4. Sediment Dynamics in a Large Alluvial River: Characterization of Materials and Processes and Management Challenges

Sediments of a large alluvial river often hold the key to its geoenvironmental behavior. Several physical and chemical behaviours of high sediment carrying river including water quality, bank line shifting, sediment budget and bank erosion are influenced by sediments. The current study carries out a partial evaluation on some of the key factors. Considering severity of bank erosion problem in the last few years, six locations on the river Brahmaputra were considered for this study. Role of individual bank material properties in bank erosion were studied with binary logistic regression using SPSS.

Young lithology, seismicity, unconsolidated sedimentary rocks of the Himalayas, steep slope of the Brahmaputra in the Himalayas, heavy rainfall in monsoon, deforestation and Jhum 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 the Brahmaputra River is high compared to most of the large rivers of the world including Amazon, Congo, Yangtze, Volga, Mississippi, Ganges and Indus. Slope of the Brahmaputra River created by using data of 175 points along the channel taken from GoogleEarth demonstrated a much clear picture with marked differences compared to the earlier figure based on a few points proposed by other researcher.

Erosion was more severe in the south bank whereas more deposition is taking place in the north bank of the Brahmaputra, particularly in upstream of Dibrugarh. Erosion during 1990–2008 in north bank and south bank of the Brahmaputra (within Assam) were 544 km² and 920 km² respectively, whereas corresponding deposition were 145 km² and 68 km² respectively.

An attempt was made to construct a sediment budget for the Brahmaputra River in Assam using a mass balance equation. Major tributaries were found to contribute 326 × 106 t suspended sediment in a year to the Brahmaputra. Considering 30 % of riverine sediments trapped in the river beds and flood-plains, 228 × 106 t sediments were considered in suspension at downstream, whereas 98 × 106 t was estimated to be deposited. From scouring and deposition data, mass of deposited sediment on river bed has been 69 × 106 t in a year. Area of land lost due to bank erosion in a year was found to be 81 × 106 m². Total sediment load in the river at downstream was estimated 869 × 106 t/year. Considering 10 % of sediment load of the Brahmaputra as bed load, suspended sediment load at downstream was 782 ×3 106 t/year. Tributaries, bank erosion and scouring of river bed were found to contribute 26 %, 54 % and 20 % to suspended load of the Brahmaputra at downstream. The findings differed somewhat from previous estimates possibly due to relatively dependable volume of currently measured data.

Among the analysed parameters pH, OC, CC, CEC, ESP, d10, d50 and d90, were more fluctuating in bank materials of erosion sites than that of non-erosion sites. Low (<2 %) organic content, dominance of silt and sand sized particles, particularly in lower layer of sediment profile were the major geochemical properties of bank materials contributing to bank erosion in sites like A, B, E and F. Decrease of SAR, ESP and CEC towards deeper depth showed similarity to results of other erodible areas elsewhere. Presence of clay pocket and high organic content at lower layer of sediment profile had no role in stability of bank profile in C area due to overall dominance of silt particles in the bank.

The present study has the potential scope of study of fate and transport of sediments of a large alluvial river using more data from tributaries as well as the main stem. Inclusion of hydrological parameters can led to reliable erosion prediction in a particular area. Consideration of bank material properties and erosion behavior can be effective in site specific protection measures of erosion problem.