History of Flood in Bihar
Bihar is India’s most flood-prone State, with 76 percent of the population, in the north Bihar living under the recurring threat of flood devastation. About 68800 sq Km out of total geographical area of 94163 sq Km comprising 73.06 percent is flood affected. 
The plains of Bihar, adjoining Nepal, are drained by a number of rivers that have their catchments in the steep and geologically nascent Himalayas. Kosi, Gandak,Burhi Gandak, Bagmati, Kamla Balan, Mahananda and Adhwara Group of rivers originates in Nepal, carry high discharge and very high sediment load and drops it down in the plains of Bihar. About 65% of catchments area of these rivers falls in Nepal/Tibet and only 35% of catchments area lies in Bihar. A review by Kale (1997) indicated that the plains of north Bihar have recorded the highest number of floods during the last 30years. In the years 1978, 1987, 1998, 2004 and 2007 Bihar witnessed high magnitudes of flood.  The total area affected by floods has also increased during these years. Flood of 2004 demonstrates the severity of flood problem when a vast area of 23490 Sq Km was badly affected by the floods of Bagmati, Kamla & Adhwara groups of rivers causing loss of about 800 human lives, even when Ganga, the master drain was flowing low.

Flood highlights during (1998-2012)

In the year 1998 maximum discharge in the first week of July in most of the rivers in North Bihar caused excessive pressure on the embankment along the rivers resulting in damages at several places. Embankments of Burhi Gandak, Bagmati, Adhwara and Kosi were partially damaged. Three hundred and eighty one persons died and public property worth rupees 9,284 lacs was damaged. There was crop damage of about rupees 36,696.68 lacs. 

In the year 1999 there was unexpected heavy rains in the month of October in the catchments in Nepal and flood level suddenly touched the 1987 HFL at JhanjharpurRailwayBridge in Kamla Balan river and the spurs in Kosi river experienced threat throughout the flood season. Crop of rupees 24,203.88 lacs, and public property of rupees 5409.99 lacs  were damaged.

In the year 2000 Kamla Balan and Bhutahi Balan catchments received heavy rainfall during first and last week of July resulting in unexpected rise of water level. In first week of August 2000 Eastern Kosi Afflux Bund was punctured.Twelve thousands three hundred and fifty one villages were affected and crop worth rupees 8303.70 lacs were damaged.  

In the year 2001 north Bihar was badly affected by flood due to heavy rain in Nepal portion of catchments of rivers. Western Kosi embankment, Bhutahi Balan right embankment, Bagmati left embankment and Burhi Gandak  left embankment  were partially damaged. Crop of rupees 26721.79 lacs and public property of rupees 18353.78 lacs were damaged.

During year 2002 North Bihar experienced serious flood and overtopping reported in Kamla Balan left embankment and Khiroi right embankment. Four hundred and eighty nine persons died. Crop damage of rupees 51149.61 lacs and public property damage of rupees 40892.19 lacs were reported.

In the year 2003 HFL at Bhagalpur surpassed the 1978 record of 34.18m and at Gandhighat, Patna the HFL surpassed the 1994 record of 50.27m in river Ganga and the  status of flood in other rivers except Ganga and Gandak remain normal.

In the year 2004 catchment area of North Bihar rivers received heavy rainfall in the first week of July itself which not only broke last three years flood record but also surpassed the 1987 flood. Flood level at Dubbadhar site on river Bagmati surpassed all time high flood level by about 1.18 m. Similarly Burhi Gandak river on 15.7.04 and Kamla Balan river on 10.7.04 touched all time high flood level. This itself speaks about the fury of flood in the year 2004. Many places in the embankments of north Bihar were breached, resulting in flood inundation in a vast area of North Bihar. Unprecedented flood in river Bagmati, Burhi Gandak, Kamla Balan and Bhutahi Balan and Adhwara group of rivers breached the embankments at many places and there was loss of life and property on a large scale. In river Kosi, situation by and large remain normal. There were altogether 53 number of breaches during 2004 flood season. Crop damage of rupees 52205.64 lacs, public property damage of rupees 103049.64 lacs and death of 885 persons were reported.

Flood situation during 2005 and 2006 remain normal but in the year 2007 the flood situation was serious in north Bihar due to heavy rainfall in catchments of alomost all rivers flood situation during 2007 was very serious in north Bihar. There were 28 breaches at different locations of the embankments during 2007 flood season. Heavy spell of rainfall (average 82.70mm) was observed in the begining of flood season. In Burhi Gandak and in Bagmati river basins there has been regular rainfall in July and August which kept the river water level continuously rising. Almost whole of north Bihar was badly affected and heavy losses of crops and public property occurred.

2008 :- an appreciable amount of rainfall was received on very first day of monsoon season i.e. 15th June ( 160mm at Chanpatia, 141 mm at Sikanderpur and 92.2 mm at Khagaria ). July was the wettest month having maximum rainy days followed by August-08.
There was an unprecedented flood due to breach near 12.9km of Eastern Kosi Afflux Embankment near Kussha village in Nepal on 18th August 2008 that took a shape of a catastrophe leading to miseries to lakhs of people in Sunsari and Saptari districts of Nepal and Supaul, Madhepura, Araria, Saharsa, Katihar and purnea districts of Bihar. River Kosi entirely changed its course from earlier one which was again tamed to its original course by Water Resources Department after a tremendous effort keeping in line with the advice of Kosi Breach Closure Advisory Committee ( KBCAT ).

2009 :- The rainfall was scanty in entire Bihar in the year 2009. The situation was so aggravated that Diasaster Management Department GoB declared 26 districts as draught hit. The first appreciable rain fall was recorded in late June-09 and early July-09. There were few isolated storms at few stations of some basin in September and October. Flood situation remained normal this year except few breaches such as Tilak Tajpur on right embankment of river Bagmati  under Runnisaidpur block of Sitamarhi district, Gobindpur site of Labha Choukia Paharpur embankment of Mahananda river and Sallehpur Tandespur site of Gandak river. The loss to life and property brought to minimum by undertaking rescue and relief measures

2012- Gandak remained in spate since the beginning of monsoon and kept on exerting pressure on both its embankments. The incessant pressure on Gandak right embankment, especially in Pipra-Piprasi reach was so enormous that round the clock vigil and protection work had become necessary. The problem was accentuated by eroded length of spur at Dhuniawapatti at 26.75 km of PP right embankment.

Heavy rain in the catchment of Burhi Gandak resulted in overbank flow in smaller rivers and rivulets causing some flash flood in West Champaran, where overtopping on railway track was reported at Sikta railway station.

Continuous high level of Ganga exerted pressure on some of the protecting structures along its left bank in Bhagalpur, Vaishali and Samastipur districts. Ismailpur Bindtoli Embankment on left bank of river Ganga between spur no. 5-6; situated downstream of Vikramshila setu in Bhagalpur district was severely threatened.  Incessant threat on the left bank of Ganga upstream of Vikramshila setu at Raghopur- Khairpur site kept the department on its toes. The dowel at left bank of river Ganga at Chandpur-Dhamaun-Rasalpur in Samastipur district had to be strengthened continuously to save it from breach/overtopping.