Each drop of rain from Hurricane Harvey proved more dangerous than the last, as inch after inch fell across the Houston suburbs that paved over what was once the Katy Prairie.
It fell onto roofs, driveways and concrete streets before running off into storm drains that flowed towards creeks that filled two aging, 1940s-era reservoirs whose dams were all that shielded Houston’s skyscrapers, historic central city neighborhoods, glitzy Galleria district and wealthy West Houston residents from a Katrina-level deluge
The runoff from Harvey’s 50 inches of rain poured into the Addicks and Barker reservoirs more quickly than the Army Corps of Engineers could release it. The giant basins filled, swamping the neighborhoods behind them. The floodwaters sloshed down one dam’s auxiliary spillways — like the emergency drain on a bathtub — in euphemistically described “uncontrolled releases.”
Harvey’s destructive rains have reignited the debate in Texas’ most populous county over the wisdom of policies that fueled decades of sprawl that provided acres of cheap housing, made developers wealthy but — environmentalists say — left a region already prone to flooding even more vulnerable.
It’s a familiar theme in Austin, which has long battled its own flooding problems as it positioned itself as the polished tech-centric counterpoint to Houston’s oil-town grit and sprawl.
“To any who dismiss need for Austin’s zoning & (impervious) cover rules, I give you Houston,” tweeted District 7 Council Member Leslie Pool on Aug. 27, the morning after Harvey lashed Houston with 20 inches of rain in the span of just a few hours. An additional 30 inches would fall across Houston over the next two days or so, before the sun finally re-emerged.
Austin restricts impervious cover — any human-made surface that doesn’t absorb rain — to about 45 percent of property in most areas of the city, and about 25 percent near Barton Springs. A draft of the city’s CodeNext zoning rewrite would require commercial properties that add impervious cover to increase flood mitigation, such as adding drainage ponds, or pay a fee.
However, it’s unclear if those regulations would ease the magnitude of the disaster that would befall Austin if the city got 50 inches of rain — about 45 percent more than the normal annual amount of 34.24 inches — over just a few days.
“It would be as big a disaster here as it was in Houston,” said Troy Kimmel, a meteorology lecturer at the University of Texas. “Thirty, 40, 50 inches of rain, you’re in trouble no matter where you are.”
It’s a fate Austin only narrowly avoided. Harvey hammered towns like Smithville and La Grange, which endured 25 inches of rain that sent the Colorado River out of its banks.
“If this thing had been 30-40 miles to the west, some of this misfortune would have been visited upon us,” Kimmel said. The major difference between what happened in Houston and what could happen here, Kimmel added, would be the type of devastating flooding Austin would endure.
Austin’s rolling terrain means flash flooding, as stormwater rushes downhill. Such floodwater can wash away people and homes, but it’s less likely to keep buildings underwater.
“It rises faster and it flows out faster,” said Kevin Shunk, an Austin Watershed Protection Department division manager. “We rarely have houses that are flooded for days.”
In hilly areas, the difference in how land is affected in a 100-year flood versus a 500-year flood isn’t as great as people might think, Shunk said. In flatter areas, such as in East Austin, the water spreads out more, but fewer homes are built in the flood plain there.
Some areas of Austin also flood badly even though they’re nowhere near flood plains. The city, for instance, bought out property near Charing Cross Road in Northwest Austin, where an old, undersized storm drain backs up. It was cheaper for the city to buy out four homes than to replace the storm drain.
Instead of evaluating flood threats based on complaints, Austin is creating models for the first time of flooding that occurs outside of flood plains.
Overall, about 10 percent of Austin’s landmass is within a 100-year flood plain. The city’s staff estimates about 4,000 houses would flood in a 100-year storm and about 10,000 would have water around them. But that assumes the whole city gets the same rainfall, whereas heavier rain tends to be localized, Watershed Protection Department spokeswoman Stephanie Lott said.
However, Houston sits on the flat Coastal Plains. When it floods, the water has nowhere to go but slowly drain into the network of creeks and bayous that course through the city and eventually flow to the Gulf of Mexico. It’s a process that can take days or longer — a slow-motion disaster.
The Bayou City’s long-running battle with its geography and climate stretches back decades. Like Austin, Houston was devastated by flooding in the 1920s and 1930s, leading to calls for a network of dams and reservoirs to protect the city. Austin got the Lower Colorado River Authority and its chain of dams along the Colorado River. Houston got Addicks and Barker, which were built in the Katy Prairie to the city’s west.
The Katy Prairie was once a vast field of tall grass and wetlands, perfect for absorbing soaking rains and slowing the flow of stormwater, and stretched to about 500,000 to 750,000 acres, said Mary Anne Piacentini, executive director of the Katy Prairie Conservancy.
