How the flooding in Southeast Texas got so dire

The Lake Livingston Dam is seen on Jan. 31 in Livingston. The Southeast Texas reservoir reached capacity during several days of intense rainstorms this week. (Maria Crane/The Texas Tribune, Maria Crane/The Texas Tribune)

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HOUSTON — Southeast Texas is used to heavy spring rains — but the widespread flooding that the region faced this week stands out because of just how much the rivers have risen.

Back-to-back storms drenched the area that includes Polk, Montgomery, Liberty and Harris counties, causing flash-flooding from heavy rain. That rain also filled creeks, rivers and reservoirs, creating a compounding, dangerous problem of too much water with nowhere to go but back out of the riverbanks.

Operators for three major reservoirs on rivers in the area have been on high alert as they deal with the slugs of water flowing into the man-made lakes. Part of their job is to calculate how much water to release downstream to protect the dams from failure, which would cause an even worse catastrophe than the swollen rivers.

Lake Livingston, a reservoir located in the East Texas Piney Woods; Lake Houston, a reservoir on the west fork of the San Jacinto River, 15 miles northeast of downtown Houston; and Lake Conroe, a reservoir located north of Houston in Montgomery County, were all at capacity and releasing water downstream.

The Trinity River’s water feeds into Lake Livingston. Meanwhile, the West Fork of the San Jacinto River contributes to Lake Conroe, which then feeds into Lake Houston, as does other runoff. All eventually empty into the Gulf of Mexico.

Lake Livingston operators said at one point they were releasing more water than they did during Hurricane Harvey, a disastrous storm that hit Texas in 2017 and dropped unprecedented amounts of rain across the Greater Houston region. Lake Conroe was releasing high amounts too — but not quite as much as during Harvey.

[Rain eases in Southeast Texas but flooding will take time to recede]

While some dams are designed for flood control, these three are not. Instead, they serve as sources of drinking water.

As the rivers swell with the reservoir releases and with other rainfall draining into them, low-lying neighborhoods are going underwater.

“You're affecting people's livelihoods downstream,” said Rick Davis, assistant project manager at the Trinity River Authority, which manages the Lake Livingston dam. “But with every release, you can only hope that the water levels coming into the reservoir stabilize and can start tapering back the releases of water and hopefully give folks some relief.”

Here’s how we got here and how this system is designed to work:

What is a reservoir?

A reservoir is a man-made lake formed by the construction of a dam across a river. The dam and gates control the amount of water that flows out of the reservoir.

Reservoirs are built to hold back a certain amount of water because the amount of water in a river can vary over time. There are different types of reservoirs; the most common are for flood control and water conservation. Lake Livingston, Lake Houston and Lake Conroe essentially serve as big pools of water that hold drinking water for the city of Houston and the growing region.

How can reservoirs contribute to flooding?

Reservoir operators control how much water gets released downstream. That’s not necessarily a bad thing. Without a dam, all the water draining into the river might flow downstream even quicker, said Katie Landry-Guyton, a senior service hydrologist with the National Weather Service Houston/Galveston office.

But reservoirs can contribute to flooding when lakes reach capacity and operators must release the water to make sure the dam can keep operating safely.

Rick Warner, a superintendent with the Coastal Water Authority, which operates the Lake Houston dam, said that when operators see the weather forecast alerting people of potential rainfall, they can open the dam’s gates to release water before the storm hits.

Opening the gates helps lower the lake’s water levels and creates room for more water. He said these so-called controlled pre-releases will hopefully alleviate flooding that can occur with rainfall.

“This gives us a little head start on releasing water before the flood water comes down to the reservoir,” he said.

Warner said that any opening or closing of the dam’s gate has to be directed by the city of Houston, which owns the lake.

That calculation might be harder for a reservoir farther north, like Lake Conroe. If the rainfall prediction is off, and neighborhoods to the south of the lake get hit with hard rainfall at the same time the river swells with an early release, that can create a major problem.

Hydrologists and engineers set guidelines for monitoring and matching inflows into the reservoir and rates of release. Each reservoir has a unique set of rules and they are based on historical river data, floods and droughts, said Davis, the assistant project manager at the Trinity River Authority.

With some dam and reservoir designs, such as Lake Conroe, when there’s heavy rainfall, lakes can begin to reach higher capacities and run the risk of overflowing. If water is not released, the dam could break.

“It could cause catastrophic damage when lakes were allowed to get too full,” Warner said.

According to the Association of State Dam Safety Officials, hundreds of dam failures have occurred throughout U.S. history and they’ve caused severe property and environmental damages and have taken thousands of lives.

How did the weather lead to flooding this week?

The flooding disaster unfolding across parts of Southeast Texas began days earlier, on Sunday, when the first rounds of heavy rain drenched the region and started to drain into lakes that were already full. The Trinity River too was elevated from the start, said Landry-Guyton, at the National Weather Service.

Another round of heavy rain on Wednesday and Thursday added even more water to roughly the same areas, meteorologists said.

Over the past week, 15 to 20 inches of water fell on the area that drains into Lake Livingston, and 10 to 15 inches on the area that drains into the East Fork of the San Jacinto River and Lake Conroe, Landry-Guyton said. More fell in some isolated spots.

That’s a whopping amount of water.

Meteorologist Matt Lanza, who helps run the much-watched Space City Weather website in Houston, said he was used to seeing 10 inches of rain falling overnight on occasion. But to see heavy rain spells twice in one week was “problematic,” he said.

By Wednesday, as the rain was still falling, Lanza realized the region couldn’t take any more water.

“This is not a normal type of spring flood,” Lanza said. “This is more of an extreme type of spring flood.”

Polk County Office of Emergency Management officials warned residents Monday that the Trinity River Authority of Texas, an agency that oversees Lake Livingston and its dam, would discharge water from the lake into the area. Officials asked residents to evacuate immediately.

By 3 p.m. Thursday, the amount of water coming out of Lake Livingston reached 124,000 cubic feet per second — equivalent to releasing 124,000 basketballs per second. This is the highest water release for the lake in its history. For context, during Hurricane Harvey, operators were releasing 110,000 cubic feet per second at the most.

Carlos Nogueras Ramos contributed to this story.

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