Hurricane Harvey, which struck Texas’s Gulf Coast in 2017, was the largest rainfall event in U.S. history, dumping more than 60 inches (152 centimeters) of precipitation in some parts of the state over the course of a week. The hurricane also moved a superlative amount of sediment. At least 27 million cubic meters of sand, silt, and other geological debris flowed through Houston during and immediately after the storm, according to a new study published in Geology. That’s up to 51 times the modern sediment volume that enters nearby Galveston Bay in an entire year.
“Everything became a source [of sediment],” said Julia Wellner, a sedimentologist at the University of Houston and coauthor on the study.
Sediment doesn’t often make headlines during a disaster, but it’s a major hurdle in the cleanup process. Streets are plastered in mud, sewers are clogged, reservoirs lose capacity, and streams fill with sand. After Harvey, the federal government spent roughly $351 million to dredge Texas’s navigational channels.
But despite the sediment’s obvious impact, no one knew until recently how much the storm churned up. Knowing that number isn’t just a curiosity; it has practical implications for how cities can prepare for future disasters.
Galveston Bay Turns Beige
With publicly available digital elevation models (DEMs), the researchers looked at land height before and after the storm to track sediment erosion and deposition. Adding the volume reportedly dredged from the Houston Ship Channel and the city’s central Buffalo Bayou Park, plus the 100 million metric tons that other researchers determined entered downstream Galveston Bay—which turned beige in satellite views during the storm—the researchers calculated the amount of sediment that moved through the two flood control reservoirs and 12 main waterways that drain the Houston metropolitan area.
Though the total amount of shifted sediment they estimated is massive, this is a minimum number, emphasized Andrew Stearns, who led the study as a graduate student in sedimentology at the University of Houston. The actual total is “much, much larger,” he said, because plenty more sediment moved underwater, where it couldn’t be detected from imagery.
This basic approach is limited to the sediment above the water, but it’s still intriguing, said Nina Stark, a coastal geotechnical engineer at the University of Florida who was not involved in the study. “By keeping the methods a little bit on the simpler side, that maybe creates larger uncertainty, but that likely doesn’t change the message,” she said.
Since the approach was straightforward, Stark added, “it makes it easy to apply somewhere else.” Lessons from the hurricane could serve as a cautionary tale for other cities. The nation’s fourth largest city, Houston’s urban footprint increased by 63% in just the 20 years before Harvey. That means plenty of pavement and other impervious surfaces, which move water through a city faster than natural covering. Faster water slices deeper into the soil.
The group also found that Houston’s most manipulated rivers and creeks—those lined with concrete—moved the most sediment.
“When you develop land, you tend to make sediment a lot easier to erode,” Stearns said.
Filling the Reservoir
The more sediment clogs a waterway, the less room there is for water to move, and the worse flooding becomes.
The deluge of sediment in Houston will have long-term effects, particularly in the Addicks and Barker Reservoirs. Built by the city in the 1940s as flood control measures, these massive basins roughly 20 miles (32 kilometers) west of downtown are designed to keep water from reaching the city. But since their construction, the reservoir watersheds went from less than 1% to 40% developed. About 20,000 homes now sit in the shadow of the reservoir dams. Harvey’s sediment load increased their flood risk.
Roughly 26% of the transported sediment ended up in just these two reservoirs, decreasing their holding capacities by ~1.2% and ~1.6%, respectively, according to the study. “That’s not huge. But that’s an amount that the next flood can’t use,” Wellner noted. A storm slightly smaller than Harvey could now have equally devastating consequences.
The authors argue it’s time more cities think about how sediment moves through their watersheds.
“Any flooding event that you’ve heard about in the last 20 years, I can’t personally remember them talking about sediment as one of the major problems,” Stearns said. It’s an easy issue to overlook because it’s largely out of sight and underwater. Sand is small, and basins are big. But knowing where sediment moves has clear benefits for policy and planning.
“If you want to better predict how reservoirs will fill up, then you really have to understand what happens during these types of events,” said Torbjörn Törnqvist, a geologist at Tulane University who was not involved in the research.
Törnqvist agrees with Stearns that few urban planners are developing with sediment loads in mind. “I think there’s a lot more intention now for how to deal with water during these extreme events, but how to deal with sediment is still kind of a frontier,” he added.
—J. Besl (@J_Besl), Science Writer