- Deva K. Borah (PhD, PE) Chesapeake, Virginia, USA

Abstract: The Brahmaputra River in Assam has several constrictions as can be seen in Goggle map. Constrictions at Majuli, Shilghat, Guwahati, and Goalpara have been playing a major role in flooding and erosion of the river and have not received enough attention. A mathematical model of the river is needed to account for effects of these constrictions, as well as, designing flood and erosion control or prevention measures, especially designing and aligning an adequate and stable channel of the river. Erosion-control measures must be designed and built immediately at active bank erosion sites where properties and human lives are at stake. For sustainable solutions to flooding and erosion, the river must be carefully studied using mathematical model(s). Adequate and stable channel reaches with sufficient capacity to convey the incoming water and sediment without damaging flood, erosion and siltation must be precisely designed and optimally aligned using the mathematical model. The stable and optimally aligned channel must be built and maintained using bank stabilization technologies and minimal dredging.

Introduction

It is encouraging to see much interest in recent years by public and private organizations or groups in addressing the serious flooding and erosion problems caused by the Brahmaputra River and finding immediate relief and permanent solutions to these problems. Investigations by Coleman (1969), Goswami (1985), Sarma (2005), and Sarkar and Thorne (2006) provide valuable information and background on the Brahmaputra River. Phukan et al. (2010) compiled all the available reports on past investigations on the Brahmaputra River and documented their findings and key issues.

As outlined in Phukan et al. (2010), there are unique and complex flow patterns in the Brahmaputra contributing to its unique flooding and erosion problems, braiding and shifting of thalweg (deepest part of the river) are among those processes. In this article, two other key factors controlling flooding and erosion in the Brahmaputra, as may be observed in the Google map of the river, are discussed. The Brahmaputra River has several constrictions along its path in Assam, mainly at Majuli, Shilghat (east of Tezpur), Guwahati, and Goalpara. These constrictions have been playing a major role in flooding and erosion of the river and have not received enough attention yet. Figure 1 shows an example of the constriction at Majuli and its effects in forming braided channels downstream of the constriction. Handique (2010) discussed the constrictions at Guwahati and Goalpara causing high backwaters and flooding, and suggested bypass tunnels through the rocky hills for fast release of the impounded water.

A mathematical model of the Brahmaputra River is urgently needed for designing flood and erosion control measures for immediate construction, and also for designing and aligning an adequate and stable channel of the river leading to sustainable solutions to flooding and erosion. Mathematical models are inexpensive versatile tools to investigate complex natural processes such as flooding and erosion in a river system and find effective measures to prevent or control these destructive natural processes. Once the models are built in the computer, calibrated and validated using physical dimensions and observed or monitored data, those are run with virtual implementation of the preventive or control measures or physical alterations to the river. By comparing results from different model runs with various alternative measures, the most effective measures or physical alterations in preventing the destructions can be determined. Because the model results (e.g., flow rate, water depth, velocity, erosion, deposition, and sediment discharge) are continuous in space and time, those provide the best data for designing control measures.

In case of the Brahmaputra, designing and aligning an adequate and stable channel is the key to sustainable solutions to flooding and erosion. The designed channel must have enough cross-sectional area and longitudinal slope (balanced) to carry the incoming flow of water without causing flooding destructions. Also, the designed channel must be in a quasi-stable, if not stable, state with just enough transport capacity to carry the incoming sediment without further erosion of the river bed and bank, and deposition or siltation. The channel will be variable throughout its length in Assam in order to match or balance variable local conditions. Such variable adequate and stable channel reaches can only be designed using mathematical models. 

Figure 1. The Brahmaputra River near Majuli with a constriction upstream (far right) and braided channels downstream (river flowing from right to the left)

Constrictions in the Brahmaputra River

As can be seen in Google map of the Brahmaputra River, the river has several constrictions along its path in Assam, mainly at Majuli, Shilghat (east of Tezpur), Guwahati, and Goalpara. The Guwahati constriction is a long stretch beginning approximately near Mayang where the river narrows considerably and slowly towards the narrowest constriction from Kharghuli Hills to Guwahati University. The river extensively spreads out again further west of Palashbari. These constrictions have been playing a major role in flooding and erosion in the river.

As indicated by Handique (2010), constriction creates a lake behind it. During monsoon flood, constrictions create bottleneck or flow congestion and are unable to pass the high flow at normal depths resulting in raising the water depth substantially high and creating large cross sectional areas to pass the incoming water. Such rise of water level at a constriction creates backwater effect on its upstream river reach raising water levels as far upstream as its effects cease and thus creating a lake. As the water level gets higher at the upstream lake, water spreads out to the entire river width submerging the sand bars, the islands, and its flood plains resulting in reducing flow velocities as well as sediment transport capacities. Monsoon floods carry enormous amount of sediment from its upstream drainage basin. Due to the low sediment transport capacity at the upstream lakes, a large portion of the sediment, specially, the larger particles get settled out building more sand bars or islands or enlarging the existing ones.

Different processes take place downstream of the constrictions. Due to sediment deposition upstream of the constrictions, relatively cleaner water with less sediment content passes through the constrictions with transport capacities in excess of the sediment being transported. As a result, the hungry for sediment water erodes river beds or banks whichever is found easier below or downstream of the constrictions. In case of the Brahmaputra having alluvial banks with fine silt materials, the banks become the primary victims.

A constriction may be envisioned as a narrow outlet to a fully flowing pipe where water comes out as a jet. The Brahmaputra constrictions add tremendous velocities as well as sediment transport capacities to the water, thus adding more forces to erode downstream alluvial banks. Further downstream from the constriction, as the water reaches the wider part of the river, water velocities and carrying capacities go down, and sediment particles start settling down beginning with the larger particles.

River stretches (reaches) between the constrictions go through combinations of both the upstream and downstream of constriction processes at different times. Sediment deposition primarily takes place at the middle of the river creating the islands and sand bars, thus pushing the main channel (flow) or the thalweg to the sides causing accelerated bank erosion. The Brahmaputra River near Majuli as shown in Figure 1 is a perfect example. Bank erosion continues during the dry season as the main flow (current) is in direct contact with the highly erodible banks. The pictures shown in Figure 2 are examples of such erosion during dry seasoned low flows. These pictures were taken by the author on Janury 10, 2010 at Bhurbandha, southern bank of the Brahmaputra between the Silghat and Guwahati constrictions. 

A Bhurbandha family is losing almost everything they own foreve

Highly erodible alluvial bank materials

Effects of the constrictions may be noticed on Goswami’s (1985) gross aggradation-degradation evaluation along the Brahmaputra using suspended sediment measurements at Ranaghat, Bessamara, Bhurbandha, Pandu, and Jogighopa during 1971-1979. He reported net degradation (erosion or loss of bed material) between Ranaghat and Bessamara, aggradation (deposition, siltation, or gain of bed material) between Bessamara and Bhurbandha, degradation between Bhurbandha and Pandu, and aggradation between Pandu and Jogighopa. These are gross evaluations as the measurements were taken too far apart. In reality, erosion and deposition are more variable along the river and across the river similar to flow depth and velocity. Appropriate mathematical models are capable of predicting such spatial and temporal variations of flow depth, flow velocity, sediment discharge, aggradation, and degradation.

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