Many of us have this idea that flood is simply, too much water around our houses, flooding is a lot more than that. It is an extremely dangerous and has the potential to wipe away an entire city, coastline or area, and cause extensive damage to life and property. It also has great erosive power and can be extremely destructive, even if it is a foot high. Flood is a natural event or occurrence where a piece of land (or area) that is usually dry land, suddenly gets submerged under water. Some floods can occur suddenly and recede quickly, others take days or even months to build and discharge. When floods happen in an area that people live, the water carries along objects like houses, bridges, cars, furniture and even people. It can wipe away farms, trees and many more heavy items.1   Floods occur at irregular intervals and vary in size, duration and the affected area. It is important to note that water naturally flows from high areas to low lying areas. This means low-lying areas may flood quickly before it begins to get to higher ground.  In this paper, we shall examine the cause(s) of flooding, the types of flooding, some effects of floods and what we can do before, during and after floods occur.


A flood is an overflow of water that submerges land that is usually dry. The European Union  (EU) Floods Directive2 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.

1. (
2. “Directive 2007/60/EC Chapter 1 Article2”.

Flooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, 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.

Flooding can have a variety of direct impacts on the environment and ecosystems contained within a flooded region. Some of these impacts are positive; flooding is a natural ecological process that plays an integral role in ensuring biological productivity and diversity in the flood plain.3 Other impacts of flooding may be less positive, and in some cases can result in environmental degradation. The most extensive and difficult to repair environmental damage usually occurs in developed areas located in the floodplain.

Flooding can directly impact, the health and wellbeing of wildlife and livestock; riverbank erosion and sedimentation; the dispersal of nutrients and pollutants; surface and groundwater supplies; and local landscapes and habitats. 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.4 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.

3. “Environmental Aspects of Integrated Flood Management.” Associated Programme on Flood Management. WMO. Published August 2006 in Geneva, Switzerland. Pg. 34.
4. Glossary of Meteorology (June 2000) Flood Archived 2007-08-24 at the Wayback Machine., Retrieved on 2009-01-09

Generally, the natural behavior of water (and flowing water) is that it moves from higher ground to lower ground. This means that if there is a higher ground adjacent a lower ground, the lower ground is a lot more likely to experience floods. Additionally, anywhere that rains fall, floods can develop. This is so because anytime there are more rains bringing more water than it can be drained or absorbed by the soil, there is a flood potential. In many cities, there are buildings springing up in many places where they have not been authorized. Some of these building are placed in waterways.  Other places also have very bad and chocked drainage systems. The danger is that, with the rains, water will find its own level if it cannot find its way. The result is flooding and homes could be under water. Any plain low-lying area adjacent a river, lagoon or lake is also more likely to have floods anytime the water level rises. This includes coastal areas and shorelines, as seawater can easily be swept inland by strong winds, tides and tsunamis.



Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either infiltrate or run off. The excess accumulates in place, sometimes to hazardous depths. Surface soil can become saturated, which effectively stops infiltration, where the water table is shallow, such as a floodplain, or from intense rain from one or a series of storms. Infiltration also is slow to negligible through frozen ground, rock, concrete, paving, or roofs. Areal flooding begins in flat areas like floodplains and in local depressions not connected to a stream channel, because the velocity of overland flow depends on the surface slope. Endorheic basins may experience areal flooding during periods when precipitation exceeds evaporation.5



Floods occur in all types of river and stream channels, from the smallest ephemeral streams in humid zones to normally-dry channels in arid climates to the world’s largest rivers. When overland flow occurs on tilled fields, it can result in a muddy flood where sediments are picked up by run off and carried as suspended matter or bed load. Localized flooding may be caused or exacerbated by drainage obstructions such as landslides, ice, debris, or beaver dams.

Slow-rising floods most commonly occur in large rivers with large catchment areas. The increase in flow may be the result of sustained rainfall, rapid snow melt, monsoons, or tropical cyclones. However, large rivers may have rapid flooding events in areas with dry climate, since they may have large basins but small river channels and rainfall can be very intense in smaller areas of those basins.

5. Jones, Myrtle (2000). “Ground-water flooding in glacial terrain of southern Puget Sound, Washington”. Retrieved 2015-07-23.

Rapid flooding events, including flash floods, more often occur on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, or normally-dry channels. The cause may be localized convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier. In one instance, a flash flood killed eight people enjoying the water on a Sunday afternoon at a popular waterfall in a narrow canyon. Without any observed rainfall, the flow rate increased from about 50 to 1,500 cubic feet per second (1.4 to 42 m3/s) in just one minute.6 Two larger floods occurred at the same site within a week, but no one was at the waterfall on those days. The deadly flood resulted from a thunderstorm over part of the drainage basin, where steep, bare rock slopes are common and the thin soil was already saturated.


