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A tropical storm drifts north and eventually stalls over a Texas town. Within 24 hours, it dumps more than two feet of water, forcing breaches in an earthen dam and sending people to seek shelter.
This wasn’t Hurricane Harvey. Instead, the torrential rain and flooding occurred in Albany, 140 miles west of Fort Worth, in 1978. The remnants of Tropical Storm Amelia, which had already wreaked havoc across the Texas Hill Country, dumped 29 inches of rain. During its week-long march across central Texas, 33 people died.
Scenes of devastating floodwaters from Harvey, which turned streets into rivers in Houston, Beaumont and other cities, have raised the question: Could something like that happen here in North Texas?
Not on that scale, experts say. But extraordinary rainfall of 20 to 30 inches is possible in smaller areas, something that has planners concerned about the risks associated with the region’s nonstop growth.
Harvey was an unprecedented storm, what some are calling a 1,000-year flood event, not just in rainfall but in terms of the geographic area it covered.
“Our distance from the coast makes a Harvey-like event extremely unlikely, if not impossible altogether,” said Dan Huckaby, a National Weather Service meteorologist. “A 50-inch plus tally, which occurred with Harvey, had never been measured before in the continental U.S. So it’s obviously a very rare event.”
By comparison, parts of Navarro and Hill counties saw 20-inch rains in October 2015 which flooded roads and sent massive flows into area lakes such as the Richland-Chambers reservoir. But that was still a drop in the bucket compared to Harvey.
“With Harvey, the 30-inch-plus area encompassed all of Harris County,” Huckaby said. “The 20-inch-plus area was over 100 miles from west to east and north to south, far greater than the Navarro/Hill flood,” Huckaby said. “Typically, when we have 10-inch plus rainfall totals in North Texas, it occurs over a very small area. While the flooding impacts can be devastating and life-threatening, it is nothing in comparison to a tropical system on the Gulf Coast.”
In the Houston area, Huckaby said there have been 16 instances of at least 10 inches of rain over the last century, while DFW’s climate record shows only three.
But while the chances are remote, such a storm is still within the realm of possibility.
A Tarrant Regional Water District study placed the maximum storm for Eagle Mountain Lake at 31 inches over a 72-hour period. That type of flood, which would be a 5,000-year event, would bring Harvey-like destruction, said David Marshall, TRWD’s director of engineering and operations support. The water district maintains the levee system that runs through Fort Worth.
“It would overtop the levees,” Marshall said. “It would be like Houston. The levees are only designed for about half the flow that would generate.” Fort Worth’s levees can handle a 800-1,000-year event.
“The economics are such you really can’t build more than that,” Marshall said. “These 800-to-1,000-year events — these incredibly rare events — are not even like once in a lifetime. They’re like once in five or 10 lifetimes.”
’49 flood brought changes
Dealing with the swings of the Trinity’s Clear Fork and West Fork has been a challenge since Fort Worth was founded. To cope, reservoirs were built to control floodwaters and create a water supply.
Several instances of widespread flooding have been documented since the 1800s.
On July 5, 1889, the Fort Worth Gazette reported “that thousands of people visited the bluff to see the huge sheet of water surrounding Fort Worth beyond the river.”
In April 1922, torrential downpours dumped 11 inches of water in two days and Trinity River levees had 17 breaches that led to 10 deaths and more than $1 million in damage, according to TRWD’s history page. That triggered calls for a water district to develop flood control and protect the water supply.
In 1938, Marine Creek flooded the Stockyards and much of the north side.
But the grandaddy of all floods came in May 1949. Considered the worst in Fort Worth history, the 1949 flood killed at least 10 people and left 13,000 homeless. It was triggered by 10 to 12 inches of rain falling along Mary’s Creek and the Clear Fork.
Lake Benbrook was under construction but didn’t start impounding waters until 1952.
“Traditionally, the Clear Fork was the toughest to tame,” Fort Worth historian Quentin McGown said in a 2015 Star-Telegram article. “Lake Benbrook came as a part of that. Had Benbrook been completed, the ’49 flood would have largely been averted.”
By the next major flood in 1957, Benbrook Lake was in place to limit floodwaters on the Clear Fork.
Following the 1949 flood, the levee system was beefed up on the Clear Fork and West Fork through downtown Fort Worth.
Since 1949, the greatest flow of water on the Trinity through Fort Worth occurred on May 3, 1990, when more than 36,200 cubic feet per second passed through. Much like the spring of 2015, that was caused by an unusually wet spring. Still, that was less than half of the flows seen in the 1949 flood. (Cubic feet per second measures the volume of water passing a specific point each second.)
The nearly $1 billion Trinity River Vision project, which will bring both flood control and economic development, is designed to let the levees come down near downtown by creating a bypass channel to divert floodwaters.
More room for floodwaters, called valley storage, has already been completed downstream from downtown Fort Worth along the West Fork of the Trinity near Samuels Avenue and Northside Drive. Work is underway at Riverside Park and Gateway Park to create room for floodwaters. That valley storage also provides upstream flood protection, said Matt Oliver, a spokesman for the Trinity River Vision Authority.
Concerns over development
One contributing factor to the Houston flooding caused by Harvey was urban sprawl. Development leads to concrete replacing wetlands and farmland, eliminating the ground’s ability to act as sponge for the floodwaters.
There is a similar concern in North Texas as development keeps pushing outward.
Programs such as the North Central Texas Council of Governments Integrated Stormwater Management Plan (iSWM) have been developed to limit increased runoff from new development and a number of North Texas cities have joined the program.
In unincorporated areas, most subdivisions face few requirements to reduce runoff.
One area that has water planners concerned is Mary’s Creek, which runs from Parker County through western Tarrant County before dumping into the Clear Fork below the Lake Benbrook dam.
One water official said Mary’s Creek can essentially become another fork of the Trinity during a major flood so concerns about increased runoff are taken seriously.
A 2014 Council of Governments report showed runoff could increase by as much as 30 percent along Mary’s Creek by 2055 from new development as homes and businesses replace farms and ranches.
The Council of Governments believes counties have the tools to enforce tighter runoff restrictions on new development through the state water code, said Edith Marvin, the Council of Government’s director of environment and development.
“We believe it is already in place,” Marvin said. “It’s up to the county attorneys in each county to determine whether that is the case as well.”
If Parker County endorses the higher guidelines, the Council of Governments would consider a Mary’s Creek pilot project to study the impact of increased development, Marvin said. Denton County just became the first county in North Texas to endorse the higher standards.
For now, Parker County has no plans to raise its runoff restrictions on new development, said Parker County spokesman Joel Kertok. Developments like the 7,200-Walsh Ranch and the new Morningstar development near Aledo fall under Fort Worth’s control, Kertok said.
Keeping an eye on development along Mary’s Creek is important going forward, Marshall said.
“If development is uncontrolled, there’s a lot of houses outside the levee system upstream that could be really be hit hard during a major flood,” Marshall said. “The levees could handle the flow. It’s upstream that’s the concern.”