Floods in Bihar

A recent fact finding report for the Kosi floods of 2008, prepared by a civil society organization by various experts like Dr. Sudhir Sharma, Dr. Dinesh Kumar Mishra, and Dr Gopal Krishna, highlighted that although India has built over 3000 km of embankments in Bihar over the last few decades, the flooding propensity has increased by 2.5 times during the same time period, not to mention that embankments failed during each major flooding event.

The Fact Finding Mission recently released a report titled Kosi Deluge: The Worst is Still to Come, in which it stressed that embankments straitjacket the river. In the case of the Kosi, it found that because of siltation the river bed was in fact several feet higher than the adjoining land. The high and low lands separated by embankments have created a situation where the low lands have become permanently waterlogged. Sixteen per cent of the land mass of north Bihar is subject to permanent waterlogging.

In 1954, when the Bihar flood policy was first introduced, Bihar had approximately 160 km of embankments. At this time, the flood-prone area in the state was estimated to be 2.5 million hectares. Upon the completion of the system of embankments, 3,465 km of embankments had been constructed and were administered by the Water Resources Department (WRD). However, the amount of flood-prone land increased to 6.89 million hectares by 2004.

Deforestation in the catchment area has led to increase in the silt content of the river flow. The total catchment area of the Kosi is 74,030 km², excluding the catchment areas of its two important tributaries, the Kamla (7,232 km²) and the Bagmati (14,384 km²). These tributaries of the Kosi are important in themselves and are generally dealt with separately. Out of the total catchment of the Kosi, only 11,410 km² are located in India and the rest (62,620 km²) lie in Nepal and Tibet. The river's catchment area at Triveni in Nepal is 59,550 km². The average rainfall in the upper catchment of the Kosi is 1,589 mm while in the lower areas it is 1,323 mm. The average annual silt load of the river is 92,400 acre feet (114,000,000 m³).

The period has caused serious interception in the dynamic equilibrium of the river hindering the natural oscillation of the river within its meandering belt. The meandering belt of Ganges in Malda and Murshidabad is 10 km wide. The water level of the Ganges rose about 8 m upstream of the Farakka barrage. The river which flowed in a South Easterly course between Rajmahal and Farakka during early decades of this century has now formed a mighty meander loop concentration to accommodate the additional discharge accumulated due to the barrage. Due to the obstruction caused by the Barrage each year nearly 640 million tonnes of silt is accumulated in the riverbed.[18] In the last three decades this has resulted in the accumulation of nearly 18.56 billion tonnes of silt.

Farakka barrage has led to following problems upstream of the barrage:

• Interception of the flow channel/ changed from straight to oblique
• Sedimentation (640 x106 metric tonnes/ year)
• Reduction of the cross-sectional area
• Declining slope of the long profile
• Widening of the river and increasing length
• Increase in flood frequency and magnitude

A Central Water Commission (CWC) report on Bihar floods in 2016 said that Farakka Barrage, even in the worst scenario can impact areas only up to about 42 km upstream, due to back water effect.[19] Patna is located about 400 km on the Ganga's upstream. The report blamed heavy banana plantation on the river bank between Patna and Bhagalpur as one of the reasons for the floods, based on the assessment of 100 years of floods in the Ganga. CWC report stated that the sedimentation in Ganga in Bihar is basically due to huge sediment load contributed from its northern tributaries- Ghaghra, Gandak and Kosi. The flood affected area in Bihar was 25 lakh hectares in 1954 when the length of all embankments in Bihar was 160 km, but the flood affected area increased to 72.95 lakh hectares in 2016 with the construction of 3731 km of embankments.[20]

North Bihar is a playfield of eight major rivers. All these rivers end up in Ganges.

• Ghaghra
• Gandak
• Budhi Gandak
• Bagmati
• Kamala
• Bhutahi Balan
• Kosi
• Mahananda