Flooding Facts
Are the death tolls from U.S. floods increasing?
On average, U.S. flooding kills more than 100 people a year--more than any other single weather hazard, including tornadoes and hurricanes. The average flooding toll has increased in recent decades while deaths from tornadoes and hurricanes have dropped. Almost half of all flash-flood deaths are connected to stream crossings or highway travel. Victims often underestimate the power of water when driving into flooded areas. It takes only 18 inches of water to float a typical vehicle.
How can a 100-year flood occur more than once in a short period?
A 100-year flood is one that has a one-percent chance of being exceeded in a given year. Few locations have rainfall records of more than a century, so 100-year flood values are estimates rather than certainties. Changes in watershed management, land use, and the like can affect stream flow characteristics and alter the likelihood of a given flood. Moreover, climatic patterns themselves can change. There is nothing to prevent more than one "100-year flood" from occurring at a given spot over a century.
What is a 100-year floodplain?
This term indicates the likelihood that a particular area will flood in a year's time. For example, a home in a 100-year floodplain has a one in 100 (or 1 percent) chance every year of being flooded. That percentage holds true every year, regardless of how many floods have occurred in previous years, or their severity.
Is global warming causing more floods?
Because flood risk is the result of both environmental and societal factors, it is difficult to single out the impact of climate variations. According to the United Nations Intergovernmental Panel on Climate Change (IPCC), the average global temperature at the surface increased just over 1.0 degree Fahrenheit in the 20th century. At the same time, according to NOAA, there has been a steady increase in the area of the United States affected by extreme precipitation events (more than two inches of rain in one day, or the equivalent in snow). However, flood reports have not increased uniformly across the country. Any alteration of global climate can bring either an increase or decrease in precipitation or flood events at a given location. Current computer models of climate are unable to project local variations with certainty.
Are societal choices increasing flood risk?
According to a report by the U.S. Congress's Office of Technology Assessment, "despite recent efforts, vulnerability to flood damages is likely to continue to grow." The factors cited include:
- Growing populations in and near flood-prone regions
- The loss of flood-moderating wetlands
- The loss of flood-moderating wetlands
- New development in areas insufficiently mapped for flood risk
- The deterioration of decades-old dams and levees
- Policies such as subsidies that encourage development in flood plains
Most people are unaware that:
- 66 percent of flood deaths occur in vehicles, and most happen when drivers make a single, fatal mistake trying to navigate through flood waters.
- Just 6 inches of rapidly moving flood water can knock a person down.
- A mere 2 feet of water can float a large vehicle even a bus.
- One-third of flooded roads and bridges are so damaged by water that any vehicle trying to cross stands only a 50% chance of making it to the other side.