Flood management orflood control are methods used to reduce or prevent the detrimental effects offlood waters. Flooding can be caused by a mix of both natural processes, such asextreme weather upstream, and human changes to waterbodies and runoff. Flood management methods can be either of thestructural type (i.e. flood control) and of thenon-structural type. Structural methods hold back floodwaters physically, while non-structural methods do not. Buildinghard infrastructure to prevent flooding, such asflood walls, is effective at managing flooding. However, it is best practice withinlandscape engineering to rely more onsoft infrastructure andnatural systems, such asmarshes andflood plains, for handling the increase in water.

Flood management can includeflood risk management, which focuses on measures to reduce risk, vulnerability and exposure to flood disasters and providing risk analysis through, for example,flood risk assessment.^[1]Flood mitigation is a related but separate concept describing a broader set of strategies taken to reduce flood risk and potential impact while improving resilience against flood events.

Asclimate change has led to increased flood risk an intensity, flood management is an important part ofclimate change adaptation andclimate resilience.^[2][3] For example, to prevent or managecoastal flooding,coastal management practices have to handle natural processes liketides but alsosea level rise due to climate change. The prevention and mitigation of flooding can be studied on three levels: on individual properties, small communities, and whole towns or cities.

Flood management is a broad term that includes measures to control or mitigate flood waters, such as actions to prevent floods from occurring or to minimize their impacts when they do occur.^[4][5]

Flood management methods can be structural or non-structural:

• Structural flood management (i.e.: flood control) is the reduction of the effects of a flood using physical solutions, such asreservoirs,levees, dredging and diversions.
• Non-structural flood management includesland-use planning, advanced warning systems and flood insurance. Further examples are: "zoning ordinances and codes,flood forecasting, flood proofing, evacuation and channel clearing, flood fight activities, and upstream land treatment or management to control flood damages without physically restraining flood waters".^[6]

There are several related terms that are closely connected or encompassed by flood management.

Flood management can includeflood risk management, which focuses on measures to reduce risk, vulnerability and exposure to flood disasters and providing risk analysis through, for example,flood risk assessment.^[1] In the context ofnatural hazards and disasters, risk management involves "plans, actions, strategies or policies to reduce the likelihood and/or magnitude of adverse potential consequences, based on assessed or perceived risks".^[7]

Flood control,flood protection,flood defence andflood alleviation are all terms that mean "the detention and/or diversion of water during flood events for the purpose of reducing discharge or downstream inundation".^[8] Flood control is part ofenvironmental engineering. It involves the management of water movement, such as redirecting flood run-off through the use offloodwalls andflood gates to prevent floodwaters from reaching a particular area.

Flood mitigation is a related but separate concept describing a broader set of strategies taken to reduce flood risk and potential impact while improving resilience against flood events. These methods include prevention, prediction (which enables flood warnings and evacuation), proofing (e.g.: zoning regulations), physical control (nature-based solutions and physical structures like dams andflood walls) and insurance (e.g.: flood insurance policies).^[9][10]

Flood relief methods are used to reduce the effects of flood waters or high water levels during a flooding event.^[11] They include evacuation plans and rescue operations. Flood relief is part of the response and recovery phase in a flood management plan.

Water levels during a flood tend to rise, then fall, very abruptly. The peak flood level occurs as a very steep, short spike; a quick spurt of water. Anything that slows thesurface runoff (marshes, meanders, vegetation, porous materials, turbulent flow, the river spreading over a floodplain) will slow some of the flow more than other parts, spreading the flow over time and blunting the spike. Even slightly blunting the spike significantly decreases the peak flood level. Generally, the higher the peak flood level, the more flood damage is done. Modern flood control seeks to "slow the flow", and deliberately flood some low-lying areas, ideally vegetated, to act as sponges, letting them drain again as the floodwaters go down.^{[14][15][better source needed]}

Where floods interact with housing, industry and farming that flood management is indicated and in such cases environmentally helpful solutions may provide solutions.^[16] Natural flooding has many beneficial environmental effects.^[17] This kind of flooding is usually a seasonal occurrence where floods help replenish soil fertility, restorewetlands and promotebiodiversity.^[18]

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 economical use of lands, such as interfering with farming. Structural damage can occur inbridge abutments, bank lines, sewer lines, and other structures within floodways. Waterway navigation andhydroelectric power are often impaired. Financial losses due to floods are typically millions of dollars each year, with the worst floods in recent U.S. history having cost billions of dollars.

