August 13, 1955 THE ECONOMIC WEEKLY Are Rivers in the Siliguri-Amingaon Section of N E Railway a Challenge to Engineers? Kumud Bhushan Ray NORTH-EAST India's ordeal by area of the river basins in Bihar, basins is not always restricted to water begins with the beginning Bengal and Assam, lying between the any one loop channel. Sometimes of the floods in July. The Assam foot-hills of the Himalayas and the the water flows through one chan­ Rail Link has again been snapped outfalls of the rivers into the Ganga nel and sometimes through another; in more place? than one. (Fig. 1). and Brahmaputra. Watershed ma­ the flow through these several loop The Assam Access Road has also nagement and afforestation in such channels is seldom kept under obser­ suffered a .similar fate (Fig. 2). desolate and sparsely inhabited re­ vation. It may happen that the How As flood damage has become an gions, would encounter colossal diffi­ through one channel goes on im­ annual phenomenon, the public ap­ culties and involve enormous costs. proving and consequently the flow through another, which was so long pears to be under the impression that Breaches and Loop Channels engineers do not fully realise the carrying the main discharge of the It may now be studied why breach­ river basin, goes on diminishing. great force of a river, which they es occur in the Assam Rail Link. try to harness. It is widely believed When the improving loop channel Fig. 2 shows in greater detail the becomes the main river of the river that deforestation in the upper Torsa and Kaljani Rivers, which catchments of the rivers in the Hima­ basin, the bridge over it may not cross the Assam Rail Link and the have sufficient waterway to pass layas is the cause of flood and that Assam Access Road. It will be seen the Governments of Tibet, Nepal, the total discharge of the river basin. that the Torsa River has developed, There is then damage to the bridge, Sikkirn and Bhutan, where these by erosion of banks, in its upper catchments lie, should be persuaded or the approach embankments are reach a channel about half a mile overtopped and breached. to take up and maintain watershed wide. From the point where it management and afforestation works emerges from the foot-hills of the The usual procedure, while re­ and only if this is done, India can Himalayas in Bhutan on to the be spared this annual ordeal. building the breached bridge, or plains, it has many loop channels— making up the breached embank­ The Hood damages this year in Mujnai, Burhi Torsa, Chhota Torsa, ments, is to provide additional spans Uttar Pradesh from rivers, whose Burha or Sili Torsa, etc. The Mujnai, to the bridge. But it is found that catchment area lies in the alluvial Burhi Torsa and a few smaller loop this does not provide a permanent plains, show that the deforestation channels have their outfalls into the safeguard. In course of time, an­ in the upper catchments of the river Jaldhaka, while the Chhota Torsa, other loop channel starts improving, cannot be the real cause of the flood. Burha Torsa and some smaller ones and ultimately the whole discharge Fig. 1 will show that the area of have their outfalls back into the of the River Basin may pass through the catchment basins in the Hima­ Torsa. The Kaljani has also many it in which case similar flood dama­ layas, lying in Tibet, Nepal, Sikkim loop channels. ges will occur. Considering that all and Bhutan, is more than twice the The flow of water in these river the rivers crossing this section of 970 THE ECONOMIC WEEKLY August 13, 1955 As the Manas Loop Channel could and breached. Successive bridges not accommodate and drain away with larger waterways were then the discharge of the Manas Basin, provided over the Beki Loop Chan­ there was backing up and rise of nel, from 1909 to 1945 ; water level in the channel upstream, (i) 4 x12' (ii) 4 x20' (iii) 9 x 20' so that a part of the water found its (iv) 1x50', 1x30', 6x19' (v) 1 x 50', way through the shallow but 2 x 40', 6 x19' (vi) 2 x 40', 3 x100', straighter Beki Loop Channel to its (vii) 7 x150'. outfall at T (Fig. 1). A cataract As a result of increasing flow action started at T, which dug back through the Beki Loop, less and less and back along the Beki Loop T S water flowed through the Manas until it reached E, the railway cross­ Loop, so that in 1945, water flowed ing, and further upstream to A where through only 4 x 100' out of the a cut-off channel was established. 