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Chapter 1 Introduction SEDIMENT PROPERTIES AND PROCESSES INFLUENCING KEY GEOENVIRONMENTAL ASPECTS OF A LARGE ALLUVIAL RIVER, THE BRAHMAPUTRA IN ASSAM A thesis submitted to IIT Guwahati in partial fulfillment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY By Lalit Saikia Roll no: 07610410 Under the guidance of Prof. Chandan Mahanta Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati - 781039, Assam, INDIA October, 2017 A | P a g e Dedicated to My Grandfather Late Dimbeswar Saikia i | P a g e TH-2074_07610410 DECLARATION I hereby declare that the thesis entitled Sediment properties and processes influencing key geoenvironmental aspects of a large alluvial river, the Brahmaputra in Assam is a presentation of my original research work done under the guidance of my thesis supervisor Prof. Chandan Mahanta in the Department of Civil Engineering, Indian Institute of Technology Guwahati. I hereby confirm that the thesis is free from any plagiarized material and does not infringe any rights of others. In keeping with the general practice and reporting of scientific observations, due acknowledgements have been made wherever the work described is based on the findings of other investigators, with due reference to the literature. (Lalit Saikia) Roll No: 07610410 Research Scholar Department of Civil Engineering October, 2017 Indian Institute of Technology Guwahati IIT Guwahati E-mail: [email protected] [email protected] ii | P a g e TH-2074_07610410 INDIAN INSTITUTE OF TECHONOLOGY GUWAHATI Guwahati 781 039, India Phone: +91-361-2582407 Prof. Chandan Mahanta Fax: +91-361-2582440 Professor, Department of Civil Engineering E-mail: [email protected] Dean, Students’ Affairs [email protected] October 23, 2017 CERTIFICATE This is to certify that the thesis entitled Sediment properties and processes influencing key geoenvironmental aspects of a large alluvial river, the Brahmaputra in Assam submitted by Lalit Saikia (Roll no: 07610410) to the Indian Institute of Technology Guwahati, for the award of the degree of Doctor of Philosophy, is a record of research work carried out by him under my supervision and guidance. The thesis, in my opinion, is worthy of consideration for the degree of Doctor of Philosophy in accordance with the regulations of this Institute. The results contained in the thesis have not been submitted elsewhere in part or full for the award of any degree or diploma to the best of my knowledge and belief. (C. Mahanta) iii | P a g e TH-2074_07610410 ABSTRACT The research attempted to evaluate key sediment properties and processes of a large alluvial river, the Brahmaputra in Assam, India and examine how different sediment properties and processes influence major geoenvironmental behavior of the river. Based on primary as well as secondary data, the thesis focusses physico-chemical properties of sediments including contamination status, sediment budget, bank line shifting, river bank erosion, role of geochemical properties of bank materials in erosion, braiding and land use & land cover of the river in post-monsoon months. One objective of the research was to understand geoenvironmental changes of Brahmaputra in Assam and seeking solution to the bank erosion problem. Suspended sediment and bed sediment samples were collected from six different locations of Brahmaputra River in Assam. For suspended sediment samples, four water samples were collected from each sampling location during pre-monsoon (January-March, 2011), monsoon (June, 2011) and post-monsoon (November, 2011) months. Suspended sediments were obtained from water samples by filtration using micro-pore filters. Bed sediments were collected by scooping of freshly deposited sediments from river bed in the months of December’ 2010 and January’ 2011. Considering severity of bank erosion problem in the last few years, six locations namely Rohmoria (A), Dibrugarh (B), Nematighat (C), Majuli (D), Gamerighat (E) and Palasbari (F) were selected for collection of bank material samples. Soil samples were collected at 10 cm depth and then at 30 cm intervals from vertical profiles of exposed steep banks of erosion sites and nearby non-erosion sites. pH, organic content (OC), carbonate content (CC), sodium absorption ratio (SAR), exchangeable sodium percentage (ESP), cation exchange capacity (CEC), mineralogy and particle size of collected sediments and bank materials were analyzed by standard methods and instruments, e.g., pH by pH meter; OC and CC by loss on ignition method; SAR, ESP, CEC using flame photometer, mineral composition and texture using XRD and SEM; particle size using laser particle size analyzer. Heavy metals in sediment samples were analyzed using the USEPA 3050B method and an atomic absorption spectrophotometer. iv | P a g e TH-2074_07610410 Modified BCR sequential extraction method was applied to understand the geochemical association and to predict the probable mobile fraction of heavy metals in sediments. Angle of internal friction and cohesion of bank materials were estimated by direct