Introduction

Floods rank as the most destructive water-related problem that faces mankind. It is also the most regularly occurring natural disaster. While the most flood-prone area in the world is Asia (UNESCO), the U.S has its own share of floods. In fact, it is estimated that whenever the U.S President announces that a particular location in the country is a natural disaster area, the culprit is flooding in 75% of the cases (Pearce & Leib).

Stages of Floods

A flood refers to a high flow of water that originates from a water body and overflows the usual restrictions and/or covers land that is usually dry. A flood takes place in 5 stages, commonly known as the ‘run off-cycle’ (Pearce et al.).

The first stage involves a period of fair weather when there is no rainfall. There exist low rivers with small dry steams or brooks called river rills, as well as slow seepage from groundwater.

The fair weather reduces water drainage from groundwater into the river rills. This causes the water table to drop, first to a level below the river rill, and then to slowly dry up altogether. The dropping of the water table reestablishes the soil’s underground storage capacity (Pearce et al.).

The second stage of the ‘run off-cycle’ involves steady, light rainfall. The river rills receive some of the light rainfall, causing the water in them to increase and making them flow again. The light rain also falls on vegetation and on the ground {where it is taken in and retained by minor surface depressions and puddles} (Pearce et al.).

The third stage involves an increase in rainfall. The vegetation becomes heavily wetted, while the water collected in the small land depressions and puddles starts to overflow. The increase in rainfall becomes runoff and penetrates the dry soil, which takes in the water freely.

The dry soil soon becomes sodden, and its rate of water absorption slows down. As the rainfall continues, the amount of water exceeds the penetration rate, causing the creation of active surface runoff. This runoff reaches the river rills.

At the same time, increasing the seepage of rainwater into groundwater causes both the water table to rise, as well as the base flow into the river rills to increase. The surface of the water in the river rills rises quickly as a result (Pearce et al.).

The fourth stage begins when the rainfall stops. The river rills are at their peak. The channel storage soon empties as the flood moves downstream. Wet vegetation, ground surface water, and soil moisture are soon dried by evaporation and transpiration.

The penetration of ground surface water into the soil still continues, causing the excess water in the soil to permeate to the water table. This causes the water table to go on rising; it reaches a peak when the river rills return to the bank-full situation (Pearce et al.).

The last stage involves the commencement of returning the stage capacity of the ground, thereby restoring Nature’s unique flood control system. As the river rills descend and are joined by river tributaries, they gather drainage from a widening catchment area. The catchment area decides the quantity of waterborne by various rivers. The flood passes from the headwaters to the lower stream.

The rivers start to intensify and overflow their banks in response to the higher demands on their capacity. The amount of water in all the river tributaries also increases. The water ultimately arrives at a drainage basin where it gets stored in the valley bottom to counter the rise in water discharge (Pearce et al.).

Types of Floods

There are five types of floods

River floods occur when water in a river overflows. This usually takes place after winter, or after spring rains or as a result of snowmelt. The water gets poured into a large stream from its draining basin. River floods also take place when a jam is caused by ice or floating fragments of broken materials (Pearce et al.).

Flash floods are caused when an extraordinarily heavy injection of water makes a river overflow its banks continuously for a period of many hours. The water injection may take the form of cloudbursts, torrential rains, thunderstorms, spring thaws, ice jams, dam bursts, or spillover of drainage basins. The huge amount of water gets directed into a small drainage basin.

Topography, state of soil {low permeability or heavy saturation}, anchor convections, and impervious ground surfaces also contribute on a lesser scale. Such floods are overwhelming, involving swirling waters that reach heights of 20 feet or more with hardly any warning. They are so powerful that they uproot trees, unearth boulders, demolish buildings, tear down bridges, and create new channels of water (Pearce et al.).

Ocean floods are caused when powerful offshore winds force water from an ocean on to the land. The low barometric pressure in such winds makes the ocean level rise above the coastal lowlands, leading to the production of storm surges.

The increase in rainfall in such coastal areas {which usually are barrier islands, swamps, or plains having several rivers flowing through them} exacerbates flooding in the adjoining low lying areas. The overflowing ocean water next races to mouths of river channels and inundates the area with water. Avenues of escape can be severed and shut out by high water (Pearce et al.).

Volcanic eruption-resulting floods are caused by two factors, both associated with volcanic eruptions: melting of snow/ice, and heavy rains. Such floods take place suddenly and can be massive. Iceland and Ecuador have experienced several melt-water floods brought on by volcanic heating.

Such floods cause more destruction than other flood types because they carry along with a huge quantity of sediment (for example, when Mount Saint Helens erupted in 1980, it caused massive mudflows as well as flash floods) (Pearce et al.).

