Flooding is a well-known natural hazard along the U.S. coastline. Nearly 40 percent of the U.S. population resides in its coastal counties.

A clear understanding of the various flood types and changes in the frequency of their occurrence is critical toward reliable estimates of vulnerability and potential impacts in the near-term as well as into the future. Given the heavy reliance on coastal zones for natural resources and economic activity, flood preparedness, and safety are key elements of long-term resilience.

In a study published in the journal Nature Climate Change, we assembled long historical records of rainfall, tide gauge readings, and hurricane tracks to assess the simultaneous occurrence of heavy precipitation on land and storm surges. The combination of these two—heavy rain and storm surges driven mainly by the wind from storms—can result in “compound” floods.

We found that along large coastline stretches around the U.S., a systematic linkage exists between the two important drivers for coastal flooding, making it more likely that the two occur in tandem. Our analysis showed that over the past century, the number of compound flood events for many U.S. coastal cities has increased.

Gaining more insight into the frequency and likelihood of compound floods can help planners better assess risk from flooding to critical infrastructure.