49ers Stadium Bioretention

by Steven H. Miller, CDT

Workers crane in a large bioretention module for installation in the San Francisco 49ers' new stadium complex. The bio-retention system collects and treats runoff for more than five acres of concrete parking lot. Because it was on the critical path of site development, before virtually any other construction could begin, there was heavy time pressure to install the system quickly. Overall, the project took five workers 320 hours to complete.

Increasingly, government legislation, city ordinances and even homeowners associations are either mandating (or encouraging) the reduction of pollution in runoff. Landscape contractors can increase their business by expanding their services into runoff mitigation projects. By understanding and using simple natural ways to clean run off water, contractors can both increase their company profits while protecting the environment. The bioretention project at new San Francisco 49ers Stadium provides an excellent example of a simple runoff mitigation system that uses the power of nature to achieve its purpose.

Ghilotti Construction and 2nd Phase contractor Turner-Devcon built a bioretention project for the new San Francisco 49ers Stadium to handle runoff for more than five acres of concrete.

A large construction project such as a major league sports stadium can include significant expanses of runoff-generating hardscape. A venue capable of seating 60,000 or 70,000 people requires large parking areas. Draining that area effectively and treating the runoff was the goal of this project.

The new San Francisco 49ers Stadium in Santa Clara, California will eventually feature vast expanses of parking for attendees and employees, but first it needed a parking lot for construction activities. The stormwater solution for that primary staging-area was the first thing that needed to be constructed for the project. To meet the needs there and in other hardscapes surrounding the facility, designers selected a low-impact development (LID) solution that also provides an aesthetic solution: a modular bioretention system that collects runoff and treats it using natural biofiltration, and includes attractive landscaping elements and customizability.

The Site
The site is on land with a high water table, and existing storm-drain lines are not very far below the surface. An infiltration system would not work.

To handle stormwater in the parking lots, access roads, and other hardscape surrounding the stadium, designers selected the Bio-Mod bioretention system, a stormwater collection and natural treatment system made by KriStar Enterprises. The stadium site will have six bioretention systems, in parking lots and in the grounds immediately adjoining the stadium itself. The first two systems needed to be installed at the outset of the project, even before the contract for stadium construction had been awarded, a factor of time pressure that added to the overall challenge.

This detail shows the cutaway elevation of a bioretention cell used on the 49ers' stadium. Runoff percolates through the layers of mulch and bio-filtration media to filter pollutants. Drain rock surrounds the perforated pipe that collects treated water at the bottom of the cell. The overflow drain (right) collects excess runoff in extreme rain events and sends it directly into the stormwater system.

Biofiltration is one of the simplest, most natural and cost-effective ways to collect runoff and treat it onsite, and is considered a best practice under U.S. Environmental Protection Agency (EPA) guidelines for low impact
development (LID). The modular system selected for the 49ers Stadium consists of precast concrete bioretention units that are installed below ground level. The fully deployed system looks to the casual observer like a flower-bed or tree-planter, with the earth-level somewhat lower than the surrounding pavement. Beneath the surface, a series of concrete cells filled with layers of mulch, biofiltration media, and drainage rock, with a pipe in the bottom carries treated water out of the system.

The surface-level of soil in one of these bioretention modules is about six inches below pavement-level, to provide ponding depth. In storm conditions, water runs into the modules, ponds, and percolates into the media, where it is naturally filtered along the way. The filtered water is collected in a perforated pipe that runs through the bottom of the bioretention system and carries treated water into a storm drain system. Filtered pollutants are naturally broken down by microbes, and provide nutrients for the plantings in the module. Plants and trees are irrigated by the rainwater. In locations where rainfall may not be frequent or consistent enough to sustain them – such as the 49ers Stadium – integral drip-irrigation lines built into the modules can deliver irrigation water. Available module-types include basic units (used with small ornamental plantings), tree modules, curb inlet pre-filters (to collect and detain large debris), grated pre-filters, swale overflow filters, trash modules, light pole modules, and custom modules for special situations. Pre-filtration modules have an upper chamber designed to collect litter and other solid debris, and detain it so it does not interfere with drainage. Pre-filtration units must be periodically cleaned to dispose of collected debris.

Overflow drains are built into selected modules to handle extremely heavy rains. During atypically high flows, when the ponding depth is filled and cannot be drained fast enough through the filtration media to keep up with rainfall, the parking lot must still be protected from flooding. Excess water then goes into the overflow drains and directly into the stormwater system. The stadium system was designed to handle a 10-year rain event. Having an integrated overflow bypass built into the bioretention system eliminates the need to design and install a separate peak conveyance system, as is often necessary with other biofiltration setups.

