Risk is defined as a combination of probability and consequence. If the likelihood of a significant flood is high and the consequences are severe, we say it is a “high” risk, as when a flood causes many victims and properties suffer from considerable damage. If the probability of a flood is low and the consequences are not considerable, then the risk is also “small”.

Real Numbers

The U.S. Federal Emergency Management Agency (FEMA) states that almost 13 million Americans currently live within a 100-year flood zone, which are areas with a 1 percent possibility of flooding in any given year. However, new studies demonstrate that the real number of Americans exposed to flood risk is about 41 million — more than three times FEMA’s estimate.

As FEMA’s flood maps have long been criticized for being outdated, for underestimating flood risk in the U.S., and for not taking into account smaller streams, this new research uses a high-resolution flood model with updated data from river, elevation, and rainfall, as well as revised population density maps, to carefully map current flood risk.

The study, carried out by scientists at the University of Bristol ( U.K.), The Nature Conservancy, and the U.S. Environmental Protection Agency, also reviewed projected population and housing trends to map future flood risk. Currently, more than 13 percent of the U.S. population lives in the 100-year flood zone, but that number could soar to 15.8 percent by 2050 and 16.8 percent by 2100. Communities in Nebraska, New Mexico, and South Dakota could see a fivefold increase in flood exposure by 2100. Not surprisingly, Florida and Texas flood risk could triple or quadruple.

The research also warns that these projections do not take into account climate change, which could put even more Americans at risk of flooding.

How to Assess the Risk of Flooding

A simple way to assess the risk of flooding is to find out the history of flooding in your area.

Your house may be at risk from flooding if:

It has flooded in the past.
It is located near an open water source, such as a river, the sea, a lake, a stream, ditches or drains.
It is in a hollow or a low-lying area.
Survey maps indicate your area is liable to floods or has flooded before.

If your property falls into any of the criteria described above, then you could be at risk from flooding caused by:

Rainfall filling streams, rivers, and ditches beyond their flow capacity.
Floodwater running over river banks and flood defenses.
Coastal storms leading to overtopping coastal flood defenses due to storm surge and wave action.
Blocked drainage ditches.
Sewers overflowing across roads, gardens and into properties.
Overloaded sewers flowing back into properties.
Rain causing groundwater levels to rise and flood.

It is important to remember also that while flood defenses provide some protection against flooding, they do not offer total safety. Flooding of areas behind such defenses can occur from water being unable to drain away or from floodwaters spilling over the top of defenses in particularly severe events. If your property is located behind a defense, it will flood less frequently, but it will still be at some risk.