Property owners may fit their homes to stop water entering by blocking doors and air vents,waterproofing important areas andsandbagging the edges of the building. Private precautionary measures are increasingly important in flood risk management.^[19]

Flood mitigation at the property level may also involve preventative measures focused on the building site, including scour protection for shoreline developments, improving rainwater in filtration through the use of permeable paving materials and grading away from structures, and inclusion ofberms,wetlands orswales in the landscape.^[20]

When more homes, shops and infrastructure are threatened by the effects of flooding, then the benefits of protection are worth the additional cost. Temporary flood defenses can be constructed in certain locations which are prone to floods and provide protection from rising flood waters. Rivers running through large urban developments are often controlled and channeled. Water rising above acanal's full capacity may cause flooding to spread to otherwaterways and areas of the community, which causes damage. Defenses (both long-term and short-term) can be constructed to minimize damage, which involves raising the edge of the water withlevees, embankments or walls. The high population and value of infrastructure at risk often justifies the high cost of mitigation in larger urban areas.

The most effective way of reducing the risk to people and property is through the production of flood risk maps. Most countries have produced maps which show areas prone to flooding based on flood data. In theUK, theEnvironment Agency has produced maps which show areas at risk. The map to the right shows a flood map for theCity of York, including the floodplain for a 1 in100-year flood (dark blue), the predicted floodplain for a 1 in 1000 year flood (light blue) and low-lying areas in need of flood defence (purple). The most sustainable way of reducing risk is to prevent further development in flood-prone areas and old waterways. It is important for at-risk communities to develop a comprehensive Floodplain Management plan.^[21]

In the US, communities that participate in theNational Flood Insurance Program must agree to regulate development in flood-prone areas.

One way of reducing the damage caused by flooding is to remove buildings from flood-prone areas, leaving them as parks or returning them to wilderness. Floodplain buyout programs have been operated in places like New Jersey (both before and afterHurricane Sandy),^[22]Charlotte, North Carolina,^[23] andMissouri.^[24]

In the United States, FEMA producesflood insurance rate maps that identify areas of future risk, enabling local governments to applyzoning regulations to prevent or minimize property damage.

Buildings and other urban infrastructure can be designed so that even if a flood does happen, the city can recover quickly and costs are minimized. For example, homes can be put on stilts,^[25] electrical and HVAC equipment can be put on the roof instead of in the basement, and subway entrances and tunnels can have built-in movable water barriers.^[26] New York City began a substantial effort to plan and build for flood resilience afterHurricane Sandy.^[27] Flood resilience technologies support the fast recovery of individuals and communities affected, but their use remains limited.^[28]

Some methods of flood control have been practiced since ancient times.^[38] 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).^[38] Other techniques include the construction of levees, lakes, dams, reservoirs,^[38]retention ponds to hold extra water during times of flooding.

Many dams and their associated reservoirs are designed completely or partially to aid in flood protection and control. Many large dams have flood-control reservations in which the level of a reservoir must be kept below a certain elevation before the onset of the rainy/summer melt season to allow a certain amount of space in which floodwaters can fill. Other beneficial uses of dam created reservoirs include hydroelectric power generation,water conservation, and recreation. Reservoir and dam construction and design is based upon standards, typically set out by the government. In the United States, dam and reservoir design is regulated by the US Army Corps of Engineers (USACE). Design of a dam and reservoir follows guidelines set by the USACE and covers topics such as design flow rates in consideration to meteorological, topographic, streamflow, and soil data for the watershed above the structure.^[39]

The termdry dam refers to a dam that serves purely for flood control without any conservation storage (e.g.Mount Morris Dam,Seven Oaks Dam).

Excess water can be used forgroundwater replenishment by diversion onto land that can absorb the water. This technique can reduce the impact of later droughts by using the ground as a natural reservoir. It is being used in California, where orchards and vineyards can be flooded without damaging crops,^[48] or in other places wilderness areas have been re-engineered to act as floodplains.^[49]

In many countries, rivers are prone to floods and are often carefully managed. Defenses such as levees,bunds, reservoirs, andweirs are used to prevent rivers from bursting their banks. A weir, also known as a lowhead dam, is most often used to createmillponds, but on theHumber River in Toronto, a weir was built nearRaymore Drive to prevent a recurrence of the flood damage caused byHurricane Hazel in October 1954.

