Austin at heightened flood risk after Atlas-14 study shows more intense rainfall

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Central Texas is already one of the most flash-flood-prone regions in North America according to Austin’s emergency management office. However, a new federal study shows more intense rainfall has exacerbated that threat in the capital city, nearly doubling the amount of structures in danger of flooding from 4,000 to 7,200.

Such changes are acutely felt in Central and Southwest Austin. According to a Community Impact Newspaper analysis of the new proposed flood maps, approximately 1,081 Southwest Austin properties are new to the flood plain, while 694 Central Austin properties previously thought to be out of harm’s way are now in the flood zone.

Officials say the properties will likely be subject to flood insurance requirements, tighter development rules and property value reductions.

“Our understanding of flood risk has changed significantly,” said Kevin Shunk, flood plain administrator with Austin’s Watershed Protection Department. “There are more people and buildings at risk of flooding than we previously thought.”

The study—called Atlas-14—was conducted by the National Oceanic and Atmospheric Administration and is the first of its kind since 1961. Shunk said the result is the most significant change to the city’s flood-mitigation efforts since flood maps were first introduced in 1983.

“Any flood-[mitigation]infrastructure in the entire city is now undersized,” Shunk said. “Every bit of it.”

Although Austin is no stranger to deadly floods, officials, environmentalists and developers agree the changes to follow Atlas-14 will be costly, especially as the city grapples with its ongoing affordability crisis and rapid population growth. However, experts say defense against the forces of Mother Nature should be looked at as a long-term affordability strategy, as doing nothing will come at a much higher cost in the future.

Austin buckles down

Shunk said since the rainfall data was finalized last fall, his department has been scrambling to educate property owners on their new flood risk and tighter development restrictions coming down the pipeline.

Atlas-14 shows a roughly 33% increase in the amount of rain that could fall in a 24-hour period. As a result, the properties experiencing greater flood risk are those in close proximity to the city’s creeks, such as Shoal, Waller, and East and West Bouldin, since more intense rain means a wider spread of creek overflow. The area over which a creek’s floodwaters can reach is called a flood plain.

Flood plains are dictated by rainfall intensity and the likelihood that a storm with such rainfall intensity could occur in a given year. A 25-year storm has a 4% chance of occurring in a year; a 100-year storm has a 1% chance of occurring in a year; and a 500-year storm has a 0.2% chance.

Before Atlas-14’s findings, a 25-year storm brought 7.5 inches of rain in 24 hours; a 100-year storm brought 10.2 inches of rain in 24 hours; and a 500-year storm brought 13.5 inches. The intensity of these storms have since jumped. After the Atlas-14 findings, a 25-year storm would now bring 2.5 more inches of rain. Properties formerly in the 500-year flood zone are now in the 100-year zone, meaning a storm that drops more than 13 inches of rain in 24 hours is five times more likely to hit in a year than previously understood.

Cities like Austin design flood infrastructure and regulate land use based on 100-year storm levels. Now that the 100-year storm has jumped in intensity and the flood plain has expanded, the watershed protection department has proposed tighter development restrictions, highlighted by a permanent moratorium on adding dwelling units in the flood plain and a requirement that the ground floor on any redevelopment or new construction needs to be lifted 2 feet above the flood plain.

District 5 Austin City Council Member Ann Kitchen said the results of Atlas-14 place even more pressure on the city to update its flood mitigation efforts. She said the city cannot rely on retroactive action.

“It’s important to plan for flood mitigation beyond just buyouts, we need to look at engineering solutions,” Kitchen said. “There is a lot of growth happening in some of these affected areas, so we need to think about how we’re growing from stormwater infrastructure and flood mitigation perspectives.”

The ordinance changes were initially scheduled for a City Council vote this spring; however, watershed protection officials wanted further public input before submitting the proposal. A vote is now expected for October.

David Maidment, professor of environmental and water resource engineering at The University of Texas, said the new efforts at mitigating flood risk are the city’s way of ensuring no repeats of the destruction of residences along Onion Creek in the 2013 and 2015 floods and the $100 million in tax money subsequently spent on buying people out of those properties.

“We’ve got to plan more appropriately in the future than what was done in the past,” Maidment said. “It’s clearly a shock to people who are now going to be in a flood zone that weren’t before, but it’s just a recognition of the risk that exists.”

A ‘new normal’

Local environmental experts say the findings of Atlas-14 only further the evidence of a shifting climate.

“We have to acknowledge that we’ve entered a new normal with extreme weather events,” said Jo Karr Tedder, founder of the nonprofit Central Texas Water Coalition, which advocates for use of scientific data in water management. “We must use current data and science to better prepare for the future.”

Maidment said city planning has to take the shifting climate into consideration, and acknowledging Atlas-14’s increased flood levels across the community is a “step in the right direction.”

Brian Zabcik, a clean water advocate with Environment Texas, said tighter development rules could not only mitigate flood risk but also provide water-quality benefits, as less development in the flood plain means less pollutive runoff entering the city’s streams. Atlas-14, he said, was long overdue.

