Kosi Could   Abandon its Course For Ever

We might  be very close to  the days  when the swollen Kosi  would permanently  abandon  its  present  course      and  it would be freely roaming  over  its vast eastern floodplain.  The damages,  that  resulted  from   the  2008 bursting of the  eastern embankment, would pale in comparison  with the great losses of life and property consequent  upon  the coming catastrophic  Kosi flooding.  Even the distant  West Bengal  and Bangladesh lying on  the  old  courses  of the Kosi might not be completely out of the Kosi flood danger. 

Lesson  from  China’s  Bitter  Experience

We  should draw  lesson  from  the  terrifying  experience  of catastrophes   when the Yellow river of the China known as the Sorrow of China just like the way the Kosi  is known  as the Sorrow of Bihar used  to burst  periodically its embankments in course of  high floods.   The Yellow river  like Kosi  now  flowing  well above the surrounding lands   burst  its embankments in  1931 and 1887    that resulted  in  loss of life  between  2 to 4 million and  1 to 2    millions respectively.    

River  Bed  Rise

Kosi   rising in the  Himalayas and emptying into  the   plain  at  Chatra  after  traversing   the  mountain   region, brings with  her  every  year   an  enormous  quantity  of  sediments estimated  at  120  million  cubic  meters  along  with  52  billion    cubic  meters  of  water.  The  annual  volume  of   Kosi   sediment  is  about   5  times  greater  than   that  of  the Sutlej  at  Bhakra, and two times   greater  than   that   of    the  Nile  at  Aswan.

Various  studies done  on  Kosi  so far  come  to  one  common  conclusion - the  Kosi carries  enormous  load  of  sediments  that  it is unable to  transport   far  beyond.  The river, therefore, deposits  huge  quantity of  sediments.  In  this  process  of  building  up  an  inland   delta,  the  river  shifted  over  120  km  from  east  to  west  during  the period   from  1736  to 1954.

In 1966 a study of the  delta cone of the  Kosi river made by Prof Gole CV Dr. Chitale  M.  appeared   in  the  ":Proceedings  of the  American  Society of  Civil  Engineers". The  study explains that  the delta building of  Kosi  was still  incomplete. The authors  have cautioned that  the  existing  measures  (embankments) may  be short  lived.

 The studies carried out by Sanyal (1980)  and  Gohain & Prakash  show that in the period between 1963 and 1974  there was high aggradations  upstream  of  the barrage.  The area just downstream of the barrage  had been marked  by a small degradation.  But further downstream, there had also been  increased  aggradations.

Alarming Reduction in Kosi Barrage Capacity

The Kosi barrage had been designed for a maximum flood discharge of 9.5 lakh cusecs. The maximum flood water level was 25 feet above the bed level. Similarly, the normal pond level was 15 feet above the bed level.  According to Gohain & Prakash  the annual rate of the sedimentation upstream barrage based on observations as of the early 1980s  is about 5 cm per annum. It implies that  within the last 60 years after the commissioning of the barrage a layer of 3 meters  thick sediments might have already deposited on the river bed. Now the maximum flood water depth might be only 60% of the original depth.

We can calculate the present maximum flood  discharge capacity of the Kosi  barrage  assuming that the maximum water depth would be only 60% of the original depth.  It can be roughly calculated  based on the formulas of  French Engineer Antoine Chezi  and Irrish Engineer Robert Manning.  We will find  that at present the barrage  might  be able to safely discharge a flow only up to 5.0  lakh cusecs.  Gohain & Prakash have reported  in early 1980s “If the discharge exceeds 8400 cumecs ( or  3  lakh cusecs), which is fairly common, the whole area between  the two embankments is submerged.”

Sedimentation Problems Will Worsen

Within the last  60 years twice the flood discharges  up  to 9 lakh cusecs had been recorded.  But after 1968 the maximum Kosi discharge never crossed the 6 lakh cusecs limit.  It is now an accepted  scientific conclusion  that  the high flood discharges resulting from heavy rainfalls are the primary cause of  increased sediment flow. The following is an excerpt from the Karnali project report prepared under the World Bank’s assistance.

“The sediment load from mountainous areas, such as the Himalayas and the Karakoram, is believed  to be derived  extensively  from localized mass wasting (landslides ) rather than  from generalized surface erosion (Carson 1985). Sediment yield should  thus be better correlated  to precipitation or runoff events which are two  of the main factors causing landslides and gully erosion.”

It is seen that within the last 45 years the Kosi flood discharges were far below the maximum  discharges already recorded twice within the last 60  years.  As a result,  we might not be wrong to  conclude that within the last  45 years  the Kosi barrage project might have been spared  to face serious sedimentation  problems.  However, there is a great  probability that  in very near future  the situation could  take an alarming  turn. 

What Comes Next?

In  2008  the Kosi had  burst the eastern embankment  and shifted its course heading to south- east   when the flood discharge was only about one lack and  sixty thousand cusecs.  This flood discharge  is quite insignificant  compared to  about  9 lakh  cusecs. flood discharges, which  have already been  recorded twice within the last  sixty years.  Despite the fact that the Kosi has changed its course at such a low flood discharge,  Bihar suffered  very heavy  losses. It is frightening even to think of the horrors if   the  Kosi would suddenly  burst the embankment at a time when the flood discharge is close to nine lack cusecs. There would be a total devastation of almost the whole north-east Bihar and  a  large area  of  Nepal’s  Sunsari   district  if  we  failed  to take in  time adequate preventive  measures.