This kind occurs within a very short time (2-6 hours, and sometimes within minutes) and is usually as a result of heavy rain, dam break or snow melt. Sometimes, intense rainfall from slow moving thunderstorms can cause it.  Flash floods are the most destructive and can be fatal, as people are usually taken by surprise. There is usually no warning, no preparation and the impact can be very swift and devastating.7

Flash floods are the most common flood type in normally-dry channels in arid zones, known as arroyos in the southwest United States and many other names elsewhere. In that setting, the first flood water to arrive is depleted as it wets the sandy stream bed. The leading edge of the flood thus advances more slowly than later and higher flows. As a result, the rising limb of the hydrograph becomes ever quicker as the flood moves downstream, until the flow rate is so great that the depletion by wetting soil becomes insignificant.

6. Hjalmarson, Hjalmar W. (December 1984). “Flash Flood in Tanque Verde Creek, Tucson, Arizona”. Journal of Hydraulic Engineering. 110 (12): 1841–1852. doi:10.1061 /(ASCE) 0733-9429(1984)110:12(1841).
7. (


Flooding in estuaries is commonly caused by a combination of sea tidal surges caused by winds and low barometric pressure, and they may be exacerbated by high upstream river flow.

Coastal areas may be flooded by storm events at sea, resulting in waves over-topping defenses or in severe cases by tsunami or tropical cyclones. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category. Research from the NHC (National Hurricane Center) explains: “Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases.”8


Urban flooding is the inundation of land or property in a built environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Although sometimes triggered by events such as flash flooding or snowmelt, urban flooding is a condition, characterized by its repetitive and systemic impacts on communities, that can happen regardless of whether or not affected communities are located within designated floodplains or near any body of water. Aside from potential overflow of rivers and lakes, snowmelt, storm water or water released from damaged water mains may accumulate on property and in public rights-of-way, seep through building walls and floors, or backup into buildings through sewer pipes, toilets and sinks.

8. “Storm Surge Overview”. http/ Retrieved 3 December 2015.

In urban areas, flood effects can be exacerbated by existing paved streets and roads, which increase the speed of flowing water. The flood flow in urbanized areas constitutes a hazard to both the population and infrastructure. Some recent catastrophes include the flooding of New Orleans (USA) in 2005, and the flooding in Rockhampton, Bundaberg, Brisbane during the 2010–2011 summer in Queensland (Australia). Flood flows in urban environments have been studied relatively recently despite many centuries of flood events. Some recent research has considered the criteria for safe evacuation of individuals in flooded areas.


Catastrophic riverine flooding is usually associated with major infrastructure failures such as the collapse of a dam, but they may also be caused by drainage channel modification from a landslide, earthquake or volcanic eruption. Examples include outburst floods and lahars. Tsunamis can cause catastrophic coastal flooding, most commonly resulting from undersea earthquakes.


Here are a few events that can cause flooding: 9

1.   RAINS
Each time there are more rains than the drainage system can take, there can be floods. Sometimes, there is heavy rain for a very short period that result in floods. In other times, there may be light rain for many days and weeks and can also result in floods. Nigerian Vanguard report “Since January, this year, no fewer than 141 lives have been lost to rainstorm, wind storm and flood disasters across the country with at least 19,369 persons displaced on account of their 5,732 houses and sources of livelihood among others destroyed. Saturday Vanguard’s  checks showed that the rainstorm and flood disaster in 2018 is the worst in the last six years after the 2012 floods that killed 363 people, displaced 2.1 million people and affected seven million people in 30 of the 36 states of the country, according to the National Emergency Management Agency, NEMA. The economic losses in 2012 were put at N2.5 trillion. The seriousness of the 2012 flooding, referred to as the most harmful in the last 40 years, was attributed to a combination of two events: very heavy local rainfall”.10

Rivers can overflow their banks to cause flooding. This happens when there is more water upstream than usual, and as it flows downstream to the adjacent low-lying areas (also called a floodplain), there is a burst and water gets into the land. The Nigerian Channels Television 11  report an updated on Over 100 Communities Submerged As River Niger Overflows. This comes weeks after the (NIHSA)12 issued a flood alert for seven states – Kebbi, Niger, Kwara, Kogi, Anambra, Delta, and Bayelsa – on the floodplains of River Niger. The agency also warned that the steady rise in water levels and weather forecast in the coming weeks have put Nigeria at risk of witnessing a recurrence of catastrophic flooding similar to what it witnessed in 2012.

