FLOODINGA flood is an overflow of water that submerges land which is usually dry. The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may occur as an overflow of water from water bodies, such as a river or lake, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries, or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry.Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighborhood or community, or very large, affecting entire river basins.PROCESS OF FLOODINGThe water cycle describes the continuous movement of water on, above and below the Earths surface. Flooding can occur at different points in the water cycle. Water in the environment is ultimately derived from rainfall and other forms of precipitation. Water flows downhill to the sea via a series of routes and stores.

This system is dynamic and changes with time as routes and the adjacent landscapes are modified by humans or by natural processes.Flooding generally occurs when the volume of water exceeds the capacity of a water route (sometimes referred to as pathway) or store (sometimes called a receptor). This causes an area of land to be submerged temporarily. Flooding may take place at any point along the course of the route and not necessarily at the place where the extra water has entered the system. In rare cases, flooding may occur when a feature fails that would normally keep water off the land, such as a reservoir embankment or levee.

WHAT ARE THE NEGATIVE SOCIAL IMPACTS OF FLOODING?Flooding of areas used for socio-economic activities produces a variety of negative impacts. The magnitude of adverse impacts depends on the vulnerability of the activities and population and the frequency, intensity and extent of flooding. Some of these factors are shown below;Loss of lives and property: Immediate impacts of flooding include loss of human life, damage to property, destruction of crops, loss of livestock, non-functioning of infrastructure facilities and deterioration of health condition owing to waterborne diseases. Flash floods, with little or no warning time, cause more deaths than slow-rising riverine floods.Loss of livelihoods: As communication links and infrastructure such as power plants, roads and bridges are damaged and disrupted, economic activities come to a standstill, resulting in dislocation and the dysfunction of normal life for a period much beyond the duration of the flooding. Similarly, the direct effect on production assets, be it in agriculture or industry, can inhibit regularly activity and lead to loss of livelihoods. The spill over effects of the loss of livelihoods can be felt in business and commercial activities even in adjacent non-flooded areas.Decreased purchasing and production power: Damage to infrastructure also causes long-term impacts, such as disruptions to clean water and electricity, transport, communication, education and health care. Loss of livelihoods, reduction in purchasing power and loss of land value in the flood plains lead to increased vulnerabilities of communities living in the area. The additional cost of rehabilitation, relocation of people and removal of property from flood-affected areas can divert the capital required for maintaining production.Mass migration: Frequent flooding, resulting in loss of livelihoods, production and other prolonged economic impacts and types of suffering can trigger mass migration or population displacement. Migration to developed urban areas contributes to the overcrowding in the cities. These migrants swell the ranks of the urban poor and end up living in marginal lands in cities that are prone to floods or other risks. Selective out-migration of the workforce sometimes creates complex social problems.Psychosocial effects: The huge psycho-social effects on flood victims and their families can traumatize them for long periods of time. The loss of loved ones can generate deep impacts, especially on children. Displacement from ones home, loss of property and livelihoods and disruption to business and social affairs can cause continuing stress. The stress of overcoming these losses can be overwhelming and produce lasting psychological impacts.Hindering economic growth and development: The high cost of relief and recovery may adversely impact investment in infrastructure and other development activities in the area and in certain cases may cripple the frail economy of the region. Recurrent flooding in a region may discourage long-term investments by the government and private sector alike. Lack of livelihoods, combined with migration of skilled labour and inflation may have a negative impact on a regions economic growth. Loss of resources can lead to high costs of goods and services, delaying its development programmes.Political implications: Ineffective response to relief operations during major flood events may lead to public discontent or loss of trust in the authorities or the state and national governments. Lack of development in flood-prone areas may cause social inequity and even social unrest posing threat to peace and stability in the region.FLOODS CAUSES, EFFECTS, PREVENTION AND CONTROL MEASURESFloods occur when excess water flows over land that is normally dry. The water does not flow through any definite channel but spread over land surface. The most common type of flood is when a river overflows its banks. This may occur in the wet reason in the tropics or in spring and summer in the temperate latitudes. Another common type of flood is when ocean water overflows its shoreline and spreads over the coastal plains.CAUSES OF FLOOD1. Heavy rainfall during the wet season.2. Rapid melting of ice in the watershed of rivers in the temperate latitudes.3. Collapse of dams and river embankments.4. Poor land drainage systems, e.g. lack of drainage ditches, blocked drainage ditches, and narrow, shallow drainage ditches. Bridges and culverts that are either too low or too narrow also add to poor drainage.5. Low relief coupled with a high water table as in flood plains, river valleys and coastal plains.6. Strong tidal waves, storm surges and hurricanes from the sea.7. Unplanned urban residential, commercial and industrial development on the flood plains of rivers.EFFECTS OF FLOODING1. Destructive of farmlands.2. Destruction of life and property.3. Destruction of transport and communication systems.4. Displacement of population and settlements.Flood Prevention and Control Measures1. Practical watershed and management such as preventing deforestation and carrying out re-forestation where necessary. Control the use of land.2. Construction of high walls and strong embankments and the reinforcement of levees along river bank.3. Uses of breaks to retain run-off water over land surfaces.4. Flood plains should be used for recreational or agricultural purposes only to avoid building houses on them.5. Construction of flood containing dams with a capacity to retain large quantities of run-off water. Such reservoirs could be used as fish ponds, sources of water for irrigation and domestic purposes.6. Stream channelisation to increase the capacity of the channel to carry more water.7. There should be good land use planning and development control in both rural and urban centres.CONTROLIn many countries around the world, waterways prone to floods are often carefully managed. Defenses such as detention basins, levees,[16] bunds, reservoirs, and weirs are used to prevent waterways from overflowing their banks. When these defenses fail, emergency measures such as sandbags or portable inflatable tubes are often used to try to stem flooding. Coastal flooding has been addressed in portions of Europe and the Americas with coastal defenses, such as sea walls, beach nourishment, and barrier islands.In the riparian zone near rivers and streams, erosion control measures can be taken to try to slow down or reverse the natural forces that cause many waterways to meander over long periods of time. Flood controls, such as dams, can be built and maintained over time to try to reduce the occurrence and severity of floods as well. In the USA, the U.S. Army Corps of Engineers maintains a network of such flood control dams.In areas prone to urban flooding, one solution is the repair and expansion of man-made sewer systems and stormwater infrastructure. Another strategy is to reduce impervious surfaces in streets, parking lots and buildings through natural drainage channels, porous paving, and wetlands (collectively known as green infrastructure or sustainable urban drainage systems [SUDS]). Areas identified as flood-prone can be converted into parks and playgrounds that can tolerate occasional flooding. Ordinances can be adopted to require developers to retain stormwater on site and require buildings to be elevated, protected by floodwalls and levees, or designed to withstand temporary inundation. Property owners can also invest in solutions themselves, such as re-landscaping their property to take the flow of water away from their building and installing rain barrels, sump pumps, and check valves.BENEFITSFloods (in particular more frequent or smaller floods) can also bring many benefits, such as recharging ground water, making soil more fertile and increasing nutrients in some soils. Flood waters provide much needed water resources in arid and semi-arid regions where precipitation can be very unevenly distributed throughout the year and kills pests in the farming land. Freshwater floods particularly play an important role in maintaining ecosystems in river corridors and are a key factor in maintaining floodplain biodiversity.[17] Flooding can spread nutrients to lakes and rivers, which can lead to increased biomass and improved fisheries for a few years.For some fish species, an inundated floodplain may form a highly suitable location for spawning with few predators and enhanced levels of nutrients or food.[18] Fish, such as the weather fish, make use of floods in order to reach new habitats. Bird populations may also profit from the boost in food production caused by flooding.[19]Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River among others. The viability of hydropower, a renewable source of energy, is also higher in flood prone regions.

