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River training, and not dredging mooted to tame Brahmaputra

By Ajit Patowary
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GUWAHATI, Oct 29 - Rather than dredging the bed of the Brahmaputra, the authorities should concentrate more on applying river training measures to tame this mighty river, which is the second highest sediment-load-carrying river of the world.

For, the dredged channel of the Brahmaputra will not sustain, it will be filled up in no time. The proposed express highways along both of its banks would also face threat of being overtopped due to the progressively elevating high flood levels (HFLs) caused by the mounting river bed. The everwidening braided river course will also erode away the express highways.

Therefore, proper and adequate management of its sediment holds the key to taming this river, said senior river engineer Uma Baruah.

Talking to this Correspondent, Baruah, a former Additional Chief Engineer of the State�s Water Resources Department, said its heavy sediment load has made the Brahmaputra to take a braided course. This is making the river widen its bed progressively. Within its 640-km-long stretch in Assam, the width of the river varies between 15 km and 22 km in most of its reaches.

However, due to the existence of the rocky hills on both its banks near Pandu, the Saraighat Bridge could be constructed with a span of 1.3 km only. A maximum discharge of 77,000 cumecs passes under this bridge without any sand shoal or sand bar formation within this constricted stretch. The rocky heads on the riverbanks are known as the �hard points.�

Similar existence of hard points has enabled construction of the 2.84-km-long Naranarayan Setu and the 3.015-km-long Kolia Bhomora Setu. In contrast, the much longer Bogibeel Bridge (4.5-km-long) is being constructed at a location where the river is about 20-km-wide. This has been made possible by guiding the discharge of the river between two artificial �hard points�� guide banks and guide bunds � in both upstream and downstream of the bridge.

�This shows that this river does not require a width of 15 km to 20 km and its discharge can be accommodated within a maximum width of 4.5 km. With these observations and understanding of the river, particularly its gigantic problem of sediment management in its entire length of 640 km in Assam, we may perhaps opt for river training measures,� said Baruah.

By training the river in a well-defined course, benefits like mitigation of floods and erosion could be availed. Because, channelising the discharge through a well-defined course, with limited width, will make the mono channel deeper as per required flow area and wetted perimeter.

This will also scour the riverbed and a degraded bed slope will maintain a uniform velocity and an obstruction-free discharge. Sooner or later, a near regime condition will prevail in the channel, ensuring control over the river�s tendency to surpass the HFLs progressively. After attaining this condition, the existing flood protection structures like the embankments on both the banks, will not require periodic raising and strengthening, barring nominal maintenance.

This will result in lowering of bed levels and degradation of bed slopes of the tributaries too, making the entire Brahmaputra Valley free from flooding in course of time.

There will be no bank erosion. Hence, there will be no requirement of bank protection work too. Only nominal maintenance work of the river training structures, including enhancement of permeable spurs/silting devices for land reclamation, will be required periodically.

On top of all the above benefits, creation of a maintenance-free large navigation channel with more than sufficient draft and reclamation of the lost land for an area of 6,400 square km{640km x (15- 5)km} will also be possible .

Additionally, stabilisation of the main stem will bring stability to the network of 100 plus tributaries and rivulets linked with the Brahmaputra, Baruah said.

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River training, and not dredging mooted to tame Brahmaputra

GUWAHATI, Oct 29 - Rather than dredging the bed of the Brahmaputra, the authorities should concentrate more on applying river training measures to tame this mighty river, which is the second highest sediment-load-carrying river of the world.

For, the dredged channel of the Brahmaputra will not sustain, it will be filled up in no time. The proposed express highways along both of its banks would also face threat of being overtopped due to the progressively elevating high flood levels (HFLs) caused by the mounting river bed. The everwidening braided river course will also erode away the express highways.

Therefore, proper and adequate management of its sediment holds the key to taming this river, said senior river engineer Uma Baruah.

Talking to this Correspondent, Baruah, a former Additional Chief Engineer of the State�s Water Resources Department, said its heavy sediment load has made the Brahmaputra to take a braided course. This is making the river widen its bed progressively. Within its 640-km-long stretch in Assam, the width of the river varies between 15 km and 22 km in most of its reaches.

However, due to the existence of the rocky hills on both its banks near Pandu, the Saraighat Bridge could be constructed with a span of 1.3 km only. A maximum discharge of 77,000 cumecs passes under this bridge without any sand shoal or sand bar formation within this constricted stretch. The rocky heads on the riverbanks are known as the �hard points.�

Similar existence of hard points has enabled construction of the 2.84-km-long Naranarayan Setu and the 3.015-km-long Kolia Bhomora Setu. In contrast, the much longer Bogibeel Bridge (4.5-km-long) is being constructed at a location where the river is about 20-km-wide. This has been made possible by guiding the discharge of the river between two artificial �hard points�� guide banks and guide bunds � in both upstream and downstream of the bridge.

�This shows that this river does not require a width of 15 km to 20 km and its discharge can be accommodated within a maximum width of 4.5 km. With these observations and understanding of the river, particularly its gigantic problem of sediment management in its entire length of 640 km in Assam, we may perhaps opt for river training measures,� said Baruah.

By training the river in a well-defined course, benefits like mitigation of floods and erosion could be availed. Because, channelising the discharge through a well-defined course, with limited width, will make the mono channel deeper as per required flow area and wetted perimeter.

This will also scour the riverbed and a degraded bed slope will maintain a uniform velocity and an obstruction-free discharge. Sooner or later, a near regime condition will prevail in the channel, ensuring control over the river�s tendency to surpass the HFLs progressively. After attaining this condition, the existing flood protection structures like the embankments on both the banks, will not require periodic raising and strengthening, barring nominal maintenance.

This will result in lowering of bed levels and degradation of bed slopes of the tributaries too, making the entire Brahmaputra Valley free from flooding in course of time.

There will be no bank erosion. Hence, there will be no requirement of bank protection work too. Only nominal maintenance work of the river training structures, including enhancement of permeable spurs/silting devices for land reclamation, will be required periodically.

On top of all the above benefits, creation of a maintenance-free large navigation channel with more than sufficient draft and reclamation of the lost land for an area of 6,400 square km{640km x (15- 5)km} will also be possible .

Additionally, stabilisation of the main stem will bring stability to the network of 100 plus tributaries and rivulets linked with the Brahmaputra, Baruah said.

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