Some floods are deliberate, from rice paddies to wartime defences
Most floods are disasters, but people also drown land on purpose. Farmers flood paddies to grow rice, Dutch engineers designate fields to take a swollen river's overflow, and armies have inundated plains to stop an enemy, from the siege of Leiden to Flanders in the First World War.
A flood is water spilling over land that is normally dry. The word descends from Old English flōd, shared across Germanic languages and related to flow and float. Natural floods come in several kinds: rivers overtopping or breaching their banks and levees, groundwater rising, coastal surges and sudden urban flash floods. Rain collecting on saturated flat ground produces an areal flood.
Rivers flood when flow outruns the channel, especially at bends. Large rivers with huge catchments usually rise slowly after prolonged rain, snowmelt, monsoons or tropical storms. Small, steep or normally dry channels can flood in minutes after intense thunderstorms or a dam failure. In desert arroyos the first water soaks into the sandy bed, so the front advances slowly while later water catches up. The very flat Red River Valley on the Minnesota, North Dakota and Manitoba border, an old glacial lakebed, suffers a hybrid form locals call overland flooding. Coasts flood when storm surges coincide with high tides and big waves, and estuaries when surges meet high river flow.
Human choices make things worse. Draining wetlands that once stored water and paving surfaces that absorb nothing send more runoff into channels than they can carry. Many people live near coasts and floodplains, and climate change is raising the risk of both coastal and river flooding. Floods damage property, can spread waterborne and mosquito-borne diseases, and may displace residents for years.
Intentional flooding is a form of hydraulic engineering. Besides paddy fields, river managers may divert floodwater onto less valuable land to protect more valuable areas, as with the Dutch overlaten. Military inundation creates obstacles. For it to count as controlled, planners must allow civilians time to evacuate, keep it reversible and limit ecological harm; uncontrolled examples include the Yser plain in the First World War and Walcheren in the Second.
Source: Flood