shear tests. Role of individual bank material properties in bank erosion were studied with binary logistic regression using SPSS. Besides scientific references, secondary data needed for the study was collected from reports of North Eastern Council, Brahmaputra Board, Water Resources Department (Govt. of Assam) and Ministry of Water Resources (Govt. of India). Braiding, erosion-deposition and land use and land cover of Brahmaputra River was studied extracting information from Landsat images using Remote sensing and GIS techniques. Brahmaputra River in Assam at three different years, i.e., 1973, 1994 and 2014 was divided into 16 reaches with equal length of 40 km (Reach 1 in the upstream and reach 16 in the downstream). Erosion and deposition during 1973 – 2014 was studied using overlay analysis in ArcGIS 10.1. Braiding indices for different reaches of Brahmaputra in 1973 and 2014 were calculated by seven different methods suggested by Brice (1964), Howard et al. (1970), Rust (1978), Mosely (1981), Ashmore (1982), Friend & Sinha (1993) and Sharma (2004). A new braiding index for an alluvial and large braided river like the Brahmaputra has been introduced in the present study using number of mid channel bars as an input parameter. LULC analysis for 1994 – 2014 was carried out by categorizing the satellite images into different classes, i.e., river/ water, sandbar, vegetation (including natural grass land) and agriculture (including human settlement), using unsupervised classification in an image processing software, i.e., ERDAS Imagine 2014. Young lithology, seismicity, unconsolidated sedimentary rocks of the Himalayas, steep slope of Brahmaputra and its tributaries in the Himalayas, heavy rainfall in monsoon, deforestation and sediment generating cultivation were the causes of sediment generation in the river, whereas decrease of slope in Assam plains was the main cause of sediment deposition. Steepness of Brahmaputra River is high compared to most of the large rivers of the world like Amazon, Congo, Yangtze, Volga, Mississippi, Ganges and Indus. Slope of Brahmaputra River created for the present study by using data from 175 points along the v | P a g e TH-2074_07610410 channel demonstrated a clearer picture with marked departure from the earlier figures based on fewer points proposed by other researcher. Average gradient of Brahmaputra River was found to be 1.52 m km-1. Steep slopes of river (slope of Brahmaputra in the reach between PE and entry to India was 8.27 m/km) and tributaries in the mountainous reaches led to high amount of sediment generation and transportation whereas a sudden decrease in slope (slope in the reach between India border and Pasighat is 1.52 m/km) results in a large amount of sediment deposition, developing a prominent braided pattern. Slope of Brahmaputra has further decreased during the course of the river in Assam plains (from 0.81 m km-1 between Pasighat and Dibrugarh at upstream to 0.09 m km-1 between Guwahati and India-Bangladesh border at downstream), causing more prominent braided pattern and river instability in terms lateral shifting and bank erosion. Average scour of 1.92 cm per year was noticed in the entire reaches with maximum scouring of 4.96 cm per year in the middle reaches (C/S 31 – 41). Decrease in average bed slope at the second reach (C/S 42 – 52) and then increase in the third and the fourth reaches (C/S 32 – 42, 22 – 32) favored more erosion in the cross-sections 42 – 52 and deposition in other reaches. The river was widened to both the banks at upstream in earlier years, i.e., 1911–28 and 1971–71. There was severe erosion in the south (left) bank of the river during 1973 – 2014 with downward shifting of the main channel. Total area of erosion and deposition during 1973 – 2014 were 1557 km2 and 204 km2 respectively. There was more erosion in the south bank (829 km2) and more deposition in the north bank (122 km2) during the period. An attempt was made to construct a sediment budget for Brahmaputra River in Assam using a mass balance approach. The tributaries were found collectively to contribute 698×106 t suspended sediment annually to Brahmaputra. Since a large portion of river- derived sediments of Tibetan rivers are trapped in low reaches, considering 40% of riverine sediments trapped in the river bed and flood-plains, 419×106 t sediments (60% of 698×106 t) were estimated to be in suspension at downstream, i.e., at India-Bangladesh Border, whereas 279×106 t (40% of 698×106 t) was estimated to be deposited in river bed and floodplains. From scouring and deposition data, mass of deposited sediment on river bed was estimated to be 69×106 t in a year. Average annual land area lost due to bank vi | P a g e TH-2074_07610410 erosion was found to be 38×106 m2 during 1973 – 2014. Considering depth of bank erosion as 4.7 m (average difference of yearly observed highest and the lowest water level of Brahmaputra for the period 1914 – 1990) and sediment density 1.36 g/cm3, mass of eroded soil was found to be 243×106 t.
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