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What is flood?

Flood, which is one of the most common geological hazards, is a temporary covering of land by water, usually as a result of heavy rainfall and quick snow melting. The probability of flooding is more predictable than this of other types of geological hazards.

How do floods occur?

A flood is a temporary covering of land by water:
A) As a result of surface water - still or flowing - escaping from its normal confines.
B) As a result of heavy precipitation.

In the first case, flooding may be caused because of storms, landslides, earthquakes, tsunamis, breaking of natural or artificial dams, etc.

In the second case, the generating reasons of floods are different: excessive rainfall, snowfall and following snowmelt, etc.

It is the nature of floods to be generated by the random coincidence of several meteorological factors but man's use of the river catchment also has an impact upon the severity and the consequences of those events.

When and how often do floods occur?

All over Europe, floods are usually the result of heavy precipitation and sudden snow melting. In northern Europe floods may also be caused by icy dams arising when ice on frozen rivers melts and flows away, meeting eventually a natural or man-made obstacle, when an icy dam arises.

Floods are divided into two broad categories:

• Spring floods, which are caused by snow - melting and breaking of icy dams on rivers.
• Summer floods, which are caused by heavy and sudden summer rainfalls.

Floods are - to some extent - a natural phenomenon, occurring regularly in river basins. They have been an integral part of the human experience ever since the start of the agricultural revolution when people built the first permanent settlements on the great riverbanks of Asia and Africa. The famous floods of the Nile had been occurring every year making the Nile valley fertile, until the Assuan dam was constructed. In Europe, heavy floods occur almost every decade. For instance, the rising of water levels in the winter of 1994-1995 affected Italy, Germany, Belgium, France and the Netherlands. In the summer of 1997, the River Odra overflowed its banks, causing serious damage in Central Europe. Very recently, the summer of 2002, floods on the rivers Elbe and Vltava caused billions of Euros damages in Germany and Czech Republic. Meteorologists and hydrologists use the 5-year, 10-year, 100-year and 1000-year flood terms in order to point out which could be the expected lapse of time between the occurrences of that specific flood.

Most of the heavy floods which occurred in Central and Western Europe in the past decades were the result of artificial restraining flood waters by dikes and artificially regulated river beds and also by expanding the cities to the river flood plains.

What are the impacts of floods in our lives?

• Loss of human lives and damages on properties.
• Collapsing of the lifelines of cities.
• Contamination of drinking water resources.
• Damages on the communication lines.
• Loss of industrial, agricultural and forest productivity and tourist revenues because of the damage of the land or the damage of facilities or the interruption of the transportation system.
• Reduced real estate values in areas threatened by floods.
• Measures have to be taken to prevent or mitigate additional flood damage.
• Loss of human or animal productivity because of injury, death or psychological trauma.

How does science cope with floods?

• Prediction of areas susceptible to flooding using various techniques
• Construction of some engineering structures such as dams, flood walls and/or embankments and sediment traps etc
• Planning the engineering structures and the settlement areas at flood free or non-susceptible regions
• Set up early warning systems
• Forestation
• Improving hydraulic capacity of river channels
• Public education on floods

Flood forecasting and warning is a prerequisite for successful mitigation of flood damage. Flood forecasting systems have been advanced to use the latest developments in monitoring, communications and computing technology. They are mostly based on precipitation forecasts, and an appropriate modelling of riverbeds, valleys and water flows. Using radar and modelling, rainfall and its consequences can be predicted more and more accurately. Floods can be mitigated via combined improvements in forecasting and river basins management. In addition, the authorities must dispose over adequate information channels and provide reliable flood warnings to the citizens on time.

Selected RTD Projects to be presented in SHIELD's Exhibitions

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