Over the decades, the land was converted to rice and grazing fields, then transformed again as the city’s suburbs marched westward, covering them in pavement. With each transformation, each acre of land retained less and less stormwater, which now travels toward the dams at even greater speed.
“It’s not that it necessarily absorbs every drop of water from the sky, it’s that it helps to hold back the water until Addicks and Barker can absorb it,” Piacentini said. “If we can hold water back for a day, a week, 10 days, it will allow Barker and Addicks to release (water) at the recommended release rate, and it won’t put pressure downstream.”
When the Addicks and Barker dams were built seven decades ago, the watersheds could absorb or hold about half of the rain that fell on it, the Sierra Club claimed in a 2011 lawsuit challenging new road construction in the area. Today, only about 150,000 to 200,000 acres of the Katy Prairie remains undeveloped, Piacentini said.
The swaths that have been paved over with subdivisions and shopping centers can only retain 15 to 20 percent of the rain that falls, estimated Larry Dunbar, the water resource engineer who drafted the watershed regulations for Fort Bend County, which borders Harris County.
“The concrete doesn’t retain anything; the grass maintains some,” Dunbar said.
Less than a decade ago, the Army Corps of Engineers noticed the growing impact from the sprawl, too.
Emails and documents uncovered by the Sierra Club during its 2011 lawsuit to stop construction on Houston’s Grand Parkway showed the Corps suspected that development around the reservoirs’ watersheds was increasing the amount and speed with which stormwater made its way to the Addicks and Barker reservoirs.
The Sierra Club argued that additional development on the toll road would increase the stress on the dams and make failure more likely.
In emails, the Corps’ natural resources division warned “construction of new infrastructure within the watershed or that encourages additional development within the watershed … further compounds issues and problems that already exist with Addicks and Barker Dams and Reservoirs,” according to excerpts contained in the lawsuit.
Its operations division sounded the alarm too, reporting that more development would increase the chance that homes and businesses behind the reservoirs are flooded for extended periods of time — the exact scenario that played out during Harvey.
Those red flags followed a study finished in December 2008 — but deemed a “draft” and never publicly released — that reported the “Corps had noticed an increase in the magnitude and frequency of storm water runoff entering these reservoirs and resultant increase in pool levels,” the Sierra Club told the court.
For Jim Blackburn, the longtime Houston environmental lawyer who represented the Sierra Club in the 2011 lawsuit, the fight over the tollway offered a perfect example of Houston’s growth “conundrum.”
“The politics of Harris County have been the politics of favoring new development over the people who are here,” he said. “And that, in a nutshell, is the dilemma Houston faces when it comes to flooding.”
But in the wake of Harvey and debilitating floods in 2015 and 2016, Harris County officials have signaled the decades of go-go suburban growth might be over.
“We can’t continue to say these are anomalies,” County Judge Ed Emmett told the Houston Chronicle on Monday. “You’ve got to say, ‘We’re in a new normal, so how are we going to react to it?’”
The proposals Emmett sketched would require billions of dollars for massive buyouts near flood-prone areas, a new flood control reservoir northwest of Houston, additional upgrades to Addicks and Barker, and stricter regulations on suburban growth.
Conservationists and environmentalists were heartened by Emmett’s call for an overhaul.
“I am very happy that they are beginning to identify that we need to do something differently,” Piacentini said.
Blackburn, an old hand in these fights, was circumspect.
“It looks like Emmett is having a serious gut check, and I’m very positive about it,” he said. “He’s talking now; let’s see what he’s going to do.”
Austin mitigates floods with buyouts
Austin has worked since the 1980s to buy out homes vulnerable to flooding.
The Onion Creek buyouts, Austin’s second-largest flood mitigation program, scooped up 801 properties, leaving only 22 remaining in the 100-year flood plain near East William Cannon Drive. That program has a cost of $169.7 million, $62.1 million of it covered by the Army Corps of Engineers and Federal Emergency Management Agency grants.
Elsewhere, the city has bought out homes near Walnut Creek, Johnny Morris Road, Shoal Creek, Jefferson Street, Carson Creek, February Drive and in the Bull Creek Watershed.
Overall, Austin has spent $297 million since 2001 on flood mitigation, not counting the $163 million Waller Creek Tunnel project. A drainage utility fund helps pay for much of that.
“You have buyouts as an option,” says Kevin Shunk, an Austin Watershed Protection Department division manager. “We’re fortunate to have the option to spend more money, whereas smaller communities may not.”
— Elizabeth Findell, American-Statesman