EFFECT ON PLANTThe answer is that it depends. Some plants are perfectly happy growing in wet places e.g. bulrushes or water lilies, and are adapted to do so. However, in generally drier areas such as fields or pastures, the presence of excessive amounts of water can cause the plants to become stressed and even die.

There are many ways that flooding can damage plants. Excessive moisture in soil decreases oxygen levels. This impedes respiration (where energy is released from sugars) in the roots leading to the build-up of carbon dioxide, methane and nitrogen gases. Ultimately, the roots can suffocate and die. Toxic compounds such as ethanol and hydrogen sulphide can also build up in the soil and damage plants. If leaves and stems are submerged, photosynthesis (absorption of energy from the sun to produce sugars) can be inhibited and plant growth can slow or even stop.

Plants that are suffering from excessive-water stress are more prone to infection by disease-causing organisms such as fungi or insects. Also, excessively wet soil tends to favour the growth of soil-microbes such as Fusarium spp., Phytophora spp. and Rhizoctonia solani, which can infect plant roots leading to diseases such as root and crown rot.Disease symptoms on bean roots infected with Rhizoctonia solani. Image by Howard F. Schwartz (CC BY 3.0) via wikimedia commons.

Disease symptoms on bean roots infected with Rhizoctonia solani. Image by Howard F. Schwartz (CC BY 3.0) via wikimedia commons.

The deposition of soil and rocks onto plants during flooding can damage plants, as can the exposure of roots to the air by the washing away of surrounding soil. Also, even after the flood waters recede the damaged plants can be more vulnerable to other stresses. For example, trees with substantial root damage are more likely to be uprooted in windy weather.

The damage caused to plants by flooding differs depending on the time of the year and the age of the plants. During winter most trees and shrubs are not actively growing and so they are more tolerant to flooding. Established healthy plants are generally more tolerant than young or very old plants. The longer that excessive water is present the more likely that damage to plants will be fatal. Even once the flood waters recede it can take weeks for the soil to dry out with plants continuing to suffer damage in the meantime. It can take weeks or months for plants to die after being damaged so it will be a while before the full impact of the UKs wet winter on our plants becomes clear.