Understanding Coastal Inundation

The first step is to understand what inundation is and why communities should be concerned.

Inundation and Coastal Communities

What Is Inundation? Water covering normally dry land is a condition known as inundation.

Impacts

Inundation events are among the more frequent, costly, and deadly coastal hazards that can impact coastal communities in the U.S. In fact, riverine and coastal inundation causes the highest number of natural-hazard-related deaths. With coastal states supporting 81% of the U.S. population and generating 83% of U.S. gross domestic product, the potential for catastrophic loss from inundation events is greater in these states than in other areas of the country.

In addition, future inundation risks may be exacerbated by local changes in climate and sea level. It is important to know current inundation risks to understand the potential effects of changing conditions.

Causes

Episodic Coastal Inundation Events

There are four primary causes of significant inundation: storm surge, tsunamis, inland flooding, and shallow coastal flooding.

Storm surge - Storm surge results from severe storms such as tropical cyclones (e.g., hurricanes, typhoons) and nor'easters, as strong winds combined with low pressure drive water onshore (NOAA, 2009). Hurricanes like Katrina (2005) and Hugo (1989), and the 1993 nor'easter (the "Storm of the Century"), caused extensive storm surge.

Tsunamis - Tsunamis are large waves generated by an abrupt disturbance of the sea surface (e.g., from an earthquake or landslide). A tsunami caused by a local event arrives minutes after generation, while a tsunami caused by a distant event arrives hours later (NOAA, 2009). The 2004 Indian Ocean tsunami is the most recent example of the devastating effects of tsunamis. Learn more about tsunamis.

Inland floods - Inland floods occur when moderate precipitation falls over several days, intense precipitation falls over a short period, or an ice or debris jam or a dam or levee failure causes a river to overflow (NOAA, 2009). Hurricane Floyd (1999), aided by Tropical Storm Dennis (1999), resulted in widespread severe flooding that caused the majority of the $3 to 6 billion in damage recorded after the storms.

Shallow coastal flooding - Shallow coastal flooding is flooding that occurs in low-lying coastal areas during extreme high tides. These tides occur a few times per year when the sun, moon, and earth align. Even relatively weak wind pushing water over land can increase the level of shallow coastal flooding, and rainfall during these events can dramatically increase the level of flooding. A coastal storm is not necessary for shallow coastal flooding to occur.

Longer-Term Coastal Inundation

Sea level rise is an increase in the mean level of the ocean. NOAA has been measuring mean sea level for over 150 years, with tide stations operating on all U.S. coasts.

Global (or eustatic) sea level rise - Global (or eustatic) sea level rise is caused by a change in the volume of the world's oceans due to expansion as the oceans warm and to the melting of land-based ice (i.e., glaciers, ice sheets).

Relative sea level rise - Relative sea level rise refers to a local increase in the level of the ocean relative to the land due to ocean rise or land subsidence (e.g., from groundwater withdrawal, tectonic activity).

View works cited and additional resources for understanding inundation.