Nepal floods: 10 Thane pilgrims untraceable; desperate families cling to faint hopes

Nepal's recent floods have highlighted a risk that extends beyond its borders, particularly for India. The two countries share numerous rivers that originate in the Himalayas, but the impact of a flood in Nepal on India is more complex than simply water flowing downstream. Factors such as rainfall, river levels, embankments, drainage, and development on the Indian side play crucial roles in determining whether a flood becomes a disaster in India.

Nepal boasts over 6,000 rivers and streams, most of which drain into India, primarily the Ganges. Three major river systems - the Kosi, Gandaki, and Karnali - dominate the region, with a dozen smaller rivers also contributing to potential flooding. Seven to nine major rivers from Nepal flow into the Indian plains, leading to flooding in various Indian states. Bihar, in particular, is highly vulnerable, with three-quarters of the state officially classified as flood-prone, with the Kosi being the most notorious.

However, it's not just the volume of water that determines the severity of flooding in India. The conditions in India, including rainfall, river levels, embankments, and drainage, also significantly impact the disaster's scale. The 2008 Kosi disaster in Bihar serves as a stark example, as the river breached an embankment carrying far less water than its design capacity due to the embankment's poor maintenance. The failure was structural, not hydrological.

For ordinary monsoon floods, India typically has 12 hours to two days of warning depending on how quickly the water travels downstream. Flash floods from the Chure rivers can arrive within hours, posing an even greater threat. While India and Nepal share real-time rainfall and river-level data, there are limitations in real-time monitoring stations, a lack of a shared forecasting model, and limited routine sharing on embankment conditions and river-channel changes.

Ultimately, effective cooperation on information sharing and early-warning systems is crucial to mitigate the risks posed by these transboundary rivers.