Fig: Erosion by Brahmaputra

Introduction

• The Brahmaputra River originates in a great glacier mass in Kailas range of the Himalayas (elevation 5300 m) and flows through China, India and Bangladesh for a total distance of 2880 km before emptying into the Bay of Bengal jointly with the Ganges (Figure 1). It drains a combined international area of approximately 580,000 km2. It is the fourth largest river in the world in terms of average discharge at the mouth and second only to the Yellow River in China in the amount of sediment transported per unit drainage area.
• In India, the Brahmaputra River flows southerly and westerly through the states of Arunachal Pradesh and Assam over a distance of approximately 800 km. In the Himalayas range before entering India, the river is known as the Tsangpo River flowing west to east, then south through the eastern Himalayas as the Dihang River. In Assam, the Dihang River is joined by other tributaries to form the Brahmaputra River. Near the western boundary of Assam, the river turns south to enter Bangladesh changing its name to Jamuna till its confluence with the Ganges from where both the Jamuna and Ganges form the Padma flowing into the Bay of Bengal. The total length of the river in Bangladesh is approximately 240 km.
• A longitudinal bank profile of the Brahmaputra reveals that the river has a gradient of 0.09 to 0.17 m/km near Dibrugarh, Assam at the head of the Valley and it is reduces to about 0.1 m/km near Guwahati. Through Assam, the long term average discharge increases from 8,500 to 17,000 cubic meters per second as flows are augmented by major tributaries. The width of the river varies from one km on an individual channel to as much as 10 km in some reaches with multiple braided channels. 
• The geology of the Brahmaputra system in Assam Plain comprises of the Higher and Lesser Himalayas. The Higher Himalayan rocks consist of schists and marbles with amphiboles at some locations. The lesser Himalaya in the Brahmaputra system drainage is composed of quartzites and schists. The Brahmaputra valley in Assam is underlain by recent alluvium (approximate 200-300 m thick) consisting of clay, silt, sand, and gravels. The present configuration of the Brahmaputra valley in Assam is known to have evolved during 2 million years of Pleistocene and recent era. The valley and its adjoining highlands constitute an extremely unstable seismic region.

River Bank Erosion

• Due to the river bank erosion and the channel migration, the Assam valley portion of the Brahmaputra River has lost approximately 7.4 % of its land area during its recent history of observations. The river bank erosion has caused major human and economic disasters than the annual flooding. The loss or the discomfort associated with the flooding is temporary but the loss of land due to river bank erosion is permanent and has a long term impact on the economy of the region and its people. Once a section of well developed land (agricultural, industrial, or residential) or productive forest land is lost due to river bank erosion, it can hardly be replaced. A mechanism to compensate citizens or the businesses impacted by bank erosion is generally not available through government rules or property insurance methods.
• The salient hydraulic and bank material factors responsible for bank erosion of the Brahmaputra system are i) rate of rise and fall of river water level, ii) number and position of major channel active during flood stage, iii) angle at which the thalweg approaches the bank line, iv) amount of scour and deposition that occurs during flood, v) variability of cohesive soil in bank material composition, vi) formation and movement of large bed forms, vii) intensity of bank slumping, and viii) progression of abandoned river courses to present-day channel.
• Due to braided characteristics, the main stem river consists of variable number of different sized channels and sand bars which change their locations and sizes each year. As indicated earlier, the most significant bank line modifications take place during the falling stages when excess sediment is deposited as bars within channel, causing a change in local flow direction and migration of thalweg.
• During floods, because of change of river hydraulics (mainly, depth, velocity, and shear stress), inducing variable sediment transport characteristics and erosive forces, the channel starts shifting at some vulnerable reaches.

