LANDSLIDE HAZARD ZONATION MAPPING, CONSIDERING GEOENVIRONMENTAL CONDITIONS, OF PARTS OF BHAGIRATHI RIVER VALLEY, U.P., INDIA

Abstract

The Himalaya, which represents technically active mountainous region, is generally characterised by steep slopes, high relief, weathered, fractured and folded rocks, in addition to unfavourable hydrogeological conditions. These factors make the Himalayan terrain more vulnerable to landslides. Moreover, there has been a sudden spurt in the development activities in the last two decades, mainly related to road construction, urbanisation, small scale industries and tourism. These development activities are often implemented without taking into consideration the existing instabilities and thereby increasing the landslide potential. In this connection, the landslide hazard zonation provides useful data on the status of instability of the area. These maps also help the planners in implementing the development schemes with minimum geoenvironmental hazards to the area. Landslide hazard may be defined as the probability of occurrence of a potentially damaging natural phenomena. Alandslide hazard zonation map depicts a division of land surface into zones of varying degree of stability based on an estimated significance of causative factors in inducing the instability. An attempt has been made to evaluate the geoenvironmental hazards of Tehri and its vicinity by preparing a landslide hazard zonation map. The detailed evaluation of unstable slopes has also been carried out to evolve possible remedial measures. VII The Study area is located in the Lesser Himalaya of Garhwal hills between latitudes 30°20' - 30' and longitudes 78°15' - 30' and falls within the administrative limits of Tehri and Uttarkashi districts of Uttar Pradesh. The area extending over 450 sq km approximately, is covered in the Survey of India topomap number 53J/7. The study area partially covers the Tehri dam reservoir also. Objectives intended in the present study are; preparation of Landslide HazardZonation (LHZ) map of the study area, analysis of stability for high hazard slopes and assessment of the geoenvironmental status of the area with particular reference to degradation of hill slopes. For the preparation of landslide hazard zonation (LHZ) map, the Landslide Hazard Evaluation Factor (LHEF) rating scheme (Anbalagan, 1992) has been adopted. This scheme is based on an empirical approach and includes the major inherent causative factors of slope instability, such as lithology, structure, slope morphometry, land use land cover, relative relief and hydrogeological condition. The reliability of LHZ map is essentially dependent on the rating scheme of causative factor adopted which has been well established in parts of Kumaun (Anbalagan, 1992) and Garhwal Himalaya (Gupta et al., 1993, Gupta and Anbalagan, 1993 and Anbalagan et al., 1993). The external contributory factors, such as rainfall and seismicity are not included, since they are regional in nature and their impact on landslide potential cannot be estimated with particular reference to a slope facet. The causative factors included in LHEF rating scheme are divided into a number of subcategories. These subcategories of each individual causative factors are arranged in their right hierarchial order and awarded a relative rating. Here the time is indicated viii in relative terms; for example, a moderate hazard slope is more vulnerable to landslide failures as compared to a low hazard slope but less vulnerable as compared to a high hazard slope. Procedures taken up initially for LHZ mapping of the study area, incorporate preparation of a slope facet map and pre-field factorial maps for individual causative factors on 1:50,000 scale. This is followed by facet-wise collection of data for causative factors and preparation of final factorial maps using field data. Next, LHZ map is produced through using TEHD value for each facet which is rendered by LHEF rating scheme. The LHZ map of the study area delineates the entire area in five hazard zones. Assessment of geoenvironmental status of the area with particular reference to degradation of hill slopes is carried out, following and exercise on the distribution of sub-categories corresponding to each individual causative factor, for whole area of study and for all the five hazard zones. Later, order of influence of causative factors has been established by applying Friedman Test and later verified by age's Test. Correlation coefficients between TEHD &causative factors and among causative factors are estimated in order to achieve corresponding relations. Further, stability analysis of high hazard facets is carried out after identifying potentially unstable high hazard slope facets, collection of structural data, preparation of cross sections and determination of strength parameters. This ultimately leads to calculation of factor of safety for plane, wedge and circular failures of potentially unstable HH slope facets. Finally, general remedial measures has been discussed.