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Shoal Creek Trail solution coming to Council

Monday, June 10, 2019 by Jessi Devenyns

After a catastrophic landslide carved a 30-foot-high cliff through residents’ backyards along Shoal Creek last year, the city sprang into action to devise a plan to stabilize what was left. However, the slope shifted again this spring, leaving Austinites wondering how much more damage is coming before a solution can be implemented.

Repairing the crumbling slope along Shoal Creek has become an even more urgent matter since a second fissure appeared in April and split off in May.

At the June 5 meeting of the Environmental Commission, Mike Kelly, a managing engineer with the Watershed Protection Department, told commission members that the department has selected a $13.6 million solution that benefits both the city and the residents whose properties were damaged. The proposal will go before Council on June 20 for a vote to allow execution of the contracts.

Once approved, the construction of the engineered solution will take about six months.

“The next time (the cliff) goes, it will start taking significant private infrastructure,” Kelly said.

Even before the second landslide, the city had been scrambling to stabilize the section of Shoal Creek that suffered damage in 2018. Just weeks after the initial incident, Council deemed the situation an emergency and the Watershed Protection Department worked to push through four contracts for design engineering, stabilization and storm drainage for a total of $1 million.

Now, Kelly says that engineers have created a solution he describes as “essentially a retaining wall.”

According to schematics presented at the Environmental Commission, the engineered solution will include drilling concrete piers 80 feet down into the bedrock limestone and then adding reinforcement cabling to stabilize the remaining slope. Rebar inserted into the cliff face will ensure that no more of the limestone cliff cleaves off from backyards.

“This is going to be a crisscrossed area of cables and walls,” Kelly said.

This solution, according to Kelly, will cost about $2 million more than a buyout of the properties in the area would have. However, purchasing the private residential properties would have left the city “hanging.” Engineering a reinforcement, Kelly explained, allows for the protection of city utility infrastructure in addition to preserving the houses on the hill.

While the commissioners did not question the necessity of the project, they did wonder if Watershed Protection had identified other areas along Shoal Creek that were at risk of similar landslides. Kelly said that the department has identified only one potential location along the hike-and-bike trail near 32nd Street. “Theoretically this could happen,” he said. “But we didn’t see the signs.”

Landslides of this nature are not unexpected with the geology of Shoal Creek, though they are unusual and occur perhaps once in a generation. Kelly told commissioners that in the area where the cliff near Shoal Creek collapsed, aerial photos from 1952 “show the exact same thing happening in the exact same spot.” As they did not find photographic evidence elsewhere of a similar event, the department concluded “there’s not an imminent danger anywhere else.”

As the engineered restoration project awaits Council approval on June 20, the city will continue to follow emergency measures to keep residents in the area safe. A staff meteorologist sends alerts for impending rain events so that the city can sandbag the area and put in temporary levees to help direct water, and also monitors Lamar Boulevard in case of flooding in the area.

Photo courtesy of the city of Austin.

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