as highly and mod- erately susceptible to landslides, respectively. It will be an arduous task to implement stringent measures such as habitation- and construction-free zones in these areas. However, some of the landslide-facilitating practices that are common, may be inadvertently so, can be averted. Kerala is predominantly an agrarian state, and the gen- eral agricultural land use seen are cash crops, with rub- ber plantations occupying the midlands and tea, coffee and cardamom in the highlands. All the three landslides occurred in the midlands, especially where rubber plan- Fig. 7 Hyetograph of Kanjirappally rain gauge, which is the nearest tation dominates the land use. The construction of rain to the landslide affected area. Note the prominent 266 mm rainfall on the landslide day pits is a common practice in almost all rubber estates. Major disturbance to the slope stability occurs when fully matured rubber trees are slaughtered after their life span Kanjirapally, approximately 10  km away from this land- of ~ 20 years, and fresh saplings are planted in a broad pit slide, which is grossly inadequate to capture the micro- of 1 m size. Rain pits are also dug here. The method of climatic conditions of the susceptible areas. Moreover, stubble mulching is not practiced here and large area of this rain gauge station is a manual one with daily rainfall land will be disturbed when the trees are uprooted using recording on the succeeding day at 8.30 am (www. imd. machinery. Hence, avoiding rain pits, planting pits, and gov. in). Having automated rain gauges that report rain promoting stubble mulching practice will help reduce information near real-time will be critical for developing the probability of landslide occurrences. Avoiding rain early warning systems. pits and planting pits in susceptible landslide areas will help increase run-off rather than infiltration. In addi - tion, all agricultural techniques on the hilly slope affects Soil thickness and soil‑rock interface plane the lower-order drainage, by obstructing it with rubble- The hilly area of the entire state of Kerala is characterized masonry walls, redirecting it to a more hazardous slope, by a thin veneer of unconsolidated soil, resting above the or by constructing houses. These lower-order courses, massive Precambrian crystalline rock except for plateau except in thickly vegetated forest areas, are usually sea- regions like Munnar and Nelliyampathy (Sajinkumar and sonal, and during monsoon season, the normal flow of Anbazhagan 2015). Usually, the glide plane of the land- water is thus disturbed by these practices. Hence, a strin- slides will be the contact plane of these two litho-units gent land use policy to avoid such practices in agricul- (cf. Istiyanti et  al. 2021). u Th s, wherever the landslide tural fields is a pressing requirement. occurs, the bedrock will be exposed, which can be seen in all these three landslides. Hence, along with the under- standing of landslide susceptibility, the soil thickness of Rainfall‑ the sole triggering factor the area and the saturation capacity of that soil column As mentioned, these three landslides were also triggered have to be investigated. The contact between these two by a sporadic-high intensity rainfall of > 266 mm in a sin- litho-units is stable in a plain or gentler slope (Fig.  8a) gle day (Fig.  7) but with a 5-day antecedent rainfall of whereas it will be in a meta-stable position when in a only 109.9 mm. The comparatively higher rainfall of 48.8 steep slope (cf. Getachew and Meten 2021; Puente-Soto- (2nd October), 45.4 (8th October and 69.6  mm (11th mayor et al. 2021) (Fig.  8b). This equilibrium will be lost October) might have saturated the soil column and the when the soil column is saturated by water during the 16th October anomalous event was sufficient enough to monsoon season (Fig. 8c). trigger landslides. In order to limit the risk of rainfall- induced landslides, an accurate and exact rainfall fore- Conclusion cast that allows for the issuance of early warnings based The three landslides that