“Folks will put their names or stickers on their trash cans so that if and when [heavy rains] happen, they can pick their bin up at the end of the street,” he said.
With old infrastructure in long-established neighborhoods and proximity to creeks, watersheds and hills, county and city officials say flood risk in Northwest Austin is nothing new.
However, concerns about Austin’s vulnerability to flooding have intensified following a mid-October storm that caused an unprecedented week-long, citywide boil-water advisory.
From Jan. 1, 2017 through Nov. 1, 2018, the city of Austin fielded 600 flood-related 311 calls originating from ZIP codes throughout the city, including over 80 from Northwest Austin, per city records.
City and county officials are now in a balancing act of meeting the needs of residents in many areas of Austin.
Re-evaluating the risk
On Sept. 27 the National Oceanic and Atmospheric Administration released a peer-reviewed study called Atlas 14 that re-evaluates flood risk in the U.S. based on rainfall data collected through 2017. The study redefines the 100-year storm.
A 100-year storm is a storm that has a 1 percent chance of occurring during any given year, according to the Federal Emergency Management Agency.
The new 100-year storm is defined as 13 inches of rain in a 24-hour period, which is nearly the same as the current 500-year storm model. The last time the models were outlined the NOAA used rainfall data collected through 1994.
Over the next two years the city of Austin will use this data to redraw its 100-year flood plain maps, which outline flood-prone areas where property owners are required to buy flood insurance and adhere to other regulations, Williamson County Floodplain Administrator Terron Everston said.
When applied to the city of Austin and Travis and Williamson counties, data from Atlas 14 could mean drainage systems will need to be larger to handle the redefined events, Everston said.
“A lot of folks think of [Atlas 14] as purely flood plain but it’s not,” he said. “As we design all of our infrastructure, whether it’s the private sector with land development or the public sector doing it with roads, bridges and tunnels, we use rainfall data to reduce the risk of flooding to whatever that infrastructure is.”
In Northwest Austin the flood plain maps will not expand significantly; however, the intensity of the rainfall predicted for those who live within and near the flood plain will increase, Everston said.
“All of the rainfall intensities, which is just another way of saying inches per hour, generally have increased,” he said. “The city of Austin on the 24-hour, 100- year duration has gone up a little over 3 [inches]. Williamson County is not as much, but it was an increase to us.”
Kevin Shunk, the engineering division manager of the Watershed Protection Department, said residents can find out if their property is located within the floodplain as it is mapped today as well as in the proposed map made with Atlas 14 data by entering in their address on the city of Austin Watershed Department’s website, www.atxfloodpro.com.
“The first step is to identify what [a resident’s] flood risk is,” he said. “Then they can identify what the next step is, which is probably contacting their insurance agent to talk about flood insurance.”
Residents do not have to live within the flood plain to purchase flood insurance, Shunk said.
Establishing priorities
The watershed department tracks 311 data to help determine where flood-mitigation projects are needed, Shunk said.
In Northwest Austin these projects have taken the form of storm drain improvements on Oak Knoll Drive and West Cow Path, culvert renovations near Little Walnut Creek and a flood plain study of Upper Brushy Creek.
Not all of these projects are within or near the 100-year flood plain. City Council Member Jimmy Flannigan, who represents Northwest Austin’s District 6, said the area is also prone to localized flooding, which has less to do with creek overflow and more with poor drainage infrastructure.
“You have very small areas of unusual topography, and it just starts to collect water in really weird ways,” he said.
Of the 600 flood-related 311 calls received by the city during the same nearly two-year period, the 78703 ZIP code—representing the Tarrytown neighborhood along Lake Austin—made over 60 calls, the highest volume in the city. The 78759 ZIP code in Northwest Austin, which contains much of the Bull Creek watershed, reported 29 calls.
The watershed department evaluates what concerns are being voiced and what action is necessary.
“Whether it be road flooding, yard flooding or home flooding, we compare that risk of flooding with the other areas of town in order to prioritize our project locations,” Shunk said.
Sometimes offering home buyouts is less expensive than putting in a new drainage system, Flannigan said. In 2017, City Council approved buying four flood-prone homes in District 10.
“This happens a lot on the south end of District 5 and District 2, and in many cases the land prices are still cheaper than building drainage infrastructure to support it,” he said. “That’s true on all the edges [of Austin].”
Looking forward
Examining Atlas 14 data has caused some officials to re-evaluate projects that are in planning or underway.
“In some cases we may be ready to permit a project, and we don’t think it is a good idea to take steps back to incorporate Atlas 14 into the design,” Shunk said. “Other projects, we may look at the Atlas 14 information, and all of the sudden we realize, ‘You know what, we can actually make this bigger to accommodate more water.’”
The watershed department has requested the city adopt temporary flood plain maps based on the 500-year data ahead of the release of Atlas 14 maps that are set to be finalized in two years.
“That future level of the 100-year flood is going to look a lot like what our current 500-year flood looks like,” Shunk said. “Our recommendation is to use the 500-year flood plain for the next two years until we get to a place where we know where the new 100-year flood plain is located.”
Travis County commissioners voted Nov. 13 to allocate an additional $12 million to road, bridge and drainage projects included in the county’s 2017 bond to accommodate Atlas 14 rainfall data. This includes an additional $715,262 to improvements on low-water crossing No. 1 on Spicewood Springs Road.
At City Hall, Flannigan said flood-mitigation requirements in CodeNEXT, the now-defunct land development code rewrite, should be included in the new code that City Manager Spencer Cronk will put before council in early 2019.
“I would expect that whatever code-rewrite process that the manager has for us for 2019 would have similar updates and changes to the flooding regulations,” Flannigan said. “Those [regulations] seemed to have broad approval across the council.”
After Austin voters passed a $184 million bond proposition for flood-mitigation funding Nov. 6, it appears residents are in support of such efforts. The bond funds will help fund current and future watershed department projects, such as ones in Northwest Austin, Shunk said.
For now Eaton said he may add additional protection to his property, such as a berm, or a raised bank, along the edge of his lawn.
“I’ve got three small kids that go to Hill [Elementary School],” he said. “I have no intentions of moving, but I do know that I need to address the potential problem at hand.”
Additional reporting by Taylor Jackson Buchanan
