Coastal flooding occurs when intense offshore storm systems push ocean water inland above the normal tide level. The rise in water is the storm surge. A storm surge can occur in just a few minutes. Hurricanes, tsunamis, and unusually high tides can cause coastal flooding.
River and stream flooding may be triggered by heavy rains, melting snows, and storm surge. River and stream flooding can also occur when a dam breaks, producing effects similar to flash flooding in minutes.
Closed-basin flooding occurs when a lake has no outlet or a relatively small outlet. Seasonal rainfall and storm systems can cause the lake level to rise faster than it can empty. Floodwaters in closed-basin lakes accumulate over long periods of time and may stay for weeks, months, or years.
Devastating floods occur throughout the United States every year. Ninety percent of all natural disasters in the U.S. involve flooding.
Floods are not all alike. Some floods develop slowly, sometimes over a period of days. Other floods develop very quickly.