San Francisco 49ers Stadium Scores Big with BioMod® Biofiltration System

Santa Clara, California



GHD Engineering

San Francisco, California

The Problem

A large construction project such as a major-league sports stadium can include significant expanses of runoff-generating hardscape. A venue capable of seating nearly 70,000 people requires large parking areas and other impervious grounds. If rainwater can turn a football field into a muddy swamp, it can also turn a parking lot into a floodplain. Draining that area effectively and treating the runoff is a basic necessity that should be provided from the very beginning of the project, but it can present challenges. The new San Francisco 49ers stadium in Santa Clara, Calif., is a good case in point. The new stadium moved the team out of Candlestick Park into a location with about double the square footage, better freeway access and about twice the parking spaces. Parking lot runoff typically includes petrochemical pollutants that drip or leak from vehicles as well as litter and other debris. The new stadium sits adjacent to San Tomas Aquino Creek, which flows into San Francisco Bay less than six miles away. The site is on land with a high water table, and existing storm-drain lines are not very far below the surface. As such, a stormwater infiltration system would not work.

The Design

To handle stormwater runoff from the parking lots, access roads and other hardscapes surrounding the stadium, project manager GHD Engineering selected the BioMod® biofiltration system, a stormwater collection and natural treatment system manufactured by industry leader Oldcastle® Precast. Today, the stadium has six biofiltration systems to naturally treat and manage stormwater runoff from the parking lots and grounds immediately adjoining the site. In total, about 2,500 linear feet of BioMod cells, or approximately 14,000 square feet of bioretention area, currently treat stormwater runoff from the complex. Although the standard bioretention modules used in most of the 49ers project are three feet wide, two systems in the second phase featured tapered plans: one three feet wide at one end and 14 feet at the other, an irregular wedge more than 300 feet long. The other is 230 feet long and rectangular over most of its length, with a subtle taper at the south end. The unusual shapes were designed by the architect with specific visual goals in mind. The main parking lot has two biofiltration systems. The one on the north side featuring the rounded modules was manufactured and installed first. The main run is nearly three feet wide and just more than 600 feet long, mostly basic curb-cut units without pre-filtration. The modules that lie perpendicular to the main run are seven feet wide. They divide the parking lot into six Drainage Management Areas (DMAs), each about 100 feet long for managing about 10,000 square feet of runoff.

The Solution

The drainage rate is determined by the surface area of the bioretention system and by the composition of the filtration media. The BioMod system doesn’t require proprietary biofiltration media. For the 49ers stadium, engineers selected a blend that yields up to 10 inches per hour of drainage in accordance with Contra Costa County requirements. Other media blends are capable of treating as much as 100 inches of rainwater per hour, as needed. Computed against the size of the impervious hardscape and the predicted rainfall, the size of the required bioretention system can quickly be calculated. However, the media must be selected not only for drainage characteristics, but also for compatibility with the intended plantings. One of the greatest advantages of a biofiltration system is that it’s easy to maintain and essentially self-sustaining. It harnesses nature to break down petrochemical pollutants, a process that needs little help from humans. Debris must be occasionally removed from the system, and plants must be maintained like any other landscaping, but little more is necessary. The San Francisco 49ers biofiltration system will largely maintain itself, keeping the grounds looking good and safe from flooding, while protecting the San Francisco Bay and other sensitive areas surrounding it from stormwater pollutants.

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