COMMENTARY The 18 August 2008 Kosi river breach: an evaluation D. V. Reddy, D. Kumar, Dipankar Saha and M. K. Mandal In the Himalayan terrain, the Kosi river surface structures like East Patna fault, in the west and along 32 km in the east. is formed by the confluence of seven Begusarai fault, Monghyr-Saharsa ridge, To channelize the river flow in the down- smaller streams, viz. Indravati, Sun Kosi, Bhawanipur fault, Malda-Kishanganj stream side, 125 km long eastern em- Tama Kosi, Likhu Kosi, Dudh Kosi, Arun fault, Madhubani graben with sediment bankment and a 101 km long western Kosi and Tamar Kosi at different places thickness of about 6 km and the shallow embankment were constructed. The in China, Tibet and Nepal, before its entry Purnea depression underneath the vicinity building up of embankments on either into the Indian state of Bihar (Figure 1). of Kosi and north Bihar flood plains may side of the river confined the river chan- In India, the Kosi river merges with the result in uplift/subsidence of the surface, nel to some extent. The Kosi river has an mighty Ganges near Kursela village, causing changes in the river course1. Of average discharge of 55,000 cusec, Katihar District, Bihar. Kamla, Baghmati late, climate change is also being thought which increases 18 to 20 times during (Kareh) and Budhi Gandak are the major to be responsible for escalating fre- peak floods. The highest flood recorded tributaries of Kosi in India, besides many quency and intensity of floods. in recent history of the river is reported minor tributaries like Bhutahi Balan. The The Kosi river is responsible for many to be 850,000 cusec on 24 August 1954. river travels a distance of 729 km from floods in Bihar. Due to the devastation Therefore, the Kosi barrage has been its source to the confluence with the caused by the Kosi floods in 1954, the designed for a peak flood discharge of Ganga, and drains a total area of about Government of India (GOI) entered into ~950,000 cusec. However, several floods 69,300 km2. an agreement with Nepal under which a have also been recorded in this area dur- The Holocene Kosi river mega fan in barrage was built about 5 km inside the ing 1963, 1971, 1984, 1987, 1991, 1995, Bihar is ~180 km long and 150 km wide, Nepal territory at Bhimnagar and was in- apart from the present flood in 2008, and is believed to be one of the largest augurated in 1963–64. The barrage con- after construction of the barrage. river-built alluvial fans in the world1. struction was aimed at taming the river This alluvial cone preserved the evi- floods and using the water for irrigation The 18 August 2008 Kosi breach dences of lateral channel shifting of the purpose. A 1149 m length barrage was Kosi river in the form of many distinct constructed along with 46 gates (each On 18 August 2008, the eastern em- palaeochannels, suggesting a dynamic gate is 18.288 m × 6.44 m). Two canals bankment of Kosi was breached near Ku- river system. Through the first maps pre- (Eastern and Western Kosi canals) were saha village, Nepal, about 12 km upstream pared by British surveyor, C. C. English later constructed on either side of the of the barrage. At the time of breach the (1779), the Kosi course could be traced barrage with six gates for left under sluice river flow was below average, considering since AD 1731. These maps indicated that and four gates for right under sluice. The the same time-period in different years. in the 18th century Kosi flowed near design discharge of the Eastern Kosi The water inundated the adjoining vil- Purnea town and then gradually moved canal is 15,000 cusec. The gross command lages of Kusha and Haripur (Nepal) westward, shifting its course from east of area under the Eastern canal is 7.45 lakh immediately and reached Bhimnagar, Purnea to west of Supaul, i.e. a shift of hectares and the cultivable command Birpur (India) within a few hours. In about 120 km in about 275 years. area is 4.4 lakh hectares. The Western Birpur, flood water rose alarmingly to a The river cuts across the Himalayas canal is under various stages of comple- height of 5–6 ft within a short time after and the Shiwalik ranges carrying huge tion. Afflux bunds were made along 12 km the breach. Subsequently, the flood water quantities of sediment. High rainfall (mean 1451.8 mm/yr) leads to extensive soil erosion and landslides in its upper catch- ment. The silt load of the river is one of the highest in the Indian sub-continent, to the tune of 80 million tonnes/yr. On reaching the plains, high aggradations of river bed and sediment bed load offer enough resistance to the water, forcing the river to find alternate paths, resulting in lateral shift of the river course. Though shifting of a river is part of its natural evolution process, other factors inducing this phenomenon could be earth- quakes, landslides and neotectonic acti- vity1. The Kosi flood plains are constantly being influenced by the compressive tec- tonic regime resulting from the collision of the Indian and Eurasian plates. Be- Figure 1. Regional map showing the course of the Kosi river (old – pre-18 August sides, the presence of various active sub- 2008, and new) and areas affected by the 18 August 2008 floods. 