Hidden Beneath the Park: Precast Reservoir Captures 6.4 Million Gallons of Stormwater
Stormwater runoff from nearly 7,000 acres in Los Angeles County carried pollutants toward local waterways, while shallow groundwater prevented the use of conventional infiltration systems. To capture and manage that runoff, the county built a 6.4 million-gallon underground stormwater reservoir beneath Adventure Park in South Whittier, California. Pre-Con Products designed, manufactured and installed a modular precast concrete system consisting of 1,419 elements, creating a watertight structure engineered for seismic loads and public park use above. The reservoir passed its required leak test on the first fill, and the renovated five-acre park reopened in June 2025.
Fast Facts
- Project Name:
- Adventure Park Multi-Benefit Stormwater Capture Project
- Location:
- South Whittier, California
- Precast Products Used:
- Manholes, Pump Station, Stormwater Management, Underground
- Precast Producer:
- Pre-Con Products
- Completion Date:
- June 9, 2025
The Challenge
Stormwater runoff from a 6,985-acre drainage area in Los Angeles County carried sediment, trash, bacteria and metals through the Sorensen Drain channel toward Coyote Creek and the San Gabriel River. The county needed a system to capture and treat runoff, with particular attention to reducing zinc pollution, while retaining water for eventual reuse.
The project site presented several engineering challenges.
Groundwater was approximately 10 feet below the surface, making infiltration impractical and requiring a sealed storage system that would prevent captured runoff from escaping into the surrounding groundwater.
The reservoir also needed to hold approximately 6.4 million gallons during a storm event while meeting seismic design requirements for a water-filled underground structure. Engineers had to account for soil pressure, groundwater-related buoyancy, earthquake forces and the movement of stored water during seismic activity.
Because the facility would be constructed beneath an existing neighborhood park, the roof needed to support substantial soil cover, maintenance vehicles and public-use loads.
The county also required an interior layout that would allow equipment and maintenance crews to access and clean the reservoir over its service life.
Finally, the project had to return the five-acre park to the surrounding community after construction.
Precast Solution
Pre-Con Products designed, manufactured, delivered and installed a modular StormPrism precast concrete reservoir beneath Adventure Park.
The underground structure consists of 1,419 precast elements assembled into a continuous storage vault with a capacity of approximately 6.4 million gallons.
The system uses stacked precast modules with tapered interior support columns rather than continuous interior walls. This creates open storage cells with a clear interior height of 13 feet while providing access for inspection and cleaning equipment.
Standard modules weigh up to 23,000 pounds. The system also incorporates perimeter wall panels, baffles, a weir wall and a sediment trap to manage water movement and the accumulation of solids.
Pre-Con manufactured the components using wet-cast concrete with a specified compressive strength of 6,000 psi and Grade 60 reinforcing steel at its Simi Valley, California, plant, approximately 59 miles from the project site.
Before production began, the company constructed a full-scale mockup for review and approval. The county also conducted structural observations during manufacturing.
Pre-Con developed the reservoir’s joint sealing system and performed the field sealing and grouting. The company coordinated deliveries with installation crews working inside the excavation.
Beyond the reservoir, Pre-Con supplied additional precast structures for pretreatment, filtration, pumping, metering, sampling, valves and access throughout the stormwater management system.
The completed facility diverts runoff from the Sorensen Drain channel, directs it through pretreatment and filtration, and stores it underground before metering it into the sanitary sewer system for downstream treatment and reuse.
- 6.4 million gallons Stormwater storage capacity per storm event
- 1,419 Precast concrete elements assembled into the reservoir
- 6,985 acres Drainage area served by the stormwater capture project
- 13 feet Clear interior height of the underground reservoir
- $7.2 million Reported cost of precast concrete
Why Precast?
Precast concrete provided the structural performance, watertightness and construction efficiency needed to build a large underground reservoir beneath an active public space.
Watertight construction: The county required a sealed structure because of shallow groundwater. Pre-Con designed and installed an engineered joint sealing system that could be tested after the reservoir was fully assembled.
Seismic performance: The reservoir was engineered for earthquake loading under both empty and water-filled conditions. The structural design accounted for moving water, surrounding soil pressures, buoyancy and other forces associated with a buried liquid-containing structure.
Structural capacity: The precast system was designed to support up to 16 feet of soil cover, maintenance vehicle loads and public assembly loads, allowing the surface to remain a usable neighborhood park.
Controlled manufacturing: Producing 1,419 components in a plant environment supported consistent dimensions, reinforcement placement and concrete quality. A full-scale mockup and county inspections provided additional verification before and during production.
Efficient installation: Completed components were delivered to the site and assembled within the excavation, reducing the amount of concrete forming, placement and curing required in the neighborhood.
Long-term access: The modular system provides open interior cells, an unobstructed floor and ceiling, and multiple access points so maintenance equipment can reach the reservoir’s interior.
These advantages allowed the county to meet demanding storage, structural and maintenance requirements without permanently converting public parkland into a surface stormwater facility.
Results
The Adventure Park stormwater capture facility was completed in June 2025, creating underground storage capacity for approximately 6.4 million gallons of runoff from a 6,985-acre drainage area.
The completed reservoir passed Los Angeles County’s required watertightness test on its first fill, without repairs or a second attempt. The test required the assembled structure to retain water over 24 hours with a maximum allowable loss of 0.05% of its volume.
The system captures runoff that previously flowed through the Sorensen Drain channel toward Coyote Creek and the San Gabriel River. Captured water is directed through pretreatment and filtration before being stored and released to the sanitary sewer system for downstream treatment and reuse.
The project also eliminated standing water that had accumulated in the Sorensen Drain channel between storms, according to the submission.
Above the reservoir, Los Angeles County reopened five acres of renovated parkland on June 9, 2025. Improvements included ball fields, restrooms, walking paths, exercise equipment, a bioswale, landscaping, benches and interpretive signage.
The project received the 2025 Project of the Year award from the American Public Works Association’s Southern California Chapter and was named a 2025 Top Project by Stormwater Solutions.
- Written by:
- National Precast Concrete Association
- Source:
- Adventure Park Multi-Benefit Stormwater Capture Project, Built for Purpose Awards submission
- Published:
- October 26, 2026
- Last Update:
- Oct. 26, 2026