The Leeds flood alleviation scheme uses movable weirs which are lowered during periods of high water to reduce the chances of flooding upstream. Two such weirs, the first in the UK, were installed on theRiver Aire in October 2017 at Crown Point, Leeds city centre andKnostrop. The Knostrop weir was operated during the2019 England floods. They are designed to reduce potential flood levels by up to one metre.^[50]

Coastal flooding is addressed with coastal defenses, such assea walls,beach nourishment, andbarrier islands.

Tide gates are used in conjunction withdykes and culverts. They can be placed at the mouth of streams or small rivers, where anestuary begins or where tributary streams, or drainage ditches connect tosloughs. 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.

The self-closing flood barrier (SCFB) is a flood defense system designed to protect people and property from inland waterway floods caused by heavy rainfall, gales, or rapid melting snow.^{[citation needed]} The SCFB can be built to protect residential properties and whole communities, as well as industrial or other strategic areas. The barrier system is constantly ready to deploy in a flood situation, it can be installed in any length and uses the rising flood water to deploy.

When permanent defenses fail, emergency measures such assandbags, inflatable impermeable sacks, or other temporary barriers are used.

In 1988, a method of using water to control flooding was discovered. This was accomplished by containing 2 parallel tubes within a third outer tube. When filled, this structure formed a non-rolling wall of water that can control 80 percent of its height in external water depth, with dry ground behind it. Eight foot tall water filled barriers were used to surroundFort Calhoun Nuclear Generating Station during the2011 Missouri River Flooding. Instead of trucking in sandbag material for a flood, stacking it, then trucking it out to a hazmat disposal site, flood control can be accomplished by using the on site water. However, these are not fool proof. A 8 feet (2.4 m) high 2,000 feet (610 m) long water filled rubber flood berm that surrounded portions of the plant was punctured by askid-steer loader and it collapsed flooding a portion of the facility.^[51]

AquaFence consists of interlocking panels which are waterproof and puncture-resistant, can be bolted down to resist winds, and use the weight of floodwater to hold them in place.^[52][53][54] Materials include marine-grade batlic laminate, stainless steel, aluminum and reinforced PVC canvas.^[54] The panels are reusable and can be stored flat between uses.^[55] The technology was designed as an alternative to building seawalls or placingsandbags in the path of floodwaters.^[55][54][56]

Other solutions, such asHydroSack, are polypropylene exteriors with wood pulp within, though they are one-time use.^[57]

There are several methods of non-structural flood management that form part offlood risk management strategies. These can involve policies that reduces the amount of urban structures built around floodplains or flood prone areas through land zoning regulations.^[58][19] This helps to reduce the amount of mitigation needed to protect humans and buildings from flooding events. Similarly,flood warning systems are important for reducing risks.^[58] Following the occurrence of flooding events, other measures such as rebuilding plans andinsurance can be integrated into flood risk management plans.^[58] Flood risk management strategy diversification is needed to ensure that management strategies cover several different scenarios and ensure best practices.^[59]

Flood risk management aims to reduce the human and socio-economic losses caused byflooding and is part of the larger field ofrisk management. Flood risk management analyzes the relationships between physical systems and socio-economic environments throughflood risk assessment and tries to create understanding and action about the risks posed by flooding. The relationships cover a wide range of topics, from drivers and natural processes, to models and socio-economic consequences.^[vague]

This relationship examines management methods which includes a wide range of flood management methods including but are not limited to flood mapping and physical implication measures.^[60] Flood risk management looks at how to reduce flood risk and how to appropriately manage risks that are associated with flooding. Flood risk management includes mitigating and preparing for flooding disasters, analyzing risk, and providing a risk analysis system to mitigate the negative impacts caused by flooding.^[60]

Flooding and flood risk are especially important with moreextreme weather andsea level rise causedby climate change as more areas will be effected by flood risk.^[61]

Flood mapping is a tool used by governments and policy makers to delineate the borders of potential flooding events, allowing educated decisions to prevent extreme flooding events.^[62] Flood maps are useful to create documentation that allows policy makers to make informed decisions about flood hazards.^[63] Flood mapping also providesconceptual models to both the public and private sectors with information about flooding hazards.^[64] Flood mapping has been criticized in many areas around the world, due to the absence of public accessibility, technical writing and data, and lack of easy-to-understand information. However, revived attention towards flood mapping has renewed the interest in enhancing current flood mapping for use as a flood risk management method.^[63]

