Flood Types
Floods result from a combination of meteorological and hydrological extremes, however in many cases floods are additionally influenced by human factors. This article explains several types of floods, without discussing the human factors.
Content Table
Riverine Flood
When surface water runoff introduced into streams and rivers exceeds the capacity of the natural or constructed channels to accommodate the flow, water overflows the stream banks, spilling out into adjacent low lying areas.
- Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.
Rainfall over an extended period and an extended area can cause major rivers to overflow their banks. The water can cover enormous areas. Downstream areas may be affected, even when they didn’t receive much rain themselves.
- Fast kinds: include flash floods resulting from convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.
The area covered by water in a flash flood is relatively small compared to other types of floods. The amount of water that covers the land is usually not very large, but is so concentrated on a small area that it can rise very high. Because of the sudden onset and the high travelling speed of the water, flash floods can be very dangerous. The water can transport large objects like rocks, trees and cars.
Coastal Flood
Coastal floods occur when the coast is flooded by the sea, as result of ocean/sea waters being driven inland, most commonly by tropical storms, or less commonly as a result of tidal waves driven by earthquakes.
When a storm surge occurs, the water surface elevation above normal tide levels increases due to low barometric pressure. The low pressure inside a storm or hurricane’s eye creates suction like a straw, creating a dome of water near the center of the storm. In the deep ocean, this dome of water sinks and harmlessly flows away. But as a storm nears land, strong winds in the storm push this dome of water toward the shore, the rising sea floor blocks the water’s escape and it comes ahore as huge waves (see picture below). Storm surge causes sea levels to rise for a relatively short period of time (typically four to eight hours, though some areas may take much longer to recede to their pre-storm levels) – often resulting in extensive coastal flooding that can weaken or destroy coastal structures.
A large and increasing fraction of the world’s population lives along coastlines and significant numbers are already vulnerable to coastal flooding. This exposure is likely to increase as sea level rises with climate change. In some locations future changes in tropical storms and storm surges may further add to the risk of flooding.
Urban Flood
Flooding in urban areas can be caused by flash floods, or coastal floods, or river floods, but there is also a specific flood type that is called urban flooding. Urban flooding is specific in the fact that the cause is a lack of drainage in an urban area. As there is little open soil that can be used for water storage nearly all the precipitation needs to be transport to surface water or the sewage system. High intensity rainfall can cause flooding when the city sewage system and draining canals do not have the necessary capacity to drain away the amounts of rain that are falling.
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