As implied by its name, Confluence Park overlooks the meeting of San Pedro Creek and the San Antonio River in San Antonio, Texas. Located about three miles south of downtown, the park acts as a gateway for the historic Mission Reach section of the San Antonio River. The $13.7 million project includes an education center and extensive landscaping that illustrates the diverse biomes of Texas.

But what most visitors will remember about the 3.5-acre park are the nearly 30-foot-tall concrete petals that emerge from the ground to form a sprawling overhead canopy. Twenty-two of these sculptural panels are clustered together to form a single, large, open-air pavilion. Another six are paired together to form three smaller gathering areas. In addition to providing relief from the South Texas sun, these panels are shaped so that when it rains, they channel water into an integrated system of rainwater collection, filtration, and dispersal.

Styrofoam molds were milled by Kreysler & Associates on their 5-axis CNC router in California. These were used to make fiberglass molds to form tilt-up concrete units. (Courtesy Lake|Flato)

All of this reinforces the stated mission of the park, which is to act as a destination for recreation while teaching important lessons about environmental science and sustainability. To that end, the design team sought to create a composition of architectural and landscape elements that used the same kind of logic found in nature. Ball-Nogues Studio, a Los Angeles–based design practice, established the park’s conceptual master plan. From there, the design was developed in close collaboration with the landscape architect Rialto Studio, Lake|Flato Architects, and Matsys, a San Francisco–based design practice that specializes in the development of new approaches to architectural design and fabrication. That particular skill set was critical in the development of the park’s concrete. Given the structural gymnastics involved, the project’s structural engineer, Architectural Engineers Collaborative (AEC), became an integral part of the design team as well.

Although petals of steel, fabric, and wood were all considered during the design process, concrete was ultimately selected for its durability and permanence. Even though the majority of funding for the project came from private donations, Confluence Park functions as a public park, and so vandalism and long-term resiliency were key considerations.

Petals being installed on-site (Courtesy Lake|Flato)

Despite the apparent complexity of the assembled petals, the design only required three unique petal shapes. These three forms were refined digitally using Grasshopper and Rhino. The resulting computer files were then provided to Kreysler & Associates and fed to their large 5-axis CNC router at their factory in California. The resulting Styrofoam “positives” were then used to manufacture the fiberglass “negatives” that were shipped to San Antonio to be used as formwork for the petals.

Each of the park’s 28 petals was cast on-site but not in place. Given their complex geometry, a portion of the petal had to be exposed during the pour. This resulted in two contrasting concrete textures: a smooth finish where the concrete was poured into the fiberglass form, and a broom finish where the concrete was left exposed. As with many other aspects of the project, a custom solution was required here, too. A special eight-inch broom was used to apply the finish consistently to the petal’s curved form and to emulate the flow of water down the petals.

Neighboring petals were joined to one another with two steel pin connections. (Casey Dunn)

After the concrete had cured for several days, the petals were lifted into their final positions. As with any tilt-up concrete structure, this was the moment when the highest stresses would be placed upon the petals. Adding to the complexity of the erection process was the fact that the petals had to be assembled in pairs: neighboring petals were joined to one another with two steel pin connections to form a determinant structure.

The result of all this effort is a unique landmark on the south side of San Antonio. Despite the weight of the concrete petals—individual petals weigh between 15 and 20 tons each—the resulting structure feels remarkably light. The space between individual petals contributes to this feeling of weightlessness, while acrylic lenses embedded in the concrete add a bit of playfulness to the overall composition.

In addition to illustrating the possibilities of contemporary concrete construction, Confluence Park demonstrates what is possible when a highly collaborative interdisciplinary design team works with an educated client to create something truly unique. It is only fitting that a park built to celebrate the confluence of diverse bodies of water be created by a confluence of diverse design professionals.

The finished structure has two finishes—a smooth finish where the concrete was poured into the fiberglass form, and a broom finish where the concrete was left exposed. (Casey Dunn)

Pavilion Design
Matsys

Landscape Architect
Rialto Studio

Structural Engineer
Architectural Eng. Collaborative

MEP
CNG Engineering, PLLC

Lighting Designer
Mazzetti

Energy Consultants
Positive Energy

Waterproofing Consultant
Acton Partners

This article originally appeared in the July/August issue of Texas Architect magazine.

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