Flood modelling is a tool used to model flood hazard and the effects on humans and the physical environment.^[65] Flood modelling takes into consideration how flood hazards, external and internal processes and factors, and the main drivers of floods interact with each other. Flood modelling combines factors such as terrain,hydrology, and urban topography to reproduce the evolution of a flood in order to identify the different levels of flooding risks associated with each element exposed.^[66] The modelling can be carried out using hydraulic models,^[67] conceptual models,^[68] or geomorphic methods.^[69] Nowadays, there is a growing attention also in the production of maps obtained withremote sensing.^[70] Flood modelling is helpful for determining building development practices and hazard mitigation methods that reduce the risks associated with flooding.^[71]

Stakeholder engagement is a useful tool for flood risk management that allows enhanced public engagement for agreements to be reached on policy discussions.^[72] Different management considerations can be taken into account including emergency management anddisaster risk reduction goals, interactions of land-use planning with the integration of flood risks and required policies.^[63] In flood management, stakeholder engagement is seen as an important way to achieve greater cohesion and consensus.^[73] Integrating stakeholder engagement into flood management often provides a more complex analysis of the situation; this generally adds more demand in determining collective solutions and increases the time it takes to determine solutions.^[72]

Wetlands are effective for flood management strategies, particularly in coastal regions, because they are able to hold excess water that flows towards inland regions.^[74] For example, the communities of multiple bays and river deltas, including theChesapeake Bay, theMississippi River delta, and theYangtze River delta, benefit from local wetlands with respect to flood management, specifically because wetlands are able to protect inland communities fromstorm surges that these regions often face.^[75] Wetlands also support flood mitigation from a financial perspective. For example, duringHurricane Sandy in 2012, coastal wetlands elicited an estimated $625 million in savings related to the prevention of damage to private property, such as homes, and public infrastructure, such as roadways.^[76]

The costs of flood protection rise as more people and property are to be protected. The USFEMA, for example, estimates that for every $1.00 spent on mitigation, $4.00 is saved.^[77]

An elaborate system of flood way defenses can be found in theCanadian province ofManitoba. TheRed River flows northward from theUnited States, passing through the city ofWinnipeg (where it meets theAssiniboine River) and intoLake Winnipeg. As is the case with all north-flowing rivers in the temperate zone of the Northern Hemisphere, snow melt in southern sections may cause river levels to rise before northern sections have had a chance to completely thaw. This can lead to devastating flooding, as occurred in Winnipeg during thespring of 1950. To protect the city from future floods, the Manitoba government undertook the construction of a massive system of diversions, dikes, and flood ways (including theRed River Floodway and thePortage Diversion). The system kept Winnipeg safe during the1997 flood which devastated many communities upriver from Winnipeg, includingGrand Forks, North Dakota andSte. Agathe, Manitoba.^{[citation needed]}

In the United States, theU.S. Army Corps of Engineers is the lead flood control agency.^[78] AfterHurricane Sandy,New York City'sMetropolitan Transportation Authority (MTA) initiated multiple flood barrier projects to protect the transit assets in Manhattan. In one case, the MTA'sNew York City Transit Authority (NYCT) sealed subway entrances inlower Manhattan using a deployable fabric cover system called Flex-Gate,^[79] a system that protects the subway entrances against 14 feet (4.3 m) of water.^[80] Extreme storm flood protection levels have been revised based on newFederal Emergency Management Agency guidelines for 100-year and 500-year design flood elevations.^[81][82]

In theNew Orleans Metropolitan Area, 35 percent of which sits below sea level, is protected by hundreds of miles of levees and flood gates. This system failedcatastrophically, with numerous breaks, duringHurricane Katrina (2005) in the city proper and in eastern sections of the Metro Area, resulting in the inundation of approximately 50 percent of the metropolitan area, ranging from a few inches to twenty feet in coastal communities.

