GEO-ENVIRONMENTAL STUDIES IN PARTS OF RAMGANGA CATCHMENT, KUMAON HIMALAYAS

Abstract

The Himalayas constitute highly sensitive zone with regard to natural environment, stability and soil erosion. The Ramganga river rising in the Lower Himalaya is a major and perennial tributary of the River Ganga making a catchment of about 3134 km2. For the investigation of geo-environmental aspects like landslides, land use, etc. and silt problems, in particular, the pr3sent study was undertaken in the Ramganga catchment. The main thrust of the study has been given on the following aspects : (i) (ii) (iii) (iv) To map geomorphological features, to evaluate their physical aspects and to know physiographic characteristics of the catchment. To investigate landslides - their distribution, pattern and relation with landforms, geology and land use. Mapping of land use pattern to establish and decipher its relationship with various geo-environmental factors and To study erosional processes in the catchment and to establish a model for sediment yield estimation considering various geo-environmental factors like hydrology, lithology, land use, landslide, basin characteristics and developmental activities etc. and to delineate erosion prone zones in the catchment for future planning of developmental activities. ii The investigations are based on the study of aerial photographs, Landsat MSS images and false colour composites, Landsat MSS data tapes, hydrometeorological records for several years on rainfall, runoff and sediment load, topographic sheets, land use data from Forest Department Offices, selected field checks and other published data and maps. In general, the entire catchment has been studied. However, keeping in view the distribution and availability of the data, more detailed investigations are carried out in six selected sub-basins. The geology of the Ramganga Catchment is mainly used, as described by various workers ,with minor modifications resulted from the present study. The entire catchment area lies over two major lithotectonic zones, viz., Sub-Himalaya and Lesser- Himalaya. The Sub-Himalaya comprises the rocks of the Siwaliks of mid-Miocene to Pleistocene molasse sediments. The Lesser Himalaya is dominantly composed of unfossiliferous sequence of low to medium grade metasediments and crystallines of Paleozoic to Mesozoic age. These are separated in nappe sheets and exhibit evidences of multiple-fold deformations and fracturing. A number of depositional and erosional landform units identified are mapped in the Ramganga catchment are depositional alluvial terraces, alluvial fans, colluvial cones, channel and point bars, channel/valley fills, lake deposits, hill segments with varying slopes, erosional terraces, scarps and unstable slopes (landslides). The physical aspects of these Ill landforms are interpreted and described. The drainage pattern and important drainage anomalies in the Ramganga catchment are also identified and studied. Further, in order to study the relationship between hydromorphometry and lithology third order drainage basins underlain by single lithological unit are taken for morphometric studies including various linear, areal, and gradient aspects. it is found that some of the parameters are dependent on the bed rock lithology. A bird eye view of the area shows that, in general, the Lesser Himalayan region is more affected by landslide activity as compared to the Sub-Himalayan region, the most abundant types of slope movement being rock slide, debris slide and complex movements. The recognition criterion is evolved to identify old and recent landslides on aerial photographs and these are mapped in the catchment. A statistical study of the shape, size and relative distribution of landslides is carried out. It is observed that landslides of medium to small in size and having elongated to equidimensional shape are most common. Moreover, the slide size is also found to be linked with lithology and land use. The landslide frequency is found to be related with relief ratio, relative orientation of rocks and hillslopes, and proximity to regional tectonic (neotectonic?) features. It is also inferred that the areas of sparse vegetation and barren lands which have generally poor soil conditions are more affected by landslides and areas underlain by low-grade metamorphics, quartzites and slates are more prone to landslides.

Based on the above interpretations, a landslide hazard index is developed which includes the parameters like land use type, rock formation, distance from major tectonic features and azimuth. A landslide map is prepared after reexamination of aerial photographs. The land use mapping in the catchment is carried out using variety of data base. The interpretations made from different data sources are compared with each other for accuracy and mutual compatibility using randomly picked test sites, subbasins as units and for the entire catchment. The various land use types are - forest area, sparse vegetation, agricultural land, barren land and water bodies. Dependence of land use on topography, lithology and depositional landforms (terraces, fans, channel/valley fills) is evolved. The various depositional landforms have been the sites of human activity in the past and presently as well. Moderate to gentle slopes on the low-grade metamorphics, crushed quartzites and weathered gneisses have also been used for agricultural purposes. The steeper slopes on the limestone, quartzites and low-grade metamorphics are mostly the sites of sparse vegetation and occasional barren areas. The Siwaliks and upper reaches of hills composed of quartzites and gneisses are mostly covered with forest. The knowledge of erosional processes and sediment yield from a mountainous basin is important for environmental management and proper implementation of water-based development projects. Excessive erosion from the area can be harmful in many ways. The main agents of erosion in the Ramganga catchment are running water (fluvial processes) and gravity (mass-wasting). An attempt is made to develop a regression model for sediment yield estimation using various geo-environmental data as input parameters. The previous work on regression models, aiving sediment yield as a function of various hydrologicalgeological- land use parameters, is reviewed at lengths. In the present study, sediment yield is linked with rainfall, relief ratio, lithology, landslide, land use, and road density using the data of the selected sub-basins. The technique of multiple regression analysis is used and the computations include correlation coefficients, regression coefficients, IS -coefficients, multiple correlation coefficients and the F-test. It is found that the sediment yield is directly related with several geo-environmental parameters like, landslideaffected area, relief ratio, rainfall, land use index (the factor given as sparse vegetation and barren land/forest area and agricultural land) and road density. It is also noted that the rock types, as such, do not seem to have any significant direct influence on the sediment yield. Out of the several equations obtained two equations, selected after screening on the basis of various coefficients and F-test, are given in the text. Furthermore, a study is made to demarcate the erosionprone zones using the grid search method. The three main factors considered for grid search are relief ratio, landslide-affected area and land use index since their correlation coefficients with sediment yield are the highest among the geo-environmental

parameters considered. Weightage is assigned to the various categories for each parameter, and depending upon the integrated value grid-cellwise, the areas are categorised as highly susceptible, moderately susceptible, and less susceptible to erosion. This map, showing the areas of relative proneness to erosion, has been reexamined and is found to be in conformity with the silt data distribution, geological map (and field observations. This map should be useful for further planning of developmental projects including roads, soil conservation measures, and other environmental programmes.