occurred on 16th October on the rainfall threshold of the area is essential (Wei- 2021 are located in the same valley, and were triggered dner et  al. 2018). The sparse density of rain gauges and by a high-intensity rainfall of 266  mm in one day. These manual operation methods make things difficult. For e.g., similarities are never the same when conditioning fac- the rain gauge station nearest to these three landslides is tors are analyzed. The steep slopes of the hilly regions A jin et al. Geoenvironmental Disasters (2022) 9:16 Page 7 of 8 Fig. 8 Sketch depicting the contact between unconsolidated soil and massive crystalline Precambrian rocks along the Western Ghats part of Kerala. (a–c) shows the different stages of stability of these two lithounits Competing interests where all three landslides occurred originally contained The authors declare that we have no financial and personal relationships with natural contiguous forests that may have held the thin other people or organizations that can inappropriately influence our work, soil and regolith layer together. The modern landscape, there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the however, is dominated by human interventions such as position presented in, or the review. the replacement of natural vegetation with plantations, highways, and human settlements. These measures facili - Author details State Emergency Operations Centre, Kerala State Disaster Management tated the triggering of the landslides by a sudden storm of Authority (KSDMA), Thiruvananthapuram, Kerala 695033, India. Sree Sank a- intense rainfall (cf. Lahai et  al. 2021). However, a closer racharya University of Sanskrit, Kalady, Kerala 683574, India. Depar tment check using ground reality and satellite photographs of Geology, University of Kerala, Thiruvananthapuram, Kerala 695581, India. Department of Geological and Mining Engineering and Sciences, Michigan revealed that the Kokkayar landslide was completely Technological University, Houghton, MI 49931, USA. Indian Institute of Sci- caused by humans, whereas the Plappally landslide was ence Education and Research, Mohali, Punjab 140306, India. also affected by geomorphic and tectonic causes. The Received: 13 February 2022 Accepted: 2 July 2022 third site, the Kavali landslide, was caused by forest frag- mentation on the forest island. Consequently, regardless of the contributing components, the common and vital feature to be researched is the rainfall dynamics, which References can be converted into early warning systems, thereby sav- Achu AL, Joseph S, Aju CD, Mathai J (2021) Preliminary analysis of a cata- ing countless lives. strophic landslide event at Pettimudi, Kerala state. India Landslides 18:1459–1463 Acknowledgements Alcántara-Ayala I, Esteban-Chávez O, Parrot JF (2006) Landsliding related to The authors thank Kerala State Disaster Management Authority (KSDMA) for land-cover change: a diachronic analysis of hillslope instability distribu- facilitating fieldwork in these areas. Jobin Sebastian, a freelance photographer tion in the Sierra Norte, Puebla, Mexico. CATENA 65:152–165 and paraglide trainer, is highly thanked for providing photos (Figs. 1d and 2). Batar AK, Shibata H, Watanabe T (2021) A novel approach for forest fragmenta- The lab work was carried out at the Laboratory for Earth Resources Informa- tion susceptibility mapping and assessment: a case study from the Indian tion System (LERIS) housed at the Department of Geology, University of Kerala. Himalayan region. Remote Sens 13(20):4090 LERIS is a collaborative initiative of Indian Space Research Organization and Escobar-Wolf RV, Sanders JD, Oommen T, Sajinkumar KS, Vishnu CL (2021) University of Kerala. A GIS tool for infinite slope stability analysis (GIS-TISSA). Geosci Front 12(2):756–768 Author contributions Getachew N, Meten M (2021) Weights of evidence modeling for landslide RSA-Field visit, manuscript writingDN-Field visit, manuscript writingAR-Data susceptibility mapping of Kabi-Gebro locality, Gundomeskel area central analysis, figure preparationTO-Data analysis, figure