A Dutch-Texan team 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 floodplains. Harvey, one of the costliest storms in US history, hit southeast Texas on 25 August 2017 causing unprecedented flooding and killing dozens. Researchers at Delft University of Technology in the Netherlands and Rice University in Texas published their results today 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,” says Sebastiaan Jonkman, a professor at Delft’s Hydraulic Engineering Department who led the new study.

Drowning caused 80% of Harvey deaths, and the research showed that only 22% of fatalities in Houston’s 4,600-square-kilometre district, Harris County, occurred within the 100-year floodplain, a mapped area that is used as the main indicator of flood risk in the US.

Flood zones, or floodplains, 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, US authorities outline floodplains for 100- and 500-year floods. These events have a 1% probability (100-year flood) and a 0.2% 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,” says Jonkman. Rainfall totals in the week after the hurricane made landfall were among the highest recorded in US history, with over 1000 mm of rain falling in just three days in large parts of both 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 floodplains given that these zones are expected to be “reasonable predictors of high-risk areas,” according to Jonkman.

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,” says 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.

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