A new study was released Monday (August 28) that explains why Houston’s flooding was so catastrophic following Hurricane Harvey—and how other coastal cities can be saved from a similar fate.

Typically, scientists focus on one of two factors—ocean surges or terrestrial flooding (which occurs inland as a result of precipitation and overflowing rivers)—when making a flood hazard assessment of an area. But according to an article published yesterday (August 29) in The Washington Post, the flooding in Houston is a result of both factors.

“What we are seeing right now in Houston is actually a compound flooding event,” Thomas Wahl, an expert on coastal risks, told The Post. “A lot of the rain that’s coming down right now can’t really drain into the open ocean as it’s supposed to do under normal sea level conditions because of the elevated water conditions.”  

Houston is not the only coastal community at risk for such devastating flooding. As The Post reports: “In many coastal areas, where rivers run out to meet the sea, both factors play a major role in the risk of regional flooding. Focusing on only one or the other can run the risk of underestimating the likelihood of a major flood.”

This new study, conducted by a group of scientists from the University of California at Irvine and the University of Salento in Lecce, Italy, created a model that takes both factors—terrestrial flooding and ocean surges—into account. They then applied it to available data for Washington, D.C., San Francisco and Philadelphia to re-evaluate flood risks. They selected these cities because, as one of the researchers told The Post, their “coastal monitoring data suggested a particularly strong relationship between ocean and river flooding.”

They concluded that flood risk is much higher using this model than the one typically used. For example, using the new model to assess D.C., “the level of flooding a typical hazard assessment might suggest would occur only once every 20 years was predicted to occur once every 16 years instead under the researchers’ new model,” reports The Post. 

The researchers also considered what role climate change could have in predicting flood risk. Per the study, “The results show that, in a warming climate, future sea level rise not only increases the failure probability, but also exacerbates the compounding effects of flood drivers.”

While this information could be beneficial to people around the world who live in potential flood zones, in the United States, coastal communities are disproportionately occupied by people of color. In addition, of the three areas studied, Philadelphia and D.C. are majority-Black cities, according to the 2016 Census.

The researchers recommended an update to the methods used to evaluate coastal risks.  Their findings were published in the journal Proceedings of the National Academy of Sciences.