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 grounds and parking areas. If rain can turn a football field to a mud pit, it can also turn a parking lot into a flood. Draining that area effectively and treating the runoff is a basic function 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., a major project in the San Francisco Bay area, faced exactly that challenge. The facility 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 need there, and in other hardscapes surrounding the stadium 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 customizability and attractive landscaping elements.
The new stadium moves the team out of Candlestick Park, in San Francisco, into a location with about double the stadium square footage, better freeway access, and more than twice the parking spaces. Parking lot runoff always carries with it 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 directly into the Guadalupe Slough and San Francisco Bay, sensitive ecologies 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. 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 engineering challenges.
Water is one of nature's most powerful forces. Hardscaping can turn that force destructive, both in terms of erosion and pollution. A stormwater collection system has three mandatory functions:
- remove rainwater from the hardscape;
- channel runoff so it does not erode the surrounding landscape; and
- treat runoff so it does not pollute landscape or waters downstream.
Biofiltration is one of the simplest, most natural, and most cost-effective ways to collect runoff and treat it onsite, and is considered a best practice under U.S. Environmental Protection Agency LID guidelines.
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 (Figure 1). Beneath the surface, it is a series of concrete cells filled with layers of mulch, biofiltration media, and drainage rock, with pipe in the bottom to carry treated water out of the system (Figure 2).
The surface level of soil in one of these bioretention modules is about 6 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 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 that are built into the modules can deliver irrigation water.
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 cleaned periodically to dispose of collected debris.
Overflow drains are built into selected modules to handle extremely heavy rains. During atypical 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 49ers' 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.
The bioretention system prevents hazardous conditions and collects water so it does not erode the surrounding landscape. Simultaneously, it is treated so it can be safely reintroduced into the landscape or drained into the creek, the natural drainage of the area. Pollutants are kept out of the bay, but no chemicals are used, there are no moving parts to maintain, no energy is consumed to do it, and maintenance is minimized.
The site's high water table 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 designed slightly shallower than the manufacturer's standard units.
Engineers compensated for the reduced filtration depth 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 to allow for easier entry to neighboring parking stalls. The shape has been described as resembling a hockey stick.
The architect designed the side walkway of one parking lot with a shallow S-shaped jog in its otherwise straight path. The biofiltration system runs along the edge of this walkway and follows the jog. This also required custom modules. None of the custom pieces caused any production delays or presented any problem to fit into the modular system.
Constructing the system
The kickoff to construction at the stadium site was to create the parking lot on the east side, next to the training field. It will serve the parking needs of construction activities plus those of the 49ers' team offices. It was begun in January 2012, months before the official groundbreaking for the stadium.
The 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 7 feet wide. They divide the parking lot into six drainage management areas, each comprising about 100 feet of the main run, and each draining 7,500 to 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. Sections weighed 10 to 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.
The drainage rate is determined by the surface area of the bioretention system and by the composition of the filtration media. The size of the lot and the predictable rainfall determine the size of the required bioretention system. The system does not require proprietary biofiltration media. However, the media must be selected not only for drainage characteristics, but also for compatibility with the intended plantings. For the 49ers' Stadium, a blend was selected that yields 5 to 10 inches per hour of drainage, in accordance with the Contra Costa County requirements. (Other growing media blends are capable of drainage as high as 100 inches per hour, where required.)
The greatest challenge of the first phase proved to be timing. The underground contractor had only 70 days to install the system and tie it into an existing storm drain, or pay liquidated damages of $25,000 per day for every day the project ran over. Use of the modular bioretention system helped meet this deadline. The modules for the first system were manufactured and delivered in fewer than six weeks. The second system was installed later, with two weeks production time for 55 modules.
The contractor noted there was a learning curve, but it was a fast one. They installed 120 feet of the system on the first day. By the fourth day, they set 480 feet in a day with a single crew.
The second, main phase of the project includes four more bioretention systems: two in a large parking lot south of the stadium, and two lining the walkway area surrounding the main entrance to the stadium. Onsite parking for the public will include about 10,000 parking spaces. A total of more than 2,500 linear feet of Bio-Mod cells, approximately 14,000 square feet of biorentention area, will drain the overall stadium site.
While the standard bioretention modules used in most of the 49ers' project are 3 feet wide, two systems in the second phase feature tapered plans – one is 3 feet wide at one end and 14 feet at the other, an irregular wedge longer than 300 feet. 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.
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. 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.
Steven H. Miller, CDT, is a marketing consultant and award-winning writer and photographer specializing in construction industry issues. He can be contacted at firstname.lastname@example.org.