Context: Bihar received 601.1 mm rainfall between June 1 and September 8, 27% below normal, yet severe flooding affected 2,157 villages across 14 districts and ~40.51 lakh people by September 9.
- The episode highlights that flood severity depends not only on cumulative rainfall but also on upstream discharge, river dynamics, drainage constraints and localised intense rainfall.
Why Flooding Occurred Despite Rainfall Deficit
- Rainfall pattern: Although cumulative monsoon rainfall remained deficient, September rainfall was 31% above normal, with intense localised events including 214.92 mm in East Champaran and 154.55 mm in Sitamarhi.
- Ganga-driven flooding: Unlike the usual pattern where rivers entering Bihar from Nepal rise first, the Ganga became the primary concern, receiving substantial upstream flows from areas including Uttar Pradesh.
- Inter-State discharge: Increased discharge from neighbouring states contributed significantly to the rise of Bihar's rivers, making Nepal alone an inadequate explanation for the floods.
The ‘Backwater Effect’
- The swollen Ganga restricted the drainage of tributaries such as the Gandak and Punpun, causing water to back up into adjoining areas; the Punpun was also affected by heavy rainfall in Jharkhand.
- Flood peaks can move downstream from one river gauge to another, meaning flood-affected areas may be much larger than locations receiving the highest rainfall.
Why Nepal Was Not the Main Cause
- During Nepal's August 26 floods, Bihar opened all 36 gates of the Valmikinagar Barrage on the Gandak; discharge reached 1,50,200 cusecs at 11 pm but subsequently declined.
- The Gandak was initially below normal levels, while flooding continued even after its discharge decreased, indicating that Ganga levels and wider upstream flows were more important drivers.
Role of Embankments and Siltation
- Bihar has over 3,730 km of river embankments, which historically protect around 3,600 sq km of land, but embankments cannot eliminate vulnerability in Bihar's exceptionally flat and sediment-rich alluvial landscape.
- Rivers such as the Ganga, Gandak, Kosi and Bagmati carry heavy sediment loads; its accumulation raises riverbeds over time, reducing channel capacity and increasing flood risk.
Farakka Barrage Debate
- Bihar's political leadership argues that the Farakka Barrage has contributed to sediment accumulation and reduced the Ganga's carrying capacity, although the extent of its role remains a matter of debate.
- The 1996 India-Bangladesh Ganga Water Sharing Treaty, associated with Farakka, is due for review as it approaches its December 2026 expiry, intensifying demands from Bihar for reconsideration.
Structural Vulnerability of Bihar
- Bihar's flat terrain and constantly shifting channels of the Ganga, Gandak and Kosi naturally increase flood susceptibility.
- High population density along river corridors further magnifies human vulnerability, as rising water frequently necessitates large-scale evacuation.
