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Hurricane Harvey could redefine risk models for extreme floods in Waco area

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503 flood

Someday, a rain storm like none in the Waco area’s recorded history could push Lake Waco to the top of its floodgates, engulf Waco Regional Airport, cascade down to the Brazos River and imperil East Waco and Baylor University with a surge of floodwater.

That apocalyptic scene comes not from the fevered imaginations of Hollywood scriptwriters but sober-minded hydrogeologists who designed Lake Waco for the worst-case scenario, called a “maximum probable flood.”

But recent events have some experts wondering if their worst-case scenarios are dire enough.

Hurricane Harvey’s assault on the Houston area last month broke records more than a century old and caused disastrous flooding well beyond designated floodplains. A record rainfall just shy of 52 inches fell near Cedar Bayou, and an area of 11,492 square miles was covered in at least 30 inches of rain.

Since then, climatologists and government hydrologists have resolved to revisit the models that communities use to assess how vulnerable they are to storms and floods.

“We haven’t analyzed this yet. It’s almost too soon,” said Allen Avance, a hydrologist with the U.S. Army Corps of Engineers Fort Worth District office. “Our plans are to take that storm and analyze it and see if it’s possible for such a thing to move inland.”

The Corps has already been working with the city of Waco and McLennan County to model how another storm — the one that dumped 29 inches on Gainesville in May 2015 and flooded the Trinity River — would have affected waterways in McLennan County.

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Lake Waco and dam

Lake Waco overflows near the FM 185 bridge in 2005.

In the meantime, the Houston flood has already worked its way into Waco politics. Opponents of a proposed new landfill on Old Lorena Road, next to the current landfill, have repeatedly asserted that a Harvey-style rainfall event could cause Lake Waco to back up into the landfill site. Waco officials say that scenario is inconceivable given the design of the lake.

Of course, Waco isn’t Houston. Waco is much better drained: Its terrain drops 240 feet between the old Hillcrest Hospital and the Brazos River, which is itself 350 feet above sea level. And being more than 200 miles from the Gulf of Mexico, this city is far less prone to hurricane-based rainfall, state climatologist John Nielsen-Gammon said.

Nielsen-Gammon, who is based at Texas A&M University, was involved in calculating “probable maximum precipitation” estimates for Texas.

“We would not have conjectured that a tropical storm like Harvey would occur as far north as Waco,” he said.

Still, Nielsen-Gammon said there’s reason to think the worst-case scenarios around the state may ultimately need to be revised upward, especially as warming ocean temperatures intensify tropical storms.

“The biggest problem with the theoretical flood is that there’s actually no theory involved with this,” he said. “It’s more like a maximum flood based on historical events and maximum moisture available. As more events happen, Harvey being an example, you expand the realm of possibility. More things that can happen actually do happen.”

And when it comes to storms and floods in the Waco area, there’s reason to think the worst is yet to come.

Extremes of rainfall

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Flood map

A mapping tool from floodmap.net shows the elevation of 503 feet above sea level. At that point, water would flow around the Lake Waco dam or overtop it.

A 1980 master’s thesis published by Baylor University’s geology department observed that Waco has been spared the extremes of rainfall that have fallen in other inland areas, such as the 36 inches that fell in Thrall, Texas, in an 18-hour period in 1921.

Based on such a storm, the author, Edward Dale Leach, calculated a maximum probable flood for Waco that would reach 68 feet — 27 feet above the flood of record on the Brazos River.

If Lake Waco and Lake Whitney were already full, an extreme storm could have disastrous consequences such as Waco has never seen, Leach wrote.

“Those conditions will someday coincide with a great storm, and a great flood will occur on the Brazos River in Waco,” Leach wrote.

“Waco City Hall will be under nine feet of water, and the county courthouse will be on the edge of the Brazos River,” he wrote. “There will not be a square foot of Baylor University that is not inundated, and all bridges crossing the Brazos River will be either washed away or covered by floodwater. … Hundreds of houses will be flooded and many washed away. There is no method of estimating the loss of life.”

Avance, the Corps hydrologist, said such a scenario is not impossible, but it would require simultaneous maximum floods on the Bosque and Brazos Rivers, and it would be what might be termed a “million-year flood.”

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Lake Waco and dam

Lake Waco backed up into some of the holes on the Twin Rivers golf course in 2016, when the Corps held back water to protect the Houston area. Lake Waco was up 22 feet over normal but still 15 feet from the top of the spillway gates.

