Rameshwar Yadav, 63, who lives in a village in Nepal’s Saptari district, sits on a platform beneath a tree, at some distance from Bridge Number 1, built on the Mahuli river. Sukhilal Khankh, 74, is with him. It had been a week since the Kosi floods, and their conversation revolves around heavy rains in Nepal in the last week of September that caused the floods. Rameshwar Yadav says about the Mahuli river, “This is a mountain river. Water comes from above and brings soil along with it. Due to this, the water levels in Kosi and Mahuli have increased and the risk of floods has increased.” Pointing at the bridge in front of him, he informs that the water flowed over the Saptari bridge this time.

Khankh says, “Ten years ago, this bridge was about 10 feet above the riverbed. Now, the distance between the two, is about two feet.” While explaining, he waves the stick in his hand in the air and explains, “Back then, even if we had gone under the bridge and raised the stick in the air, it would not have touched the roof of the bridge.”

After heavy rains in many parts of Nepal and Bihar in the last days of September 2024, the Kosi region faced a devastating flood after decades.

Mahuli is an important tributary of the Kosi before it enters India and hits the Kosi Barrage. It carries silt, which also gets mixed with the Kosi water, increasing the quantity of the silt in Kosi. In fact, many tributaries of Kosi in Nepal add to the amount of silt and sediment in the river.

Like Nepal, in India too, the presence of excessive sediment and silt in Kosi is leading to many problems. Every year, especially during the rainy months, large areas of agricultural land get inundated with silt. Excessive rainfall and the intensity of floods, augment the issue further.

Mohammad Sayeed, who lives in Mungrar chhint (an island located in the middle of Kosi river) in Supaul district, points at the silt accumulated in his field and says, “We had sown paddy in our 2.25-bigha field. The entire crop got washed away in the floods. Now, all you can see is silt.” He works as a migrant labourer in Delhi but had to return to his village after he heard about the floods. The floods also washed his house away and there was visible debris all around.

After the flood in the Kosi river in the last days of September this year, silt accumulated in the villages of Bagaha, Khukhnaha, and Belagote. A considerable amount of silt was also seen at the spot where the western embankment of Kosi broke near Bhubhaul village in Darbhanga district. Farmers in the region are apprehensive about rabi farming due to the excessive accumulation of water and silt in the fields, even in October. Water and silt damaged the buds of paddy crops in some parts.

Kosi barrage crisis aggravated by silt

After heavy rains in Nepal in the last days of September, the water level of Kosi and other rivers coming to India from Nepal increased. On September 29, the maximum discharge level of the Kosi Barrage reached 6.61 lakh cusecs, and all 56 gates had to be opened. However, water started flowing over the barrage only when the discharge level reached 6.15 lakh cusecs. The maximum discharge level of the Kosi Barrage at Bhimnagar near Birpur is 9.50 lakh cusecs.

In conversation with Mongabay India, Varun Kumar, Chief Engineer (Flood Control and Drainage) at Bihar Water Resources Department in Birpur says, “Siltation is an issue and a lot of sediment is getting accumulated in Kosi due to which the riverbed is getting elevated and the discharge level of the barrage has reduced by 1-1.5 metres. The government is trying to deal with the problem of siltation.” He adds, “Desiltation (silt removal) is a complex process. Disposal of silt is another problem. But we are considering all the possible measures.”

When asked to explain why the water crossed over the barrage at a maximum flow of 6.15 lakh cusecs on September 29, he adds, “The barrage was designed to hold a capacity of 9.5 lakh cusecs, so that was its maximum or 100 percent capacity. No structure works to its full potential. Assuming it will work at 75 percent capacity, the maximum flow from Kosi Barrage can be between 7.0 and 7.20 lakh cusecs. In such a situation, water overflowing at a discharge of 6.15 lakh cusecs is definitely a matter of concern.”

After this year’s Kosi floods, efforts to address silt accumulation have intensified at the state and central levels. A Pune-based institute is researching barrages. Options such as cutting the silt by operating the gate are also being considered. Experts say that silt has different levels of thickness in different parts of Kosi. These are estimated to be four to eight feet or more. For now, the methods to deal with the increasing risk of floods due to silt accumulation, are being explored. These include strengthening the embankment and making arrangements to ensure that there is no seepage in it.

