Downhill Battles

IN 1839, WATERLOO, a riverside village of 856 people, was renamed Austin and designated the capital of Texas. What began as a small trading post for ranchers along the Colorado River is today home to more than 2.2 million people, making the Austin metro area the 11th most populous in the United States.

Rapid population growth has forced Austin’s developers to look east along the Colorado River or to build on any available piece of land.

As developers build near the river, which runs through the center of the city, and its hundreds of square miles of creeks and tributaries, the likelihood of geological hazards increases. Two devastating landslides occurred in 2018 and 2019 in the Shoal Creek area, a 13-square-mile watershed west of downtown Austin that flows directly into the Colorado River. Several feet of dirt fell from the top of the banks into the creek area below, the erosion moving steep slopes uncomfortably close to several houses.

The damage from those events is estimated to have cost the city $20 million to rebuild trails and reinforce the creek’s banks to protect against future landslides.

Shoal Creek is a speck in an urban area consisting of more than 4,000 square miles of land. No one has determined yet whether additional areas of Austin might be at risk of landslides.

A team of geologists at TCU’s College of Science & Engineering is aiming to learn more about landslide activity in and around the capital of Texas. This team is also developing a tool to help predict where and when a landslide might occur in the city.

Rosbeidy Hernandez, who earned a master’s degree in geosciences from TCU in 2022, conducted a study to determine the factors contributing to Austin-area landslide activity for her thesis.

Esayas Gebremichael, assistant professor of geology at TCU, gave her the idea for the project. He also acted as Hernandez’s adviser throughout the study.

“Working with Esayas was a great experience,” Hernandez said. “He researched other papers that would help us with our project. He was always with me in the field to help collect data” and aided in the other processes in their work.

STUDYING SHOAL CREEK

Hernandez and Gebremichael zeroed in on the Shoal Creek area. The level of landslide activity over recent years provided an ideal location for the study.

Rosbeidy Hernandez, who earned a master’s degree in geosciences from TCU in 2022, studied the factors contributing to Austin-area landslides for her thesis. Courtesy of Rosbeidy Hernandez

They began by compiling imagery of the area acquired by satellites using synthetic aperture radar. Unlike standard imaging, this type of radar can measure a surface’s motion or displacement using energy waves.

Hernandez and Gebremichael used radar data to construct a seven-year chronology of landslide activity in and around Shoal Creek. With the timeline, they could identify and track the ground motion to determine triggering events, such as the intense rainfall that coincided with the two landslide incidents.

Using 3D modeling data from an aerial drone, they identified geologic and topographic variables that might be contributing factors to landslides, such as geological structures and the slope of terrain.

“By understanding what is going on in that particular site, we can then use that data … to see what will happen in other parts of the city with similar geological features,” Gebremichael said.

Once completed, the map outlining Shoal Creek’s landslides was cross-referenced with the data collected from the radar to determine which variables were associated with the highest levels of landslide activity.

CRITICAL CLAY

One important variable is the stratigraphic layer of Del Rio clay beneath a layer of topsoil and limestone. Due to its low permeability, the clay is prone to breaking apart and shearing, or sliding horizontally, when excessive water runoff occurs. The Shoal Creek terrain consists of moderate to steep slopes, which turns the Del Rio clay into a wet surface for the layers of limestone and topsoil above to slide along.

Another critical factor is that the Shoal Creek area is in a constant state of motion.

“Using geographical data ranging from 2015 to the end of my project [in 2022], we found that the land was continually moving little by little,” Hernandez said.

This phenomenon is referred to as a slow-moving landslide. The surface of Shoal Creek moves as much as 4.8 millimeters per year. Those millimeters are not destructive on their own, but the continual motion makes the area susceptible to triggering events that could result in larger, more destructive landslides.

Rainfall also plays a critical role. “We have confirmed that the Shoal Creek landslides of 2018 and 2019 were triggered by intense rainy seasons,” Hernandez said.

After excessive rainfall, water runs down the steep slopes and passes over the Del Rio clay fast enough to increase the risk of a landslide. Also, Gebremichael said, “The loss of vegetation, which could have anchored the landslide material … over the past few decades may have worsened the conditions that led to the occurrence of landslides in the area.”

WARNING SYSTEM

Hernandez and Gebremichael’s work may ultimately contribute to developing an early warning system for landslides. Future studies will look at additional variables that may be tied to shifting land in the Austin area.

“After our research into the Shoal Creek area, we can identify other areas of the city that are at risk if they have factors similar to what we found contributes to landslide activity,” Hernandez said.

The warning system could be used by Austin city officials to identify areas that are the most at risk for landslides. They can then reinforce those areas to reduce landslide activity by, for example, building retaining walls along creeks. The warning system can also serve as a tool for real estate developers to assess a plot of land’s risk for landslides before starting construction.

The team’s research has drawn the interest of the Texas chapter of the Association of Environmental and Engineering Geologists. The organization, which advocates for environmentally conscious legislation, building codes and standards, has invited Hernandez and Gebremichael to do a presentation on their findings.