New research suggests Hurricane Harvey was deadlier outside the 100- and 500-year floodplains drawn by the federal government.

"It was surprising to me that so many fatalities occurred outside the flood zones," Sebastiaan Jonkman, a professor of hydraulic engineering at Delft University of Technology in the Netherlands, said in a news release.

At least 88 deaths have been blamed on Harvey. Some 80 percent were caused by drowning. Surprisingly, just 22 percent of the 37 deaths recorded in Houston's Harris County -- the epicenter of the disaster -- occurred inside the 100-year flood zone.

The 100- and 500-year flood zones define the probability of the land within the contour lines being flooded. A neighborhood within the 100-year zone can be expected to flood once a century. The odds of flooding within the 500-year zone are smaller -- a 0.2 percent probability of flooding during any given year. Hurricane Harvey was larger than a 500-year flood, affecting land outside the flood zones.

Unprecedented rainfall totals lasting more than a week triggered flooding across an area of southeast Texas roughly the size of the Netherlands.

Researchers at Delft and Houston's Rice University began analyzing fatalities within days of the storm. They found 19 of the deaths in Harris County occurred outside the flood zones.

"The number of fatalities outside of the floodplains highlights how widespread flooding from Harvey really was," Jonkman said.

Scientists found that most of the drowning deaths occurred in cars or when people trapped in cars tried to get out and were swept away. The second leading cause of death, after drowning, was electrocution by downed power lines. Several deaths were also blamed on an inability of medical personnel to reach those in need of treatment.

Researchers published their analysis of Harvey deaths in the journal Natural Hazards and Earth System Sciences.

The authors of the study say flood maps are still useful. However, researchers believe zoning efforts should be improved and officials should identify high-risk areas outside of the 100- and 500-year floodplains.

"Better communication of their purpose and limitations would help reduce risk," said Jonkman.

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