The Kosi embankment and barrage had been designed for a maximum flood discharge of only 9.5 lakh cusecs. Within the last 50 years twice the flood discharge close to 9 lakh cusecs have been recorded.<br><EM><STRONG>-&nbsp;Dr. A.B. Thapa</STRONG></EM>

May 16, 2011, 5:45 p.m. Published in Magazine Issue: Vol.: 04 No.-22 May 13-2011 (Baisakh 30,2068)

 Governments of India and Nepal were completely taken by surprise when  year before last  the eastern embankment was  suddenly breached and the Kosi shifted its course to the east.   Despite  that horrifying  flood  disaster experience  both  the governments are not  as yet seen  taking  much interest  to find  satisfactory ways   to  protect the life and property of millions from the oncoming Kosi floods  feared  to strike again anytime soon . The year before last’s flood disaster is indeed the forewarning of a far greater catastrophe.

Hanging River

Kosi river held back between the embankments since the last  half a century and now flowing well above the adjoining lands has already turned into the world’s deadliest river.  World might be heading for the rerun of the worst  flood  disasters reminiscent of the past Yellow river disasters when each  time the flood  took  the life of a record number of people.   Already the peoples of Nepal and India are in grave danger.  Unfortunately  the governments of both countries are seen even until now unable  to get to the root of the  Kosi  flood problems.   Indian Government was even telling just few years back that the Kosi flood control problem does not exist.  During  the run up to the 1997 Indo-Nepal joint committee meeting on Kosi  several  letters  were received from the Govt.  of  India explaining our government that the Kosi flood control problems  have already been solved once for ever after the completion of the construction of the embankments

Analogy with Yellow River

The  Yellow River of China known as “the Sorrow of China” resembles  to a considerable  extent  the Kosi River similarly known as “the Sorrow of  Bihar”.  The killer Yellow River has already been tamed after the completion of the construction of several storage dams  and it has completely relinquished its aggressive character, whereas  the greatly feared  Kosi River held back  by side embankments and now flowing well above  the adjoining  surrounding  lands  remains a great danger to life and property of millions of our region.

Judging from  the angle of  similarity between these two rivers the way they roamed rampaging their vast floodplain  quite often completely changing their course,  it appears,  the world  might be very close to the deadliest catastrophe triggered by the failure  of the Kosi  embankments. Now the safety of the peoples living in the Kosi floodplain must be at the top  in priority.

World’s  Worst Natural Disasters

Among the world’s  top five worst natural disasters are the 1931  Yellow River flood in China, the 1887 Yellow River flood, the 1938 Yellow River flood,  the Bhola Cyclone of 1970 in East Pakistan (now Bangladesh)  and the 1556 Shaanxi earthquake in China.  The next five in order of severity are the 1839  Cyclone in Coringa India,  the 1642 Kaifeng flood  in Henan province China,  the 2004 Indian Ocean Earthquake and Tsunami in various nations, and  the 1138 Allepo Earthquake in Syria.

The 1931 Yellow River  flood  devastated  the China  in summer. It took the lives of  people numbering between  one to four millions.  It is the worst natural disaster  ever to have been recorded  of  any  time.   The 1887 Yellow River flood resulted  from a long period  of rains  which caused the river  water to rise above the man made dikes.  When that happened, it resulted  in the worst natural  disaster ever recorded prior to the 1931 flood.  It is believed that  between 900,000-2,000,000 people  had lost their lives in that  horrible disaster.  The 1938 Yellow River flood disaster claimed the lives of people numbering between 500,000 and 900,000.

Existing Kosi Project

The Kosi  embankment and barrage  had  been designed for a maximum flood discharge of  only 9.5 lakh cusecs.   Within the last 50 years twice the flood discharge close to 9 lakh cusecs have  been recorded.  Surprisingly, the maximum flood discharge recorded within the last 40 years was only around 5  lakh cusecs. As a result, it was not necessary to face up to the problems of the breach in  embankment.

