Study: Most Harvey fatalities occurred outside flood zones

A team from Rice University and Delft University of Technology in the Netherlands found that most Houston-area drowning deaths from Hurricane Harvey occurred outside the zones designated by government as being at higher risk of flooding: the 100- and 500-year flood plains.

A map of the Netherlands illustrates the size and extent of flooding rains from Hurricane Harvey, an August 2017 tropical cyclone that inundated southeast Texas. Harvey’s highest recorded rainfall of 60.58 inches (1,539 mm) was in Nederland, Texas. (Image courtesy of SSPEED Center/Rice University)

Harvey hit southeast Texas Aug. 25 and caused unprecedented flooding and killed dozens. The Delft-Rice team published its results this week in the European Geosciences Union journal Natural Hazards and Earth System Sciences.

“It was surprising to me that so many fatalities occurred outside the flood zones,” said Sebastiaan Jonkman, a professor at Delft’s Hydraulic Engineering Department, who led the study.

The research showed drowning caused 80 percent of Harvey deaths, and only 22 percent of fatalities in Harris County occurred within the 100-year flood plain, the mapped area that is used as the main indicator of flood risk in the U.S.

Flood plains, or flood zones, are low-lying areas surrounding rivers and streams that are subject to flooding. To assess flood risk for insurance purposes and to set development standards, U.S. authorities outline flood plains for 100- and 500-year floods, which have a 1 percent probability (100-year flood) and a 0.2 percent probability (500-year) of occurring in any given year.

“Hurricane Harvey was much larger than a 100- or 500-year flood, so flooding outside of these boundaries was expected,” Jonkman said.

Rainfall totals in the week after the hurricane made landfall were among the highest recorded in U.S. history, with over 39 inches of rain falling in just three days in large parts of Harris and surrounding counties. As a result, a report by Delft University found that “unprecedented flooding occurred over an area the size of the Netherlands.”

Nonetheless, it was surprising for the researchers to find that so many of Harvey’s fatalities happened outside the designated flood plains, given that these zones are expected to be “reasonable predictors of high-risk areas,” according to Jonkman.

Antonia Sebastian (Photo by Brandon Martin/Rice University)

The research began within days of the storm. “We wanted to identify lessons that could be learned, for both Texas and the Netherlands, from Harvey’s impact and the local and government response to the flooding,” said study co-author Antonia Sebastian, a postdoctoral research associate at Rice University’s Severe Storm Prediction, Education and Evacuation from Disasters (SSPEED) Center, who was based at Delft University when Harvey struck.

The team compiled a database of fatalities by using official government records and media sources, which they analyzed in the Natural Hazards and Earth System Sciences study.

The researchers concluded that at least 70 deaths occurred as a consequence of Hurricane Harvey, including 37 in Harris County. Of the Harris County deaths, eight were in the 100-year flood plain, 10 more fell within the larger 500-year flood plain and 19 were outside the 100- and 500-year zones.

“The number of fatalities outside of the flood plains highlights how widespread flooding from Harvey really was,” Sebastian said.

Hurricane Harvey on Aug. 24. (Photo courtesy of NASA)

The study also showed that most fatalities – over 80 percent – were drownings, many occurring either in vehicles or when people were swept away while trying to get out of their cars. Six people died when their boat capsized during a rescue. The second-largest causes of death were electrocution and lack of medical treatment, each of which was responsible for 6 percent of fatalities.

About 70 percent of those killed by Harvey were men. The team thinks that men tend to show more risk-taking behavior, such as driving through flooded crossings or taking part in rescues.

The researchers hope their findings encourage authorities to identify high-risk areas outside of the designated flood plains and to take preventive measures to reduce the number of victims in future floods, including closing low-water crossings and underpasses during extreme flood events.

Jonkman said that the current flood maps will need to be improved, but that flood plains should not be abandoned as an indicator of high-risk areas.

“Better communication of their purpose and limitations would help reduce risk,” he said.