This story has been updated as of 4:45 pm EDT on Tuesday, August 29.
Hurricanes are classified according to their wind speed. But a truer measure of their destructive potential would also include their moisture level. Just before making landfall on Friday night, Hurricane Harvey jumped up to become a category 4 hurricane, with sustained winds of 130 miles per hour. More dangerously, though, it’s also packing enough moisture to drop 20 to 50 inches of rain across Texas’ gulf coast.
As early as Wednesday, August 23, Governor Greg Abbot declared a state of emergency so the gulf region would be eligible for federal disaster relief. They’ll likely need it. The current forecast predicts that Harvey, which was downgraded to a tropical storm on Saturday, will stall out over east Texas for days. Yet, while seven counties had ordered evacuations by Friday, Houston—home to 2.3 million people—didn't. Its mayor told residents to weather it out, rather than clog highways leading out of the city.
And by Sunday evening, Harris County—Houston’s encircling jurisdiction—became one of 18 counties across Texas that had been declared a federal disaster area. (Those counties contain 7 million people, nearly a quarter of the state's population.) In the city of Houston, nine shelters have opened up to take in people fleeing their homes.
This isn’t Houston's first rodeo: The city designed its layout for drainage, with copious reservoirs and dams, to store and shed the downpour without inundating residents. But the region’s front line of defense are its wetlands. These soak up moisture that would otherwise flood Houston. And this week’s deluge threatens to overrun both the built and natural systems.
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“Just to give you a lowdown on the area, it’s wet, it’s flat, and it’s gonna be really wet in the next couple days,” says John Jacob, the director of the Texas Coastal Watershed Program at Texas A&M University. Any rain that falls in Harris County can run to sea through any of the county’s 22 watersheds. That might sound like a lot of drainage potential, but the highest point in the county is only 125 feet above sea level, which means the water can pool up pretty quickly.
To mitigate the buildup, Houston turns to two main engineered systems: the Addicks and Barker dams and reservoirs. “These were designed to protect the downtown area,” says Randy Cephus, a spokesperson for the Army Corps of Engineers Galveston Division. “They are dry reservoirs, meaning the dams are wide open and water is flowing freely until the heavier rains arrive.” The flood-prone city has one of the most sophisticated networks of gauges in the country. Once those reach critical levels, the dam’s safety officer will order the floodgates closed, and the reservoirs will fill up.
Combined, the Addicks and Barker reservoirs can hold 410,000 acre-feet of water. Twenty inches of rain across Harris County adds up to about 1,894,980 acre-feet. Don’t let that number scare you: Harris County has all those other watersheds, plus attendant flood control schema (which I’ll get into a little later). But neither should you be relieved. Addicks and Barker are on the northwestern end of Houston, which means a lot of the water they take up will come from outside Harris County. Acts of God don’t abide by earthly borders.
Addicks and Barker are also in the middle of a $75 million safety upgrade. The system was built in the 1930s, and the Army Corps of Engineers recently finished a coffer dam that will function in place of the regular flow. And the construction crews still on the ground might actually help things out. “We were able to modify the contract on the heavy equipment and resources like the quarry, so they can help us out, moving rocks and sandbags in a more expeditious manner,” says Cephus.
The Houston suburbs have their own protective measures. One of the biggest projects is Brays Bayou, the watershed of which encompasses 700,000 people. Parts of this watershed flooded in 2015 and 2016. The Brays Bayou Project—a joint endeavor of the Army Corps and Harris County Flood Control District—is several years behind schedule on a $480 million upgrade. Project manager Gary Zika says the upgrade is almost complete. For this flood, his workers will focus on keeping drains clear of rubbish and keeping the public aware of the flood levels.
Houston, and much of east Texas, was built over a wet coastal plain. “Historically, when we consider the undisturbed landscape, 30 percent of it was covered with wetlands,” says Jacob. “That gave us about ⅓ an acre-foot of water retention in every acre of land.” That’s pretty significant, considering Harris County currently requires every paved acre of land to have ½ acre foot of water retention. Paving them over just means humans have to spend more money creating building artificial means to store and route storm runoff to the sea. All that free water storage has been lost to agriculture and urban sprawl.
The wetlands are important, but they aren’t magic. “If you get 16, 17, 18 inches, the prairie is going to flood,” says Jacob. But he points out that the built retention is only meant to hold 100-year flood event type rainfall. By Monday morning, after more than 25 inches had fallen in areas around Brays Bayou and near both reservoirs, area officials were saying that Harvey has the potential to exceed a 1,000-year flood plain threshold.
Jeff Lindner, with Harris County Office of Emergency Management, said in a press conference Monday that Brays Bayou was among the hardest hit, with water levels reaching four feet over the banks Sunday night. But in a bit of positive news, around midnight the rain over Brays let up, and by 10 am the water had receded to only one foot over the banks. Whether or not that trend continues depends on how much rain Harvey still has in store for Houston. Near the reservoirs, the picture is not so rosy.
