Floods are the most frequent type of natural disaster worldwide. They may occur inland or along the coast. Coastal floods, which involve the inflow of the tide, may be caused by a heavy storm, a high tide, a tsunami, or a combination thereof. Since many urban communities are located near the coast, floods are a major worldwide threat. Coastal areas are occasionaly flooded by high tides caused by severe winds on ocean surfaces, or by tsunami waves caused by undersea earthquakes.
Inland flooding occurs when heavy rain causes a river to overflow its banks. A flood that rises and falls rapidly with little or no advance warning is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area. Periodic floods occur naturally on many rivers, forming an area known as the flood plain.
When attempting to quantify how much rainfall is needed to produce a major flood, one has to account for the fact that the amount of rainfall transformed to runoff depends to a large degree on three factors: soil type, the underlying geology and existing soil moisture conditions controlled by the rainfall intensity of preceding storms.
An additional factor—and one often neglected even by hydrologists when estimating the risk of flood—is relative rainfall intensity compared to the average precipitation received by a river basin over a year. It is well known among geomorphologists that natural streams adjust their shape and conveyance to average annual precipitation.
Very roughly, this means that a river with a 1000 km2 upstream drainage area that receives, say, 2000 mm of rainfall a year will have twice the conveyance (and cross-sectional dimensions) of a reach with the same contributing area and slope, but receiving only 1000 mm of rainfall a year. This connection between the channel conveyance capacity and the precipitation regime is made more complex by the soil type and underlying geology. Roughly speaking, these two factors control how much of the rainfall goes directly into the river, potentially producing flood, and how much penetrates down to groundwater and later drains to the river network gradually and benignly over long periods of time.