The types of floods include areal flooding, riverine flooding,
river-management purposes. For example, agricultural flooding may occur in preparing paddy fields
for the growing of semi-aquatic rice in many countries.
Flooding may occur as an overflow of water from water bodies, such as a
domestic animals
.
Floods can also occur in rivers when the flow rate exceeds the capacity of the
fertile
. Also, the rivers provide easy travel and access to commerce and industry.
Flooding can damage property and also lead to secondary impacts. These include in the short term an increased spread of
vector-bourne disesases, for example those diseases transmitted by mosquitos. Flooding can also lead to long-term displacement of residents.[7] Floods are an area of study of hydrology and hydraulic engineering
.
Types
Areal flooding
Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either
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.[8]
Slow-rising floods most commonly occur in large rivers with large
tropical cyclones
. However, large rivers may have rapid flooding events in areas with dry climates, since they may have large basins but small river channels, and rainfall can be very intense in smaller areas of those basins.
Rapid flooding events, including
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.[citation needed] 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.[9]
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.
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.
Coastal areas may be flooded by storm surges combining with high tides and large wave events at sea, resulting in waves over-topping flood 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. A storm surge is "an additional rise of water generated by a storm, over and above the predicted astronomical tides".[10] Due to the effects of climate change (e.g. sea level rise and an increase in extreme weather events) and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected.[11]
Flooding in
barometric pressure
and large waves meeting high upstream river flows.
may accumulate on property and in public rights-of-way. It can seep through building walls and floors, or backup into buildings through sewer pipes, cellars, toilets and sinks.
There are several types of urban flooding, each with a different cause. City planners distinguish pluvial flooding (flooding caused by heavy rain),fluvial flooding (caused by a nearby river overflowing its banks), or coastal flooding (often caused by storm surges). Urban flooding is a hazard to both the population and infrastructure. Some well known disaster events include the inundations of Nîmes (France) in 1998 and Vaison-la-Romaine (France) in 1992, the flooding of New Orleans (United States) in 2005, and the flooding in Rockhampton, Bundaberg, Brisbane during the 2010–2011 Queensland floods in Australia, and the 2022 eastern Australia floods.
Intentional floods
The intentional flooding of land that would otherwise remain dry may take place for agricultural, military or river-management purposes. This is a form of hydraulic engineering. Agricultural flooding may occur in preparing paddy fields for the growing of semi-aquatic rice in many countries.
Flooding for river management may occur in the form of diverting flood waters in a river at flood stage upstream from areas that are considered more valuable than the areas that are sacrificed in this way. This may be done ad hoc,
Linden, North Brabant
.
Military inundation creates an obstacle in the field that is intended to impede the movement of the enemy.
Grebbe line
in that country).
To count as controlled, a military inundation has to take the interests of the civilian population into account, by allowing them a timely evacuation, by making the inundation reversible, and by making an attempt to minimize the adverse ecological impact of the inundation. That impact may also be adverse in a hydrogeological sense if the inundation lasts a long time.[17]
Examples for uncontrolled inundations are the
Second World War
).
Causes
See also:
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.
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.
This has been exacerbated by human activities such as draining wetlands that naturally store large amounts of water and building paved surfaces that do not absorb any water.
stream channels or retained in natural ponds, lakes, and human-made reservoirs. About 30 percent of all precipitation becomes runoff[21]
and that amount might be increased by water from melting snow.
River flooding is often caused by heavy rain, sometimes increased by melting snow. A flood that rises rapidly, with little or no warning, is called a flash flood. Flash floods usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation.
Periodic floods occur on many rivers, forming a surrounding region known as the
shorelines of lakes and bays can be flooded by severe winds—such as during hurricanes
—that blow water into the shore areas.
Upslope factors
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.[22]
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.[23]
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.[24]
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.[25]
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.
Downslope factors
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[24] 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.
Climate change
NOAA predicts different levels of sea level rise for coastlines within a single country.[27]
floods are likely to become more severe when they do occur.[28]: 1155 The interactions between rainfall and flooding are complex. There are some regions in which flooding is expected to become rarer. This depends on several factors. These include changes in rain and snowmelt, but also soil moisture.[28]: 1156 Climate change leaves soils drier in some areas, so they may absorb rainfall more quickly. This leads to less flooding. Dry soils can also become harder. In this case heavy rainfall runs off into rivers and lakes. This increases risks of flooding.[28]
: 1155
Coincidence
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
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.[31] These considerations ignore further the risks associated with large debris entrained by the flow motion.[32]
Negative impacts
Floods can also be a huge destructive power. When water flows, it has the ability to demolish all kinds of buildings and objects, such as bridges, structures, houses, trees, and cars. Economical, social and natural environmental damages are common factors that are impacted by flooding events and the impacts that flooding has on these areas can be catastrophic.[33]
There have been numerous flood incidents around the world which have caused devastating damage to infrastructure, the natural environment and human life.[33] Flood risks can be defined as the risk that floods pose to individuals, property and the natural landscape based on specific hazards and vulnerability. The extent of flood risks can impact the types of mitigation strategies required and implemented.[34]
Floods can have devastating impacts to human societies. Flooding events worldwide are increasing in frequency and severity, leading to increasing costs to societies.[33] A large amount of the world's population lives in close proximity to major coastlines,[35] while many major cities and agricultural areas are located near floodplains.[36] There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions.[37]
Catastrophic riverine flooding can result from major
, most commonly resulting from undersea earthquakes.