Sea water can be carried by massive winds and hurricanes onto dry coastal lands and cause flooding. Sometimes this is made worse if the winds carry rains themselves. Sometimes water from the sea resulting from a tsunami can flow inland to cause damage.


(Embankments, known as levees, are built along the side of a river and are used to prevent high water from flooding bordering land)
Dams are man-made blocks mounted to hold water flowing down from a highland. The power in the water is used to turn propellers to generate

electricity. Sometimes, too much water held up in the dam can cause it to break and overflow the area. Excess water can also be intentionally released from the dam to prevent it from breaking and that can also cause floods.
February 26, 1972 – Buffalo Creek Valley, West Virginia
The failure of a coal-waste impoundment at the valley’s head took 125 lives, and caused more than $400 million in damages, including destruction of over 500 homes.13 

In many cold regions, heavy snow over the winter usually stays un-melted for sometime. There are also mountains that have ice on top of them. Sometimes the ice suddenly melts when the temperature rises, resulting in massive movement of water into places that are usually dry. This is usually called a snowmelt flood.

9. Livingstone, Andrew. (2013, June 24). Alberta Floods: Assessing the human, environmental and economic impacts. The Toronto Star. Retrieved from
10. August 11th, 2018
11. Channel Tv: September 13, 2018
12. Nigeria Hydrological Services Agency


The amount, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow at downstream locations. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily sequestered as snow or ice, and some may produce rapid runoff from surfaces including rock, pavement, roofs, and saturated or frozen ground. The fraction of incident precipitation promptly reaching a drainage channel has been observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain on accumulated snow.14

Most precipitation records are based on a measured depth of water received within a fixed time interval. Frequency of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to intensity by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if the duration of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction. Short duration precipitation is more significant to flooding within small drainage basins.15

The most important upslope factor in determining flood magnitude is the land area of the watershed upstream of the area of interest. Rainfall intensity is the second most important factor for watersheds of less than approximately 30 square miles or 80 square kilometres. The main channel slope is the second most important factor for larger watersheds. Channel slope and rainfall intensity become the third most important factors for small and large watersheds, respectively.

Time of Concentration is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel controlling flooding of the area of interest. The time of concentration defines the critical duration of peak rainfall for the area of interest.16 The critical duration of intense rainfall might be only a few minutes for roof and parking lot drainage structures, while cumulative rainfall over several days would be critical for river basins.


Water flowing downhill ultimately encounters downstream conditions slowing movement. The final limitation in coastal flooding lands is often the ocean or some coastal flooding bars which form natural lakes. In flooding low lands, elevation changes such as tidal fluctuations are significant determinants of coastal and estuarine flooding. Less predictable events like tsunamis and storm surges may also cause elevation changes in large bodies of water. Elevation of flowing water is controlled by the geometry of the flow channel and, especially, by depth of channel, speed of flow and amount of sediments in it. Flow channel restrictions like bridges and canyons tend to control water elevation above the restriction. The actual control point for any given reach of the drainage may change with changing water elevation, so a closer point may control for lower water levels until a more distant point controls at higher water levels.

Effective flood channel geometry may be changed by growth of vegetation, accumulation of ice or debris, or construction of bridges, buildings, or levees within the flood channel.

14. Babbitt, Harold E. & Doland, James J., Water Supply Engineering, McGraw-Hill Book Company, 1949            
15. Simon, Andrew L., Practical Hydraulics, John Wiley & Sons, 1981, ISBN 0-471-05381-3
16. Urquhart, Leonard Church, Civil Engineering Handbook, McGraw-Hill Book Company, 1959


Extreme flood events often result from coincidence such as unusually intense, warm rainfall melting heavy snow pack, producing channel obstructions from floating ice, and releasing small impoundments like beaver dams.[14] Coincident events may cause extensive flooding to be more frequent than anticipated from simplistic statistical prediction models considering only precipitation runoff flowing within unobstructed drainage channels.[15] Debris modification of channel geometry is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats and railway equipment. Recent field measurements during the 2010–11 Queensland floods showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by velocity and water depth fluctuations.  These considerations ignore further the risks associated with large debris entrained by the flow motion.

Some researchers have mentioned the storage effect in urban areas with transportation corridors created by cut and fill. Culverted fills may be converted to impoundments if the culverts become blocked by debris, and flow may be diverted along streets. Several studies have looked into the flow patterns and redistribution in streets during storm events and the implication on flood modelling.


