There are several kinds of floods. These can usually be predicted with considerable accuracy, providing adequate warnings that result in saving lives and reducing loss of property.
Flash floods are a different matter.
They usually result from rapidly changing weather situations, such as the sudden development of an intense local storm over the drainage basin of a small stream or river. Rivers can rise way above flood stage in a matter of hours if not minutes. But not all flash floods are caused directly by heavy rain. Ice and log jams can suddenly let loose huge torrents of water. Natural or constructed dams can collapse due to earthquakes or mudslides.