. A quantitative procedure for estimating landslide risk to life and property is presented and applied in a mountainous area in the Nilgiri hills of southern India. Risk is estimated for elements at risk located in both initiation zones and run-out paths of potential landslides. Loss of life is expressed as individual risk and as societal risk using F-N curves, whereas the direct loss of properties is expressed in monetary terms. An inventory of 1084 landslides was prepared from historical records available for the period between 1987 and 2009. A substantially complete inventory was obtained for landslides on cut slopes (1042 landslides), while for natural slopes information on only 42 landslides was available. Most landslides were shallow translational debris slides and debris flowslides triggered by rainfall. On natural slopes most landslides occurred as first-time failures. For landslide hazard assessment the following information was derived: (1) landslides on natural slopes grouped into three landslide magnitude classes, based on landslide volumes, (2) the number of future landslides on natural slopes, obtained by establishing a relationship between the number of landslides on natural slopes and cut slopes for different return periods using a Gumbel distribution model, (3) landslide susceptible zones, obtained using a logistic regression model, and (4) distribution of landslides in the susceptible zones, obtained from the model fitting performance (success rate curve). The run-out distance of landslides was assessed empirically using landslide volumes, and the vulnerability of elements at risk was subjectively assessed based on limited historic incidents. Direct specific risk was estimated individually for tea/coffee and horticulture plantations, transport infrastructures, buildings, and people both in initiation and run-out areas. Risks were calculated by considering the minimum, average, and maximum landslide volumes in each magnitude class and the corresponding minimum, average, and maximum run-out distances and vulnerability values, thus obtaining a range of risk values per return period. The results indicate that the total annual minimum, average, and maximum losses are about US$ 44 000, US$ 136 000 and US$ 268 000, respectively. The maximum risk to population varies from 2.1 × 10−1 for one or more lives lost to 6.0 × 10−2 yr−1 for 100 or more lives lost. The obtained results will provide a basis for planning risk reduction strategies in the Nilgiri area.

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Quantitative estimation of landslide risk from rapid debris slides on natural slopes in the Nilgiri hills, India

High intensity rainfall leads to slope failures that are initially unsaturated. Rainfall is the main triggering factor for the slope failure; however, the denseness of the soil with associated soil properties has not received adequate attention on the stability of the slopes. In this study, the finite difference analysis of transient water flow through unsaturated–saturated soil is carried out to study the effects of rainfall intensity and duration on pore pressure generation, degree of saturation, slope stability and shear strain increment during rainfall event. A parametric sensitivity analysis is performed considering three rainfall intensities applied over the soil slopes consisting of three different soil states namely loose, medium and dense. Results indicate that the rainfall affects the stability of loose and medium soil slopes much as compared to the dense soil slope. The rain-induced instability is much more in loose soil slope as compared to the medium soil slope.

Numerical modelling of rainfall effects on the stability of soil slopes

Rainfall-Induced Landslides: Case Study of the Marappalam Landslide, Nilgiris District, Tamil Nadu, India

The present paper aims at to produce the building vulnerability and its spatial distribution in mountainous regions of the Nilgiris District in the Western Ghats, India. The landslide-susceptible areas were identified based on the existing landslide-susceptible maps. The landslide-prone slope was identified based on the historical and recent landslide information’s collected from various authenticated sources as well as the field investigations made on the recent landslides. The high-to-severe landslide hazard-prone areas were selected for the present study of building vulnerability analysis. The areas were divided into ten segments based on the landslide inventory, built-up areas, and transportation corridors in the vicinity of landslide locations. Building foot print map was prepared for each segments using ArcGIS software visual interpretation for 1627 buildings and infrastructures. Different thematic layers viz, building type/material, surrounding wall, sloping side details, warning, and number of floors which contribute to landslide vulnerability in vector formats are used for the present study. Using the formula proposed by Papathoma-Kohle et al. (Nat Hazards Earth Syst Sci 7:765–779, 2007) the vulnerability score were calculated by scripting in ArcGIS software. Based on the vulnerability score, the buildings were grouped under three categories viz, low, medium, and high vulnerability. The spatial distribution of vulnerability of buildings was prepared and presented. The present study can be useful data for preparation of regional landuse plan as well as evacuation plans and warning systems to safeguard measure for population living in the vulnerable buildings in The Nilgiris District of Western Ghats in India.

Use of GIS in assessing building vulnerability for landslide hazard in The Nilgiris, Western Ghats, India

Landslides represent a major threat to human life, property and the environment. Landslide hazard and risk assessments seek to inform the policy and practice of landslide hazard risk management, for example, by identifying high-risk areas so that appropriate policy and private actions could be taken in terms of preventive and mitigative measures. We examine whether a decentralized risk assessment system leads to better assessment outcomes compared to a centralized risk assessment system. The paper is based on a comparative study of two countries—India and Italy—and their responses to landslide risk. Our results indicate a causal relationship between decentralization and three outcomes. First, decentralization appears to be conducive to the more rapid and more complete assessment of risks in local places, through mapping at an appropriate scale. Second, decentralization appears to foster greater and more transparent communication of risk assessment products, such as maps. Third, decentralization appears to lead to a more open, and at times contentious, public discourse over how to interpret and respond to the information contained in the risk assessments and maps. However, in practice, decentralization faces serious institutional resistance. Our analysis does not preclude other risk assessment outcomes or competing explanations for differences in risk assessment and management outcomes. Rather, it provides an understanding of the direction in which the institutional change may be driven for bringing about more effective risk assessments and their use.

The effects of decentralization on the production and use of risk assessment: insights from landslide management in India and Italy

Landslide is one of the major natural hazards that are commonly experienced in hilly terrains all over the world. Landslides are affect at least 15 per cent of the land area of Indian area which exceeds 0.49 million km2. In India the incidence of landslides in Himalayas and other hill ranges is an annual and recurring phenomenon. There is a variation in the degree of landslide incidences in various hill ranges. For example, the landslide incidences are high to very high in Himalayas, high in Northeastern hill ranges, high to moderatein Western Ghats & Nilgiris and low in the hill ranges of Eastern Ghats & Vindhyas. The landslide hazard zonation atlas of India published by Building Materials and Technology Promotion Council (BMTPC), Government of India reveals that the Nilgiris district of Tamil Nadu state is one of the severe to very high landslide hazard prone areas of India.

G. P. Ganapathy

Papers

Need and Urgency of Landslide Risk Planning for Nilgiri District, Tamil Nadu State, India