- There are few places on Earth where people need not be concerned about flooding. Any place where rain falls is vulnerable, although rain is not the only impetus for flood.
- A flood occurs when water overflows or inundates land that’s normally dry. This can happen in a multitude of ways. Most common is when rivers or streams overflow their banks. Excessive rain, a ruptured dam or levee, rapid ice melting in the mountains, or even an unfortunately placed beaver dam can overwhelm a river and send it spreading over the adjacent land, called a floodplain. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.
- Most floods take hours or even days to develop, giving residents ample time to prepare or evacuate. Others generate quickly and with little warning. These flash floods can be extremely dangerous, instantly turning a babbling brook into a thundering wall of water and sweeping everything in its path downstream.
- Disaster experts classify floods according to their likelihood of occurring in a given time period.
- A hundred-year flood, for example, is an extremely large, destructive event that would theoretically be expected to happen only once every century. But this is a theoretical number. In reality, this classification means there is a one-percent chance that such a flood could happen in any given year. Over recent decades, possibly due to global climate change, hundred-year floods have been occurring worldwide with frightening regularity.
- Moving water has awesome destructive power. When a river overflows its banks or the sea drives inland, structures poorly equipped to withstand the water’s strength are no match.
- Bridges, houses, trees, and cars can be picked up and carried off. The erosive force of moving water can drag dirt from under a building’s foundation, causing it to crack and tumble.
News of flooding in countries
1) Two residents of Dresden, Germany, pedal through floodwaters from the Elbe River as water levels continue to rise. Cities in Germany and throughout Europe have been battling severe flooding after several days of heavy rainfall.
2) Causes and effects of flooding in Boscastle, UK (2004)
- Over 60 mm of rainfall (typically a month’s rainfall) fell in two hours.
- The ground was already saturated due to the previous two weeks of above average rainfall.
- The drainage basin has many steep slopes, and has areas of impermeableslate causing rapid surface run-off.
- Boscastle is at the confluence (where tributaries meet) of three rivers – Valency, Jordan, and Paradise. A large quantity of water all arrived within a short space of time causing the rivers to overflow.
- The flooding coincided with a high tide, making the impact worse.
- Homes, businesses and cars belonging to more than 1,000 people were swept away.
- Income from tourism was lost. This had an impact on livelihoods and the local economy.
- There were vast numbers of subsequent insurance claims.
- No lives were lost, partly due to the rapid response of the emergency services.
Causes of flooding
A flood occurs when a river bursts its banks and the water spills onto the floodplain. Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channel, the more likely it is to flood. The nature of the landscape around a river will influence how quickly rainwater reaches the channel.
The following factors may encourage flooding:
- A steep-sided channel – a river channel surrounded by steep slopes causes fastsurface run-off.
- A lack of vegetation or woodland – trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.
- A drainage basin, consisting of mainlyimpermeable rock – this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
- A drainage basin in an urban area – these consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.
Flood management techniques include river engineering, afforestation and planning controls to restrict urban development on floodplains.
The impact of flooding
- Floods can cause damage to homes and possessions as well as disruption to communications. However, flooding can also have positive impacts on an area.
- Flooding deposits fine silt (alluvium) onto the floodplain, making it very fertile and excellent for agriculture. People living on or near floodplains may rely upon regular flooding to help support their farming and therefore provide food.
- LEDCs tend to be affected more than MEDCs by the effects of flooding. This is partly because LEDCs have more farms, and farming communities are attracted to fertile flood plains. LEDCs often do not have the resources to prevent flooding or deal with the aftermath of flooding.