An objective of the research was to understand the geo-environmental changes of the Brahmaputra in Assam and seek solutions to the problem of bank erosion. However, the increased area of the Brahmaputra in Assam is not necessarily related to riverbank erosion in every case.
ACKNOWLEDGEMENT
Acknowledgments I would like to thank Jonali Saikia, Kaustoov Acharya and Payodhar Pathak for their help in the laboratory. I am grateful to the staff of Civil Engineering Department (especially Juri Jyoti Hazarika, Rajib Gogoi, Kumud Deka, Dipak Deka and Tapan Das), Academic Section (especially Pradip Sinha) and Siang Hostel (especially Farid Ali) for their help in need .
CONTENTS
Physico-chemical properties of sediments of Brahmaputra 85-98
Introduction
- Introduction to sediments
- Sediments: physical and chemical characteristics
- Role of sediments in river morphology
- Objectives of the study
- Organization of the thesis
- Chapter 3 outlines the study area, methodology used for collection and analysis of the samples, instruments used and tools and procedures followed to arrive at the results and
- Chapter 4 synthesizes findings on various aspects of sediments of Brahmaputra obtained from secondary sources. Causes and sources of high sediment, aggradation and
- Chapter 5 covers physico-chemical properties of sediments of Brahmaputra based on analysis of suspended sediment and bed sediment samples collected from different
- Chapter 6 covers geochemical evaluation of bank materials of both the erosion sites and non-erosion sites of Brahmaputra River. Role of geochemical properties of bank materials
- Chapter 7 discusses braiding and land use and land cover of Brahmaputra River in post- monsoon months using remote sensing and GIS tools
- Chapter 8 discusses existing flood and erosion management measures. Based on understanding of sediment properties and processes of Brahmaputra, a few strategies are
- provides a summary and conclusions of the research work with future scope
The stability of a bank and its characteristic failure mode depend on the geotechnical and geological properties of the bank materials (Thorne, 1991). The sediment load of Himalayan rivers is disproportionately (~4 times) high compared to the surface area.
Review of literature
- Sediment properties of Brahmaputra and other large rivers .1 Physical properties
- Chemical properties: metal concentration in river sediments
- Sediment dynamics of the major fluvial basins
- River bank erosion in large rivers
- Geochemical properties of bank materials as a facilitator of erosion
- Land use and land cover change of large rivers
In general, the estuary is the main depositional center of the river sediment (Wang et al., 2007). As a result of the construction of the Aswan Dam, 98% of the sediment goes to the bottom of the Nasser Reservoir.
Materials and methods
Study area: The Brahmaputra River in Assam
- River bank erosion in Brahmaputra
The average width of the Brahmaputra valley is 80 km, of which the river itself covers about 1.5 km to 25 km. More than 50 percent of the rural people of Assam are employed in the agricultural sector.
Sources of secondary data
Collection of sediments
High quality 1 liter wide mouth plastic bottles were used, which were dipped into the river water and after being filled, were capped in the river water itself. In the months of December 2010 and January 2011, bottom sediments were collected by scooping up freshly deposited material.
Collection of bank materials
This channel is now strengthened by the flow of the Lohit, which joins it at Balijan. Constant and extensive soil erosion, especially on the southern and downstream edge due to the mighty Brahmaputra, is the biggest problem that threatens the very existence of the island and the lives and property of its inhabitants (Figure 3.10). Among the 25 Vulnerable and Severe Bank Erosion Zones identified by the Water Resources Department, Govt.
Soil samples were collected at 10 cm depth and then at 30 cm intervals from the vertical profile of the exposed steep banks of the erosion sites of the six sites.
Evaluation of physico-chemical properties of sediments/ bank materials
The hydrogen peroxide digestion method has several limitations, including incomplete oxidation of the organic matter and varying degrees of oxidation from one soil or sediment to another (Robinson, 1927). Assuming 44 g mol-1 for carbon dioxide and 60 g mol-1 for carbonate, the weight loss by LOI at 950°C multiplied by 1.36 should theoretically equal the weight of the carbonate in the organic sample (Bengtsson & Enell , 1986). Additional certified reference material from Japanese Geological Survey, JSd-3 and JLk-1 were used for calibration and intermediate checking in the analytical procedure to evaluate the effectiveness of the digestion procedure.
Approximate relative abundance ratios of major minerals were estimated from the relative intensities of the most intense and specific peak for each mineral from the XRD diagram.
