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Study: Deadly Wimberley floods rooted in weather patterns, intensified by development

Researchers at the University of Texas at San Antonio have found that development along the Blanco River intensified the impact of a 40-foot wall of water that gushed down the waterway on Memorial Day weekend 2015.

An overturned vehicle on the banks of the Blanco River on May 26, 2015. The area saw record-breaking flood levels two days before.

The 40-foot wall of water that gushed down the Blanco River in May 2015, wiping out parts of Wimberley and killing more than a dozen people, was largely a natural phenomenon. But a new study shows that development along the waterway made its impact on the fast-growing Central Texas community that much worse. 

While the path, size and timing of a freak rainstorm were the primary drivers of the unprecedented swell that killed 13 people in the area on Memorial Day weekend, researchers at the University of Texas at San Antonio found that the removal of floodwater-corralling vegetation along the river and the proliferation of non-absorbent pavement near the waterway aggravated the unprecedented swell.

“Urbanization is a big part of this,” said Hatim Sharif, a civil and environmental engineering professor who co-authored the study, which is based on extensive storm and land modeling. “When there is urbanization, we have a street, a building, a parking lot, and this reduces the chance that the water will be absorbed. It also means the flood moves even faster and can be more destructive.”

Still, Sharif said the floods would have been destructive enough without these factors. Southern Blanco County received a record 10 to 13 inches of rainfall in a 4- to 6-hour period that weekend. And the ground already was saturated from smaller storms that fell earlier that spring. The storm also moved downriver, adding to the torrent.

“If the same exact storm had been moving in the opposite direction it would have resulted in a much smaller peak and the water would have risen much slower,” Sharif said. “Everything lined up the way it needed to for the water to rise so quickly.”

Wimberley is no stranger to floods; It's located in "Flash Flood Alley" — one of the most flood-prone regions on the continent, according to the Texas Water Resources Institute. It's also located in Hays County, one of the fastest-growing counties in the nation.

As the area has boomed, riverside developers and landowners have increasingly removed the natural vegetation along the riverbanks that slows runoff, said Ryan McGillicuddy, a conservation ecologist with Texas Parks and Wildlife. This can intensify already dangerous floods as there is nothing to slow the flow of the water or prevent natural sediments from being carried away with the flood, he said. 

“It’s well studied that if you remove the amount of cover, water will not enter the ground nearly as quickly, and it will contribute to these high peaks during floods,” McGillicuddy said. “If it isn’t there to hold on and slow down these rains and prevent them from quickly entering creeks and rivers, then we’ll see more destructive flooding.”

A 2016 investigation by The Texas Tribune and ProPublica found that unchecked development has greatly exacerbated flooding in the Houston area. The investigation also found that climate change has contributed to the frequency and severity of rainstorms. 

The impact of future floods may be lessened by updating flood maps to better reflect flood risk, said Sharif. Official flood maps, which are drawn by the Federal Emergency Management Agency, serve as the basis for everything from flood insurance requirements to building rules. 

“Texas has so much urbanization because it’s growing so fast and that changes the map,” Sharif said. “We are adding more buildings and more parking lots, so floods are going to change. We need to be prepared for that.”

Wimberley councilman Steve Thurber, who was serving as mayor in 2015 when the flood hit, said residents are still trying to shake the trauma of the event three years later. But he also said the city has taken big steps to protect from future floods. 

It has upgraded gauges and sensors that warn residents of flooding upriver, and it has sponsored programs to educate developers about the benefits of leaving intact riparian zones along the river.

The expectation is that another big flood is in the cards, Thurber said.

“Obviously, we hope it doesn’t happen, but you never know," he said. "With climate change it’s hard to predict."

The University of Texas at San Antonio has been a financial supporter of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune's journalism. Find a complete list of them here.

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