The annual rate of the sedimentation upstream barrage based on the observations as of the early 1980s  is about 5 cm per annum. It implies that  within the last  50 years after the commissioning of the barrage a layer of  2.5  meters  thick sediments might have already deposited on the river bed.  At present the  capacity of the embankments to  safely  hold back the maximum flood  might  have  been  reduced to the extent as low as only about 5 lakh cusecs, and even such greatly diminished  capacity  is  decreasing  further year by year due to continued  bed level rises.

A New Cycle of Disasters

It  is pretty obvious  that in future the breaching of the Kosi embankments  even during   minor  floods would be a regular phenomenon and on such occasions there would be a rerun of the year  before last’s flood disaster episode.  If there would be a breach in embankment at the time of maximum flood discharge close on nine lakh cusecs., a discharge already recorded twice within the last fifty years,  there could be a total devastation of the north-east Bihar.  Most of the low lying areas would be quickly submerged way deep inside the water particularly if  the breach  coincides  with prolonged incessant rain.  Too many people would not have time to reach the safety of the high grounds.   No wonder in similar circumstances  the loss of life in China used to be in astronomical number!

Physical Process

When the Kosi bursts the embankment at extremely high river discharge several  very adverse hydraulic factors  would be brought into play.  All of them are compounded to result in  the most dreadful flood disaster.   In this process  the relegated old Kosi channels would be reactivated  one after another.

If there is a breach in embankment,  say  at a flood discharge of 9 lakh cusecs, perhaps the initial  flow channeled through the newly  reactivated  Kosi old channel  could even reach 18 lakh cusecs, which would be the combination of the base flood discharge and the flow from  the negative surge  moving  upstream. These hydraulic phenomena would result in very rapid current of the positive surge  heading downstream all along the fully reactivated  old channel expected at that time to be full  of water drained from the local catchment.

Sediments Play Havoc

Sediments previously heaped up on the river bed of the Kosi  would  be quickly  flushed down to the reactivated   channel by the fast running flood water gushing out from the original   course.  Soon the upper reach of  the newly  reactivated channel would be throttled with sediments.  Thereafter the bulk of the flood  water would be concentrated  on the next adjacent reactivated channel. This channel too would be choked with sediments. This process of reactivating the old channels   would be repeated again and again until the Kosi flood water  reaches  the far end of its floodplain unless the flood  quickly subsides. Thus the sediments would be playing  havoc.    Prevention of the breach in embankment must be at the top in priority.

Buffer Channel

At present the capacity of the present Kosi river channel to hold back big floods is greatly reduced.    It would not anymore be possible to confine always the river within the boundaries of the embankments  to prevent the rerun of the last  disaster.  However, the loss of life and property can be greatly reduced by  preventing the Kosi to roam freely to rampage  its vast floodplain.   For this purpose one of the relegated old Kosi channels must be developed as a buffer channel for the diversion of the high flood water in excess of the carrying capacity of the present Kosi course.  A  spillway must be provided   to discharge safely the  flood water across the embankment into the buffer channel,  which could also help to hold back the sediments already accumulated on the river bed.

Postponement of Kosi Dam Project

It would be a big mistake to opt to implement at the very beginning the mammoth  Kosi  dam project to resolve  the  rapidly worsening flood situation.   It will take a very long time to complete the detailed study and  construction of the optimum sized Kosi dam project, that might  have   a dam perhaps the highest in the world and a hydropower plant capable to generate about 16,000 MW  in accordance with further studies based on more correct recent data and  well tested evaluation procedures  recently applied in the Karnali project study

Recommended Method of Kosi Flood Control

The implementation of relatively  small  Sun-Kosi  Project and  the Tamar-1 Project  provided with adequate flood control storage  would suffice  to  resolve the present Kosi flood problems at least for a period of next 50 to 60 years.  There could   be a possibility to incorporate to a  limited extent the provision  for  the Arun river flood storage  also by raising  the present dam height  of the  Arun-3  project.

The useful life of the above proposed reservoirs could be prolonged to last for ever  by applying the technology widely  used in China to vent sediments as a density current.  Further  aggradations  of  the  Kosi river bed could also be prevented if it is found commercially feasible to transport in  big barges  the sediments drawn from the Kosi  river near Chatra for use in construction  by using the proposed Kosi canal waterway.

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