Floods can have devastating consequences and can have effects on the economy, environment and people. The Nigerian Tribune17 report the flood in kano with a screaming headlines “Kano flood victim cry out for help” NAN18 reported that the state government had on September 17, confirmed the death of 31 people and destruction of 10,000 houses by flood in 15 local areas of the state.

  1. EFFECT ON THE ECONOMYDuring floods (especially flash floods), roads, bridges, farms, houses and automobiles are destroyed. People become homeless. Additionally, the government deploys firemen, police and other emergency apparatuses to help the affected. All these come at a heavy cost to people and the government. It usually takes years for affected communities to be re-built and business to come back to normalcy.  The monetary cost of all floodings in the USA in 2011 was $8,640,031,956 (approx 8.5B USD)19
    The environment also suffers when floods happen. Chemicals and other hazardous substances end up in the water and eventually contaminate the water bodies that floods end up in. In 2011, a huge tsunami hit Japan, and sea water flooded a part of the coastline. The flooding caused massive leakage in nuclear plants and has since caused high radiation in that area. Authorities in Japan fear that Fukushima radiation levels are 18 times higher than even thought. Additionally, flooding causes kills animals, and others insects are introduced to affected areas, distorting the natural balance of the ecosystem.

17. Wednesday, 10 October, 2018
18. News Agency of Nigeria

    Many people and animals have died in flash floods. Many more are injured and others made homeless. Water supply and electricity are disrupted and people struggle and suffer as a result. In addition to this, flooding brings a lot of diseases and infections including military fever, pneumonic plague, dermatopathia and dysentery. Sometimes insects and snakes make their ways to the area and cause a lot of havoc.

    PRIMARY EFFECTS: Apart from the basic and direct consequence on the economic, environment and the people and animal, there are equally primary, secondary and long time effect of the flooding to mankind and the environment.20

The primary effects of flooding include loss of life, damage to buildings and other structures, including bridges, sewerage systems, roadways, and canals.

Floods also frequently damage power transmission and sometimes power generation, which then has knock-on effects caused by the loss of power. This includes loss of drinking water treatment and water supply, which may result in loss of drinking water or severe water contamination. It may also cause the loss of sewage disposal facilities. Lack of clean water combined with human sewage in the flood waters raises the risk of waterborne diseases, which can include typhoid, giardia, cryptosporidium, cholera and many other diseases depending upon the location of the flood.

Damage to roads and transport infrastructure may make it difficult to mobilize aid to those affected or to provide emergency health treatment.

Flood waters typically inundate farm land, making the land unworkable and preventing crops from being planted or harvested, which can lead to shortages of food both for humans and farm animals. Entire harvests for a country can be lost in extreme flood circumstances. Some tree species may not survive prolonged flooding of their root systems.

20. Stephen Bratkovich, Lisa Burban, et al., “Flooding and its Effects on Environment”USDA Forest Service, Northeastern Area State and Private Forestry, St. Paul, MN, September 1993.


Economic hardship due to a temporary decline in tourism, rebuilding costs, or food shortages leading to price increases is a common after-effect of severe flooding. The impact on those affected may cause psychological damage to those affected, in particular where deaths, serious injuries and loss of property occur.

Urban flooding can lead to chronically wet houses, which are linked to an increase in respiratory problems and other illnesses.  Urban flooding also has significant economic implications for affected neighborhoods. In the United States, industry experts estimate that wet basements can lower property values by 10–25 percent and are cited among the top reasons for not purchasing a home.  According to the U.S. Federal Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster.


There is also something good about floods, especially those that occur in floodplains and farm fields. Floodwaters carry lots of nutrients that are deposited in the plains. Farmers love such soils, as they are perfect for cultivating some kinds of crops.

Floods (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.21 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.22 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.23 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.

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.

21. WMO/GWP Associated Programme on Flood Management, “Environmental Aspects of Integrated  Flood Management”, 2007.
22. Extension of the Flood Pulse Concept, Retrieved on 2012-06-12
23. Eychaner, J.H. (2015) Lessons from a 500-year record of flood elevationsAssociation of State Floodplain Managers, Technical Report 7, Accessed 2015-06-27


In many countries around the world, waterways prone to floods are often carefully been managed as well as other causes of flooding. Defenses such as detention basins, levees, 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 areas prone to urban flooding, one solution is the repair and expansion of man-made sewer systems and storm water 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. 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 storm water 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.

Sometimes there is no warning of flash floods, and that is why it is important to think of them and prepare for them before they happen. Here are a few things you can do in preparation or anticipation of flood.

