Atmosphere And Weather

A Scientific Explanation of how Floods Occur

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Flood is defined as overwhelming of usually dry land by a large amount of water that comes from an overflowing river or lake, exceptionally high tide, melting snow or excessive rainfall [1]. Although this definition doesn't describe the mechanism of flood creation, it lists most major causes of flooding.

Floods can be caused by several hydrological mechanisms, which vary according to the type of flood [3]. From this point of view, floods can be divided into coastal and estuarine floods, riverine (fluvial) floods (these can include flash floods and slow-onset floods), groundwater floods and catastrophic floods (caused by human-made or natural disaster, e.g. an earthquake or a dam breakage).

The same weather event can cause different types of flood depending on the lay of the land and other geographical features. Floods are natural occurrences and in many cases the designation of a “flood” depends on the impact on the human settlements rather than actul severity of it. Many floods occur regularly, some with a great seasonality; especially river flooding, of which the most famous example is the flooding of the Nile which created foundations of the ancient Egyptian civilization.

Hydrologists recognize several distinct processes that can cause floods, but overall most flooding is caused by rivers, in connection with rainfall, snow and ice melt, landslides, ice blockages; or by sea, in connection with storm surges and tsunami as well as heavy storms including typhoons and cyclones [7].

Riverine floods are caused by the overflowing water bursting the banks of the river and spilling out on the floodplain [2]. This is usually caused by heavy rain and depends on how fast the rainwater accumulates in the river channel. Long-rain floods [5] result in the saturation of the catchment area and gradual filling up of the river channel. The rains usually cover large area and are not only long-lasting but high in intensity. Short-rain floods [5] from very high intensity and relatively short rainfall can cause large surface run off and fast flow under surface. The land is usually quite wet before such floods occur. Flash-floods [2,5] can occur even if the land is dry. They are caused by short but extremely high intensity rain-falls (usually from convective storms), and most of the flow occurs on the surface.

Certain factors tend to exacerbate the magnitude of the floods [2]. If the banks of the river are steep, the runoff is faster, and the flooding is more likely. Lack of vegetation will also increase the probability of flooding, as plants (especially trees) intercept water. A drainage basin i.e. the area of land that the river gathers its water from) that is mostly impermeable rock (or artificially constructed concrete, as in human cities) will also encourage water to flow quickly over the surface rather than seep through the ground and thus will cause fast surface run off, and consequently overflowing of the river channel and flooding.

In addition to rainfall, other mechanisms may be involved in flood creation. Snow-melt floods and rain-on-snow floods [5] occur where there is snow cover on the ground. Snow-melt floods are typically spring floods, and are especially common in mountain areas where the winter snowfall is high and where snow accumulates and remains on the ground for a long period. If the melting is fast (due to sudden onset of high temperatures), the water runs off quickly from the higher altitudes, saturates the ground and, often as it reaches lower ground, bursts the banks. Rain-on-snow floods are significantly more serious that pure snow-melt floods, and can cause enormous run-off when rain falling on existing snow causes intense melting that can saturate large areas and cause widespread floods when compounded by rainfall.

Coastal and estuarine floods are caused by markedly different mechanisms. The rainfall is of lower significance (though can contribute to the compounded flood levels), and the floods are caused by the sea encroaching on the areas of the land not normally covered by tides. Coastal floods can be a result of one or a combination of the following [6]: storm surge and/or seiche reaching land, heavy surf and tidal piling. A storm surge is a dome /bulge of water, over and above normal surf or tidal action, caused by a combination of wind and pressure conditions. A seiche is a similar even on a lake. Storm surges are caused by strong coastal storms and occur when low air pressure allows the sea water to raise above its normal level (by as much as 2 feet). Thus created dome or bulge influences the wind and can cause significant flooding as well as wave damage exerted by the sheer force of the surf. The surge flood can be significantly compounded by high tides, as well as the geography of the coast.

Climate change, if warming, is likely to increase the incidence of extreme weather events in general and flooding in particular [4], especially the floods caused by heavy rainfall. But floods have been a part of human history from time immemorial, and are among the most catastrophic events that humanity knows [7]. Increased population in coastal regions and on river floodplains means that a large proportion of humanity now live in the areas in danger of flooding. It is of utmost importance to understand how floods occur and how people can predict them and protect themselves from flood damage.

[1] Clark, A. The Penguin Dictionary of Geography (2003). Penguin books, London.

[2] River flooding and management issues. BBC Bitesize. Retrieved on 16 Feb 2011 from

[3] Types of Flooding. A guide by the UK Environmental Law Association. Retrieved on 16 Feb 2011 from

[4] Kundzewicz, Zbigniew W. (2006) Climate change and floods. Retrieved on 16 Feb 2011 from

[5] Gunter Bloschl, Dan Rosbjerg, Stewart Franks, and Michio Kumagai (2003). Water Resources Systems - Hydrological Risk, Management and Development (IAHS Proceedings & Reports). P51-52. Retrieved on 16 Feb 2011 via GoogleBooks.

[6]  Coastal Flooding. A factsheet.

[7] Doe, R (2006). Extreme floods. A history in a changing climate. Sutton Publishing, Stroud.

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