In the context of river systems, flooding can be defined as the overflowing or inundation of a river. This is caused by varying factors such as abnormal rainfalls, physical factors such as nature of surface or by human activities such as deforestation or building of concrete surfaces.
A flood can also occur as sea or ocean encroaches the land caused by factors such as tsunamis and melting ice caps which increase the volume of the ocean.
Another cause of flood can occur when dams burst causing water to overflow. This can also be caused by abnormal rainfalls or tectonic activities such as earthquake which creates cracks for a dam to burst.
Causes of Floods(Physical Factors)
Abnormal Rainfalls
Rainfall is the primary factor causing most floods. Rainfall abnormalities are usually caused by long term climate changes or short climate changes such as La Nina events.
These events increase the discharge and volume of a river causing the river to overflow.In addition,some rainfall abnormalities are caused by tropical cyclones.
Zimbabwe experienced high abnormal rainfalls in the 2016-2017 rain seasons which were caused by the Dineo cyclone.
Relief
Relief of a drainage basin determines if flooding can occur. Catchments characterised by steep terrains are likely to flood than flat terrains that promote infiltration and reduce runoff.
Nature of Surface
Impermeable surfaces hinders infiltration and encourage more runoff. In deserts baked rocky surfaces impedes water from infiltrating and promotes more runoff. The same is true in ice covered surfaces that creates an impermeable layer which discourage infiltration.
In addition, porous soils such as sand favour infiltration than cohesive clays that impedes infiltration and favours runoff.
Soil Moisture
Soils that are already filled with water tend to refuse infiltration of more water thus any rain that falls on these surfaces will not infiltrate therefore runs off on the surface(saturated overlandflow)
Vegetation
Densely vegetated catchments are less likely to experience flooding as more water is intercepted from reaching the surface than sparsely vegetated catchments.In addition,plant roots absorb water and encourage infiltration.
Drainage Basin Morphology
Elongated drainage basins with lots of tributaries are likely to experience floods as the volume of water is high. In contrast, large drainage basins may not experience flooding as the lag time tend to be long. Conversely small catchments can be prone to flooding as they have short lag times .
However, drainage density large or small both can experience flooding. This will depend on varying factors in the drainage basin such as how permeable the ground is or how steep the catchment is.
If a drainage basin is small but characterised by flat terrains, permeable surfaces or dense vegetation, the basin is likely to inexperience flooding or if a basin is long but have impermeable surfaces, sparse vegetation or steep slopes, that basin is more likely to flood.
Human Activities
Deforestation
Deforestation is perhaps the number one human cause of deforestation. Lack of trees means that more water readily runs off on the surface without being intercepted.
In addition, lack of trees results in more soil movement which in turn deposits in rivers and cause siltation. With lots of sediments the river has no choice but to overflow its banks.
Urbanisation
As lots of people are moving in urban areas, the need for more road and pavement construction results in more flooding. Concrete and asphalt surfaces are impermeable thus impedes infiltration and encourage more runoff.
Mining
Mining results in high amounts of unconsolidated soil which can find its way to rivers causing siltation, choking the river and eventually river flooding.
Furthermore, mining involves the removal of trees which again encourages high risks of flooding. Small scale miners mining for gold along rivers can also cause siltation which can lead to flooding.
Agriculture
Farming can involve the removal of vegetation which contributes to flood. In addition, overgrazing results in sparse vegetation and unconsolidated soil which can bes washed away into rivers.
Farming activities such as stream bank cultivation(farming along the banks of a river) results in the silting of rivers as more soil readily reaches the river.
Dumping
Dumping commercially or domestically in rivers can also lead to flooding as waste and sediments builds up leading to siltation and eventually flooding.
River Modification
Channel straightening, channel diversion, artificial levees can have grave consequences, even though its done in order to reduce floods.
For example, straightening a river on a particular area may reduce flooding on that area but discharge increases which can cause floods downstream. Letting the river flow natural can minimise flooding than artificially modifying it.
Effects of Floods
Floods impose devastating impacts on both humans and the environment.Furthermore,the economy is crippled as a result of floods.The most disastrous effects are experienced in LEDCs due to the inability to cope and lack of technology to implement protective and mitigatory measures.
Some effects of floods are:
Social Effects
Loss of livelihoods e.g the 2016-2017 floods in Zimbabwe claimed nearly 250 lives and injured 128 people.
Settlements are destroyed
Endemics such as typhoid or cholera resulting from sewage bursts
School drop-out
Separation of families.
Economic
Infrastructure such as road networks are cut-off rendering them inaccessible
Buildings, schools and hospitals are destroyed. Unemployment can rise sharply
Dam or oil plant bursts.
Power lines are destroyed
Farming lands are destroyed.
Environmental
Natural ecosystem is destroyed
Silting of rivers as water drawn back to the river carries large amounts of sediments
Animal habits and aquatic life is destroyed
Even though floods pose some of the worst effects, they are also beneficial. Flood water can render farming lands fertile. Floods can create wetlands for new animal speices.
Preventing and Mitigating Floods
Flood prevention includes soft and hard engineering techniques. Soft engineering techniques involves natural ways of preventing floods eg deforestation while hard engineering mainly consist of artificial practices eg dam constructions or artificial levees.
Hard Engineering Techniques
Dam Construction
Dams collect water from upstream and impede discharge downstream. Water is allowed to pass through a controlled point (floodgates).
However, dams are expensive to construct and require constant monitoring. Dam construction means an area has to be destroyed for the dam destroying the natural ecosystem. Furthermore, if the dam is to burst, widespread floods will again be experienced.
Artificial Levees
Humans can build raised embankments made of concrete called levees which blocks water from overflowing and retain it in the channel. In contrast, levees can occur naturally as sediments are deposited along the banks of a river.
Wing Dykes
Slats placed in a river channel at right angles or adjacent to the banks which causes sediments to collect behind them. This improves the flow of the water at the particular area where wing dykes are placed.
Source:Alex Jackson under the Creative Commons Attribution-NonCommercial 4.0 Int license
River Diversion
This involves the creation of new channels in which a river permanently flows . This helps to divert water from areas prone to flooding. However, this can lead to flooding in areas where the diverted stream passes.
Building High and Wider bridges
Higher and wider bridges causes water to flow freely and reducing the accumulation of sediments. Too much sediments chocks a river
Channel Straightening
This improves river flow by removing and preventing things like meanders, pools or deposition.
Building Permeable Surfaces
Permeable surfaces encourages water to infiltrate and reduce high amounts of runoff.
Storm Drains
Constructing storm drains helps by removing excess water on the surface.
Soft Engineering Techniques
Afforestation
Planting trees help to intercept more rainwater and therefore reduce excess runoff. In addition, plant roots absorb water from the surface reducing runoff. Furthermore, plant roots stabilise soil movement which then reduce the possibilities of soil entering river systems.
Dredging
This involves removing excess sediments within the river improving its flow and lessening flooding.
Isolate Flood Prone Areas
This involves the complete removal of people from a flood prone area.This helps by reducing damage and can be less costly than having to estabilish or build expensive structures such as dams.However, finance will be needed to build new houses for the evacuated people.
Placing Dumping Restrictions
Imposing laws,jail penalties and restrictions on waste dumping ensures that rivers are free from being choked by sediments and flow is improved.
Improving Standards of Living
By improving people’s standards of living through education on better farming techniques,settlements,allocation of resources and land etc will help to mitigate some causes which leads to floods.
River Restoration
Sometimes it is better to restore a river to its natural course.Modifications such as river straightening and diversion may cause some long term river problems.As such letting a river take its natural routes may lessen the likelihood of flooding.
