As the Blanco recedes, officials wonder aloud about a dam

May 30--WIMBERLEY -- Fifty years ago, the U.S. Army Corps of Engineers and the Guadalupe-Blanco River Authority proposed a $42.2 million reservoir that would have dammed the Blanco River upstream of Wimberley.

The Cloptin's Crossing Reservoir project, designed as a water source and flood-control measure, was abandoned following local opposition. But after the Memorial Day weekend flooding along the Blanco River that claimed the lives of at least seven people, Hays County Judge Bert Cobb said the idea should be revisited.

"If we had built the Cloptin dam as it was already designed and funded, we probably would not have had the life-threatening flooding that we just had in Wimberley, Texas," Cobb said.

Cobb's view is not universal. Opponents say large reservoirs -- like those Texas built across the state in the middle of the last century -- come with their own environmental costs, flooding property and enabling development in a delicate part of the state.

The renewed debate over whether to dam the Blanco plays into a larger question of what should be done to prevent another disaster in a Hill Country region known as "flash flood alley."

As climate change causes extreme weather events to become more frequent and the Hays County towns downstream of the Hill Country continue to grow, both the severity of the flooding and the number of people in harm's way are expected to increase. The week before the floods devastated northeastern San Marcos, the Census Bureau named it the fastest-growing city in the country for the third year in a row.

Other solutions include further restricting development in flood-prone areas, widening river channels so they can better absorb inundations and investing in more precise gauges to monitor river flows and alert systems to warn residents of coming danger.

For Cobb, the answer is all of the above.

"Once you've had a flood like a 42-foot wall of water hitting the city, it should change minds, and it should change hearts," Cobb said. "I think the biggest change will have to be attitudinal -- about what people are allowed to do and what people are not allowed to do, and where you build things and how you build things."

"Flash flood alley"

The terrain and geographic position of the Hill Country make it vulnerable to the sort of flash floods that wrought disaster in Blanco and Hays counties last weekend.

Many of the world-record rates for rainfalls of less than 48 hours have occurred along Balcones Escarpment, the geologic uplift that characterizes the Hill Country, says Raymond Slade, a retired U.S. Geological Survey Texas water specialist.

Partly because the Hill Country sits at a crossroads between moist Pacific air from the west and moist Gulf air from the south and east; and partly because, as air heads over the escarpment, it lifts and cools -- leading moisture in it to condense. If a system stalls over the Hill Country, it "just dumps" water, Slade said.

On Saturday night, nearly 6 inches of rain fell over a mere four-and-a-half hours in Blanco, on soils already saturated with rains from weeks of storms. The Blanco rose about 34 feet in three hours.

Unlike the flatter land east of Interstate 35, which spreads out and slows down water heading to creeks, the steep Hill Country means that "a lot of water runoff gets to the (river) channels at the same time," Slade said. "The Blanco has a steep slope -- three to four times steeper than the areas to the east -- leading to flash flood events."

Texas, wrote engineer Jonathan Burnett in his 2008 book Flash Floods in Texas, "has some of the most flash flood-prone land in the world."

And such extreme floods are likely to become more frequent in the future because of global warming, said John Nielsen-Gammon, the state's climatologist and a Texas A&M University professor.

"The rate of rainfall is limited by how much moisture the air can carry. The amount of moisture the air can carry is limited by its temperature. So the warmer the ocean and overlying atmosphere, the greater the amount of water that will be transported into Texas," he said. "If a region is vulnerable to flash flooding because of steep terrain, shallow soils and an increasingly built-up environment, the risk of flooding is going up."

Damming the Blanco

As long ago as 1961, engineers proposed building a dam upriver of Wimberley, at a spot variously known as Cloptin's or Clopton's Crossing. It didn't go over well.

"The large landowners fought that thing tooth and nail," said Bill West, who is now general manager of the Guadalupe-Blanco River Authority but wasn't with the authority at that time. "Dams today are the most controversial water issues, not only in Texas but across the country."

The plan was ridiculed because there were few apparent buyers of the reservoir's water and because its costs nearly exceeded the river authority's entire budget.

