Effects of Boulder Lifting on the Fluvial Characteristics
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EFFECTS OF BOULDER LIFTING ON THE FLUVIAL CHARACTERISTICS OF LOWER BALASON BASIN IN DARJEELING DISTRICT, WEST BENGAL A Thesis submitted to the University of North Bengal For the Award of Doctor of Philosophy in Geography & Applied Geography BY Lakpa Tamang GUIDE Dr. Deepak Kumar Mandal Associate Professor Department of Geography & Applied Geography University of North Bengal March, 2013 DECLARATION I declare that the thesis entitled “ Effects of Boulder lifting on the fluvial characteristics of the lower Balason basin in Darjeeling District, West Bengal ” has been prepared by me under the guidance of Dr. Deepak Kumar Mandal, Associate Professor, Department of Geography & Applied Geography, University of North Bengal. No part of this thesis has been formed the basis of award of any degree or fellowship previously. (Lakpa Tamang) Department of Geography & Applied Geography, University of North Bengal, Rajarammohanpur, Darjeeling – 734 013 Date: i CERTIFICATE I certify that, Lakpa Tamang has prepared the thesis entitled “ Effects of boulder lifting on the fluvial characteristics of the lower Balason basin in Darjeeling District, West Bengal ”, for the award of PhD degree of the University of North Bengal under my guidance. He has carried out the work at the Department of Geography & Applied Geography, University of North Bengal. (Dr. Deepak Kumar Mandal, Associate Professor) Department of Geography & Applied Geography, University of North Bengal, Rajarammohanpur, Darjeeling – 734 013 Date: ii ABSTRACT Almost all the rivers originating from the Darjeeling Himalayas have witnessed changes in its behaviour either due to natural (tectonic, catastrophe) or by increasing human interferences with changes in land use pattern. Degradation, channel shifting, lateral erosion, changes in bed form and sediment distribution are common amongst these rivers, thereby forcing the rivers to adjust within the changing environment. Human interferences with increasing pressure over the existing natural resources of the region has also been responsible for the change in the competency and capacity of these rivers. The use of region’s water resources for various human needs and also the gradual encroachment of adjacent lands is a common scenario in this region. Although the available water resources of the region are adjusting with the growing demand from increasing population, the equilibrium in this supply- demand is sometimes disrupted in order to fulfil the never ending needs of human beings. From the source area till its confluence, the Balason river faces acute anthropogenic problems, ranging from deforestation, landslides, unplanned and unscientific construction of settlement and roads to innumerable reckless usage of land and water resources. The fluctuating discharge, excessive bed load supply and highly erodible banks have resulted in braided nature with ephemeral bars in its lower course. This has restricted the free passage to an excessive amount of run-off during heavy and concentrated rainfall causing floods in its lower segments. High intensity rains induced landslides and high surface run-off in its upper catchment also provides huge quantities of weathered materials as sediment load which are being deposited in its lower course. These materials can easily be extracted directly from the river bed and due to its corrosive nature do not require much processing. Hence, the bed materials from the lower Balason river has become an ideal source of demand for growing construction industries of the region as well as the neighbouring districts and states. The common practice of extracting the bed materials along the lower Balason river is a traditional one with labours still using their physical strength iii and few tools for extraction process is dependent upon its size, amount and quality of bed materials available. During the dry period, the entire stretch of lower Balason river seems to be like a mining site with the entire river bed as well as adjacent terraces and flood plains are excavated up to 2 m for extracting the deposited bed materials. But during monsoon period, this activity ceases to the near bank locations, since the high flow of the river disrupt the in-bed extraction. Although the near bank extraction continues, mostly boulders and sands are extracted, but during this period the labours collects the larger materials brought down with monsoon flows and are broken into different grades. Presently, the extent and intensity of bed material extraction in the lower Balason river have taken such a dimension that it needs immediate attention to manage the associated effects on its form and processes. The problem though seems local, proper management is very much essential both in the lower course as well as the upper catchments of the Balason river. The continuous extraction inducing lowering of bed elevations and resultant bed erosion and also lifting specified size gravels and leaving behind the smaller one thus disrupting the sediment transportation and distribution have resulted into head ward erosion which is being transformed into its upper segments beyond the margins of the lower plains. The adjustment by the river in the form of scouring and filling have also resulted into undercutting below bridges and embankments due to narrow channel width causing retention of flows. Besides, human settlements near the banks and adjacent flood plains for easy access to river bed and also construction of roads near banks for transportation of extracted bed materials have reduced the vegetation thus triggering the exposed banks to retreat further resulting into loss of nearby agricultural lands, tea garden and even residential areas in few sites. Although the concerned authorities have formulated different prohibitions but their proper implementation has not taken place due to callousness of the authority and the role played by the permit-holders as middle man. The people dependent upon such activities are not concerned about the ultimate effects and with more demand from local markets; the illegal extraction is a common practice. Extraction of bed materials from below the bridges and iv embankments although have been prohibited but in reality the extraction are done as per the convenience of loading and transporting of the extracted materials. Hence, an overall assessment should be under taken based on the river adjustment of its fluvio-geomorphic characteristics. Such an integrated assessment of river modifications will help resource management decisions including protection of its fluvial characteristic so that the boulder lifting activities does not jeopardise the sustainability of other appropriate river users and values. This could be possible by implementing long term management plans with proper understanding and management of sediments at basin level and a wider application of the available scientific knowledge, particularly of fluvial geomorphology and hydraulics. v PREFACE River systems are complex entities that adjust their channel geometry and shape to different hydrologic and sediment regimes in order to establish or maintain an equilibrium state. This equilibrium state is maintained by adjustments in multiple factors, but mainly channels morphology, discharge, sediment transport and capacity at varying spatial and temporal scales. Stream channels rarely achieve this state of equilibrium due in part to constantly having to adjust to variations in its form and processes and human- induced changes. Balason basin has been degrading due to increasing human transformations but, if proper management planning is formulated, it has lots of potentialities and scope for development. In its upper catchments, the acute anthropogenic problems ranging from deforestation, landslides, unplanned and unscientific construction of settlement and roads to innumerable reckless usage of land and water resources followed by extensive bed material extractions and human occupancies in its lower course has been largely responsible for the changing fluvial characteristic. The continuous extraction inducing lowering of bed elevations and resultant bed erosion and also lifting specified size gravels and leaving behind the smaller one thus disrupting the sediment transportation and distribution have resulted into head ward erosion which is being transformed into its upper segments beyond the margins of the lower plains. As such the researcher has made an attempt in this dissertation to correlate the existing fluvio-geomorphic adjustment of the Balason river for evolving the long term management strategies for maintain sustainability of the available resources and its fluvial environment. For the proper presentation of this long pursuit, I would like to thank my supervisor Dr. Deepak Kumar Mandal for the real autonomy he has given to me, so that I could explore the topic and enrich my own ideas. I express my sincere thanks to my parents, my wife and all the family members for all their sacrifices they have made in supporting and motivating me through all the difficult times. vi I sincerely thank my brother Dr. Mozzam Ali, Department of Chemistry, SMIT for his unconditional help during data analysis and thesis writing. I also thank Prof. Subir Sarkar, Late Prof. Subash Ranjan Basu, Prof. Auribindo Ghosh, Prof. Leszek Starkel, Dr. Lukasz Wei Ja Wiejaczka, Dr. Pawel Prokop, Mr. Dominik Ploskonka , Mr. Kausik Ghosh and Mr. Binod Bhattarai for their valuable suggestions which greatly helped me in understanding my research needs. I would like to thank University