A single rainfall figure cannot reliably “completely submerge” Kathmandu. The result would depend on where the rain fell, how quickly it arrived, river levels, drainage capacity, soil absorption, and the height of each neighborhood. A citywide blanket of water would require an extreme, physically unusual event, not merely a very large daily rainfall total.
Why there’s no simple number
Kathmandu lies in a basin with substantial differences in elevation. Rainwater would collect first in low-lying areas, floodplains and neighborhoods beside the Bagmati and its tributaries, while higher ground could remain above water. Water would also drain out of the valley rather than remain as a uniform layer. Rainfall totals are therefore not equivalent to flood depth. For example, 100 millimetres of rain means 100 litres falling on each square metre, but only part of that becomes surface runoff. Some infiltrates into the ground, some is stored temporarily, and some enters streams and drainage channels. In heavily built-up areas, however, paved surfaces can send a large share of rainfall rapidly into already strained drains.
What recent floods show
Kathmandu Valley has experienced severe flooding from much smaller totals than would be needed to cover the entire city. In September 2024, some locations reportedly recorded as much as 323.5 millimetres in 24 hours, producing destructive floods and landslides, but not a complete submergence of Kathmandu.
Research on Kathmandu’s flood risk estimates that, under a 100-year flood scenario, areas inundated by more than one metre could expand from about 11.7 square kilometres to 23 square kilometres under projected future conditions. That describes serious flooding in vulnerable zones, not water covering the whole metropolitan area.
A useful way to think about it
If “submerge” means water reaching one metre across a particular low-lying district, the required rainfall might be on the order of several hundred millimetres in a short period, though the exact amount could be lower or higher depending on runoff and river flooding. If it means placing one metre of water over the entire Kathmandu Valley, a rough volume calculation would require many billions of cubic metres of water. Rainfall sufficient to produce that volume would be extraordinarily large, and much of it would run off through rivers or leave the basin rather than form a permanent citywide layer. The practical danger is not total submergence. It’s rapid, uneven flooding caused by intense rain, blocked or undersized drainage, river encroachment and uncontrolled urban growth. Recent assessments also find that three-day heavy- rainfall events in Nepal have become more intense and more than twice as likely, although the precise estimates carry substantial uncertainty.