Special Conditions
The high water table of the site was a primary challenge. Even though closed-bottom modules would be able to filter runoff without interference from existing groundwater, the runoff system would have to tie into the existing, shallow storm-drain lines, dictating that the drain system could not go very deep into the ground. Modules for the 49ers' stadium had to be custom designs that are slightly shallower than the manufacturer's standard units. The reduced filtration depth available had to be compensated for in the overall design of the drainage system by increasing bioretention surface area. The first system installed includes special units for that purpose. These sections are more than twice as wide as standard modules, and are laid out perpendicular to the main system, with a large, rounded end for aesthetic purposes and easier entry to neighboring parking stalls. The shape has been described as resembling a hockey-stick.

The installed bioretention system has been filled with drainage rock (bottom layer, not visible here) and planting media. The upper edge of the system has precast holes for dowels to connect to paving. The second system in the east parking lot of the 49ers Stadium site, includes an overflow drain with the Flo-Gard Overflow Filter. This unique device retains floatables within the system, further protecting downstream waterways.

These special wide modules were custom designed and fabricated for the 49ers' project. They increase the surface area of the filtration system to compensate for site conditions. Each drains 7,500-10,000 square feet of parking lot. Custom fabrication did not delay the tight production schedule for this system.

The architect designed the side walkway of one parking lot with a shallow S-shaped jog in its otherwise straight path. The bio-filtration system runs along the edge of this walkway, and follows the jog. This required custom modules. None of the custom pieces caused any production delays or presented any problem to fit into the modular system.

Workers laid in 600 feet of perforated pipe in the bottom of the system, and then connected them to the local storm-water system. The planting modules are then filled with a non-proprietary blend of filtration media designed to conform to local codes. Over all, there is 15,000 square feet of bioretention surface area.

Constructing the System
The parking lot has two bioretention systems. The one on the north side, featuring the wide, rounded special modules, was manufactured and installed first. The main run is 2.5 feet wide and just over 600 feet long, mostly basic curb-cut units without pre-filtration. The hockey-stick modules that lie perpendicular to the main run are seven feet wide. They divide the parking lot into six drainage management (DMAs) areas, each comprising about 100 of the main run, and each draining 7,500-10,000 square feet of parking and walkways.

Installation of the bioretention modules was relatively simple. The excavation was cut to the proper depth and leveled, and the units set in place. The sections weighed 10-13 tons. Joints between sections were sealed with mastic and a specialized tape. The tops of the sections include holes for dowels to attach to adjacent pavements. (At the ends of the U-shaped sections, a small amount of fresh concrete was required to fill around curves.) Perforated pipe was laid into the bottom and connected to the overflow drains, and the system was ready to be filled with drainage rock and filtration media.

At this stage the modules are in place, but unfilled. The system runs more than 600 feet long. There are 2,500 lineal feet of bioretention cells.

The drainage rate is determined by the surface area of the bioretention system and by the composition of the filtration media. The system does not require proprietary biofiltration media. For the 49ers' stadium, a blend that yields 5/10 in/hr of drainage in accordance with the Contra Costa County requirements is used. (Other growing media blends are capable of drainage as high as 100 inches per hour, where required.) Computed against the size of the lot and the predictable rainfall, the size of the required bioretention system can be calculated. However, the media must be selected not only for drainage characteristics, but also for compatibility with the intended plantings.

Challenges
The greatest challenge of the first phase proved to be timing. The underground contractor had a mere 70 days to install the system and tie it into an existing storm-drain, or pay liquidated damages of $25,000 a day for every day the project ran over. Use of the modular bioretention system helped the company meet this deadline. The modules for the first system were manufactured and delivered in under six weeks. The second system was installed later, with a mere two weeks production time for 55 modules. While the standard bioretention modules used in most of the 49ers project are three feet wide, two systems in the second phase feature tapered plans: one is 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 main entrance to the stadium runs between the two systems, and the area they drain is immediately outside the luxury boxes. The unusual shapes were designed by the architect with specific visual goals, helping to define the focus of a key space in the stadium's grounds.

Workers excavated a series of trenches and leveled, then bedded them with drainage rock.

The Long Term
One of the great advantages of biofiltration is that the system is easy to maintain and essentially self-sustaining. It harnesses the method by which nature breaks down petrochemical pollutants, a process that needs little help from us. Debris must be removed from the system, and plants must be maintained like any other landscaping, but little more is necessary. The San Francisco 49ers' bioretention system will largely maintain itself, keeping the grounds looking good and safe from flooding, and protecting the San Francisco Bay and sensitive areas surrounding it from contamination.