Eliza Spring Daylighting
Re-Engineering a Home for Salamanders through Aquatic Ecosystem Restoration
Barton Springs attracts over 1.5 million annual visitors and has long been one of Austin’s most visible outdoor green spaces. When Andrew Zilker deeded the land to Austin in 1918, construction of a concrete amphitheater around one of the springs — Eliza Spring changed the character of the site and eventually led to enclosing the spring-fed stream within a buried concrete pipe.
Once propagating the spring, the concrete pipe severed the natural connection to Barton Creek, resulting in loss and fragmentation of surface habitat by eliminating hydraulic surface connections and degrading aquatic habitat quality. This proved consequential for the habitat of the highest known concentration of endangered Barton Springs Salamanders that call the Eliza Spring home.
Nearly a century later, the City of Austin and HDR have removed the pipe, restored the stream, and reduced the anthropogenic flow regime and habitat modifications of the past. The Eliza Spring Daylighting supports long-term species recovery and offers the community a highly visible place to view and learn about Austin’s unique environment and its iconic endangered species.
The endangered salamanders now have a surface stream at Eliza Spring and reduced hindrance of aquifer flow to surface habitat. The complex geotechnical and structural design provides a stable site for in-stream habitat enhancement, native riparian landscaping, and improved water quality with hydraulic control.
Historic Structure Prevents Traditional Engineering Methods
The century-old, National Register of Historic Places-listed amphitheater does not have steel reinforcement and risked collapse during construction. Due to the sensitive nature of the site, the team was prohibited from performing a geotechnical investigation. Without drawings or plans for the amphitheater, the team relied on the Texas Historical Commission and forensic engineering to develop design assumptions.
Construction marked the first time the team would see what was under the amphitheater. Therefore, the team designed a micropile and hydraulic concrete structural system that was adaptable to differing site conditions and constructed via a top/down method as the material was excavated. In the limestone below the excavation is the aquifer and salamander habitat. Shallow 4-inch exploratory borings identified the depth of limestone and the hydraulic connections to the amphitheater floor and endangered habitat. With strong hydraulic connections but no structural foundation discovered, the design team had to modify drilling procedures and redesign concrete headwalls to avoid failure of the existing structure, damage to habitat, and construction delays.
Modeling a Home for Salamanders
The team relied on several types of modeling, including geotechnical, hydrologic, hydraulic and aquatic organism passage design, to design a suitable habitat. These efforts brought engineers and biologists together to deliver a multi-objective project. Hydraulic modeling and statistical analysis of spring flow allowed the team to evaluate depth and velocity, especially at the substrate level where the salamanders live. Composition and placement of the substrate were designed to allow salamander passage to/from the amphitheater and to/from the daylighted stream. Lower-velocity hydraulic refuge was provided behind larger boulders. The design of a gate system provided wetted surface habitat during low aquifer discharge, an objective not previously possible prior to the project. A secondary benefit of the gate system is the protection of the spring-fed stream from Barton Creek flows, which are diverted around the pool in a bypass tunnel.
Every proposed material was scrutinized for potential impact on salamander habitat. Native substrate was harvested from a nearby stream for use in the project, fly ash-free concrete was specified, gasoline-powered pumps were prohibited, construction equipment used vegetable-based hydraulics, locally-sourced native landscaping was pesticide-free, all imported fill was tested in a lab, and Material Data Safety Sheets were required in every shop submittal for dual review by biologists and engineers.
Care of Water During Construction
The design team had to maximize construction activities under a spring flow gravity bypass as opposed to a pumped bypass. The team developed a care of water plan that addressed more than 6,000 gallons per minute and included six phases of construction but only two short phases of pumped bypass. By minimizing pumped bypasses, the risk of environmental and structural damage in the event of a mechanical or electrical failure was reduced.
Studies on Salamanders and Habitat
While the Barton Springs Salamander was first described in 1993 and the Austin Blind Salamander in 2001, there is still a lot to learn about both endangered species. This project provides a study site for the City’s biologist to better understand what makes a suitable habitat for the salamanders and the effects of varying spring flow on wild-born populations. This project is also a great example of using sophisticated engineering design to support natural and beneficial ecological functions. By daylighting the historical spring, this project reminds us that we can mitigate previous thinking that did not consider multiple objectives.
Social, Economic and Sustainable Development Considerations
If salamanders in the Barton Springs complex are thriving, then we know we are protecting water quality in Central Texas. The Edwards Aquifer and its springs are the drinking source for over 1.5 million Texans. The Eliza Spring project increases habitat for these indicators of our water quality. The project also helps the City fulfill the Barton Springs Pool Habitat Conservation Plan, which was put in place so that the City could renew its U.S. Fish and Wildlife permit and continue to operate Barton Springs as a public swimming pool. With over 1 million annual visitors to Barton Springs Pool, the Eliza Spring project keeps the pool open, helps Austin secure its strong ecotourism industry and increases the quality of life for residents and visitors alike.
The new walkways around the facility were constructed with lookouts and interpretive materials, making Eliza one of the premier interpretive and educational destinations in the park. The new boards enhance the parkgoers’ enjoyment and play a significant stewardship role by helping sensitize people to the fragility of this precious ecosystem.
The revitalized Eliza Spring offers a new view on Austin’s iconic, endangered salamanders. The project created new habitat for the first time in a century, enhances long-term species recovery, and provides the community and researchers with greater educational opportunities.