        Fig: Braided Channel of Brahmaputra

Key factors in causing the river extremely unstable at many vulnerable reaches, such as:

• Nagaghuli, Maijan, Majuli, Bhairabpur, Balikuchi, Kaziranga, Howlighat, and Palasbari, are aggradation of the riverbed, intense braiding, large water discharge, and heavy sediment load since the 1950 flood. Moreover, there is a tendency of the river to shift southward within the valley reach. The tendency has become more prominent after the great earthquake of 1950, which raised the whole landmass of the northeastern part of the valley, particularly north of the river including the Himalayan foothill region by 3 to 4 meters. This southward thrust has initiated widespread erosion in the south bank near the Dibrugarh town and is still continuing at different reaches in spite of implementation of aggressive bank protection measures.
• The records of the last century show a general trend of widening of the Brahmaputra in Assam. The widening trend is clearly visible when comparing erosion and accretion rates over different periods.
• In general, the losses due to erosion show an increasing trend. Reports available with Water Resources Department indicate that 3,860 km2 of land were lost since 1954, with about 80 sq km per year. The erosion wiped out more than 2,500 villages and 18 towns, including sites of cultural heritage and tea gardens affecting the lives of nearly half a million people.

Properties of River Bank Material and Erosion Mechanism

• The fine sand and silty nature of the river bank material and the unstable bank line along the most part of the river create a highly favorable environment for bank erosion.
• Bank failures are composed of violent actions of the flowing water and the weak geotechnical properties of the bank materials.
• Overhanging cantilevered blocks are produced during the undercutting which leads to eventual falls and over steepening causes steep slopes due to migration of thalweg closer to the bank during the falling stages.

Erosion Control Measures and Restoration

• An extensive network of earthen flood embankments was erected all over the state of Assam in the main stems of the Rivers Brahmaputra, Barak and their tributaries as immediate and short-term measures under the “food for work” program.
• The short and medium term measures taken up under this initiative include erosion control and river training works that mostly comprise of bank revetments, stone spurs, boulder deflectors, timber dampeners, pile screens, reinforced concrete porcupines, Leet Fanci and other pro-siltation devices.
• The Water Resources Department of Assam also constructed 86 major sluice gates, 539 medium and minor sluice gates, and about 855 km of drainage channels to provide drainage and dewatering facilities.
• The emergency situations arising in flood seasons were mostly taken care of by installation of temporary dowel bund with empty cement bags, back filling with bamboo pallisading, A-Type spurs, bamboo porcupines, breach closing works, bamboo cribs etc.
• Need for a holistic strategy away from present piecemeal approach.
• The population density in the floodplains indicates the need for adequate flood and erosion management (structural – non-structural) measures.

Summary, Conclusions, and Recommendations

The Brahmaputra River has destroyed nearly 4000 km2 since last five decades at a rate of 80 km2 per year and erosion also wiped out more than 2500 villages affecting nearly 500,000 people. Though severe erosion caused by the Brahmaputra at various locations such as Majuli Island, Rohmoria, Kaziranga area etc. are well known, limited efforts has been made for the erosion control measures. An effective road map for long term solutions of flooding and riverbank erosion of this big river is yet to be developed.

• Examine successful erosion control measures in major rivers of the world.
• Strengthen and monitor anti-erosion measures already built at Majuli Island and other severely eroded towns along the river.
• Armor existing embankments located at urban and other strategic locations.
• Phase-wise solutions for the mitigation of erosions may include a combination of measures including strategic dredging, protection of erodible bank materials with anchored bulkhead or tie back sheet piles, spurs, toe, and bank revetments.
• Improve data quality and quantity by extending rain, flow, and sediment monitoring network using state-of-the-art equipments.
• Develop advanced and efficient computational tools (numerical models) capable of utilizing the detailed hydro-meteorological data and predicting real-time flooding and hydraulic characteristics of the river for planning and designing effective flood and erosion control measures.
• Consider physical modeling to study severe and potential scour sites and their control.
• Take advantage of modern technologies such as Satellite Image based morphological studies to warrant perspective on changes to river bank lines, river movement and indication of erosion risk along the river.

             Fig: Earthen/Rocky Embankment