Flooding has been an environmental menace in most urban centres worldwide. In well planned urban centres however, the problem is minimized. It is more pronounced in urban centres of developing countries due to poor planning and negative human activities. This article will attempt a definition of flooding; examine the causes, impacts and control of flooding.
What is flooding?
Flooding is the inundation of land beyond the normal confines of a channel or coastline, either by overflow of excess water or its influx via shallow subsurface or low-lying routes (Smithson et al, 2002).
Flooding can also be defined as a process whereby a river outflows its natural channel or the artificial channel constructed for it.
Periodic floods occur on many rivers, forming a surrounding region known as the flood plain. During times of rain or snow, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, and the rest travels over the land as surface runoff.
Floods occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water. Water then runs off the land in quantities that cannot be carried within stream channels or retained in natural ponds, lakes, and man-made reservoirs.
Causes of Flooding
There are several causes of flooding, prominent amongst which are meteorological factors, poor development planning and poor maintenance. Flooding can however, be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce the supply of vegetation that can absorb rainfall.
Most floods are the result of severe meteorological or climatologically conditions. They may follow severe local thunderstorms or more widespread rain falling on a saturated landscape.
Snowmelt, particularly when associated with further rain, can cause major flooding. Storm surges onto the coast as a result of tropical cyclones can cause even more damage when reaching delta where rivers are in flood.
Floods may also occur as a result of individual disasters such as landslides or dam bursts. Although these conditions may be very important, major floods are usually the result of flood-intensifying conditions which worsen the original meteorological problem.
For example, the basin characteristics may aid the movement of rainwater by having unvegetated steep, impermeable slopes, variable altitude and a basin shape, which focuses the tributaries on to a particular part of the catchment.
Floods often occur where there is a sudden change of channel gradient, causing the flow of water to decrease its velocity and perhaps spillover the flood banks.
Severe winds over water bodies can also result to flooding, even when rainfall is relatively light. For example, the shorelines of lakes and bays can be flooded by severe winds-such as during hurricanes-that low water into the shore areas. In addition, coastal areas are sometimes flooded by unusually high tides, such as spring tides, especially when steep compounded by high winds and storm surges.
Tsunamis have also been identified as a trigger for flooding. Tsunamis are high large waves, typically caused by undersea earthquakes or massive explosions, such as the eruption of an undersea volcano.
A recent example of a tsunami triggered flood happen in Japan some months ago, as a result of a massive earthquake, which claimed the lives of over 10,000 people, with property worth billions of US dollars destroyed.
Apart from meteorological factors, anthropogenic (human) factors are also very important in determining the severity of floods. Poor development planning and maintenance are responsible for some of the flooding episode so far recorded in the world.
In the urban areas of most parts of the world, there are reported cases of inappropriate developments in flood plains due to pressure on available land. Inappropriate constructions of roads, car parks, buildings etc, in such a way that prevents rainfall from draining away naturally, can increase the risk of flooding from rainwater runoff.
Furthermore, poor or insufficient drainage networks, inadequate maintenance of watercourses, faulty sewer networks, failure of dams, levees, retention ponds, or other structures that retain water, due to poor design, construction or maintenance all contribute to flooding.
Impacts of Flooding
Flooding has many impacts. It damages property and endangers the lives of humans and other species. Rapid water runoff causes soil erosion and concomitant sediment deposition elsewhere (such as further downstream or down a coast).
The spawning grounds for fish and other wildlife habitats can become polluted or completely destroyed. Some prolonged high floods can delay traffic in areas which lack elevated roadways. Floods can interfere with drainage and economic use of lands, such as interfering with farming.
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Structural damage can occur in bridge abutments, bank lines, sewer lines, and other structures within floodways. Waterway navigation and hydroelectric power are often impaired.
Financial losses due to floods are typically in millions of dollars each year. The impacts of flooding will be discussed further under three board sub-heading – health, economic and social impacts.
Specifically, health effects caused by a flood event may result from: the event itself; the disruption and problems arising from trying to recover; and from the worry or anxiety about the risk of flood re-occurring. The potential health effects can be considered at three time periods:
Immediate: Death by drowning, injuries due to being knocked over by flood waters or struck by falling trees, over-exertion during the event, hypothermia, electrocution, exposure to contaminants, and the stress of the event itself.