1668 CURRENT SCIENCE, VOL. 95, NO. 12, 25 DECEMBER 2008 COMMENTARY spread over a great extent covering Now the dilemma is whether to plug ing monsoon. Groundwater development Supaul, Saharsa, Madhepura, Araria and the breach or leave it as such. The chances also helps in tackling waterlogging prob- Purnea districts, causing severe damage of plugging the more than 2 km breach lem in low-lying areas. A detailed ground- to life and property. In fact, after the 18 near Kusaha are bleak, till the water re- water investigation, including surface August breach, the Kosi river has alto- cedes enormously. Even if this is done, water and groundwater interaction may gether taken a new course, that of the chances of breaching at the same point or help assess the groundwater recharge 1930s, as reported by local residents near at any other are high as the river bed is a few potential in detail. Muraliganj. metres higher than the surrounding area. This area requires close interval (0.5 m) To get first-hand information, assess The advantage of allowing the newly topographic mapping, which would en- the present situation and suggest possible formed course is that it is a straight and able the flood management authorities to scientific studies/steps to be taken to shortened new river course with hydrau- quickly assess and identify probable areas mitigate future floods, we visited the lic gradient nearly double that of the ear- susceptible to inundation, and accord- affected area including the breach point lier course, hence having more sediment- ingly take necessary remedial measures. during the 12–16 September 2008. As carrying capacity. The newly formed In order to understand the problem mentioned by several workers earlier, course is likely to be more stable and from different angles, besides monitoring since the river is flowing from the high- hence flood risk can be reduced at least the surface hydrological data, structures altitude mountains, due to its high gradi- for a few tens of years. present beneath the flood plains, and the ent, it carries heavy bed load. Once it The new course necessitates rehabilita- tectonic movements should be measured reaches the plains its velocity reduces to tion of the population from the entire precisely with the help of sub-surface a great extent and the river is not able to stretch of the new course and more im- geophysical mapping. Identification and carry its load. Apart from that, the river portantly, the existing barrage and canal delineation of active structures are also in the downstream of the barrage till its system will become defunct. Further, a essential to understand the river shift. confluence with the Ganga (at Kursela) new embankment has to be constructed The past is the key to the future. The past is flowing about 250 km length in a along the new course. lateral movements of the Kosi should semi-circular way with a low hydraulic However, if we make a balance bet- also be reconstructed making use of OSL gradient (1 : 5000 m). Due to this, the ween the old and new courses and make dating to give an idea about any such river is not able to carry its suspended use of both of them, the flood risk and movement in future. load even after the barrage. The river expenditure may be minimized. Though deposits its load wherever possible and this area has good groundwater potential, 1. Aggrawal, R. P. and Bhoj, R., Int. J. Re- changes its course to lower levels. After because of availability of surface water, mote Sensing, 1992, 13, 1891–1901. the barrage construction, due to its chan- groundwater exploitation is minimum. nelized flow, the river started building up Waterlogging is a natural phenomenon in the stream bed, resulting in considerable the north Bihar plain because of high ACKNOWLEDGEMENTS. We acknowl- edge the keen interest taken and suggestions elevation difference between the stream rainfall and lesser topographic gradient. provided by the Director, NGRI, Hyderabad bed and the surrounding areas (in parti- The groundwater table in these areas is and Chairman, Central Ground Water Board, cular in the east).