Flood prevention and management of hazards to key infrastructure, a case study of Majuli Island, River Brahmaputra, Assam, India.

The Brahmaputra is the ninth largest river in the world in terms of mean annual discharge (19,800 m3/s). The river carries a heavy load of sediment, varying between about 500 million and over 1 billion tonnes per year depending on the magnitude and duration of the annual monsoon flood. The river Brahmaputra is a multi-channel, braided, anastomosed river that changes almost constantly and periodically exhibits rapid transformations in channel morphology. These channel morphological changes are generally favoured by the very large and highly variable discharge, heavy sediment load and easily eroded, non-cohesive riverbanks. Majuli Island, which is located in the upstream of the River Brahmaputra, is the world’s largest riverine island, though its area decreased during the twentieth century (from a reported 1,255 km2 in 1901 to just over 420 km2 in 2001) due to erosion by Brahmaputra. Majuli Island is proposed to make into a UNESCO World Heritage Site with a view to better protecting and conserving the island and its rich culture.

Figure 1 : Geo-morphological changes in River Brahmaputra near Majuli Island 1917 - 2010.

Flooding is a major hazard threatening the communities on Majuli Island and flood prevention relies primarily on the protection provided by 12 embankments surrounding the Island. Bank material is a significant contributing factor in accelerating bank erosion of Majuli Island. The bank material in the Island varies mostly from coarse sand to fine sand and silt. The earthen embankments, which is known as the ‘lifeline of Majuli Island’ comprises medium sand and is non-cohesive highly prone to erosion or breaching.

With respect to future flood risk management, it must be borne in mind that eight of these twelve embankments have already exceeded their engineering design lives and these embankments are known to have lost some of their capability to withstand floods due to their age and condition (Society for Socioeconomic Development, 2013).

Before 2004 the river was flowing following the natural depression (caused due to northward tilting of the riverbed) present near the southern tip of the Majuli island which is near the confluence of the river Brahmaputra and river Subansiri. During this process, the river started eroding the riverbanks and sandbars which are adjacent to the river channel. Therefore, to protect these vulnerable areas Govt. of India in January 2004 installed RCC porcupines, spurs and dampeners in this area as a measure to protect the banks from erosions. However, this resulted in more damage to the riverbed as the river became uncontrollable and started eroding those areas which were previously never eroded, creating the river wider and shallower, thus highly braided. Hence, ‘train’ the great river and protect its northern bank from erosion have not only proved inadequate but appear to have even made matters worse. 

Figure 2: Bank of the river Brahmaputra during 2016 flood with the RCC porcupine erosion protection structure laid to protect the bank from erosion.

The ‘protection’ provided by the porcupine screens, spurs and dampeners constructed in 2004 have proved inadequate and short-lived and, in December 2017, the Brahmaputra Board framed a new scheme for the protection of Majuli Island from flooding and erosion by the Brahmaputra. The major elements of this new scheme include construction of further hard engineering structures such as:

(i)             Bank revetments made of geo-bags filled with earth/sand.

(ii)           More RCC porcupines;

(iii)         A sluice gate at the mouth of the Tuni river.

(iv)          A pilot channel with a length of 3.5 km (not much of detail has been given on the pilot channel proposal, which was published by Press Information Bureau, Government of India, Ministry of Water Resources, 28th December 2017).

Based on these proposals it may be concluded that the Brahmaputra Board are intent on sticking with conventional bank protection and river training approaches, even despite experience gained since the 2004 works which would suggest otherwise.

Notwithstanding these comments regarding renewed attempts to stabilise the north bank of the Brahmaputra and train its course away from the flood embankments at critical locations, one aspect of the newly-proposed works that may be advantageous is the plan to build a sluice at the mouth of the River Tuni.While interacting with local people, it emerged that they do not support of creating continuous embankments to prevent flooding to keep out the river. According to them, when floods overtop or breach the embankments (which often happens) standing water is trapped on the landward side, inundating the land for periods of 10 to 15 days even after water levels in the river have subsided. This is due to the absence of spillways within the embankments to allow flood water to flow back out of the island after a flood.

Figure 3: Image showing the River Tuni, which has been blocked at its outlets.

 Historically, the River Tuni (which is an inland river on Majuli Island draining to the Brahmaputra near Kamalabari) used to fulfil the function of draining floodwater back to the main river. This was the case because high monsoon floods in the Brahmaputra caused backflow in Tuni River, which inundated the interior of Majuli island. On these occasions, the Brahmaputra and Tuni became a single sheet of water with an average width of 9.5 km that inundated that part of the island to depths of 2 to 4 m. However, on the falling limb of Brahmaputra floods (as well as during locally generated rainwater floods), the Tuni used to efficiently convey floodwater out from the Island and into the Brahmaputra. In recent years, the inlet and outlet of River Tuni have been blocked by embankments, creating longer-lasting floods because inland floodwater is unable to drain from the island and inundates it for longer periods. In this regard, how effective is the proposal to build a sluice gate at the mouth of the River Tuni can only be found out only when it will become operational.

Consideration of the sheer scale, the complex nature of the multiple hazards posed by the huge river, and the failure of conventional measures suggest that a new and radical approach may be required. One sustainable approach to reducing future flood and erosion risks, or at least maintaining them at current levels, could be to create a 200 m wide ‘green buffer’ between the River Brahmaputra and homes, properties and key infrastructure on Majuli Island.

While, this approach differs radically from the conventional bank protection and river training measures employed to date, there is a precedent for flood and erosion buffering. The Ministry of Forest and Environment, Govt. of India, has created the Coastal Regulation Zone (CRZ), which is intended to prevent damage to coastal plains from coastal flooding and erosion driven by waves and extreme high sea levels associated with spring tides, storm surges and tropical cyclones. The CRZ occupies coastal land up to 500 m from the normal High Tide Line (HTL). The CRZ extends in land along coastal creeks, estuaries, backwaters, and rivers in the form of 100 m wide zones on both banks. Within these demarcated areas, special steps are taken to safeguard the people, property and wildlife living in all the coastal plains of India.

While interactions with the local people of Majuli Island revealed that the great majority would be in favour of the introduction of this kind of zonation in river floodplains along the margins of the island, which in terms of topography, physical scale and exposure to flood and erosion hazards, are actually not that different to coastal floodplains. That said, this kind of zonation has never been adapted to or applied in an inland river in India.

Based on the sheer size of the Brahmaputra, the huge volume of floodwater it discharges during high monsoon runoff years, and its capacity for extensive bank erosion, even during normal runoff year, the width of the ‘River Regulation Zone’ (RRZ) should be no less than that of the 200 m. Development of a ‘green buffer’ on the floodplain, braid bars and islands within the RRZ will not only help to safeguard the island from flooding and reach-scale bankline retreat, but will also reduce rates of local bank erosion and deformation in near-bank and bank-attached sandbars. The evidence of the potential for vegetation to stabilise banks and bars at Majuli Island comes from the work of a dedicated individual named Jadav Payeng, who has devoted 30 of his 55 years of life to the cause of using trees to create habitat and protect part of Majuli Island from erosion by the Brahmaputra.

 Figure 4: 3D view of the proposal for stabilizing the sandbar and the banks of river Brahmaputra near Majuli Island

 It is likely that such radical changes can only come about if forecasts provide decision makers with the foresight they need to embrace innovation and move away from technical and institutional lock-in to traditional approaches to bank protection and river training.

 Dr Prasujya Gogoi

Water Resource Scientist