What is flood

Flood is defined as a relatively high flow of water released from river and streambeds, which sets the riverbank margins to overflow and lead to the runoff of low land areas surrounding the river catchments.

Flood is also defined as the accumulation of excessive quantity of water in an area without flowing away easily. Flood is a major environmental concern that has devastated many parts of the world on a regular basis.

Flood is essentially a physical phenomenon, it usually occur as a result of abnormally heavy rains, dam failures, snow melts, river blockages. Flood disasters have differing impacts depending on their magnitude and the character of the affected area (For example, a heavily populated area would be more affected compared to sparsely settled area). Floods rank second only to droughts in the total number of people affected worldwide; many houses and farmlands have been destroyed due to excess flooding in many parts of world.

Characteristics of flood

• Intensity of damage is governed by the time interval of standing floodwaters .
• Manmade structures and forest vegetation exhibits different levels of tolerance towards effects of floods.
• Frequency of occurrence estimated over a length of period would determine the kind of activities the flood plain should be put to.
• High velocity of running water may pull out or weaken foundations of buildings.
• The rate of rise and discharge of a river is important as a basis for flood control.
• Generally the rainy season is always associated with the floods during which agricultural economy suffers a huge loss.

Types of floods

Floods can be classified into three categories such as:

River flood

Rivers get full due to heavy rains over large catchments areas or by melting of snow or sometimes both especially in the mountainous tracts. The floods take place in river systems with tributaries that may drain into large geographic areas and encompass many independent river basins. Amount of flooding depends on moisture in the soil, vegetation cover, and depth of snow and size of catchments basin.

Coastal flood

This type of flooding is associated with tropical cyclones (harsh winds arising at the ocean surface). Coastal floods are often aggravated by wind induced storm surges along the coastline. Sea and ocean water floods the inland coasts affecting kilometres of tracts. Ocean tides, storm surges or tsunamis play a definite role. Prolonged and indefinite rains in the rainy season marked from June-September results in extreme flood in coastal river basins.

Flash flood

Flash floods occur within six hours of the beginning of rainfall and are characterized with rising clouds, thunderstorms and tropical cyclones. These result from surface runoff from a torrential downpour, particularly if the catchments slope is not able to absorb and hold a significant amount of water. Other causes of flash floods include dam failure, sudden break up of glaciers etc. These offer potential threats in the areas where the terrain is steep, surface runoff is high, water flows through canyons and where severe rainstorms are likely.

Causes of flood

1. Flooding occurs mostly due to heavy rainfall in an area.
2. Dumping of refuse is a major cause of flooding, when refuse are dumped along culverts which eventually blocks the channel of flow.
3. Due to poor drainage system within flood prone area.
4. Inadequate urban planning.
5. Erecting of buildings along the drainage channels for example culverts, road and on water sheds.
6. Establishment of settlements along river channels.
7. Poor construction of embankments and dams may result in flood as in the case of Ogunpa river in Ibadan in Nigeria

Effects of flood

1. Rising water, erosion and the force damages the residential and commercial building.
2. They are dangerous for villages lying in the coastal areas as it sweeps away everything, which comes into its path. In mountainous areas flood is the chief cause of landslides.
3. Floods can carry pollutants from the land or urban areas into the rivers and streams that are used by human communities.
4. Floods also make soil infertile, as the topsoil is lost due to erosional activity,
5. Immense flooded areas reduce the habitat for tropical organisms and restrict the use of the land for other purposes.
6. Flooding can extend into traditional hunting grounds for indigenous groups, who have little political power to restrict this type of development.
7. Flooding causes serious damage to cropland by eroding soil and washing away seeds or ruining crops every year hence making it difficult for farmers to provide food for its growing population.
8. Fisherman, local people, cattle, animals and vegetation suffer a great loss of life and property. Most of the deaths are reported to be from drowning.
9. Fresh water supplies by all sources are nearly destroyed and contaminated hence the areas falling under the impact of flooding are exposed to water borne diseases.
10. The destruction of food and fodder crops lead to acute food shortage.
11. It makes human and vehicle movement very difficult.
12. flooding leads to diversion of public fund.

Prevention of flood

1. There should be a legislation against indiscriminate dumping and erection of buildings in swampy areas.
2. Depth and width of the riverbed could be increased as its capacity to carry larger loads increases manifold and thus reduce the area of the flood plain.
3. A network of canals can be established from the river systems, which generally leads to floods. This would also benefit the agricultural economy/ section. Proper care must be taken in the design and construction because of the possible environmental impact and necessary safety features.
4. Reservoirs should be made for storing floodwater and releasing them at manageable rates for example, artificial dams and canals should built along flood areas to help control the flooding, this also will provide water for irrigation of the farms especially during the dry season.
5. Proper urban planning can be encouraged to avoid urban flooding and public enlightenment campaign on the effects of flooding.
6. Through the construction of bridges and culverts and establishment of environmental sanitation to drain culverts.
7. Rivers and streambeds should be matted with stone, masonry or vegetation at the banks. This should strictly be followed where rivers pass through cities, most especially near bridges.
8. Newly constructed residential as well as commercial buildings should have foundations, that are strong enough to withstand flood conditions.