LANDSLIDE HAZARD EVALUATION OF HILL SLOPES AROUND NAINI LAKE, NAINITAL, INDIA

Abstract

Landslides are the most common type of mass wasting process in hilly terrain. Planning civil constructions has always been considered challenging due to occurrence of such geoenvironmental hazards. Among the physiographic divisions of India, the Himalayan terrain experiences the maximum of slope instabilities. The problems of hill slope instabilities many times get aggravated because of human interference. Past incidences indicate that improperly planned constructions resulted in large scale slope failures in the Himalayan region. For the purpose of systematic town planning in hills, preparation of Landslide Hazard Zonation (LHZ) map may help the planners and engineers to a large extent. In this context, the present research work is focused on the development of a meso-scale LHZ technique which may find useful application for both town planning and expansion of existing urban settlements. The hill town of Nainital, which is a famous tourist resort of the state of Uttarakhand, has been taken as a case study for application of this technique. Geologically, Nainital area falls within Mussoorrie Group of rocks, which in turn forms a part of Krol Nappe. The Mussoorrie Group of rocks, exposed at Nainital area consists of three Formations namely, Blaini, Krol and Tal. The town resembles a saucer shaped depression because of the location of the Naini Lake which lies at the centre of the town. The lake is bordered by Sher-ka-Danda Hills onthe east and northeast, while Naina Hills are located on the north and northwest. Ayarpatha-Deopatha Hills borders the lake from west and south-west. Balia stream is the only outlet of the lake, which emerges in the southeast direction. Nainital had witnessed slope instabilities from the early days of human settlement, with large scale events reported from almost all of the lake facing hills. At the same time the urban population of this town has increased many folds in the last two decades. Taking into account the past evidences of slope failures and increasing urban pressure, a systematic study on hill slope i stability has been carried out for planning future urbanization in Nainital, using meso-scale LHZ technique. Meso-scale (1:5,000) LHZ technique is an empirical approach which categorizes hill slopes into different hazard classes, based on the inherent and external factors of slope instability. The technique is inherited from the existing macro-zonation approach of Anbalagan (1992) with suitable modifications to fit in the purpose of urban planning. The major application of such hazard zonation is to delineate the hill slopes which are prone to failure, within town limit. The technique may be considered as the first step of systematic town planning and should be followed up with further detailed studies. The LHZ study of Nainital has been carried out after dividing the hill slopes into a number of slope facets. The ratings for inherent and external factors are added facet wise to derive the hazard class of individual facets. The hazard zonation map of Nanital ranks the hill slopes into various degree of instability. To validate the approach and the prepared meso-scale LHZ map, detailed study has been carried out on the hazard prone slopes. This includes method of ground validation by visually observing the areas of instability and matching their occurrence with the high hazard zones. In addition, the status of stability of these slopes is also estimated by calculating the Factor of Safety (F) values using limiting equilibrium approach. On the basis of inferences from stability analysis, suitable control measures have been derived considering the site condition. From the experience gained after conducting the entire gamut of work, it will be fair to anticipate that most of the hill towns can be saved from future risk of landslide activity, if town planning is conducted in a meaningful way incorporating the inputs from meso-scale LHZ map and other studies carried out subsequently. In other words such approach will definitely serve the cause of sustainable development in hills with minimum disturbance to the natural geoenvironmental equilibrium.