Why flash floods are so common – and deadly – in Texas's Hill Country

Globe and Mail

3 days ago

The devastation caused by the past week's flash flooding in Texas has sent shock waves, with local officials insisting that it was an extreme event nobody saw coming.
But when flood researcher Nasir Gharaibeh first saw the news about the natural disaster, he found himself feeling upset but unsurprised.
'Nothing in terms of the likelihood of something like this happening was surprising to me,' said the professor at Texas A&M University, who studies flash floods and their causes, in an interview.
'That kind of storm, in that kind of location, will happen. And it will result in human casualties.'
On Friday, an intense downpour dumped months' worth of rain over Texas's Hill Country region. It caused destructive, fast-moving waters to rise 26 feet on the Guadalupe River, washing away trees, homes and vehicles.
Authorities and elected officials have told reporters that they did not expect such an intense downpour and were taken aback by the ferocity of the flood. But flash floods are a known risk in central Texas, which the National Weather Service has identified as the most flash-flood-prone area in the country.
And no state has more flood-related deaths than Texas. More than 1,000 Texans died in floods between 1959 and 2019, according to research conducted by Hatim Sharif, a flood expert with the University of Texas at San Antonio. (It's a total that far outstrips Louisiana's 693, the state with the second-highest death toll.)
The vast majority of flood-related deaths occur in flash-flooding events, according to Prof. Gharaibeh. And within Texas, nowhere is more susceptible than Flash Flood Alley, the swath of land that includes Hill Country and encompasses dozens of counties stretching from north of Dallas to the region west of San Antonio.
The National Weather Service defines a flash flood as flooding that begins within six hours of heavy rainfall. Prof. Gharaibeh says he's heard it described as 'a 'rain bomb,' which is really what it is.'
There are three factors that put localities at risk of flash flooding, he noted: steep terrain, intense storms and soil conditions that exacerbate flood risk. Hill Country sits at the intersection of all three.
As its name suggests, the region's topography is defined by its steep-sloped hills, which are interwoven with shallow, winding creeks. This landscape creates a magnetic draw for campers and outdoor enthusiasts, but also increases the risk of flash flooding. In heavy downpours, water will barrel down the steep hillsides and flood the creeks that feed the Guadalupe River.
When those creeks converge on a river, it can create a surge of water powerful enough to sweep away houses and cars: 'You can get walls of water coming down,' local meteorologist Cary Burgess said in a 2017 flood-awareness video.
The region is also home to the Balcones Escarpment, a row of cliffs and steep hills created by a geologic fault, Prof. Sharif, who is a hydrologist and civil engineer. When warm air rushes up from the Gulf of Mexico, it condenses and dumps moisture into the area, he explained.
Meanwhile, the soil is dry and shallow, with just a few inches sitting on top of limestone, he added: 'It gets soaked very quickly. There's no room for the soil to absorb a lot of water.'
A look at some of the deadliest floods in the U.S. in the last 25 years
Mr. Burgess, in the 2017 video, warned that just three or four inches of rain at one time can cause a 'real serious problem' in Hill Country. But in Friday's early hours, the communities along the Guadalupe River received at least 10 inches.
A river gauge at Hunt, Texas near Mystic Camp shows how rapidly the water levels rose, Prof. Sharif wrote in The Conversation. At 3 a.m., water was rising by one foot every five minutes; within an hour and a half, it had already risen more than 20 feet.
A 2017 paper published by the Bulletin of the American Meteorological Society analyzed flash-flood fatalities between 1996 and 2014, noting that late-evening events were the most devastating in terms of injuries and deaths.
But while fewer flash flooding events were associated with campsites and recreational areas, they lead to more losses per event compared with other scenarios, the study found. In 1987, another flash flood in Hill Country also killed summer campers after 10 teenagers died while being evacuated from Pot O' Gold Christian camp.
Prof. Sharif said he hopes that data will be collected from this latest flash flood to investigate what went wrong so that future disasters can be mitigated.
He also believes that early warning systems need to be improved, incorporating rainfall forecasting as well as sophisticated hydrologic models that can better predict the impact on the ground.
'If we have six inches [of rain], which areas will be flooded? How wide will the river become?' he said, adding that perhaps this type of flood modelling could establish rainfall thresholds that would trigger evacuations in specific areas.
The National Weather Service said it did issue a flood watch for the area on Thursday afternoon, which it upgraded to a warning overnight, meaning floods were imminent or occurring. But those notices would have arrived while many people were sleeping, Prof. Sharif noted.
And Kerr County judge Rob Kelly told CBS News that his region does 'not have a warning system' for alerting local residents.
This should no longer be acceptable in high-risk regions such as Flash Flood Alley, Prof. Gharaibeh said.
'You know that something like this has happened in the past, and it will happen again. Why don't we have a better warning system to people in this region?' he said.
'It needs to be quick, and it needs to have different warning signs so people can tell: This is serious, we need to do something.'
With a report from the Associated Press