Dr. Nitish Priyadarshi
The swollen Kosi River breaking its embankments and changing course has ravaged many villages on its way to join the river Ganges affecting over a million people in Bihar.
Kosi River flows from Nepal to India and is known as the ''Sorrow of Bihar'' for the havoc it wreaks in many districts of the eastern state each year. The Kosi River, called Koshi in Nepal, is a transboundary river between Nepal and India, and is one of the largest tributaries of the Ganga. The river, along with its tributaries, drains a total area of 69,300 sq. km up to its confluence with the Ganga in India.
The river basin is surrounded by the ridges separating it from the Brahmaputra in the north, the Gandaki in the west, the Mahananda in the east, and by the Ganga in the south. The river is joined by major tributaries, approximately 48 km north of the Indo-Nepal border, breaking into more than twelve distinct channels with shifting courses due to flooding.
India is second in the world after Bangladesh in deaths due to flooding, accounting for one fifth of global flooding deaths.
Every year during the monsoon season, the Bihar region appears in the headlines because of large scale flooding. In general, this is also the time to go through the annual ritual of accusing the peasants of Nepal and the nearby mountain regions for sending down floods in even-higher volumes.
The Kosi river fan located in the northern part of India (in northeast Bihar and eastern Mithila) is one of the largest alluvial cones built by any river in the world. This 180 km long and 150 km wide alluvial cone shows evidence of lateral channel shifting exceeding 120 km during the past 250 years through more than 12 distinct channels. The river, which used to flow near Purnea in the 18th century, now flows west of Saharsa in Bihar.
It is said that due to large scale of deforestation in the Himalayas the rivers bring high influx of water and sediments and create havoc in the Bihar fields just like Kosi river did today. Kosi area is devoid of vegetation. The nature and extent of vegetation have strong control on runoff characteristics of the river catchments. The vegetation generates greater infiltration, and consequently a lesser amount of runoff. in North Bihar except Champaran where forest is significant (10.92%) most districts including Saharsa, Purnea, Supol etc the forest cover is very thin.
Even the areas affected today have very thin forest cover. It is now essential to create more forest cover which I am sure will minimize the flood calamity.
The other important solution to prevent this flood is to build big dams in Bihar bordering Nepal. Seeing the area coming under seismic zones proper precaution must be taken in building the Dams.
The Dam built on the Kosi river in Nepal has now become old and ill maintained. So it has now become essential to build more bigger dam in Bihar compared to Nepal to prevent such calamity.
All the tributaries of Kosi river encircle Mt Everest from all sides and are fed by the world's highest glaciers. These melting of the glaciers contribute high volume of water to the Kosi river with rain water during monsoon. High sediment load with water flow multiply the menace. Every year Kosi contributes heavy sediment in Bihar plains. Owing to extensive soil erosion and landslides in its upper catchment by factors both natural and human, the silt yield of the Kosi is about 10 cubic yard /acre/yr, one of the highest in the world.
Several workers have reported neotectonic movements in north Bihar. On the basis of experimental result it was showed that subsidence in an area causes flooding in the downstream reaches of a braided stream. In case of meandering channel, subsidence results in flooding in downstream reaches at the axis of deformation because of cut offs. The major parts of north Bihar plains are actively subsiding. Further, the neotectonic activities in the Himalayan region have resulted in unstable slopes, loose and fragmented rocks and higher river gradients. All these effects contribute to high sediment load of these rivers and, as a result, the beds of these rivers are rising rapidly. The bed of Kosi in Bihar is now at a higher level than the flood plain. It seems therefore that rapid subsidence combined with very high sedimentation rates have resulted in reduction in carrying capacity of channels in downstream reaches which experience frequent and severe flooding.
Big dams are constructed for the storage of more water so that the stored water is available in non-monsoon months. If the stored water is not used in non-monsoon months, then during the high monsoon with intense bouts of rain water level in these reservoirs will definitely increase the possibility of destructive floods in the river basins. Kosi dam may have faced the same phenomenon as this year Kosi river catchment area in Himalayas received heavy rainfall building pressure on the Dam in Nepal. Most of the big dams in India and Nepal are old and due to lack of proper maintenance they favour such types of devastating floods. As in the case of Kosi river engineers have cautioned about the damage being created due to high influx of water in the dam.
Further, the mountain fed rivers have mixed contributions from snowmelt runoff and monsoonal rainfall. Even during lean periods (summer), runoff contribution due to snowmelt is quite significant. The cumulative effect of monsoon rainfall and snow melt runoff results in sudden increase in the river discharge mounting pressure on the ill maintained dams. The shallow alluvial channels between the narrow banks and embankments cannot effectively carry this sudden jump in discharge consequently resulting in breaches and spilling of banks.
Further, river Kosi and other rivers of Bihar are characterized by high sediment load, mainly wash load which causes rapid aggradation of the river bed. It was observed earlier that at many locations water level in the channel within the embankments is significantly higher than the general ground level in the surrounding areas. Deposition of sediments either within the channels or on the channel margin restricts the passage of water and thus reduces the carrying capacity of the channel. This again results in over spilling of water on the adjacent plains- Kosi being the best example.