Urban flooding happens when water covers land or buildings in cities or other developed areas. This can occur when heavy rain or storm surges from the ocean cause water to overflow drainage systems, such as storm sewers. Urban flooding can happen anywhere, even in areas not near rivers, lakes, or floodplains. It is often caused by rivers or lakes overflowing, sudden heavy rain, or melting snow. During a flood, water from rain or broken pipes can collect on streets, sidewalks, and in public areas. It may also seep into buildings through walls, floors, or enter through sewers, basements, toilets, and sinks.

There are different types of urban flooding, each with a unique cause. Pluvial flooding happens when heavy rain causes water to build up. Fluvial flooding occurs when a nearby river overflows its banks. Coastal flooding is usually caused by high waves or storm surges from the ocean. Urban flooding can harm people and damage buildings and roads. Examples of major flood events include the 1998 flooding in Nîmes, France; the 1992 flooding in Vaison-la-Romaine, France; the 2005 flooding in New Orleans, United States; and the 2010–2011 floods in Rockhampton, Bundaberg, and Brisbane, Australia. Other events include the 2022 floods in eastern Australia and the 2024 floods in Rio Grande do Sul, Brazil.

In cities, flooding can be worse because of roads and streets made of pavement. These surfaces prevent water from soaking into the ground, leading to more water running over the surface than drainage systems can handle. Climate change can also affect how often and how badly urban flooding happens. This is especially true for coastal cities, which may face rising sea levels and heavier rain.

To reduce urban flooding, city planners can use several methods. These include building gray infrastructure, such as pipes and drains, using green infrastructure like parks and green spaces, improving drainage systems, and planning how land is used. Overall, managing water resources in cities in a coordinated way can help reduce the risk of urban flooding.

Causes

There are three main types of urban flooding, each with a different cause:

• Pluvial flooding happens when heavy rain falls on cities and cannot be absorbed quickly enough.
• Fluvial flooding occurs when rivers near cities overflow their banks.
• Coastal flooding is often caused by storm surges, which push large amounts of water onto coastal areas.

Each type of urban flooding has different effects and needs different ways to reduce its impact.

Activities that increase the amount of hard, impermeable surfaces in cities—like roads, buildings, and parking lots—can raise flood risk. These surfaces are created when soil is covered by construction, which limits where floodwater can drain. As cities grow faster worldwide, more people may be affected by urban flooding.

Some studies have looked at how water moves in cities with transportation systems built using cut and fill methods. If underground pipes (culverts) used in these systems become blocked by debris, water may flow onto streets instead. Researchers have studied how water spreads through streets during storms and how this affects flood predictions.

Common causes of urban flooding, such as storm surges, heavy rain, and river overflow, are likely to happen more often and be more severe as climate change continues. This is because rising ocean and river levels, along with unpredictable rainfall patterns, can worsen pluvial flooding (when heavy rain overwhelms drainage systems) and fluvial flooding (when rivers overflow due to heavy rain). Extreme weather events, like hurricanes, are also expected to grow stronger. Because many cities are being built in areas at risk of flooding, more land may be exposed to climate-related flooding in the future.

Coastal cities may face greater risks from rising sea levels and heavier rainfall.

As heavy rainfall becomes more common, floods are likely to be more severe when they occur. The relationship between rainfall and flooding is complex. In some areas, flooding may become less frequent. This depends on factors like changes in rainfall, snowmelt, and soil moisture. In some regions, climate change may make soils drier, allowing them to absorb rain more quickly and reduce flooding. However, dry soil can also become harder, causing rainwater to run off into rivers and lakes, which may increase flood risks.

Impacts

Urban flooding has clear effects on people's lives and property. In 2020, floods were linked to about 6,000 deaths and caused US$51.3 billion in damage worldwide. People living in low-lying areas are often at greater risk of flooding, which can lead to financial loss and even loss of life.

Urban flooding also affects important public services, such as transportation. Floods can increase traffic congestion in cities.

According to the IPCC (as of 2022), future surface water flooding in cities and towns may cause major economic risks. This is connected to how urban systems and climate changes interact.

Urban flooding has major economic effects. In the United States, experts say that wet basements can reduce property values by 10% to 25%, and this is a common reason people avoid buying homes. The U.S. Federal Emergency Management Agency (FEMA) reports that nearly 40% of small businesses do not reopen after a flood. In the UK, urban flooding is estimated to cost £270 million each year in England and Wales, and 80,000 homes are at risk of flooding.

A study of Cook County, Illinois, found that 177,000 insurance claims for property damage were made across 96% of the county’s ZIP codes between 2007 and 2011. This means about one in six homes in the county filed a claim during that time. On average, each claim paid out $3,733, and total claims reached $660 million over five years.

Urban flooding can also disrupt supply chains, causing delays in the availability of goods and services and leading to financial losses for businesses.

From 1961 to 2020, nearly 10,000 flood-related cases were reported, resulting in 1.3 million deaths and at least US$3.3 trillion in financial losses. This equals a loss of about US$1,800 every second. On average, about 23,000 people died each year worldwide from flooding over the past 60 years, which is roughly one death every 24 minutes.

Modeling

Flood modeling is often done in a local way, with water-related models created for each city. These models include details about buildings, roads, plants, how land is used, and drainage systems. This local approach can be helpful, especially when combined with past flood data, in predicting which specific places, such as streets or intersections, will be most affected during a flood. This information can help create flood control systems that meet the needs of local areas.