Urban floods occur in areas where rain-absorbing land is transformed from fields and woodlands into roads and parking areas. Such urbanization renders the land incapable of absorbing rainfall, resulting in runoff 2 to 6 times more than what would take place in natural terrain. Urban flooding transforms streets into fast-moving bodies of water while building basements become death-traps as they rapidly fill with water (Pearce et al.).

Effects of Floods

Floods have had one beneficial and two adverse effects on mankind

Floods have resulted in a benefit to the ecosystem as well as to human activities. The wetlands created in floodplains promote the diversity of plant and animal life. Fertile floodplains are the primary mainstay of countries – especially developing nations – that provide food security while also creating and maintaining means of livelihood for mankind.

Not only do the floodplains encourage low-tech agricultural techniques, but they also provide employment for millions of people {the U.N estimates that nearly 1 billion people, or 16% of the world’s population, live on floodplains}. This aspect has caused significant economic development in places at risk from floods (UNESCO).

The first adverse effect is a huge loss of human lives (UNESCO). Floods are rated as the premier killer of mankind as far as natural disasters are concerned (Pearce et al.). The U.N estimates that each year, around 520 million individuals all over the world are affected by floods (UNESCO), out of whom nearly 1.6 million lose their lives (Pearce et al.).

While floods have been widespread all over the world, Asia has suffered the most; it had 1,200 floods during the last century, which caused 11.6 million deaths (UNESCO). In the U.S, floods caused the death of nearly 10,000 during the last century; as many as 80% of them perished in vehicles (Pearce et al.).

The second adverse effect is massive economic losses. Destruction of property and infrastructure by floods has been immense. It is estimated that with the exception of droughts, nearly 90% of worldwide destruction caused by natural disasters is perpetrated either directly or indirectly by floods. The U.S bears a $ 3.1 billion flood-related cost every year (Pearce et al.). Asia suffered economic losses in the region of $ 207 billion during the last century (UNESCO).

Flood Risk Reduction Methods

Despite the devastation caused by floods, as well as the increasing knowledge of mankind to predict the onset of this natural disaster, the number of people risking their lives to floods goes on increasing.

This is on account of 6 factors: increasing urbanization, rising population in flood- risk locations, land use transformations, climate change, heightening sea levels (UNESCO) and the heavy cost of building and maintaining flood protection frameworks {which discourages communities from putting forward proposals to tackle floods} (Pearce et al.).

It is important that all flood risk reduction methods be aimed at creating the required capacity to tackle these six factors (UNESCO).

In the U.S, river flood forecasts are regularly compiled by the National Weather Service {NWS} river-forecast centers and made available to the people through nationwide NWS offices. Such forecasts are prepared after meticulously creating and calibrating complicated mathematical models of rivers in the U.S reacting to rainfall and snowmelt.

When flooding takes place, the NWS centers compile forecasts on vital factors like the height of flood crests, the day and time when it is anticipated that the river will pour over its banks, and the day and time when it is anticipated that the river water will retreat to within its confining banks (Pearce et al.).

In addition to local and national schemes, countries all over the world can seek the help of the U.N’s Integrated Flood Management {IFM}. IFM is an initiative that considers all risk-based aspects before formulating flood management solutions after detailed consultations with all involved stakeholders (UNESCO).

Flood Prevention Methods

There are four methods of preventing floods. The first method is to construct dikes and levees to block water from overflowing on to land. Dikes, the world’s premier flood protection technique, was initially constructed in the upper floodplain areas but were later built higher and nearer the rivers.

The building of levees results in an increase in the flow speed of water through the restricted areas, thereby increasing scouring and reducing the deposit of impurities. The second method is to construct dams. These structures, which cut off flood peaks, have artificially built reservoir basins into which floodwater is stored, thereby blocking it from causing floods in the rivers.

This method is particularly popular in the U.S, whereas many as 50,000 dams have been built across almost all major rivers in the country. The third method is the natural channel advancements.

The overall aim is to increase channel discharge capacity by clearing vegetation and executing functions like cutoffs, straightening, expanding, and deepening.

This method is not popular because it does not foster aquatic life in the water body. The last method is the floodwater diversion. This involves digging ditches to create a channel into which floodwater is directed. This method is an old one that is not popular nowadays as it is greatly dependent on natural topography (Pearce et al.).

Conclusion

Given the huge volume and excellent quality of flood-related knowledge and flood prevention information, it is surprising that this natural disaster still continues to plague the world regularly, leaving an ever-increasing trail of death and destruction in its wake.

The main cause of this is the unrestrained movement of people into flood-prone areas -,, particularly in developing countries.

While acknowledging that such movement is dictated by the necessity to survive, the people must be made to realize that flood warnings should be heeded, and basic recommended precautions should be taken properly and serious. The only solution in the present scenario is to conduct and maintain a strong, widespread, and well supported educational campaign aimed at preventing more losses caused by floods.

References

.” UNESCO. (N.d).

Pearce, Katy & Leib, Deborah. “Floods & Society.” University of Michigan. 1998.

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