HYDROMORPHOGEOLOGICAL INVESTIGATIONS IN PARTS OF MIDDLE GANGA BASIN, USING REMOTE SENSING DATA

Abstract

The major natural waterways on the earth have provided sites for civilization, trade and culture from ancient times. The River Ganga is one of the most important rivers in the Indian subcontinent and the Ganga plains provide vast natural resources. However, recurring seasonal floods and channel migration processes have brought about significant modification in landform, landuse and other natural resources from time to time. The Middle Ganga Basin is especially prone to such activity. Keeping the above in view, an area in the Middle Ganga Basin around Monghyr, Bihar, has been selected for detailed investigations using remote sensing data. The main objectives have been i) landform mapping of the area to study the geomorphological setup, (ii) digital image processing of the Landsat MSS and TM data for feature enhancement, (iii) study of the changes in channel patterns; reconstruction of paleocourse of the rivers to understand the trend and the lateral extent of channel migration, and (iv) hydrogeological investigation in alluvial and hard rock regions for ground water resources appraisal. The study has been based on a variety of data, viz., aerial photographs, Landsat MSS and TM data products providing multidate coverages (1975-1986), topographic maps, lithologs, water level and water quality data, pumping test data and selected field observations. The geological formations in the study area comprise of alluvium of Pleistocene to Recent age and metasedimentary rocks of Proterozoic age. The alluvium covers the northeastern and the southwestern parts of the area and also occurs as alluvial fills in the hard rock terrain. The alluvial material dominantly is the result of aggradational processes of Ganga and its tributaries. The southern part of the area is characterized by the metamorphic rocks (Kharagpur hills). These rocks comprise slates, phyllites and schists interbedded with quartzites. The rocks are intensely deformed. The geophysical data reveals that there exists an important subsurface ridge called Monghyr-Saharsa subsurface ridge in this area. Neo-tectonic activities are manifested by some intense earthquakes. A detailed geomorphological map of the study area has been prepared with the help of aerial photographs, Landsat images and field observations. In the alluvial terrain, the various landforms mapped are channel bars, channel islands, point bars, meander scrolls, natural levees, flying levees, crevasse-splays, backswamps, abandoned channels, meander scars and oxbow lakes. The present day and old flood plains of the Ganga and BurhiGandak rivers have been identified. In the hard rock terrain, various landforms such as structural hills, inselbergs, pediments, buried pediments, erosional valleys and valley fills have been identified. An attempt has been made to deduce the geomorphic evolution of the area. The evolution of the alluvial terrain is governed by the history and migration of rivers. It is inferred that the Ganga has migrated over a large distance of about 20 km during the pre-historical times. Its present course is almost on the southern limit of the alluvial plains adjacent to the

peninsular shield boundary. It is observed that the northern part of the Monghyr and Begusarai districts is marked by abundance of palaeochannel features and water-logging. These represent the vestigial remains of the chain of lakes that once were within the Ganga valley depression. The geomorphic evolution of the hard rock terrain is controlled by tectonic-lithologic features and denudational processes. Thus, the geomorphic features of the area have evolved as a result of combination of a number of processes. The Landsat MSS and TM data on tapes have been computer processed to enhance subtle variations in the spectral patterns of ground objects. The nature of Landsat MSS and TM data, its spectral, radiometric and temporal resolution, data formats etc. have been briefly mentioned. To get an idea of the data distribution, first, some statistical analysis of Landsat MSS data was carried out. Suitable test sites were selected considering the various landforms, their composition and distribution. Based on the digital outputs of the MSS data scatterograms, bar diagrams and spectral curves have been plotted and analysed for object discrimination. For feature enhancement purposes, two enhancement techniques viz.,,linear contrast stretching and band ratioing have been used. Some of the feature which were not seen on the unprocessed image could be detected after image enhancement, e.g. meander scars, abandoned channels, chain of water bodies and high moisture rich areas - which all are important for channel migration and hydrogeological studies. The channel migration studies have been based on the 'historical methods' i.e. information from multidate data have been derived for understanding the changes in channel pattern in time and iii space. It has been observed that prominent changes in channel patterns have occurred in the case of the Ganga and BurhiGandak rivers. Reconstruction of palaeocourses has been made by studying oxbow lakes, meander scars and abandoned channels. Digitally processed and enhanced Landsat MSS and TM data products have facilitated improved detection of these features and of those associated with river flooding. The interpretation from different types of data has been brought on to a common scale using control points, for inter-data comparison and deducing the channel changes. It is inferred that the Ganga flowing from W to E in this area has migrated over a distance of about 20 km from N to S, to occupy its present day course. The migration of the Ganga has occurred in several stages, some involving rapid and some gradual shifting. In all, four major episodes of rapid shifting can be identified. Each of these was followed by a stage of flow in which the river possessed a locally oscillating character, as indicated by numerous meander scars and palaeochannels. The large scale migration of the Ganga River has converted many backswamps into high lands and elevated masses into swampy or water-logged patches as is also confirmed from the well log data. The palaeofeatures of the BurhiGandak indicate that the river has shifted from N to S over a distance of about 30 km. Three main stages of shifting of this river can be distinguished. Further, it is observed that the BurhiGandak River had a larger channel width and higher discharge in earlier times and was highly active. It is inferred that the now northerly flowing Baghmati River was, at a time in the past, draining into the BurhiGandak River. As the Baghmati migrated and occupied a

different channel in the north, the link between the Baghmati and BurhiGandak was cut-off thereby reducing the discharge of the BurhiGandak River. The paucity of the abandoned channels in the vicinity of the Baghmati River further confirmed that the present day course of the Baghmati is a relatively new one. To get an appraisal of the intensity and the areal extent of the floods, an analysis of the pre and post-monsoon data coverages has been carried out and flood prone areas have been demarcated. Further, mention has been made of the problems of river bank erosion in this area. Hydrogeological investigations have been carried out for assessing ground water potential in the area. The study area can be divided into two broad units: alluvial terrain and hard rock terrain. The nature of ground water occurrence in relation to landforms, both in the hard rock (structural hills, inselbergs, pediments, buried pediments, erosional valleys and valley fills) and in the alluvial terrain (natural levees, backswamps, abandoned channels, meander scars, point bars and meander scrolls etc.) has been described. In the hard rock terrain the aquifers are formed by weathered rock and fractured rocks. The alluvial aquifers are relatively more uniform in character. An analysis of the ground water level data has been carried out; the variation being more prominent and abrupt in the hard rock terrain than in the alluvial terrain. For ground water exploration purposes, a lineament-tectonic map of the hard rock area has been prepared from aerial photographs. The lineament map has been interpreted in conjunction with the geomorphological map to assess the degree of fracturing and weathering in different parts of the area. The subsurface lithology based on lithologs has been helpful in interpreting the landform map. The ground water occurrence distinctly controlled by fracture intersections. The diversity in structural pattern, weathering and uneven distribution of ground water call for detailed interpretation of various features in smaller test sites. Analysis of limited pumping test data has been carried out and the results of aquifer characteristics are presented. Finally, an evaluation of the ground water resources has been carried out based on various landforms, well log data, water level data and lineament tectonic map. A ground water resource map has been prepared indicating the potential ground water zones in this region.