Hills, rivers and rocky terrain: Why the Hill Country keeps flooding

Yahoo

3 days ago

When floodwaters tore through the Texas Hill Country on July Fourth weekend, killing more than 100 people — including campers and counselors at an all-girls summer camp along the Guadalupe River — Kerr County Judge Rob Kelly was quick to voice shock.
'We didn't know this flood was coming,' Kelly said. Yet in nearly the same breath, he acknowledged that the region is 'the most dangerous river valley in the United States' — one that deals with floods 'on a regular basis.'
That contradiction — an expectation of danger paired with apparent surprise — has become tragically familiar in Central Texas.
Despite being part of a wide swath of Texas nicknamed 'Flash Flood Alley,' this part of the Hill Country continues to suffer devastating losses — both in human lives and property — after floods that scientists and emergency planners have warned about for decades.
The region includes several Texas river basins: the Colorado, the Guadalupe and the San Antonio.
Between 2 and 7 a.m. July 4, the Guadalupe River in Kerrville rose 35 feet, according to a flood gauge in the area. The flooded river swallowed roads, bridges, entire RV parks and structures along the Guadalupe's banks.
The region has a history drenched in loss, marked by some of the state's most deadly floods.
Nearly a century ago in 1932, hard rains pushed the Guadalupe River out of its banks. That destructive flooding drowned seven people and property losses exceeded $500,000 — equivalent to $11.8 million today. A blog post by Kerrville Mayor Joe H. Herring Jr. recounted the story of a teen trapped in a tree for 23 hours during that flood and the men that tried to save him.
'The story of July 1, 1932 is a story of warning, and a story with heroes,' Herring wrote.
In 1978, a tropical storm stalled over the headwaters of the Guadalupe and Medina Rivers. The resulting flood drowned 33 people, causing millions of dollars in property damages, ravaging roads, bridges and ranchland.
Less than a decade later, in 1987, an intense summer storm dumped about 11.5 inches of rain in mid-July near the headwaters of the Guadalupe River, sending a massive flood wave through Ingram, Kerrville and Comfort. As the wall of water rushed through a church camp near Comfort, a bus and a van attempted to evacuate campers but stalled in rapidly rising water. Ten teenagers drowned and 33 other people were injured — a tragedy that some officials alluded to in recent days when defending the lack of evacuations before the July 4 flood.
Most recently, on Memorial Day weekend in 2015, heavy rainfall upstream on the Blanco River caused flash flooding in Wimberley, uprooting centuries-old trees and damaging or destroying nearly 400 homes along its banks, displacing hundreds of residents. The river rose approximately 5 feet every 15 minutes, cresting near 50 feet. Thirteen people died in the flood.
'People new to the area may not know the history. The climate doesn't look like a place where flooding happens often. It's hot, semi-arid. It's deceptive,' said Todd Votteler, a longtime water policy expert and former executive manager at the Guadalupe-Blanco River Authority.
As water expert and environmental consultant Matthew Berg put it: 'Rivers have a lot longer memory than we do.'
People are drawn to the Texas Hill Country for its natural beauty. It's a place where families camp under starry skies, fish in spring-fed creeks, and cool off in deep swimming holes carved into limestone.
But the very features that make this region so appealing — its hills, rivers, and rocky terrain — also make it one of the most flood-prone areas in the country.
On topographical maps, the terrain resembled elephant skin, with countless folds worn into the hills by centuries of runoff. The hilly land has dramatic elevation changes caused by the Balcones Escarpment, a major geological feature that cuts across Central Texas.
Tropical storms routinely hit the escarpment and dump heavy rain, said Avantika Gori, a flood risk expert and civil and environmental engineering professor at Rice University. Last week, the storms that caused the flash floods in the Hill Country were intensified by the moisture from the remnants of Tropical Storm Barry.
And when storms roll in, water rushes downhill fast, gaining speed and force as it moves.
There's little to slow it down — thin, rocky soil doesn't absorb much water, and exposed bedrock and sparse vegetation offer no buffer. Clay-rich soils in parts of the region also prevent infiltration, meaning rain turns to runoff almost immediately.
'It's like the region's been paved over with concrete,' said Gori. 'So the water falls, and it just runs off. And then, because of the steep slope, you get these really fast-moving waves of water.'
Robert Mace, a hydrologist and executive director of the Meadows Center at Texas State University, calls it 'a recipe for catastrophic floods.'
On July 4, the floods struck at perhaps the worst possible moment — in the early morning hours at the start of a holiday weekend that had drawn large numbers of people to the river.
The National Weather Service issued a flood watch Thursday afternoon, predicting isolated rainfall of up to seven inches. It issued a flash flood warning that included Kerr County after 1 a.m. Friday, when most people were asleep, and declared a flash flood emergency — the most severe alert possible — around 4 a.m.
'Many storms form at night,' said Votteler, the water policy expert.
The huge loss of life from Friday's flood — which is likely to grow higher with at least two dozen people still missing in several counties — has raised new questions about what local, state and federal officials could have or should have done to better warn people near the river.
John Nielsen-Gammon, the state's climatologist, said the severity of storms is also changing. Climate change has caused warmer air that can hold more water, leading to more intense rainfall. Ocean heat fuels stronger tropical systems.
Mace, the hydrologist, said what was once a 500-year flood is increasingly happening every few decades.
'Floodplain maps are based on historical data,' said Mace. 'They don't reflect the current — or future — risk. It's backward-looking in a forward-moving crisis.'
Disclosure: Rice University 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.