Scientists Warn Flooding Will Be More Common In Central Texas

Oct. 20--When he visited Austin in 2013 to give a talk at the University of Texas, Kerry Emanuel, a prominent Massachusetts Institute of Technology meteorology professor, offered the audience a stark statistic.

The chances that a 15-inch rainfall might hit Central Texas in any given year had long been about 1 in 1,000. But with the warming of air that scientists expect over the century, some predict those chances might jump to 1 in 50.

"If the ground is really dry and parched, much of it becomes runoff," Emanuel warned at the time.

The rain that has soaked Central Texas the last few weeks has not packed the one-time intensity of a hurricane-level rain. But climate scientists say the steady, above-normal rainfall -- nearly 15 inches have fallen in Central Texas since Sept. 1, more than double the average for that period -- that has led to flooding across the region is a foretaste of the sort of deluge and destruction that Austin might come to expect.

In September, the U.S. National Oceanographic and Atmospheric Administration released an analysis that found significantly higher rainfall frequency values in parts of Texas, redefining the amount of rainfall it takes to qualify as a 100-year event.

In Austin, for example, the 100-year rainfall amounts for 24 hours increased as much as three inches, up to 13 inches. Precipitation previously classified as 100-year events are now 25-year events.

The new classifications will have real consequences for infrastructure design, flood insurance and flood plain development.

The "new rainfall frequency values for Texas will help state and local authorities better understand their flood risk and more accurately plan and design infrastructure to minimize the threat of flooding," Thomas Graziano, director of the NOAA Office of Water Prediction, said in September.

Already, county officials are planning to spend $1.1 million to study how to reduce flood damage.

The new federal analysis, hammered home by the rain and flooding of the last several weeks, suggests Travis and other counties will have to spend more on resiliency measures for road, sewer line and other infrastructure projects. Among the destruction the floods wrought over the past week: the two-lane RM 2900 bridge over the Llano River in Kingsland was washed away. It was the third bridge to be destroyed by floodwaters in Central Texas since 2015.

"The science of climate change is not in question," Travis County Commissioner Brigid Shea said earlier this month. "All one has to do is look at the multiple severe weather events Travis County has experienced in the past few years. Travis County must be prepared to protect our constituents and make certain our projects are climate change resilient. By doing this as early as possible, we can maintain fiscal responsibility and prioritize safety."

A changing climate

But state agencies and the Lower Colorado River Authority, the quasi-state utility that oversees management of the river flowing through Austin, tend not to take climate change into account in their water planning.

The Texas Water Development Board looks, instead, at historic data for its State Water Plan, a projection of water demands in coming decades.

Asked this week whether water planners should take warnings from scientists that future climate conditions might depart from ones seen over the past century, John Hofmann, LCRA's executive vice president for water, said: "What we see in Central Texas -- we have a lot of variability in our weather patterns, the drought cycles we see and the wet cycles we see. We have to be ready for that drought cycle and that flood cycle."

With "global warming" and "climate change" seen as politically charged terms -- Gov. Greg Abbott has said the science is still out on whether industrial emissions contribute to a changing climate -- they rarely, if ever, appear in official state documents.

But experts say intense rains are likely to increase -- and already have.

"Intense precipitation has increased in Texas as well as in the rest of the U.S. and most of the world," John Nielsen-Gammon, the Texas state climatologist and a Texas A&M University professor, told the American-Statesman.

He said three factors are involved:

--Temperatures: Higher temperatures mean a greater amount of water vapor can be contained in the atmosphere. "When you've got precipitation there's that much more water in the air that can fall to the ground," he said.

--Storm structure: For certain types of storms, such as hurricanes and some large-scale, low-pressure systems, rainfall tends to become more concentrated near the center of the storms, leading to increases in extreme amounts of precipitation.

--Weather patterns: Climate change affects the position of the jet stream and the frequency of hurricanes.

Scientists suggest the climate in Austin will change in other ways. A 2014 presentation to city of Austin officials by Katharine Hayhoe, director of Texas Tech University's Climate Science Center found that:

--The number of cold nights -- in which temperatures dip below freezing -- will drop from a historical average of 15 times per year to just under 11 times a year over the next few decades; by the end of the century, there might be as few as four freezing nights a year in Austin.

--Very hot days -- over 110 degrees -- are now very rare but could occur twice a year by 2040.

Flash flooding

Already, federal statistics show that annual precipitation in Central Texas from 1991 to 2012 was 5 to 15 percent higher, on average, than it was between 1901 and 1960.

All that rain translates into flooding.

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

The steep terrain, shallow soils and an increasingly built-up environment add to the risks. As climate change causes extreme weather events to become more frequent and as areas of Travis and Hays counties downstream of the Hill Country continue to grow in population, both the severity of the flooding and the number of people in harm's way are expected to increase.

Many of the heaviest rainfalls ever recorded during a 48-hour period have occurred along the Balcones Escarpment, the geologic uplift that characterizes the Hill Country, Raymond Slade, a now-retired U.S. Geological Survey Texas water specialist, noted in his report "Major and Catastrophic Storms and Floods in Texas."

That is 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 told the Statesman in 2015, after Memorial Day weekend flooding along the Blanco River led to the deaths of at least a dozen people.

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