Flood management or flood control are ways to reduce or stop the harmful effects of flood water. Flooding can happen because of natural causes, like heavy rain upstream, or because of human actions that change water systems and how water flows. Flood management methods can be structural, which means they use physical barriers, or non-structural, which do not. Building strong structures, like flood walls, can help control flooding. However, experts recommend using natural systems, such as wetlands and floodplains, to manage water more effectively.

Flood management includes flood risk management, which involves steps to lower the chances of flood damage, protect people and places from harm, and study flood risks through methods like flood risk assessments. Flood mitigation is a similar but different idea that includes a wide range of strategies to reduce flood risks and damage while helping communities become more prepared for floods.

Because climate change has made flooding more frequent and severe, managing floods is an important part of adapting to climate change and building resilience. For example, to manage coastal flooding, practices must address natural processes like tides and rising sea levels caused by climate change. Flood prevention and management can be studied at three levels: on individual homes, in small communities, and across entire towns or cities.

Terminology

Flood management refers to ways to control or reduce the effects of flooding. This includes actions to stop floods from happening or to lessen their impact when they occur.

Flood management methods can be structural or non-structural:

• Structural flood management, also called flood control, uses physical solutions to reduce flood effects. Examples include building reservoirs, levees, dredging rivers, and creating diversions.
• Non-structural flood management includes planning how land is used, setting up warning systems, and offering flood insurance. Other examples are zoning rules, flood forecasting, flood-proofing buildings, evacuating people, clearing water channels, fighting floods, and managing land upstream to reduce flood damage without physically blocking water.

There are other related terms connected to flood management.

Flood management includes flood risk management, which focuses on reducing the risk, harm, and exposure to flood disasters. This involves analyzing risks through methods like flood risk assessments. In the context of natural disasters, risk management includes plans, actions, strategies, or policies to lower the chance or severity of negative outcomes based on known or suspected risks.

Terms like flood control, flood protection, flood defense, and flood alleviation all describe methods to hold back or redirect water during floods to reduce water flow or flooding downstream. Flood control is part of environmental engineering, which involves managing water movement, such as using floodwalls and flood gates to redirect floodwater away from specific areas.

Flood mitigation is a separate but related concept. It includes a wide range of strategies to reduce flood risk and impact while improving preparedness for floods. These strategies include prevention, prediction (which helps with warnings and evacuations), proofing (such as zoning rules), physical control (like nature-based solutions or structures such as dams and flood walls), and insurance (such as flood insurance policies).

Flood relief methods are used during floods to reduce the effects of high water levels. These include evacuation plans and rescue operations. Flood relief is part of the response and recovery steps in a flood management plan.

Causes of flooding

Floods can happen because of many reasons, such as long periods of heavy rain (either in one area or across a whole region), very fast melting snow, strong winds over water, unusually high tides, tsunamis, or when dams, levees, retention ponds, or other water-holding structures break. Flooding can get worse when there are more surfaces that do not let water soak in, like pavement, or when natural dangers like wildfires reduce the amount of plants that can take in rainwater.

During rain, some water is stored in ponds or soil, some is taken in by grass and plants, some turns into vapor, and the rest flows over the land as runoff. Floods happen when ponds, lakes, riverbeds, soil, and plants cannot absorb all the water.

This problem has gotten worse because of human actions, such as draining wetlands that naturally hold a lot of water and building paved surfaces that do not absorb any water. Water then flows off the land in amounts that streams or natural and human-made water storage areas cannot handle. About 30 percent of all rain becomes runoff, and this amount can increase further from melting snow.

During a flood, water levels often rise and fall quickly. The highest point of the flood, called the peak, happens as a sudden, sharp increase in water. Features like marshes, winding rivers, plants, porous materials, rough water movement, or rivers spreading out over floodplains can slow the flow of water in some areas more than others, spreading the flood over time and reducing the sharp rise. Even a small reduction in the peak flood level can lower the amount of flood damage. Modern flood control methods aim to "slow the flow" and intentionally flood some low areas, especially those with plants, to act like sponges that soak up water and then release it as the flood waters recede.

Purposes

Floods can affect homes, industries, and farms. In these situations, flood management is needed. Natural flooding can help the environment. This kind of flooding often happens seasonally and helps make soil richer, restore wetlands, and support many types of plants and animals.

Floods cause many problems. They can damage buildings and harm people and animals. Fast-moving water can erode soil and move sediment to other places, like farther down a river or along a coastline. Floods can pollute or destroy areas where fish and animals live. Long, high floods can slow down traffic in places without raised roads. Floods can also stop water from draining properly and make farming harder. Floods can damage parts of bridges, riverbanks, sewers, and other structures in flood-prone areas. They can also harm water transportation and power from hydroelectric dams. Floods often cost millions of dollars each year, and some of the worst floods in U.S. history have cost billions of dollars.

