IntroductionFloods occur when a drainage basin (or watershed) receives so much water that stream and river channels cannot handle the flow. As a result it overflows into areas with human settlements, infrastructure facilities and economic activities. Floods become a disaster when such areas become exposed to the hazard without adequate warning and/or without means of taking defensive actions. Thus, the community suffers loss of life, assets, livelihood, and environmental security.
It has been noted that climate change affect flooding. As a result of global warming, the type, frequency and intensity of extreme events, such as tropical cyclones (including hurricanes and typhoons), floods, droughts and heavy precipitation events, are expected to rise even with relatively small average temperature increases. Changes in some types of extreme events have already been observed, for example, increases in the frequency and intensity of heat waves and heavy precipitation events. Flooding affects more people on an annual basis than any other form of natural disaster. A variety of climatic and non-climatic processes influence flood processes, resulting in river floods, flash floods, urban floods, sewer floods, glacial lake outburst floods and coastal floods. These flood-producing processes include intense and /or long-lasting precipitation, snow melt, dam break, reduced conveyance due to jams or land-slides or by storm.
Causes of flooding
Meteorological Cause:Most flood damages are the result of extreme, intense and long duration floods caused by meteorological phenomena such as:
· Prolonged and intense rainfall
· Typhoons, storms and tidal surges
Flooding can also be caused by increased run off due to:
· Ice and snow melt
· Impermeable surfaces
· Saturated land
· Poor infiltration rates
· Land erosion
Anthropogenic Cause:Mankind plays a very important role in the magnitude and frequency of floods in many different ways. Actually, it is the human activities in water catchments, which drastically intensify floods. In this connection, human actions associated with land use change are the most important.
· Population growth.
· Land use change, deforestation, intensive agriculture, over grazing, unplanned flood control measures
· Socio economic and development activities
· Climate change
· Global warming
Human Influences on FloodingHuman activities along waterways can increase flooding inadvertently. Paving and building on floodplains and surrounding areas decrease infiltration of rainwater into the soil and, as a consequence, increase runoff.
Runoff also increases when forests are cleared or when wetlands are destroyed by construction or infilling. Agriculture decreases the ability of soil to retain water and therefore increases runoff. Rapid runoff causes soil erosion. Sediment-clogged streams cannot support normal levels of aquatic life, and wildlife habitats are destroyed. Sediment deposition in stream channels also leaves little room for water and leads to the likelihood of flooding.
Types of Flooding
According to Duration: Slow-Onset Flooding, Rapid-Onset Flooding, Flash Flooding.
According to Location: Coastal Flooding, Arroyos Flooding, River Flooding and Urban Flooding.
The urban area is paved with roads etc. and the discharge of heavy rain can't absorbed into the ground due to drainage constraints leads to flooding of streets, underpasses, low lying areas and storm drains.
River flooding: occurs when heavy rainfall or spring thaws (melting snow and ice) increase water levels in a drainage basin, or sometimes by both. River floods normally build up slowly. These floods are often seasonal and may continue for days and weeks.Riverine floods also deposit rich fertile sediments that enrich the nutrients lost through the intensive agricultural practices of deltaic farmers downstream. This natural action provides livelihood security for farmers and food security for the people. The surrounding watershed ecosystem also depends on flooding to enable the development of life.
There are two types of river flooding:
- Slow onset Flood: Occurs slowly and can last weeks or even months. Causes may consist of snowmelt or steady ongoing rainfall. Rising flood levels can be forecasted giving people the opportunity to evacuate the areas at risk. There are often extensive damages and losses.
- Rapid onset Flood: Occurs mainly in steep rivers with small and steep mountainous catchments after periods of intense rainfall. These floods are accompanied by a rapid rise and fall in water levels. The sudden onrush of water from mountains and high flow velocity causes intense damages to crops and property and greater direct loss of life than slow onset floods.
Flash floods: are associated with intense rainstorms that release large amounts of rain into small drainage basins in a relatively short period of time. Flash floods occur with little or no warning and can reach peak levels within minutes, carrying a deadly cargo of rocks, trees, and other debris. Floodwater collects in particular areas and may remain for a long duration of time. Flash floods are potential threats particularly where the terrain is steep, surface runoff is high, water flows through narrow canyons and where severe rain storms are likely.Coastal floods: occur when onshore winds and hurricanes cause the sea level to rise over low-lying coastal areas. If this happen during high tide, leading to a tidal surge, the devastation can be catastrophic. Sometimes during hurricanes coastal areas are affected simultaneously by storm surges and river floods. Coastal flooding can also occur as a result of a tsunami or seismic sea wave following an earthquake
Importance of FloodplainsMost streams are naturally bordered by flat, low-lying areas known as floodplains. Floodplains have been carved into the landscape by stream erosion and are covered in fine-grained sand, silt, and clay deposited by floodwaters.
- Agriculture: people are attracted to floodplains because floods deposit nutrient-rich topsoil, eroded from upstream, producing fertile land for agriculture. In Egypt, for example, floods and deposition of nutrient-rich sediment from the Nile River have increased agricultural yields for perhaps five thousand years. Floodplains tend to be flat, making them easy to cultivate, and near water, making them easy to irrigate. In addition, the nearby source of water is useful for transportation of agricultural products.
- It is beneficial to streams: It serves to maintain both local and regional environmental balance, affecting water quality and aquatic life. Floods also recharge groundwater supplies. Ironically, the same river or stream that provides sustenance to the surrounding population also renders these populations vulnerable to disaster by periodic flooding.
Effect of FloodingFloods can be considered human-caused disasters in that people build on floodplains, refusing to consider the risk. Dangers of flooding include:
- Losses of human from drowning and heart attack. Also victim may suffer injuries;
- Landslides are a major threat each year to human settlements and infrastructure, and cause more property loss. Landslide occurs as a result of changes in water contents, removal of lateral support by erosion and rise in ground water levels.
- Structural damage to bridges, buildings, roadbeds, sewer system, roads, railway lines, canal, dams, and utilities;
- Agricultural losses: food shortage may lead to price increase;
- Severe soil erosion and land degradation: soil erosion removes valuable top soil which is the most productive part of the soil profile. This results in lower yields and higher costs of production;
- Property destruction;
- Indirect health effect as a result from Epidemic Diseases, Poisoning, Post-traumatic stress disorder, Unhygienic conditions;
- Flooding may shut roads, and affect traffic in and out of a region;
- Schools and other public institution may close due to the problem;
- Water pollution: Clean drinking water becomes scarce. (e.g. lakes, rivers, oceans, groundwater). Water pollution affects plants and organisms living in these bodies of water;
- Land value: houses located within a floodplain have lower market value than an equivalent house located outside the floodplain;
- Loss of resources - Impact on capital stock and machineries.
Flood ControlFloods can be controlled in two ways: by controlling the waters or by controlling floodplain development. To minimize the effects of flooding, engineers build dams, levees, and floodwalls along rivers. Dams can store water during periods of heavy runoff and release it gradually during periods of low flow. Artificial levees and floodwalls are built along streams to confine floodwaters and to keep them from covering the floodplains. Artificial levees must be heightened because of rising water levels over time.
Other methods of flood control include restoring vegetation, instituting soil conservation measures, constructing floodways to divert floodwaters, widening rivers to accommodate more water, and purposely flooding certain areas to prevent flooding in others.