Economic impacts
The primary effects of flooding include loss of life and damage to buildings and other structures, including bridges, sewerage systems, roadways, and canals. The economic impacts caused by flooding can be severe.[36]
Every year flooding causes countries billions of dollars worth of damage that threatens the livelihood of individuals.[38] As a result, there is also significant socio-economic threats to vulnerable populations around the world from flooding.[38] For example, in Bangladesh in 2007, a flood was responsible for the destruction of more than one million houses. And yearly in the United States, floods cause over $7 billion in damage.[39]
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.[40]
Flooding in areas where people live 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.[41] According to the U.S. Federal Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster.[42] In the United States, insurance is available against flood damage to both homes and businesses.[43]
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.
Health impacts
Fatalities connected directly to floods are usually caused by
Injuries can lead to an excessive amount of morbidity when a flood occurs. Injuries are not isolated to just those who were directly in the flood, rescue teams and even people delivering supplies can sustain an injury. Injuries can occur anytime during the flood process; before, during and after.[44] During floods accidents occur with falling debris or any of the many fast moving objects in the water. After the flood rescue attempts are where large numbers injuries can occur.[44]
diarrheal diseases, to mention a few. Gastrointestinal disease and diarrheal diseases are very common due to a lack of clean water during a flood. Most of clean water supplies are contaminated when flooding occurs. Hepatitis A and E are common because of the lack of sanitation in the water and in living quarters depending on where the flood is and how prepared the community is for a flood.[44]
When floods hit, people lose nearly all their crops, livestock, and food reserves and face starvation.[45]
Floods (in particular more frequent or smaller floods) can also bring many benefits, such as recharging
fisheries
for a few years.
For some fish species, an inundated floodplain may form a highly suitable location for
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.[51]
Flooding can bring benefits, such as making the soil more fertile and providing it with more nutrients. For this reason, periodic flooding was essential to the well-being of ancient communities along the
In areas prone to urban flooding, one solution is the repair and expansion of human-made sewer systems and stormwater infrastructure. Another strategy is to reduce impervious surfaces in streets, parking lots and buildings through natural drainage channels,
In some areas, the presence of certain species (such as
beavers) can be beneficial for flood control reasons. Beavers build and maintain beaver dams which will reduce the height of flood waves moving down the river (during periods of heavy rains), and will reduce or eliminate damage to human structures,[53][54] at the cost of minor flooding near the dams (often on farmland). Besides this, they also boost wildlife populations and filter pollutants (manure, fertilisers, slurry).[53] UK environment minister Rebecca Pow stated that in the future the beavers could be considered a "public good" and landowners would be paid to have them on their land.[55]
Flood control (or flood mitigation, protection or alleviation) methods are used to reduce or prevent the detrimental effects of flood waters.[56][57] Flooding can be caused by a mix of both natural processes, such as extreme weather upstream, and human changes to waterbodies and runoff. Flood control methods can be either of the structural type and of the non-structural type. Structural methods hold back floodwaters physically, while non-structural methods do not. Building hard infrastructure to prevent flooding, such as flood walls, is effective at managing flooding. However, best practice within landscape engineering is more and more to rely on soft infrastructure and natural systems, such as marshes and flood plains, for handling the increase in water.
, rather than trying to prevent floods altogether. It also involves the management of people, through measures such as evacuation and flood proofing properties. The prevention and mitigation of flooding can be studied on three levels: on individual properties, small communities, and whole towns or cities.
Flood safety planning
In the United States, the National Weather Service gives out the advice "Turn Around, Don't Drown" for floods; that is, it recommends that people get out of the area of a flood, rather than trying to cross it. At the most basic level, the best defense against floods is to seek higher ground for high-value uses while balancing the foreseeable risks with the benefits of occupying flood hazard zones.[59]: 22–23 Critical community-safety facilities, such as hospitals, emergency-operations centers, and police, fire, and rescue services, should be built in areas least at risk of flooding. Structures, such as bridges, that must unavoidably be in flood hazard areas should be designed to withstand flooding. Areas most at risk for flooding could be put to valuable uses that could be abandoned temporarily as people retreat to safer areas when a flood is imminent.
Planning for flood safety involves many aspects of analysis and engineering, including:
observation of previous and present flood heights and inundated areas,
statistical,
hydrologic
, and hydraulic model analyses,
mapping inundated areas and flood heights for future flood scenarios,
long-term
land use planning
and regulation,
engineering design
and construction of structures to control or withstand flooding,
intermediate-term monitoring, forecasting, and emergency-response planning, and
short-term monitoring, warning, and response operations.