“If we build in flood plains, if we put more pavement over land that flows into our streams, we’re going to increase flooding and make the problem worse,” Zabcik said. “If we redefine our flood plains according to Atlas-14, we can pull back from streams and channels and reduce the runoff flowing into them.”

In Austin City Council Member Leslie Pool’s District 7, which stretches from Central to North Austin, many properties will enter the flood plain for the first time as a result of Atlas-14. Pool said her constituents have been talking about the potential flood plain changes for the last two years.

Pool said the 1981 Memorial Day flood that killed 13 people was a wake-up call for Austin and resulted in several property buyouts along Shoal Creek in what is now her district. She said something similar could happen again without proper preparation, especially as the region experiences more intense rainfall.

“My bottom line is I want folks to be safe and keep themselves out of dangerous situations,” Pool said. “We need to increase our resiliency and ensure our housing situation is more sustainable.”

Kitchen said considering  the ongoing worldwide battle against climate change, she was not surprised to see the results of the Atlas-14 study.

“It’s incumbent upon us as a city to be prepared and address these issues as best we can,” Kitchen said.

Balancing affordability

Felicia Foster is an architect and member of the Infill Builders Council, a group that advocates and educates on infill development issues. According to Foster, many in the development community believe the pending regulations could “ruin” building in Austin.

Although Foster said she appreciates the city’s efforts to educate the community about the upcoming changes, the new regulations—such as upgrading storm infrastructure to bigger pipes and on-site detention ponds on commercial projects to raising new homes 2 feet above the floodplain—will add costs for both the homebuyers and sellers.

David Glenn, director of government affairs with the Home Builders Association of Greater Austin, said the cost associated with requiring new construction to be raised 2 feet above the flood plain could run up to a few thousand dollars, which will trickle down to the listing price. Building an accessory dwelling unit, or ADU, on a property and renting it out is a key way for both homeowners and renters to fight affordability challenges. However, the proposed density moratorium would prohibit ADU construction in the flood plains.

“We understand it’s a good effort and you’re trying to protect the area, but you have to look at the bigger picture and realize how this death by 1,000 cuts is killing affordability,” Glenn said. “Austinites love their things—trees, creeks, et cetera. The problem is, they don’t have a clear vision of what the top priority is. What is the thing that Austin needs to sacrifice the most for? We think that’s affordability.”

On the other hand, homes that are new to the flood plain and its regulations will likely take a hit to property value, especially if City Council approves the prohibition of additional density, said Gordon Gorychka, a realtor with 40 years of experience in the Austin area.

“If it’s a single-family unit now, it’ll have to stay single-family unit; I think that will have a dramatic effect on property value,” Gorychka said.

However, Shunk said the proposed flood regulations are part of an affordability strategy, just one that takes more of a long view.

“We are, in a way, making things more affordable because we’re limiting the flood damage that could happen,” Shunk said. “It’s the flood damage that is hard overcome financially, so if we protect houses now then we make it more affordable in the future.”

Click here to find the city of Austin’s FloodPro website, which includes additional information and a map of properties added to the flood plain.

Additional reporting by Taylor Jackson Buchanan. 

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COMMENT
  1. Great Story, Christopher Neely, Sally Grace Holtgrieve and Taylor Jackson Buchanan as well as Community Impact News. With so many new folks in Austin it is great to educate about past flooding and water quality issues as what you don’t see you don’t consider.

  2. Caroline Reynolds

    This article is false an misleading. While Kevin Shunk and Watershed Protection may have told you that 19 inches is the new potential 24-hour max rainfall, that number has been outdated since the 27-inch rainfall in Thrall, TX in 1921. The Thrall number was taken from a 1963 document
    U.S. DEPARTMENT OF COMMERCE
    LUTHER H . HoDGES, Secretary
    WEATHER BUREAU, F. W. REICHELDERFER, Chief
    TECHNICAL P APER NO. 2
    Maximum Recorded
    United States Point Rainfall For 5 Minutes to 24 Hours
    at 296 First-Order Stations

    The new maximum is now nearer 38 to 40 inches that occurred in Houston during Harvey. City of Austin cannot complacently plan for the last catastrophy, but for the future of the City. The City Council Members and the Mayor are fooling themselves and misleading their constituents if they think that planning for a 19-inch rainfall even is adequate. Austin has already had two 100-year rainfall events since 1980 and a near 100-year event just two years ago. Buying out the million dollar homes on Shoal Creek and Johnson Creek will be a fools errand, but consoling the families that lose their loved ones and their homes will be near impossible.

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Christopher Neely
Christopher Neely is Community Impact's Austin City Hall reporter. A New Jersey native, Christopher moved to Austin in 2016 following two years of community reporting along the Jersey Shore. His bylines have appeared in the Los Angeles Times, Baltimore Sun and USA Today. He is a graduate of the University of Maryland's Philip Merrill College of Journalism.
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