Urban flooding occurs when city landscapes cannot absorb excess water after prolonged periods of intense rainfall, river overtopping, or storm surge.
Changes in the environment have already increased flooding across the United States, particularly in coastal and low-lying areas. As these trends continue into the future, 1.7 million more American properties will be at risk, and the damage and cost of flooding will continue to add up.
Urban Flooding on Risk Factor
The First Street Foundation Flood Model considers the risk of flooding from rain, river, and storm surge. Urban flooding is incorporated into the model with special consideration given to the infrastructure, development, and water management systems that exist in urban areas. Use Risk Factor™ to learn if your home is threatened by urban flooding so you can prepare for and mitigate risks before they become a reality.
Non-environmental factors like development patterns and structural failures of dams and levees are also exacerbating flood risks. Expanded development in coastal areas and floodplains simultaneously puts more people and community assets at risk, increases the amount of impervious (non-absorbent) surfaces like concrete, and destroys natural flood protections like wetlands. Flood control measures like levees, seawalls, gates, and pumps also pose a serious threat if not regularly maintained and upgraded.
Sea levels are rising
Most of America’s coast is experiencing rising seas caused by global ice melt, thermal expansion, changes in ocean circulation, and local land sinkage. Sea level rise makes high tides higher, which puts more pressure on coastal drainage systems. Higher seas also lead to higher storm surge from hurricanes and other storms, resulting in deeper floods that travel farther and last longer.
An overflowing levee near Lake Providence, Louisiana May 18, 2011. (Reuters / Eric Thayer - stock.adobe.com)
Extreme rain events are increasing
As the atmosphere warms, there is more evaporation and more water available for rain, which contributes to changing weather patterns and flood risks. Extreme rain events are increasing in duration, intensity, and frequency, which causes more urban and flash floods, and more flooding from overflowing rivers and streams.
The impact of extreme rain
Within a 22 month period between 2016 and 2018, Ellicott City Maryland saw two extreme rainfall events that caused flash floods, as the grounds were unable to absorb the sudden, high-volume of rain that fell within hours. Each event had a 0.1-0.2% chance of occurring in a given year.
Ellicott City, MD 2018 after flash floods turned it's main street into a raging river. (Libby Solomon/The Baltimore Sun via AP)
Flood Model Methodology - Calculating property-level risk
Tidal flooding - you could be at risk even on sunny days
Flood Factor probabilities - The likelihood of water reaching a home