A flood is an overflow of water which temporarily covers land not normally covered by water. Floods occur whenever there is too much rainfall, meltwater, storm surge, or other release of upstream water for the rivers to handle and the ground to absorb within that period of time. A series of smaller storms may cause more flooding than a single heavy storm, because the ground stays saturated throughout, leaving nowhere for the water to go except into the river or other drainage system. For practical purposes, flooding only matters if the overflow affects areas used by people.
Flooding of rivers and streams occurs whenever water is added to the river system faster than the river can move that water safely downstream. The river's capacity over time is a product of the size of the river channel and the speed at which water is moving downstream.
Riverine floods can be slow or fast. Both kinds of riverine floods recede shortly after the exceptional condition no longer exists.
Slow riverine floods are usually caused by sustained heavy rain or upstream melting of a large snowpack. In the spring, many rivers which receive meltwater regularly break their banks and spill into the surrounding floodplain.
Fast riverine floods, or flash floods, are usually caused by thunderstorms which produce a lot of rain over a very short period of time. If the river is not able to move the water downstream fast enough, it will flood. A special type of riverine flood is the arroyo flood, where heavy rain, usually from thunderstorms, quickly fills a dry riverbed.
Riverine floods can also be caused by a sudden upstream release of water. This may be caused by a controlled release of water from a dam upstream to avert worse flooding later. An uncontrolled released of water may be caused by a rockslide, a jokulhlaup from a melting glacier, or when a dam fails. If a river is reinforced by a levee or dike, flooding occurs whenever the new inflow of water overflows or breaks the barrier.
Estuarine flooding occurs whenever a storm surge adds more seawater to the estuary than it can hold. It is usually caused by a combination of high tide, high wind blowing into the estuary, and a strong storm coming in from the sea.
Estuaries which open onto shallows or have large plains and tidal flats of low-lying land around them are especially vulnerable to estuarine flooding. Many rivers which open onto estuaries already have tidal bores, which can become a flood when combined with storm and wind. New moon and full moon high tides are higher than usual, so they can increase the amount of estuarine flooding.
Coastal flooding may also occur as a result of large ocean waves crashing directly against low-lying shorelines, especially when the coastal area is already saturated with previous rainfall. Its effect is limited by the reach of the ocean waves.
Tsunamis are a special type of ocean wave which is caused by a strong offshore earthquake or by a large amount of land collapsing into the ocean. They are at their most dangerous when approaching shallow coastal areas, because the energy of the wave is forced upwards into a higher wave. Tsunamis can cause widespread coastal flooding, which can penetrate for miles inland over low-lying areas.
Paved areas do not absorb much water. Thus, large amounts of concrete and other paved areas increases the amount of runoff into the river system. As a result, flooding is more likely to occur during periods of heavy rain.
Localized urban flooding often occurs when heavy rain falls too fast for the storm drainage systems to handle. Storm drainage systems are more likely to be overwhelmed when the heavy rain comes in fall or winter, when storm drains may be blocked by fallen leaves or snow.
Wetlands act as natural flood control systems. The high-saturation soils of a wetland quickly absorb excess water during snowmelt and heavy rains and spread it out over a large but limited area. During dry periods, the same soils gradually release the stored water. In this way, wetlands help to keep downstream water stable.
A series of connected wetlands is even more effective. Because each wetland receives its water from the wetland above it, water does not reach the downstream wetland until the upstream wetland is fully saturated. This enables the connected wetlands to absorb and release a great deal more water than they could individually.