Homeowners can protect their homes by sealing doors and vents, waterproofing important areas, and using sandbags to block water. Personal safety steps are important for managing flood risks.

At the property level, flood prevention can include protecting shorelines from erosion, using materials that let water drain better, and grading land away from buildings. Adding raised areas, wetlands, or swales to the landscape can also help.

When many homes, businesses, and structures are at risk from flooding, the cost of protection may be worth it. Temporary flood barriers can be built in areas that often flood. Large rivers in cities are often controlled with channels. If water rises above a canal’s normal level, it can spread to other areas and cause damage. Long-term and short-term defenses, like levees, embankments, or walls, can be built to reduce damage. In cities with many people and expensive infrastructure, the cost of flood protection is often justified.

The best way to reduce flood risks is by creating flood risk maps. Many countries have maps showing areas likely to flood. In the UK, the Environment Agency has maps showing flood-prone areas. For example, a map of York shows areas at risk from a 1-in-100-year flood (dark blue), a 1-in-1000-year flood (light blue), and areas needing flood defenses (purple). The most sustainable way to reduce risk is to avoid building in flood-prone areas or old waterways. At-risk communities should create detailed floodplain management plans.

In the U.S., communities in the National Flood Insurance Program must follow rules to control development in flood-prone areas.

One way to reduce flood damage is to move buildings out of flood-prone areas and turn them into parks or natural spaces. Floodplain buyout programs have been used in places like New Jersey (before and after Hurricane Sandy), Charlotte, North Carolina, and Missouri.

In the U.S., FEMA creates flood insurance maps that show future flood risks. These maps help local governments use zoning rules to prevent or reduce property damage.

Buildings and city structures can be designed to help cities recover quickly after floods. For example, homes can be built on stilts or raised platforms. Electrical and heating systems can be placed on rooftops instead of in basements. Subway entrances and tunnels can have water barriers that can be moved. After Hurricane Sandy, New York City started planning for flood resilience. Technologies that help cities recover from floods are useful but not widely used.

Flooding can happen in cities (urban flooding) or along coasts (coastal flooding). Rising sea levels can make coastal flooding worse. In some places, there are also risks from floods caused by melting glaciers.

There are many ways to adapt to flooding:

More heavy rain may require bigger stormwater systems. These systems separate stormwater from wastewater to prevent pollution during heavy rains. An example is the SMART Tunnel in Kuala Lumpur.

After Hurricane Sandy, New York City created a detailed plan for rebuilding and resilience. The plan includes making buildings less likely to flood and reducing problems that happened during and after the storm, such as long fuel shortages, flooded hospitals, higher insurance costs, damage to power systems, and flooded subway and road tunnels.

Structural methods

Some ways to control floods have been used for a long time. These include planting plants to hold water, building steps on hills to slow water flow, and creating floodways (man-made paths to move floodwater away). Other methods include building levees, lakes, dams, reservoirs, and retention ponds to store extra water during floods.

Many dams and their reservoirs are built to help control floods. Some large dams have areas where water levels must stay low before rainy seasons so that floodwater can fill the space. Dams also help with making electricity, saving water, and providing places for people to enjoy. Rules for building dams and reservoirs are set by the government. In the United States, the US Army Corps of Engineers (USACE) creates guidelines for dam and reservoir designs. These guidelines include how much water can flow through the dam, based on weather, land shape, river flow, and soil in the area above the dam.

A dry dam is a type of dam that only helps control floods and does not store water for other uses, like the Mount Morris Dam and Seven Oaks Dam.

Flood control channels are large, empty areas where water can flow but is not kept there (except during floods). Some channels are built below streets in cities so that floodwater can drain into rivers or other water sources. These channels are sometimes built along old natural water paths to help reduce flooding.

In the 1960s, people often made flood control channels by straightening rivers into concrete paths. However, this sometimes made flooding worse. Now, some areas are changing back to natural paths with curves, plants, and materials that let water soak in.

During a flood, water levels rise and fall quickly. The highest point of water is a sharp, short rise. Things like wetlands, winding rivers, plants, and rough ground can slow water flow, spreading it out over time and reducing the height of the peak. Slowing the peak even a little can lower the flood’s height. Higher flood peaks cause more damage. Smooth, straight channels made of concrete speed up water flow and can make flooding worse. Modern flood control aims to slow water by letting floodwater enter low areas, especially those with plants, to absorb the water.