Analysis of data
Images were acquired for the same season of the year to reduce inconsistencies in data. The images are then pre-processed for image enhancement techniques such as blur reduction, brightness and contrast to make the process of information extraction easier. Images were then stitched to create a single seamless mosaic image for the entire stretch of the river which was used to visually interpret and extract the shoreline, river midline, main channel, braided channels and sandbars as vector data created by ArcGIS 10.1 with the earth model and date WGS84.
The analysis of land use and land cover was carried out by categorizing satellite images into different classes, i.e. river/water, sandbar, vegetation (including natural grasslands) and agriculture (including human settlement), using unsupervised classification in image processing software (ERDAS Imagine).
Genesis and deposition of sediments in Brahmaputra
Causes of high sediment load in Brahmaputra
The collision of the Indian and Tibetan plates, initiated ∼55 Ma ago, resulted in the formation of the Himalayan orogeny (Kundu et al., 2012). Annual observed lowest water level Annual observed highest water level. iii) Unconsolidated sedimentary rocks of the Himalayas. The Brahmaputra River and its tributaries on the north bank traverse through the unconsolidated sedimentary rocks of the Himalayas.
The steepness of the Brahmaputra River is high compared to most of the world's great rivers.
Sources of sediments in Brahmaputra
- Tributaries of Brahmaputra
- Scour and deposition of sediments in Brahmaputra
- Shifting of Brahmaputra and river bank erosion
The north bank shifted up to 9 km inward and the south (left) bank shifted up to 5.2 km outward with deposition on the north bank and erosion on the south bank downstream of the river. Thus, the river was shifted downstream to the south with a length of 46 km. For example, the south bank experienced stronger erosion, while the deposition was more prominent on the north bank.
But in the Brahmaputra, higher erosion on the southern bank is mainly due to a downward displacement of the main channel in most of the stretches (Figure 4.22).
Slope variation
The topographic subsidence resulting in reduced headwater velocity and increased sediment deposition as observed in the Mekong, which flows through rock channels, is applicable to the Brahmaputra alluvial river. The decrease in average bed slope in the second stretch (cross-sections 42-52) and then the increase in the third and fourth stretches (cross-sections favored more erosion in sections 42-52 with river widening and deposition in other Results similar were observed in the lower part of the Yellow River where sedimentation caused the channel bed to rise continuously as a 'hanging river', exerting great pressure on flood protection (Xu, 2002).
A sediment budget of Brahmaputra River in Assam
General conclusions about the sediment loads carried by the Brahmaputra river system are almost impossible due to large diurnal, seasonal and annual fluctuations in the river's sediment carrying capacity (Subramanian and Ramanathan, 1996). Thus, 90% of the total suspended sediment load at the catchment outlet can be explained by the total amount of sediment coming from upstream nested catchments (Gay et al., 2014). The calculated sediment load of 733×106 t y-1 downstream of the Brahmaputra (India-Bangladesh border) is comparable to findings from previous studies (Table 4.8).
But sediment load of Brahmaputra cannot be compared with that of other Tibetan rivers such as Yellow and Yangtze in case of sediment input from bank erosion.
Physico-chemical properties of sediments of Brahmaputra
Size distribution of bed materials
In most alluvial rivers, the average size of bed materials decreases downstream. Sorting occurs due to the variation in flow transport capacity for different sediment sizes. Sternberg (1875) assumed that the change in weight of a sediment particle is proportional to the weight of the particle itself and the distance traveled.
This is mainly due to the fact that these sediments are very fine, and are mainly transported as suspended load.
Heavy metal concentrations in suspended sediments
The elemental composition of the suspended sediments during the monsoon and non-monsoon seasons are shown in Figure 5.8 and 5.9 respectively (in terms of weight. The flat, sloping and bladed shape of the suspended sediments (Figure 5.10 and Figure 5.11) suggest immature sediments or derived from cleaning bankero with short transport history The content of organic matter in the sediments of the Brahmaputra bed is less than 1.5% (Figure 5.13), which may be due to the dominance of the coarser fraction in the sediments.
The mineralogy of Brahmaputra bottom sediments is characterized by a predominance of quartz and muscovite (Figure 5.15 & 5.16).
Geochemical evaluation of bank materials of
Brahmaputra
Geochemical properties of bank materials .1 Analysis of bank materials of erosion site A
- Analysis of bank materials of erosion site B
- Analysis of bank materials of erosion site C
- Analysis of bank materials of location D
- Analysis of bank materials of erosion site E
- Analysis of bank materials of erosion site F
Values for SAR, ESP and CEC were found to vary with depth; however, continuous decline at lower layers was evident (Figure 6.1c-e). Results of analysis of various parameters for bank materials of erosion site B are shown in figure 6.2. Results of analysis of various parameters for bank materials of the ferrosion site are shown in Figure 6.10.