The politics of building a major flood-control dam are "immensely difficult," said Todd Votteler, who oversees science, intergovernmental relations and policy at the river authority, as landowners would fight to prevent their property around the dam from being flooded or acquired through eminent domain.

The Blanco reservoir "probably would have been dry most of the time," Votteler said. "The yield on it is pretty iffy. Events like this one are so rare."

Such a project would likely require federal funding. Jake Pickle, the longtime congressman for the area, backed Cloptin's Crossing for several years but gave up as local officeholders opposed to it held their ground.

If a new proposal arises, U.S. Rep. Lamar Smith, R-San Antonio, who represents the area now, would be "open to discussing with local officials," Smith spokeswoman Laura Crist said in an email.

Although he was supportive of the Cloptin's Crossing plan at the time, West said there would have to be "overwhelming support" for the river authority to resurrect the project or one like it. Still, he said, there's a reason the nearby Guadalupe River, which is dammed by the Canyon Lake reservoir, didn't cause as much destruction over Memorial Day weekend.

"Canyon has done its job," he said. "Had Canyon not been in place, you would have had massive destruction in New Braunfels."

Over the last 15 years, the Blanco has crested in Wimberley above major flood stage -- 26 feet -- at least three times: 28.89 feet in 2001; 26.74 feet in 2013; and, over Memorial Day weekend 2015, at just over 40 feet, according to National Weather Service records.

But Dianne Wassenich, executive director at the San Marcos River Foundation, said she opposes the construction of large reservoirs and believes their value as flood-control tools are overblown.

"There are many factors that you have to think about when you start talking about dams -- the destruction of many acres of land and taking it from landowners, usually unwillingly," she said. "Then you have the added danger of a dam that could collapse and, if it did, would have a deadly impact downstream."

A new reservoir would also be a significant water source for the growing region, solving a problem Cobb and other pro-development politicians have been working on for the past few years. Conversely, a reliable water source could further fuel the construction of homes and businesses in an area that nature reminded this week is very much in harm's way.

Other options

Rather than build a dam, Wassenich suggested local governments crack down on development in flood-prone areas. The city of San Marcos has in recent years green-lighted several controversial projects in the 100-year flood plain -- the area in which construction is restricted -- including a student apartment complex on a part of the San Marcos River called Cape's Camp.

Given the record-breaking nature of the recent floods and the possibility of climate change fueling worse inundations in the future, some are suggesting that policymakers now use 500-year floods as a benchmark.

"We're looking at the new normal now," Travis County Commissioner Brigid Shea said. "We should really be looking at 500-year flood plains if we're going to protect people."

Like building a dam, such a shift is easier said than done. West said that any changes that restrict property rights or impact building plans could be as politically difficult as building a dam.

"You start moving flood plains around and you start impacting economic development, boy, you stir up a big hornet's nest," he said.

Michael Segner, the Texas Water Development Board's coordinator for the federal flood insurance program, said the state defaults to local government preferences when tackling flood control. Aside from dams, he said, options include widening the river channel, building detention ponds and "making sure that they're keeping development to a minimum in those flood-risk areas."

Hays County and the Guadalupe-Blanco River Authority are in the final phase of a three-year study on local flood conditions and possible remedies, West said.

The Blanco's peak crest in Wimberley of 40 feet raced above the previous record of 33 feet, set in 1929.

Hydrologist Joanna Curran, who wrote a 2007 paper about rainfall scenarios on the Blanco River, said there was only a 1 to 2 percent chance of a repeat of such a flood, saying that it depended on a micro-burst of rain in just the right place.

"Your grandmother isn't crazy when she calls and says, 'Is it raining where you are?'" Curran said.

She warned that the prospect of flooding could increase if the Blanco watershed becomes much more urbanized. (The watershed is roughly 2 to 3 percent urban cover, she estimated; it might have to reach 10 percent before it'll be much of a factor.)

"If rain falls and can't infiltrate (the soil), more of it is going to run off," she said. "If you take out ranches and put in ranchettes, instead of soil, it's going to hit rooftops, cars, driveways" and make its way to the river.

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