TheMorganza Spillway provides a method of diverting water from theMississippi River when a river flood threatensNew Orleans,Baton Rouge and other major cities on the lower Mississippi. It is the largest of a system of spillways and floodways along the Mississippi. Completed in 1954, the spillway has been opened twice, in 1973 and in 2011.^{[citation needed]}

In an act of successful flood prevention, thefederal government offered to buy out flood-prone properties in the United States in order to prevent repeated disasters after the 1993 flood across the Midwest. Several communities accepted and the government, in partnership with the state, bought 25,000 properties which they converted intowetlands. These wetlands act as a sponge in storms and in 1995, when the floods returned, the government did not have to expend resources in those areas.^[83]

InKyoto, Japan, the Hata clan successfully controlled floods on the Katsura River in around 500 A.D and also constructed a sluice on the Kazuno River.^[84]

InChina flooddiversion areas are rural areas that are deliberately flooded in emergencies in order to protect cities.^[85]

The consequences ofdeforestation and changing land use on the risk and severity of flooding are subjects of discussion. In assessing the impacts of Himalayan deforestation on theGanges-Brahmaputra Lowlands, it was found that forests would not have prevented or significantly reduced flooding in the case of an extreme weather event.^[86] However, more general or overview studies agree on the negative impacts that deforestation has on flood safety - and the positive effects of wise land use andreforestation.^[87][88]

Many have proposed that loss of vegetation (deforestation) will lead to an increased risk of flooding. With natural forest cover the flood duration should decrease. Reducing the rate of deforestation should improve the incidents and severity of floods.^[89]

InEgypt, both theAswan Low Dam (1902) and theAswan High Dam (1976) have controlled various amounts of flooding along theNile River.

Following the misery and destruction caused by the1910 Great Flood of Paris, the French government built a series of reservoirs calledLes Grands Lacs de Seine [fr] (or Great Lakes) which helps remove pressure from theSeine during floods, especially the regular winter flooding.^[90]

London is protected from flooding byThames Barrier, a huge mechanical barrier across theRiver Thames, which is raised when the water level reaches a certain point. This project has been operational since 1982 and was designed to protect against a surge of water such as theNorth Sea flood of 1953.

In 2023 it was found that over 4,000 flood defence schemes in England were ‘almost useless’ with many of them in areas hit byStorm Babet.^[91]

TheSaint Petersburg Dam was completed in 2008 to protectSaint Petersburg fromstorm surges. It also has a main traffic function, as it completes aring road around Saint Petersburg. Eleven dams extend for 25.4 kilometres (15.8 mi) and stand 8 metres (26 ft) abovewater level.

The Netherlands has one of the best flood control systems in the world, notably through its construction of dykes. The country faces high flooding risk due to the country's low-lying landscapes.^[92] The largest and most elaborate flood defenses are referred to as theDelta Works with theOosterscheldekering as its crowning achievement. These works in the southwestern part of the country were built in response to the North Sea flood of 1953. The Dutch had already built one of the world's largest dams in the north of the country. TheAfsluitdijk closing occurred in 1932.

New ways to deal with water are constantly being developed and tested, such as the underground storage of water, storing water in reservoirs in largeparking garages or onplaygrounds.^[93] Rotterdam started a project to construct a floating housing development of 120 acres (0.49 km²) to deal with rising sea levels.^[94] Several approaches, fromhigh-tech sensors detecting imminentlevee failure tomovable semi-circular structures closing an entire river, are being developed or used around the world. Regular maintenance of hydraulic structures, however, is another crucial part of flood control.^[95]

Flooding is the greatest natural hazard inNew Zealand (Aotearoa),^[96] and its control is primarily managed and funded by local councils.^[97] Throughout the country there is a network of more than 5284 km oflevees,^[98] while gravel extraction to lower river water levels is also a popular flood control technique.^[99][100] The management of flooding in the country is shifting towardsnature based solutions,^[101] such as the widening of theHutt River channel inWellington.^[102]

• Bioswale – Landscape elements designed to manage surface runoff water
• Coastal management – Preventing flooding and erosion of shorelines
• Constructed wetland – Artificial wetland to treat wastewater, greywater or stormwater runoff
• Disaster risk reduction – Preventing and reducing disaster risk factors
• Flood Control Act of 1936 – 1936 United States act of Congress to control floods
• Flood risk assessment – Type of risk assessment with respect to floods
• Rain garden – Runoff reducing landscaping method
• Stormwater detention vault – underground structure designed to manage excess stormwater runoff on a developed sitePages displaying wikidata descriptions as a fallback
• Stormwater harvesting
• Sustainable drainage system – Designed to reduce the potential impact of development
• Tidal barrage – Dam-like structure
• Tree box filter – Stormwater treatment system
• Water-sensitive urban design – Integrated approach to urban water cycle
• Wetland – Type of land area that is flooded or saturated with water

• Flood articles – BBC News