Each topic presents distinct yet related questions with varying scope and scale in time, space, and the people involved. Attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia.[60][page needed]
In the United States, the Association of State Floodplain Managers works to promote education, policies, and activities that mitigate current and future losses, costs, and human suffering caused by flooding and to protect the natural and beneficial functions of floodplains – all without causing adverse impacts.
disaster mitigation in the United States is available from the Federal Emergency Management Agency.[62]
Flood clean-up safety
Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers include
life jackets, and watertight boots with steel toes and insoles.[63]
A series of annual maximum flow rates in a stream reach can be analyzed
statistically to estimate the 100-year flood and floods of other recurrence intervals there. Similar estimates from many sites in a hydrologically similar region can be related to measurable characteristics of each drainage basin to allow indirect estimation
of flood recurrence intervals for stream reaches without sufficient data for direct analysis.
Physical process models of channel reaches are generally well understood and will calculate the depth and area of inundation for given channel conditions and a specified flow rate, such as for use in floodplain mapping and
channel) or 2D models (variable flood depths measured across the extent of a floodplain). HEC-RAS,[64] the Hydraulic Engineering Center model, is among the most popular software, if only because it is available free of charge. Other models such as TUFLOW[65]
combine 1D and 2D components to derive flood depths across both river channels and the entire floodplain.
Physical process models of complete drainage basins are even more complex. Although many processes are well understood at a point or for a small area, others are poorly understood at all scales, and process interactions under normal or extreme climatic conditions may be unknown. Basin models typically combine land-surface process components (to estimate how much rainfall or snowmelt reaches a channel) with a series of reach models. For example, a basin model can calculate the runoff hydrograph that might result from a 100-year storm, although the recurrence interval of a storm is rarely equal to that of the associated flood. Basin models are commonly used in flood forecasting and warning, as well as in analysis of the effects of land use change and climate change.
In the United States, an integrated approach to real-time hydrologic computer modelling uses observed data from the
Saint Lawrence Seaway
.
The Global Flood Monitoring System, "GFMS", a computer tool which maps flood conditions worldwide, is available online.[70] Users anywhere in the world can use GFMS to determine when floods may occur in their area. GFMS uses precipitation data from NASA's Earth observing satellites and the Global Precipitation Measurement satellite, "GPM". Rainfall data from GPM is combined with a land surface model that incorporates vegetation cover, soil type, and terrain to determine how much water is soaking into the ground, and how much water is flowing into streamflow.
Users can view statistics for rainfall, streamflow, water depth, and flooding every 3 hours, at each 12-kilometer gridpoint on a global map. Forecasts for these parameters are 5 days into the future. Users can zoom in to see inundation maps (areas estimated to be covered with water) in 1-kilometer resolution.[71]
Anticipating floods before they occur allows for precautions to be taken and people to be warned[72] so that they can be prepared in advance for flooding conditions. For example, farmers can remove animals from low-lying areas and utility services can put in place emergency provisions to re-route services if needed. Emergency services can also make provisions to have enough resources available ahead of time to respond to emergencies as they occur. People can evacuate areas to be flooded.
In order to make the most accurate flood forecasts for
stream flows to measured past rainfall events.[73] Coupling this historical information with real-time knowledge about volumetric capacity in catchment areas, such as spare capacity in reservoirs, ground-water levels, and the degree of saturation of area aquifers
is also needed in order to make the most accurate flood forecasts.
Radar estimates of rainfall and general weather forecasting techniques are also important components of good flood forecasting. In areas where good quality data is available, the intensity and height of a flood can be predicted with fairly good accuracy and plenty of lead time. The output of a flood forecast is typically a maximum expected water level and the likely time of its arrival at key locations along a waterway,[69] and it also may allow for the computation of the likely statistical return period of a flood. In many developed countries, urban areas at risk of flooding are protected against a 100-year flood – that is a flood that has a probability of around 63% of occurring in any 100-year period of time.
myths and the primeval waters which appear in certain creation myths, as the flood waters are described as a measure for the cleansing of humanity, in preparation for rebirth. Most flood myths also contain a culture hero, who "represents the human craving for life".[75]
Germanic languages (compare GermanFlut, Dutchvloed from the same root as is seen in flow, float; also compare with Latinfluctus, flumen), meaning "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave,".[76][77] The Old English word flōd comes from the Proto-Germanicfloduz (Old Frisianflod, Old Norsefloð, Middle Dutchvloet, Dutch vloed, German Flut, and Gothicflodus derives from floduz).[76]
^Urquhart, Leonard Church, Civil Engineering Handbook, McGraw-Hill Book Company, 1959
^US Department of Commerce, National Oceanic and Atmospheric Administration. "What is high tide flooding?". oceanservice.noaa.gov. Archived from the original on 16 October 2020. Retrieved 12 October 2020.