Speaking to Mongabay India, Nishant Kumar, Supaul’s Additional District Magistrate (Disaster Management) states, “There has been some change in the flow of Kosi which has developed more curves in the path or made it C-shaped. That’s why this time, the embankment broke, and such a disaster occurred.” He adds, “This river brings a lot of silt with it. Also, this time, there was a lot of rain in the catchment areas of Kosi, Bagmati, Gandak, and Mahananda. Hence, the damage was large-scale. About 10 feet of silt can accumulate in Kosi. The challenge is where to dump it. There is no demand for silt. But we are working on it and a team will take stock of the situation soon.”

According to a report by the International Center for Integrated Mountain Development (ICIMOD) on the problem of siltation in the Kosi, before flowing into the alluvial parts of Nepal and India, the Kosi crosses the world’s highest peaks like Mount Everest and the Kangchenjunga and passes through the eastern-central Himalayas. It is a highly mobile and sedimentary river. Therefore, managing its sediment or silt is difficult. The report links the high sediment concentrations to downstream river hazards such as flooding and channel shifting.

The study highlights a conservative estimate and suggests that over the last four to five decades, 408 and 1,080 million cubic metres of sediment may have accumulated within the embankments between Chatra (Nepal) and Birpur and from Birpur to the Batlara stretch, respectively. This has increased in the recent years due to several interventions by the water resources department, including barrages at Chatra and Birpur and embankments on both sides of the river, which were completed around 1965.

Professor Bindhy Wasini Pandey, the director of the Center of Himalayan Studies, Delhi University, told Mongabay India, “The Himalayas are divided in three parts — the Great Himalayas where there is not much rainfall and snowfall and there is no erosion (fragmentation of rocks), the Central Himalayas which receive the maximum rainfall, but erosion is comparatively less, and the third is the Shivalik range, which is the youngest mountain in the world. It is called the Baby of the Mountains. Its rocks have not yet hardened, and erosion occurs because water falls there.” He adds, “The rocks dissolve in water and flow down, after which landslides also occur there. One bank of the river causes erosion while the other bank also causes deposition, and both banks are at opposite ends. The problem of silt and sediment in Kosi is related to this.”

Silt raises the river bed

In February 2023, a committee comprising Nepal’s river expert Ajay Dixit, Medha Patkar, a leader of the Narmada movement, Professor Rajiv Sinha of IIT Kanpur, and several prominent river and environmental experts released a report, presenting facts and views on the entire situation in Koshi. This report states that Kosi originates from the highest peaks of the Himalayas and flows through weakly layered rocks of Shivalik, which is the main reason for siltation. The report notes that about 94 percent of the sediment comes in during the monsoon months—from mid-June to mid-October. According to the recordings registered from 1948-81, 19 percent of the weight consists of coarse sediments, 25 percent of medium sediments, and 56 percent of fine sediments.

The report also finds that a stroll on the eastern embankment of the Kosi river clearly shows that the riverbed or belly has become higher than the embankment and the situation is similar in upstream Nepal. The commission has suggested adopting a multi-pronged policy to deal with the challenges related to Kosi River.

Dixit shares in a conversation with Mongabay India, “Heavy rains caused widespread damage in Nepal and India this year. Kosi always brings with it a large amount of sediment, but human interference increased during British time. Measures were taken to control it, and embankments and other structures were built, but this is not a permanent solution. He says, “Kosi receives 100 to 120 million cubic metres of silt yearly. We must remember that this is silt and not sand that can be used in construction activities.”

Rahul Yaduka, from Ambedkar University, Delhi, who has conducted research on the Kosi river says, “It is the nature of the Kosi river to bring silt and change direction. This is being termed a problem. The plains of North Bihar are made of this silt. Till the 1970s, people would expect this silt to flow from the river.” Experts say that long-term and multi-dimensional initiatives are necessary, not immediate solutions.

This story was reported by Mongabay’s Hindi team and first published on our Hindi site on October 24, 2024.

Banner image: Silt accumulated in Mahuli, a tributary of river Kosi. This has contributed to the flooding. Image by Rahul Singh for Mongabay.

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Aditi Tandon Senior Production Editor

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