10 x 100' spans, while the river bed More and more water flowed through had risen 14 feet in the Manas Loop this cut-off channel in the Beki Loop, Channel. which went on. improving, while less and less water flowed through the In the Beki Loop Channel, there Manas Loop, which went on deterio­ was also erosion of river banks and rating. development of a tortuous channel C D E F G, lengthening its course As the Beki Loop gradually im­ and flattening its S W S while the proved, and its channel became widening and shallowing of its chan­ the N E Railway and the Assam deeper, the 4 x 12' span bridge with nel gradually decreased the HMD. Access Road have several loop chan­ shallow foundation was undermined With consequent decreasing velocity, nels, breaches are likely to go on occurring, in the process of improve­ ment of one loop channel and the consequent deterioration of another. Vagaries of Rivers The cause of these variations in a river channel may now be studied. Fig. 3 shows the several loop chan­ nels in the Manas Basin—Manas, Bholukadoba, New Hakua, Old Hakua, Beki, etc. At the time of railway construction in Assam in 1909, the main flow in the Manas River Basin was through the Manas Loop Channel. The bridges provided at the time over the three Loop Channels were: Manas 10 x 100' Bholookadoba 2 x 75' Beki or Mara (dead) Manas 4 x 12' Between the years 1901) and 1945, more and more water started flow­ ing through the Beki Loop Channel and consequently less and less water flowed through the Manas Loop Channel. It may now be examined why this happened. The Manas had a more or less straight channel between A and K in 1948, (See Fig. 3). Below K, due to erosion of river banks, a tortuous channel K L M N developed, which continued below the railway bridge. The channel also became wider and shallower and had islands and sand banks. The greater length of the tortuous channel flattened the Slope of Water Surface (S W S), while the wider and shallower channel de­ creased the Hydraulic Mean Depth (HMD). Thus the velocity of flow and consequently the discharging capacity of the Manas Loop Channel gradually decreased 971 August 13, 1955 THE ECONOMIC WEEKLY 972 THE ECONOMIC WEEKLY August 13, 1955 the discharging capacity of the Beki Channels exist not only in the Manas nel, which will improve and become Loop decreased, there was backing River Basin, but also elsewhere. the NEW KOSI. up and rise of water level in the Beki Mention has already been made The Yellow River Basin Loop Channel upstream, and water of Loop Channels in the Torsa and It has been mentioned that in the found its way through the Bholuka- Kaljani River Basins in Bengal. Manas River Basin, a former aban­ doba Loop Channel to its outfall at Similarly in Bihar, there are several doned eastern Loop Channel—the V (Fig 1). A cataract action from loop channels in the Kosi River Beki or the Mara (dead) Manas— V upstreamwards gradually improved Basin. Loop Channels also exist in became the New Manas, while the the Bholukadoba Loop, deepening its the Yellow River Basin in China. Old Manas Loop Channel gradually channel. Successive bridges with Can all these River Basins be classed deteriorated from 1909 to 1945. larger waterways had to be provided as undeveloped? in the Bholukadoba Loop Channel: Similarly in the Yellow River Basin The Kosi River Basin in China, a former abandoned loop (i) 2 x 75', (ii) 3 x 75', (iii) 1 x 250', The Kosi has several loop channels channel has at present become the 2 x 150'. (Fig 1). There is considerable ero­ main river. A careful record is kept After the breaches in the bridges sion of river banks in the alluvial of the many loop channels in the over the loop channels in the Manas plains of Bihar, resulting in the deve­ Yellow River Basin, as shown in River Basin, it was decided to stop lopment of a tortuous course and a Fig. 4. The water of the Yellow the How of water through the Beki wide and shallow channel. The velo­ River Basin (lowed through the loop Loop Channel, by the construction city and consequently the discharg­ channel A from 2278 B C to 602 of the Motaguri Bund at A (Fig. 3). ing capacity in such a channel gra­ B C, through B from 601 B C to The aim was that there would be no dually diminishes. There is backing 11 A D, C from 12 A D to 1048 A D, flow through the Beki Loop Channel, up and rise in water levels and over­ D from 1049 A D to 1194 A D, E so that the waters of the Manas flow away from the channel, so that from 1195 A D to 1494 A D, F from River Basin would have to river-side lands are submerged and 1495 A D to 1855 A D, G from flow through the Manas Loop there is flood damage.