Extra water can be used to refill underground water sources by directing it to land that can soak it up. This helps reduce droughts later. In California, orchards and vineyards are sometimes flooded without harming crops. In other places, natural areas are changed to act like floodplains.

In many countries, rivers are managed carefully to prevent flooding. Defenses like levees, bunds, reservoirs, and weirs are used to stop rivers from overflowing. A weir, or low-head dam, is often used to create ponds for mills. For example, a weir on the Humber River in Toronto was built to prevent damage from a flood caused by Hurricane Hazel in 1954.

The Leeds flood alleviation scheme uses movable weirs that are lowered during high water to reduce flood risks. Two weirs on the River Aire in Leeds were installed in 2017 and helped during the 2019 floods. These weirs can lower flood levels by up to one meter.

Coastal flooding is managed with defenses like sea walls, adding sand to beaches, and using barrier islands.

Tide gates work with dykes and drainage tunnels. They close during high tides to stop water from moving inland and open during low tides to let water drain out. The gates move based on water level differences on either side.

A flood barrier, surge barrier, or storm surge barrier is a type of floodgate that stops storm surges or high tides from flooding an area. These barriers are part of larger systems that include floodwalls, levees, and natural land features. Flood barriers can also protect individual buildings from floodwater.

A self-closing flood barrier (SCFB) is a system that uses rising floodwater to deploy and protect homes, communities, or industrial areas. It can be built in any length and is always ready to use. However, it is not perfect. For example, a water-filled barrier around a nuclear plant during the 2011 Missouri River flood was damaged by a machine and collapsed.

AquaFence is made of waterproof, strong panels that lock together. They can be fixed to the ground to resist wind and use floodwater weight to stay in place. Materials include special plastic, metal, and reinforced fabric. These panels can be reused and stored flat. This system is an alternative to building seawalls or using sandbags.

Other solutions, like HydroSack, are made of strong plastic with plant material inside. However, they are only used once.

Non-structural methods

There are several ways to manage flood risks without building structures. These methods include rules that limit the number of buildings in areas likely to flood, such as floodplains. These rules help reduce the need for flood protection measures. Flood warning systems also help lower risks by alerting people before floods happen. After floods occur, plans for rebuilding and insurance can be added to flood management strategies. Using different flood management methods ensures that plans can handle many types of flood situations and follow best practices.

Flood risk management focuses on reducing harm to people and communities from flooding. It is part of a larger effort to manage risks. This process studies how natural systems, like rivers, interact with human communities. It looks at causes of flooding, how floods affect people and the environment, and how to reduce risks. It includes planning to prevent floods, preparing for them, and analyzing flood risks to reduce damage.

Flood mapping is a tool that helps governments and planners understand where flooding might happen. This information helps make better decisions about where to build and how to avoid flood damage. Flood maps also help businesses and the public understand flood risks. However, some flood maps are hard to understand because they use technical language and lack clear information. Recently, there has been more interest in improving flood maps to better help manage flood risks.

Flood modeling is a way to study how floods affect people and the environment. It considers factors like land shape, water flow, and city layouts to predict how floods might spread. This helps identify areas at risk and how to protect them. Flood models can use different methods, such as computer simulations or maps made from satellite images. These models help decide where to build and how to reduce flood damage.

Flood hazard maps are important because they show areas most likely to flood. For example, in Bangladesh, where many people live in low-lying areas, these maps help identify places that flood often due to heavy rains. These maps use information about land height, river locations, and population density to help plan for emergencies. They help governments decide where to focus flood protection efforts and how to use resources to save lives and protect buildings.

Knowing flood risks is important, but knowing where to build emergency shelters is also key. In Bangladesh, fewer people have died in cyclones because shelters were built in safer places. In areas where shelters are scarce, maps consider flood risks, road access, and land use to find safe spots for shelters. These maps help ensure shelters are built on high ground and are easy to reach, making it easier to save lives during floods.

Involving people in flood planning helps create better flood management plans. This process allows communities, governments, and experts to work together on flood policies. It considers goals like reducing flood damage and planning land use. Including people in planning makes solutions more complete, even though it takes more time and effort.

Wetlands help manage floods, especially in coastal areas, by absorbing extra water during heavy rains. For example, wetlands near the Chesapeake Bay, Mississippi River delta, and Yangtze River delta protect inland areas from storm surges. Wetlands also save money by reducing flood damage. During Hurricane Sandy in 2012, wetlands saved about $625 million in damage to homes and roads.

