Kosi's devastating floods
742 million tonnes of silt has been deposited in the Chhatra to Birpur stretch of the Kosi River in the last 54 years.
Kosi, the sorrow of Bihar, has remained an enigma – for experts ranging from engineers to policymakers – for the number of avulsions it has had till date causing massive and devastating floods each time. It was as recent as in 2008 when it breached the embankment in Kushaha (Nepal) and shifted 108 km eastwards.
The cause of the woes for those living along the banks of Kosi could be the 1,082 million tonnes of silt that has been deposited in the river in the last 54 years. This has come out in a study conducted by a professor of earth sciences at IIT Kanpur, Rajiv Sinha. "Our first order estimates suggest that the total mass of sediments accumulated between Chhatra and Birpur (two gauge stations of the Kosi in the upstream region) during the post-embankment period could be approximately 1,082 million tonnes. This translates into 408 million cubic metres in terms of volume and this may have accumulated at a rate of 5.33 cm per year."
This is the highest amount of silt deposition in any river of the Ganga Basin, Professor Sinha said from Patna after the formal release of the report there in the presence of all concerned ministers. The work of constructing embankments on both sides of the Kosi was completed in 1955-56 and the period after that is known as the post-embankment period. Sinha calculated these figures by sediment budgetting—averaging the decadal sediment data of the last 54 years.
There are three gauging stations available in the Kosi river stream—Chhatra to Birpur (upstream) and Birpur to Baltara (downstream). While Birpur and Baltara lie in Bihar, Chhatra falls in Nepal. The gauging stations have equipment installed to note and study height, discharge, chemical properties of the water and other variants.
From silt to floods
"The silt causes the level of the riverbed to rise. As a result, the natural longitudinal (straight) course of the river is disturbed. Therefore the river searches for a lateral path (left or right). As a result, it changes its course and breaches the embankments on the new path it has created. The breach of embankments causes floods," explains Sinha.
The study titled 'Scoping Study of Siltation in Kosi river of Bihar' also tried to find out the scale of siltation using satellite images. The Professor and his team studied the pictures for every year available from 1972 and found that the sediment deposition along the Chatra-Birpur stream was to the tune of 742 million tonnes (280 million cubic metres in terms of volume) and 1,590 million tonnes along the Birpur-Baltara stretch (600 cubic metres in terms of volume). "For policymaking, I would suggest that the data collected by us using satellite images must be used as it gives a complete year-on-year picture," says Sinha. It was the first such comprehensive study on the siltation of the Kosi riverbed conducted at the behest of the Bihar government supported by the UK Department of International Development's Action on Climate Today.
Hotspots of silt collection
His team also identified the hotspots of silt collection by dividing the entire river stretch into 37 reaches. Most of them lie between the Supual and Madhubani districts of Bihar. "We have classified the entire stretch between Chhatra (upstream) and Baltara (downstream) into five zones, according to the amount of silt deposition: (I) very low aggradation, (II) low aggradation, (III) moderate aggradation, (IV) high aggradation, and (V) very high aggradation. Zone IV comprises reaches 2, 12-13, 17, 19-21. Zone V, which comprises reaches 9-11, 14-16, and 18, has the worst siltation. Almost all of them are downstream of the barrage falling in the Supaul district. Further downstream, the reaches of 22-37, falling in the Saharsa district, are classified as low to very low aggradation zones and both the channel and bar areas have remained stable over the last 54 years," the report says.
The significance of identifying the hotspots lays in the fact that these areas deserve major attention in terms of dredging, Sinha emphasises.
Endorsing the idea of dredging
This study is significant as it also tries to settle the debate over dredging as a method of silt management. One of the most common refrains against dredging, a project that usually involves hundreds of crores, is that once the silt is extracted from the river, it is dumped on the river bank itself because engineers don't know what to do with it. Soon it goes back into the river stream, and thus, the money literally goes down the drain. This report, on the other hand, spells out various commercial uses of the silt that is accumulated.
The samples of Kosi silt collected from different regions were sent to the Central Glass and Ceramics Research Institute (CGCRI) based in Kolkata to study their chemical composition. "The Kosi silt consists of fine to very fine sand and is dominated by quartz and significant amounts of muscovite mica. Chemically, this is composed of 72-76 per cent of silica followed by aluminium oxide (10-11 per cent), iron oxide (3-4 per cent), potassium oxide, sodium oxide (3-4 per cent), calcium oxide, magnesium oxide (< 2 per cent) and minor amount of titanium oxide(<1 per cent)," the study says.
The study also highlights that the silt can be used as a backfill material during the construction of roads. Besides, various studies have been quoted to suggest that the dredged material can be substituted as a raw material for manufacturing fired bricks for their use in the construction of buildings. The dredged sediments can also be used as a replacement of the raw material used for manufacturing Portland Cement.
The report also speaks voluminously about the agricultural uses of dredged materials. "The fine dredged sediments can rejuvenate poor agricultural soils. The fine dredged material is used to supply organic content and nutrients to the deficient soils to increase their productivity. The fine materials also help hold water and promote the retention of soil moisture needed by the crops," says the report. Not only this, the silt can also be used for reclaiming the land disturbed and/or contaminated by industrial activities and thereafter, restoring it to its natural condition.
"The whole idea behind dedicating a large part of our study to sediment management is that if we send suggestions to governments to undertake dredging, the officials must not jump from their seats due to the poor knowledge of silt management and, instead, use it judiciously so that it can be a win-win situation for them," concludes Sinha.
(The views expressed are strictly personal)
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