river flooding; flooding from rivers overflowing
Technical term used in disaster prevention and hydrology; refers specifically to flooding caused by river water overflowing or breaching levees, as opposed to inland flooding (内水氾濫).
See also: 河川氾濫
The levee broke due to river flooding caused by the typhoon.
Please explain the difference between river flooding and inland flooding.
内水氾濫 refers to inland flooding caused by rainwater that cannot drain away, while 外水氾濫 is flooding from rivers overflowing their banks.
河川氾濫 is a broader term for river flooding; 外水氾濫 is a more technical term often used in contrast with 内水氾濫.
Compound of 外水 (gai-sui, 'outside water', i.e., water from rivers) and 氾濫 (hanran, 'overflow, flooding').