Unprecedented Rainfall, Unprecedented Flooding
At 8:00 pm on Friday night the Corps ordered both dams closed. Two hours later, Harvey made landfall near Rockport, Texas. Over the next 24 hours, the storm dumped more than 24 inches of rain over sections of Houston, according to the National Weather Service. The NWS declared a flash flood emergency Sunday evening, following reports of water reaching the second stories of houses and apartment buildings in the southern parts of the city. The weather service is warning that over the next few days rainfall could reach as much as 50 inches in some places, which would be the highest levels ever recorded in the state of Texas.
“I know for a fact this is the worst flood Houston has ever experienced,” an NWS meteorologist told a Houston Chronicle reporter.
What does that mean for the city's dams? As of Sunday evening, the Addicks reservoir had risen to just over 98 feet, and Barker to 93 feet, according to Corps statistics and the Harris County flood warning system. Corps officials announced Sunday evening that because of the extreme nature of the ongoing weather in the area, they would likely have to release intermittent amounts of water from both reservoirs to reduce the risk to the heart of downtown—home to the city's largest hospital, as well as the George R. Brown Convention Center, which is preparing to shelter 1,000 people Sunday night. Late Sunday night, local officials issued voluntary evacuation notices for residents around the reservoirs, but urged them to wait until daylight to leave the area, if they chose to do so.
Shortly after the notices went out, the water in both reservoirs began to rise dramatically. So at 2:30 am Monday morning, Corps engineers opened the gates and began releasing water at about 2,500 cubic feet per second from Atticks and about 2,000 c.f.s from Barker. The plan is to continuously let out water, increasing the combined release levels to a maximum of about 8,000 c.f.s. by Monday afternoon. The Corps said that water will flow east into Buffalo Bayou and down toward the city of Houston. But as a result of the release water will also spread out into adjacent neighborhoods—like Bear Creek Village and Canyon Gate—to the west and north.
Authorities expect about a thousand homes to flood due to these gradual releases. To complicate matters, several major roads and highways that run through both reservoirs were already underwater as of Monday morning, and those routes are expected to remain impassible for weeks to months.
And early Tuesday morning the Addicks Reservoir—located about 19 miles west of downtown Houston—breached its banks. While the uncontrolled spillways are an intentional part of the dam's design, it's the first time in its 60-plus year history that it's ever happened. “Typically we just close the gates until it stops raining,” says Mario Beddingfield, a hydraulic engineer with the Army Corps of Engineer’s Galveston District, reached by phone Sunday night. “It still may go over the ends, but at least we can reduce the amount that goes uncontrolled downstream into downtown.”
After another night of heavy rain, water levels rose above the dam’s 108-foot barrier around 8:00am and began overflowing into non-government land at both ends of the dam. At a press conference Tuesday morning, local officials listed off the main neighborhoods already affected by the spill. Thousands of homes in those areas have already been flooded upstream of Addicks. But no one seems to know what to expect next.
“This is something we’ve never faced before, so we are trying our best to wrap our hands around what exactly this water is going to do,” says Jeff Lindner, a meteorologist with Harris County Flood Control District. He explained that the water is going to flow out first into the surrounding stormwater drainage systems, then into roadside ditches, and eventually into the streets. But exactly how exactly the water interacts with all those systems on its way east toward downtown is a big unknown.
Around noon, US Army Corps of Engineers Galveston District Commander, Col Lars Zetterstrom, gave a live update from the top of nearby Barker Reservoir, where levels have risen to just above 101 feet, still three feet shy of its embankment wall. According to Corps models, he said they don’t expect Barker to breach its banks until Wednesday morning. But their models also hadn't expected Addicks to overflow until Thursday. Complicating monitoring efforts, a number of rain sensors are currently out of service due to flooding.
At a 1:30 pm press conference, Corps officials updated their estimates on how much water will be escaping uncontrolled out of Addicks. Edmond Russo, a Corps spokesman, said they expected the worst of it to come on Thursday, with water levels reaching 110.4 feet and rushing over the barrier at about 4,500 cubic feet per second. That’s in addition to the water the Corps is continuing to release through controlled gates into Buffalo Bayou. At this point, Russo said they were looking at September 7 as the earliest that waters would be back within the reservoirs boundaries. Russo maintained that both reservoirs are designed to handle overflows and “can withstand them.”
In a slight bit of good news, the latest forecasts suggest the worst of the rain will be moving east of the city by Wednesday morning, which should at least slow the rate of water entering the reservoirs. About nine more inches of rain are currently expected to fall upstream of Barker and Addicks through the end of the week. The National Weather Service announced Tuesday that 49.2 inches of rain had drenched Houston since Harvey arrived, a new US record.
The Corps expressed utmost confidence that their structures are working the way they're supposed to. But in an unfolding natural disaster of unprecedented scale, even they don't know what's going to happen next. They're working with state, county, and federal officials to get the best information they can to residents near the reservoirs and in downtown. But everyone is still just taking it an hour (and an inch) at a time.
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