  1. Know about your local relief centers and evacuation routes.
  2. Keep emergency numbers and important information handy, as well as emergency supplies, kits, first aid items. These may include water, canned food, can opener, battery-operated radio, flashlight and protective clothing.
  3. Fold and roll up anything onto higher ground (or upper floors of your home), including chemicals and medicines.
  4. Make sure everything that is of importance is secured (jewelry, documents, pets, and other valuables).
  5. Plant trees and shrubs and keep a lot of vegetation in your compound if you are in a low-lying area as that can control erosion and help soften the speed of the flowing water.

Then during the floods, it is important to take note of the following in order to survive after the occurrence of flood irrespective of which type of flood.

  1. Flash floods occur in a short spate of time. As soon as they start, be quick, keep safe and ensure that children and elderly are safe by leaving the house to a higher ground.
  2. Turn off all electrical appliances, gas, heating and the like if there is a bit of time.
  3. Leave the area before it gets too late. Do not drive through the water as moving water can sweep you away.
  4. Stay away from power lines or broken power transmission cables.
  5. Try to keep away from flood water as it may contain chemicals or other hazardous materials.

After the floods, note that most people do not remain the same. These are the few step to take.

  1. Make sure you have permission from emergency officers to get back inside your house.
  2. Keep all power and electrical appliance off until the house is cleaned up properly and an electrical personnel has confirmed that it is OK to put them.
  3. Make sure you have photographs, or a record of all the damage, as it may be needed for insurance claims.
  4. Clean the entire home, together with all the objects in it very well before you use them again. They may be contaminated.
  5. Wear appropriate gear (mask and gloves) before cleaning begins.

Flood control methods are used to reduce or prevent the detrimental effects of flood waters. Flood relief methods are used to reduce the effects of flood waters or high water levels. Humans cannot stop the rains from falling or stop flowing surface water from bursting its banks. These are natural events, but we can do something to prevent them from having great impact. Here are a few.


Coastal flooding has been addressed with coastal defences, such as sea walls, beach nourishment, and barrier islands. Tide gates are used in conjunction with dykes and culverts. They can be placed at the mouth of streams or small rivers, where an estuary begins or where tributary streams, or drainage ditches connect to sloughs. Tide gates close during incoming tides to prevent tidal waters from moving upland, and open during outgoing tides to allow waters to drain out via the culvert and into the estuary side of the dike. The opening and closing of the gates is driven by a difference in water level on either side of the gate. Sea walls and tide gates have been built in some places to prevent tidal waves from pushing the waters up ashore. In some areas too, sand bags are made and placed in strategic areas to retain floodwaters.

24. Guillermo R. Giannico; Jon A. Souder (2004). “The Effects of Tide Gates on Estuarine Habitats   and Migratory Fish” (PDF). National Sea Grant College Program, Oregon State University, Corvallis, OR. Product No. ORESU-G-04-002.

In some places, retaining walls levees, lakes, dams, reservoirs or retention ponds have been constructed to hold extra water during times of flooding.

It is important that builders acquire permission before buildings are erected. This will ensure that waterways are not blocked. Also, drainage systems must be covered and kept free from objects that chock them. This way, water can quickly run through if it rains and minimize any chance of town flooding. Drainage systems should also be covered to prevent litter from getting into them.

Trees, shrubs and grass help protect the land from erosion by moving water. People in low-lying areas must be encouraged to use a lot of vegetation to help break the power of moving flood water and also help reduce erosion.

In many developing countries, drainage systems are chocked with litter and people have little knowledge of the effects that can have during a rain. When it rains, waterways and culverts are blocked by massive chunks of litter and debris, and water finds its way into the streets and into people’s homes. Education is therefore very important, to inform and caution people about the dangers of floods, what causes floods, and what can be done to minimize its impact.


These are small reservoirs built and connected to waterways. They provide a temporary storage for floodwaters. This means in an event of flooding, water is drained into the basin first, giving people more time to evacuate. It can also reduce the magnitude of downstream flooding.

25. US Army Corps of Engineers. (1997). Hydrologic engineering requirements for reservoirs. EM 1110-2-1420. Retrieved from


Floods are caused by many factors or a combination of prolonged heavy rainpour (locally concentrated or throughout a catchment area), highly accelerated snowmelt as a result of global warming, severe winds over water, unusual high tides, tsunamis, or failure of dams, levees, retention ponds, or other structures that retained the water. Flooding can 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.

Periodic floods occur on many rivers, forming a surrounding region known as the flood plain. During times of rain, 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. About 30 percent of all precipitation becomes runoff and that amount might be increased by water from melting snow. River flooding is often caused by heavy rain, sometimes increased by melting snow.


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