The mineral composition of bank materials at different depths (100 cm and 160 cm) of the vertical profile are shown in XRD (Figure 6.10i).
Synthesis of observations and interpretation from different experiments and test results
This can be attributed to the high content of silt particles (mean value 71%) in the bank material. A low amount of exchangeable Na (8%) in bank materials from erosion site C is a geochemical factor contributing to the loose structure of the bank. However, the mean velocities corresponding to a pre-monsoon discharge and peak monsoon discharge are two to one hundred fold higher than those required for threshold movement of sediment (Table 6.3), as obtained from the Hjulström diagram.
Correlation matrices (Table 6.4) showed that pH, organic content, carbonate content, SAR had negative correlation with erosion and particle size (d50 and d90) had positive correlation with erosion.
Braiding indices of Brahmaputra River
4 BI = Total length of channels / Main channel length Mosley (1981) 5 BI = Average number of active channels per inter-valley. The lengths of centerline, main channel, sandbars and center channels at the sixteen ends, referred to in section 3.7, p. It is noted that none of the methods for determining the braiding index used the strip number of the middle channel, which is a key factor of a braided channel.
Therefore, a new braiding index has been introduced that includes a part of the area covered by sand bars, the number of middle channel bars and the maximum width of the stretch (Tables 7.3 and 7.4) using the formula:.
Land use and land cover of Brahmaputra River
Land use and land cover of the entire Brahmaputra River during the post-monsoon season in 1994 and 2014 is shown in Figure 7.6. Land use and land cover change of Brahmaputra River during the two decades is shown in Figure 7.7. Area covered by sandbars generally increases from October with decrease in discharge and drying up of small streams in post-monsoon months.
Identification of sandbars/islands is necessary for better agricultural utilization or other activities in post-monsoon months.
Bank erosion management
Existing flood and erosion management practices in Brahmaputra
It has been observed that in rivers with silt factor of 0.7 to 1.50 (grain size 0.158 mm to 0.725 mm), the porcupine performs very satisfactorily as a silt inducing device. The siltation factor of Brahmaputra ranges from 0.9 to 1.0, as such the siltation inducing performance of porcupines is excellent. Sand for filling and installing the geo-textile bags/mattresses/tubes is available in abundance at the site.
However, the performance of the geotextile technology in Assam has yet to be seen at the site for its designed life of 50 years.
Promising erosion management options for Brahmaputra
Different ranges of the river can be categorized into two main groups based on braiding, displacement of the main channel and bank material characteristics to select suitable river training works. For sites eroded by direct attack of the main channel, e.g. Rohmoria upstream, liners along with flow deflectors in immediate upstream areas may be a suitable option. In places of erosion in highly braided area where secondary channels attack the banks, pro-siltation devices can be used to induce silting in banks and channelization of the main channel.
The large width of the super levee also reduces seepage (Arakawa – Karyu River Office and MLIT, 2006).
Summary and conclusions
- Genesis and deposition of sediments in Brahmaputra
- Physico-chemical properties of sediments of Brahmaputra
- Geochemical evaluation of bank materials
- Braiding and LULC of Brahmaputra river
- Understanding of geoenvironmental changes in Brahmaputra and solution to the bank erosion problem
- Future scope of research
The widening of the river has resulted in the loss of land in many locations due to the process of bank erosion. The distinct dynamic nature of the Brahmaputra in its valley section due to the flat topography following the high gradient of the river in China and Arunachal Pradesh (India) facilitates sediment deposition and river spreading (Chapter 4). The uneven decrease in the average slope of the river bed, especially in the middle course, causes instability due to bed migration and bank erosion.
Causes and mechanisms of bank erosion differ in different parts of the river due to variations in flow direction of the main channel, topography and bank material properties.
Extent and variation in the contribution of bank erosion to the suspended sediment load of the River Severn, UK. Bank Breach Hazards in the Lower Yellow River, Destructive Water: Water-Induced Natural Disasters, Their Mitigation and Control, Conference Proceedings (held in Anaheim, CA, June 1996), IAHS Publ. Headwaters of the Yamuna River System: Chemical Weathering, Its Temperature Dependence and CO2 Consumption in the Himalayas.
Bankline change and the facets of river hazards in the Subansiri-Ranganadi floodplain, Brahmaputra Valley, India.