Floods in cities have been studied more recently, even though floods have occurred for centuries. Some researchers have discussed the "storage effect" in urban areas, which refers to how water is held back in certain spaces. Other studies have looked at how water moves through streets during heavy rain and how this affects flood predictions. Recent research has examined what makes it safe for people to leave flooded areas. However, field measurements during the 2010–2011 Queensland floods showed that rules based only on water speed, depth, or movement cannot fully explain the dangers caused by sudden changes in these factors. These rules also ignore the risks from large objects, like debris, that are carried by floodwater.

The curve number (CN) rainfall–runoff model is often used. However, it has been shown to not work well at predicting water flow consistently around the world. Unlike older methods that consider how wet the ground was before a storm, a recent study kept the basic structure of the CN model but adjusted it based on different conditions in watersheds, using statistical methods. This study found that the CN model was not reliable without being recalibrated. The model can be adjusted using local rainfall and water flow data to predict sudden urban floods.

Modeling the effects of climate change is often done from a "top-down," global perspective. These models can help predict worldwide effects of global warming and raise awareness about large-scale changes. However, they often lack detail, with resolution limited to 25 km or more, making them less useful for local planners who need information about specific streets.

Some people support combining local flood models with larger climate models. This approach could use the strengths of both types of models and help predict how climate change might cause flooding in ways that allow planners to create local solutions to reduce its effects.

Scientists have studied how climate change might affect urban flooding and found that, for example, in the UK, yearly flood damage from surface water could increase by £60–200 million if global temperatures rise by 2–4°C. Better planning could help manage flooding up to a 2°C temperature increase but may not be enough if temperatures rise further.

Mitigation and management

Floods in cities have been studied more recently, even though cities have experienced floods for many years. Recent research has looked at how to safely move people out of flooded areas.

One traditional way to manage urban flooding is through gray infrastructure. This includes structures like dams and seawalls made of concrete or other materials that do not let water pass through. Gray infrastructure can help prevent flood damage and save money. However, some models suggest that gray infrastructure may not work as well in the future because climate change could cause more frequent and severe floods.

An alternative to gray infrastructure is green infrastructure. This involves using natural methods to absorb and store rainwater where it falls. Green infrastructure includes plants, open spaces with surfaces that allow water to soak in, and rainwater collection systems. These methods can help reduce urban flooding. An example of green infrastructure is the concept of "sponge cities."

Another common way to manage urban flooding is through urban drainage systems. These systems move stormwater away from streets and businesses to storage or drainage areas. While these systems help cities manage flooding and can be expanded as cities grow, they may not be enough to handle future flooding caused by climate change.

Sustainable drainage systems (SuDS) are a group of water management methods that try to make drainage systems work more like natural water processes. They are part of a larger green infrastructure plan. SuDS help urban drainage systems work better with natural water processes, such as water soaking into soil or being filtered by plants. These systems aim to reduce how human development affects the natural water cycle, especially issues like water running off surfaces and pollution.

SuDS have become more popular in recent years because people understand better how cities affect the environment and are more concerned about climate change and sustainability. SuDS often use structures that copy natural features to help urban drainage systems blend with natural drainage systems quickly. SuDS are a key part of the Blue-Green Cities project in Newcastle upon Tyne.

The balance between surfaces that let water pass through (pervious) and those that do not (impervious) is important for managing floods. Planning how land is used and how much land is set aside for different purposes is important for flood management. Increasing the amount of open, vegetated space, such as parks or golf courses, can help absorb stormwater. Using more pervious surfaces, like green walls or roofs, or materials that let water soak in, can also help reduce flood risks linked to climate change.

Integrated urban water management (IUWM) is a way to manage freshwater, wastewater, and stormwater as part of a plan for an entire river basin. It builds on existing water supply and sanitation plans by including urban water management in the broader river basin plan. IUWM is often used to achieve goals of Water Sensitive Urban Design. It aims to change how cities affect the natural water cycle by managing the urban water cycle as a whole. This approach can lead to better use of resources, economic benefits, and improved social and environmental results. One method is to create a local water cycle loop by reusing water. This requires understanding both the natural water balance before development and the water balance after development. Comparing water flows in these systems helps reduce the impact of cities on the natural water cycle.

Examples

• Worldwide: List of floods
• Africa: Floods in Africa
• Australia: Floods in Australia
• The Netherlands: Floods in the Netherlands
• United States: Lists of floods in the United States
• North Sea: Storm tides of the North Sea

One of the most well-known areas at risk for flooding in the United States is New Orleans. The city is near the coast and sits in a low area, making it likely to flood during tropical storms, including hurricanes and cyclones. It is also at risk from rising sea levels or more frequent storms. In 2005, Hurricane Katrina caused more than 1,800 deaths and $170 billion in damage. After Hurricane Katrina, new flood protection systems were built to prepare for climate changes. These systems have worked well to reduce damage from later severe weather events, such as Hurricane Ida.

During the summer of 2021, Hurricanes Henri and Ida caused major flooding in many cities along the east coast of the United States. New York City, in particular, experienced the highest rainfall levels ever recorded. This led many people to ask if the city should build more flood protection systems to prepare for future flooding. In September 2021, the New York City mayoral office released a new plan to help the city prepare for heavy rain events.