Costs

As more people and property need flood protection, the cost of flood protection increases. For example, the US FEMA estimates that every $1.00 spent on mitigation saves $4.00.

Examples by country

In the Canadian province of Manitoba, a complex system of flood defenses helps protect the area. The Red River flows north from the United States, passing through Winnipeg, where it joins the Assiniboine River, and then flows into Lake Winnipeg. Like other rivers in the temperate zone of the Northern Hemisphere that flow north, snowmelt in southern areas can raise river levels before northern sections fully thaw. This can cause severe flooding, as happened in Winnipeg in 1950. To prevent future floods, the Manitoba government built a large system of diversions, dikes, and floodways, including the Red River Floodway and the Portage Diversion. This system protected Winnipeg during the 1997 flood, which caused major damage to areas upstream, such as Grand Forks, North Dakota, and Ste. Agathe, Manitoba.

In the United States, the U.S. Army Corps of Engineers manages flood control. After Hurricane Sandy, New York City’s Metropolitan Transportation Authority (MTA) started projects to protect subway systems in Manhattan. For example, the MTA used a flexible fabric cover system called Flex-Gate to seal subway entrances, which can block up to 14 feet (4.3 meters) of water. Guidelines from the Federal Emergency Management Agency (FEMA) have updated flood protection standards for 100-year and 500-year flood levels.

In New Orleans, 35% of the area lies below sea level and is protected by hundreds of miles of levees and floodgates. This system failed during Hurricane Katrina in 2005, causing about half of the metropolitan area to flood, with water levels ranging from inches to 20 feet in coastal regions.

The Morganza Spillway helps divert water from the Mississippi River during floods that threaten New Orleans, Baton Rouge, and other cities. It is the largest of several spillways along the Mississippi and was completed in 1954. It has been opened twice, in 1973 and 2011.

After the 1993 Midwest floods, the U.S. government offered to buy flood-prone properties to reduce future disasters. Some communities accepted, and the government purchased 25,000 properties, converting them into wetlands. These wetlands help absorb stormwater, and in 1995, when floods returned, the government avoided costly damage in those areas.

In Kyoto, Japan, the Hata clan controlled floods on the Katsura River around 500 A.D. and built a sluice on the Kazuno River.

In China, rural areas are sometimes intentionally flooded during emergencies to protect cities.

The effects of deforestation and land use changes on flooding are studied. Research on Himalayan deforestation showed that forests might not prevent extreme flooding but generally agree that deforestation increases flood risks, while reforestation and careful land use reduce them.

In Egypt, the Aswan Low Dam (1902) and Aswan High Dam (1976) have helped control flooding along the Nile River.

After the 1910 Great Flood of Paris, the French government built reservoirs called Les Grands Lacs de Seine to reduce flood pressure on the Seine River, especially during winter floods.

London is protected by the Thames Barrier, a large mechanical structure across the River Thames that is raised during high water levels. It has been in use since 1982 and was designed to prevent floods like the North Sea flood of 1953.

In 2023, it was found that over 4,000 flood defense projects in England were nearly ineffective, with many located in areas affected by Storm Babet.

The Saint Petersburg Dam, completed in 2008, protects the city from storm surges and serves as part of a ring road. It includes 11 dams stretching 25.4 kilometers (15.8 miles) and standing 8 meters (26 feet) above water level.

The Netherlands has one of the world’s best flood control systems, including dykes. Two-thirds of the country is at risk of flooding due to its low elevation. The Delta Works, with the Oosterscheldekering as a key part, were built after the 1953 North Sea flood. The Afsluitdijk, completed in 1932, is one of the world’s largest dams.

New flood control methods are being tested, such as storing water underground or in parking garages and playgrounds. Rotterdam has built a floating housing area to address rising sea levels. Other solutions include sensors to detect levee failures and movable structures to block rivers. Regular maintenance of flood defenses is also crucial.

Flood control is vital in the Netherlands because two-thirds of the land is vulnerable to flooding. Natural dunes and man-made dikes, dams, and floodgates protect against storm surges. River dikes prevent flooding from the Rhine and Meuse rivers, while drainage systems, including canals and pumping stations, keep low-lying areas dry. Water control boards manage these systems locally.

Modern flood control efforts, combined with technology, have helped reduce flood risks in the Netherlands. These projects have shaped the country’s geography and economy by improving infrastructure.

In New Zealand, flooding is the greatest natural hazard, managed by local councils. The country has over 5,284 kilometers of levees, and gravel extraction is used to lower river levels. Flood management is moving toward nature-based solutions, such as widening the Hutt River channel in Wellington.