preparation, manuscript Ethiopia. Geoenviron Disasters 8(1):1–22 writingYPA-Data analysis, figure preparation, manuscript writingKSS-Field visit, Hao L, Rajaneesh A, van Westen C, Sajinkumar KS, Martha TR, Jaiswal P, McA- Data analysis, figure preparation, manuscript writing, supervision doo BG (2020) Constructing a complete landslide inventory dataset for the 2018 Monsoon disaster in Kerala, India, for land use change analysis. Funding Earth Syst Sci Data 12(4):2899–2918 The author declare that there is no funding in the manuscript. Hao L, van Westen C, Rajaneesh A, Sajinkumar KS, Martha TR, Jaiswal P (2022) Evaluating the relation between land use changes and the 2018 landslide disaster in Kerala, India, for land use change analysis. CATENA 216:106363 Declarations Istiyanti ML, Goto S, Ochiai H (2021) Characteristics of tuff breccia-andesite in diverse mechanisms of landslides in Oita Prefecture, Kyushu Japan. Ethical approval Geoenviron Disasters 8(1):1–14 The data employed in this work is accessible to the public upon request and Lahai YA, Anderson KF, Jalloh Y, Rogers I, Kamara M (2021) A comparative therefore ethical approval is not required. This paper is not being submitted to geological, tectonic and geomorphological assessment of the Charlotte, any other journal at the same time. Ajin et al. Geoenvironmental Disasters (2022) 9:16 Page 8 of 8 Regent and Madina landslides, Western area Sierra Leone. Geoenviron Disasters 8(1):1–17 Lan H, Wang D, He S, Fang Y, Chen W, Zhao P, Qi Y (2020) Experimental study on the effects of tree planting on slope stability. Landslides 17:1021–1035 Puente-Sotomayor F, Mustafa A, Teller J (2021) Landslide susceptibility map- ping of urban areas: logistic regression and sensitivity analysis applied to quito Ecuador. Geoenviron Disasters 8(1):1–26 Ramachandra T, Kumar U (2011) Characterisation of landscape with forest fragmentation dynamics. J Geogr Inf Syst 3(3):242–253 Reichenbach P, Busca C, Mondini AC, Rossi M (2014) The influence of land use change on landslide susceptibility zonation: the Briga catchment test site (Messina, Italy). Environ Manage 54:1372–1384 Sajinkumar KS, Anbazhagan S (2015) Geomorphic appraisal of landslides on the windward slope of Western Ghats, southern India. Nat Hazards 75(1):953–973 Sajinkumar KS, Oommen T (2020) Rajamala landslide: continuation of a never- ending landslides series. J Geol Soc India 6:310 Sajinkumar KS, Arya A, Rajaneesh A, Oommen T, Ali P, Yunus RVR, Avatar R, Thrivikramji KP (2022) Migrating rivers, consequent paleochannels: the unlikely partners and hotspots of flooding. Sci Total Environ 807:150842 Sajinkumar KS, Oommen T (2021) Landslide atlas of Kerala. Geol Soc India, p Vishnu CL, Sajinkumar KS, Oommen T, Coffman RA, Thrivikramji K, Rani VR, Keerthy S (2019) Satellite-based assessment of the August 2018 flood in parts of Kerala, India. Geomat Nat Hazards Risk 10(1):758–767 Vishnu CL, Rani VR, Sajinkumar KS, Oommen T, Bonali FL, Pareeth S, Thrivikramji K, McAdoo BG, Anilkumar Y (2020) Catastrophic flood of August 2018, Kerala, India: partitioning role of geologic factors in modulating flood level using remote sensing data. Remote Sens Appl Soc Environ 2:100426 Wadhawan SK, Singh B, Ramesh MV (2020) Causative factors of landslides 2019: case study in Malappuram and Wayanad districts of Kerala. India Landslides 17:2689–2697 Weidner L, Oommen T, Escobar-Wolf RV, Sajinkumar KS, Rinu S (2018) Regional scale back-analysis using TRIGRS: An approach to advance landslide hazard modeling and prediction in sparse data regions. Landslides 15(12):2343–2356 Yunus AP, Fan X, Subramanian SS, Jie D, Xu Q (2021) Unraveling the drivers of intensified landslide regimes in Western Ghats, India. Sci Total Environ 770:145357 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in pub- lished maps and institutional affiliations.

Journal

Geoenvironmental Disasters – Springer Journals

Published: Jul 13, 2022

Keywords: Landslides; Human interventions; Kokkayar; Kottickal; Western Ghats; Kerala

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