Any worst-case scenario for Waco would involve both the Brazos River, which drains a huge swath of Texas, and the much smaller Bosque River, which feeds Lake Waco before flowing into the Brazos at Cameron Park.

The U.S. Army Corps of Engineers designed Lake Whitney to contain a maximum probable flood on the Brazos and Lake Waco to do the same for the Bosque River and tributaries.

Those dams have been so effective that the Brazos has not flooded any significant part of Waco since the completion of Lake Waco in 1964. Few remember the days, most recently in 1957, when the Brazos would periodically sprawl across East Waco and rescuers would paddle boats down Elm Avenue.

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Lake Waco and dam

Water is released from Lake Waco in 2005. The record level for Lake Waco is 488.5 feet above sea level on Christmas Eve 1991, followed by 486 feet in July 2007 and 484.5 feet in June 2016.

The two dams can hold back a vast amount of water. Lake Whitney can triple in size to 1.6 million acre-feet of storage, with an acre-foot representing 325,851 gallons.

In a maximum flood, Lake Waco can rise 38 feet to hold an additional 553,300 acre-feet of stormwater above its normal level. Even without discharging water, the dam could hold back two floods the size of the record 1936 flood on the Bosque and still have room to spare.

The dam was designed based not just on that 1936 flood but on other historic rainstorms in the region, including an 1899 flood at Hearne. The lake has a normal level of 462 feet above sea level and a maximum height of 500 feet.

Long before the lake fills up, the dam gates would release unprecedented amounts of water at a rate of up to 563,000 cubic feet per second. If the lake ever topped the 500-foot mark, it is designed to drain onto Waco Regional Airport and China Spring Road so it doesn’t spill over the dam and risk a catastrophic dam failure.

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503 flood

A closeup of the 503-foot-elevation easement around Lake Waco, extending slightly into a site where the city of Waco is considering a landfill. Above that level, floodwater would flow around the dam or top it.

In building Lake Waco, the Corps designated a no-build easement around the lake up to the 503-foot elevation, and a small finger of that easement reaches into the site of the proposed Old Lorena Road landfill. Bradford Holland, head of Citizens Against the Highway 84 Landfill, has warned that the lake could back up into the proposed new landfill.

“Should water levels reach the 503 flowage easement floodwater level, our drinking water reservoir will absorb all leachate that has EVER run off the landfill property,” he wrote in an email to the Tribune-Herald.

‘Totally illegitimate’

But Deputy City Manager Wiley Stem called that concern “totally illegitimate.” First, the new landfill would skirt the 503-foot easement. The bottom of the lowest cell at the current landfill is at 519 feet, and he expects similar minimum elevations at the Old Lorena Road site.

“We would have to be 16 feet above the top of the spillway for that to happen,” Stem said. “The landfill is above the top of the dam. In a case like that, the water is going to top the dam and go over the dam, not back up into the landfill. … We would have much bigger issues, the loss of life, people drowning downstream. … I’m very confident that in any event that our landfill or any other new landfill will be able to handle any flood event.”

The record level for Lake Waco is 488.5 feet above sea level on Christmas Eve 1991, followed by 486 feet in July 2007 and 484.5 feet in June 2016.

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Lake Waco and dam

Water is released from Lake Waco after a heavy storm in 2005. The U.S. Army Corps of Engineers and emergency management officials are re-examining flood risk models in light of recent heavy rains.

The true worst-case scenario would be for water to top the dam and wash it out, unleashing a tsunami-like wall of water downstream.

The Corps of Engineers and the Waco-McLennan County Office of Emergency Management regularly collaborate for “table-top exercises” to rehearse how they would respond to such an event, unlikely as it may be.

Local emergency management director Frank Patterson said the lakes would release a huge amount of water to prevent a dam breach.

“I think the Corps has a good handle on the dam and keeping it from failing,” Patterson said. “Their job is to protect the dam.”

But he said releasing so much water could cause damaging floods downstream. And he sees increasing challenges in managing floodwater in general.

“I tell you, it’s not going to get any better,” he said. “The more things we build, the more rooftops we put in, the more risk we have of flooding.”

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Lake Waco

U.S. Army Corps of Engineers and emergency management officials are re-examining flood risk models in light of recent extreme rains.

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Lake Waco and dam

Lake Waco flooded in 2016, when the Corps held back water to protect the Houston area. The lake was up 22 feet over normal but still 15 feet from the top of the spillway gates.

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