Mediumterm:Gastrointestinal illnesses, cardiovascular disease from over- exertion during recovery/clean-up processes, lacerations, sprains/strains, dermatitis, respiratory illnesses and carbon monoxide poisoning.
Longerterm:Longer term effects are mostly psychologically related. Sewer flooding and the health issues associated with it is a key issue for urban flooding, whereas concern is growing over the effects of diffuse pollution in rural flooding.
In terms of density and sparsity issues, because of the close proximity of people to one another within the urban environment, there are more chances of an outbreak of epidemic, especially if sewer flooding is involved (Twigger-Ross, 2005).
This includes the cost of damage done to a property by a flood, the cost of clean-up (eg paying for the house to be dried out), the costs of living in temporary accommodation, and possibly the costs of having a house that is harder to re-sell because it has been flooded or is in a defined flood plain.
In rural areas where there are few facilities, the loss of services and shops could have a disproportionately large economic impact on small communities. There could also be a negative impact in deprived urban neighbourhoods where business confidence is vital to regeneration.
If businesses fail because of the impacts of flooding this could damage the perception of the area as an attractive place for investment.
Social impact includes every other kinds of discomfort, which is not directly economic or health related. Social impacts may include household disruption, community and neighbourhood changes, and associated impacts of evacuation and temporary accommodation.
Household disruption, include the stress of cleaning up the house, dealing with builders, and dealing with living in a damp environment. These activities are rated as something that is very stressful for a lot of people (Defra, 2004) as quoted from (Twigger-Ross, 2005).
In both rural and urban areas, there is likely to be pressure on services to aid the clean-up of the flooding. In urban areas, if a large area has been flooded, then there may be difficulties in finding workmen to repair flood damage.
Impacts associated with evacuation and temporary accommodation include both the effects of having to leave home and those of having to live away from home.
In urban areas, if there are large numbers of people to be relocated then this will put pressure on services, and may mean that people have to live in cramped and overcrowded conditions, or have to move a distance from their homes.
For people in both rural and urban centres, difficulties will be experienced if they are relocated to areas, which is some distance from their homes.
Specifically, it will be harder to keep in touch with repairs being carried out on their homes, and harder to maintain normal routines because of greater travelling distances and new routes to schools, work and services.
Flood control refers to all methods used to reduce or prevent the detrimental effects of flood waters. Some methods of flood control have been practiced since ancient times.
These methods include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, and the construction of floodways (man-made channels to divert floodwater). Other techniques include the construction of levees, dikes, dam; reservoirs or retention ponds to hold extra water during times of flooding.
Many dams and their associated reservoirs are designed wholly or partially to aid in flood protection and control. In many countries, rivers prone to floods are often carefully managed. Defenses such as levees, bunds, reservoirs, and weirs are used to prevent rivers from overflowing their banks.
When these defenses fail, emergency measures such as sandbags or portable inflatable tubes are used. A weir, also known as low head dam, is most often used to create millponds, but on the Humber River in Toronto, a weir was built near Raymore Drive to prevent a recurrence of the flood damage caused by Hurricane Hazael in 1954.
Coastal flooding has been addressed in Europe and the Americans with coastal defenses, such as sea walls, beach nourishment, and barrier islands.
Other methods of flood control include the stoppage of further developments of flood plains; proper channelization of drainage systems; discourage indiscriminate to disposal of waste (especially rubber bags and cans), which are not biodegradable and have the capacity to block drainage systems, leading to flooding.
In summary, floods are the most common of all environmental hazards. Each year many thousands of people die as a result of flooding and millions are affected by indirect consequences such as damage to crops, housing, transport, etc.
Most flood problems are found on riverine flood plains, in both developed and developing countries, though the impact is often greater in the latter, where high populations are to be found on extensive and often relatively fertile flood plains such as those of the ganges and the Yangtse.
Having gone through this unit, you should be well informed of the causes, impacts and control measures of flooding. This knowledge should therefore, be transform to positive actions in contributing to solving the flooding menace in our environment.
Flooding is a process whereby a river outflows its natural channel or the artificial channel constructed for it.
The major causes of floods are meteorological or climatological conditions.
However, human activities tend to exacerbate its effects.
Flooding has noticeable impacts on health, economic and social well-being of the affected people.
Flooding can be control by several coordinated measures which include planting vegetation, construction of dams, levees etc, and avoidance of areas prone to floods such as flood plains.
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