Project Case Study: Out-of-sight stormwater treatment

November 2006 » Feature Articles
Concerns about stormwater runoff are easily lost on football fans caught up in the excitement of game day at the new Arizona Cardinals’ Stadium in Glendale, Ariz.
A.J. Margetis and W. Scott Gorneau, P.E., CPSWQ

Underground system gives football fans more space for parking, parties, and picnics

Project

Project
Arizona Cardinals' Stadium stormwater treatment system, Glendale, Ariz.

Civil engineer
CMX Group, Inc., Phoenix

Contractor
CS&W Contractors, Inc., Phoenix

Product application
Underground stormwater treatment units save 14 acres of land for parking, grassy areas, trees, and fans.

Concerns about stormwater runoff are easily lost on football fans caught up in the excitement of game day and overwhelmed by the sheer size and smooth curves of the new Arizona Cardinals' Stadium in Glendale, Ariz. (named University of Phoenix Stadium). That's because the system handling runoff is mostly invisible to them, buried under the blacktop that fans cross from the parking lot to the front gate. Except for two areas of grass and trees, the 120-acre stadium site is mostly asphalt and includes parking for 16,000 cars.

When they arrive for a game, Cardinals' fans are struck by the enormity of the smoothly curved stadium. It is 206 feet high and covers 1.7 million square feet. To construct it, contractors poured enough concrete to build a 900-mile sidewalk connecting Phoenix to San Francisco. Inside are 73,000 seats. As fans wind their way to find their seats among thousands at the stadium, usually they have a clear view of the sunny Arizona sky, because the stadium roof is usually open. In inclement or hot weather, two retractable roof panels covered in Birdair fabric protect the football field and stadium seats and provide climate control. Closed, the domed roof covers more than 13 acres. But the more than 300 days of sunshine and just 8 inches of rain a year give Cardinals' fans little worry about bad weather.

System design

Arizona state regulations called for controlling runoff from the 200-foot-high stadium roof and the adjacent asphalt before it flows off-site into the abutting drainage channel that is also used for recreation. With more than 100 acres of roof and hard surface, the task of managing the site's stormwater runoff involves collecting, treating, directing, and then discharging the storm flow into nearby Bethany Home Outfall Channel.

One potential solution was to hold site runoff for a 2-hour, 100-year storm event in a surface detention pond and let it trickle into the channel nearby. But a pond would use 14 acres of valuable land—almost half the footprint of the stadium—and so was too costly. A detention pond also would have left little space for the grass and trees that the team and building owners wanted.

Looking for a more cost-effective solution that also preserved green space, building owners and project managers sought a below-grade alternative. According to Jim Jones, CMX Group project engineer, his company interviewed four stormwater system manufacturers that presented designs. The CMX group then designed the project around two of these, leaving the final design decision to CS&W Contractors, Inc.

"CONTECH was proactive, explained their solution, and later provided helpful technical assistance during installation," said Gary Graham, chief estimator for CS&W Contractors. "We chose Vortechs systems due to their cost-effective nature and ease of installation."

Working with the CMX Group and CS&W Contractors, Inc., CONTECH stormwater consultants designed a site-specific solution for the stadium using 13 Vortechs hydrodynamic separation units. CONTECH prepared a flow-based design according to the CMX Group specifications for holding and treating the 100-year, 2-hour runoff event. The method used third-party validated, laboratory-generated performance data to ensure accurate long-term performance.

"The plan was to capture the 'first flush,' or the first half-inch of rain that falls," said Jones. "The first flush is enough to dislodge the trash and oils that will accumulate in the parking lot. The runoff from additional rains in heavy storms, although more pure, also will be caught and treated through the cleansing units."

The hydrodynamic separators provide effective removal of sediment, floating trash, and hydrocarbons in the required small footprint. Traffic-rated for underground installation, the stormwater solution included a combination of Vortechs systems, including two 11000 models, four 16000 models, and seven PC1319 models for treating runoff before discharging it into the channel.

Releasing the runoff directly into a channel is unusual in Arizona. To do so, the developers had to clean any storm runoff to meet federally mandated water quality regulations. They also provided proof to the city that the stadium runoff wouldn't affect the channel during a 100-year storm. They showed mathematically that the volume of stadium runoff would be treated and spent before the rest of the runoff from the dozen square miles of drainage area reached the stadium site.

Planning and installation

After proving the stormwater management solution to the city, the installation had to be planned and scheduled perfectly said Brad Hall of CS&W Contractors, the project manager overseeing the stormwater system installation and now with McCarthy Building Companies, Inc. Every aspect of the entire project focused on meeting the Cardinals' opening day, Aug. 12, 2006.

According to Hall, to meet that deadline, planning for the stormwater system installation began months in advance. Hall said timing and trucking constraints were the biggest installation challenge because the construction crew could only move in the equipment between 9 a.m. and 3 p.m. to avoid rush hour traffic. Obtaining the necessary state permits to bring in the equipment that included a 300-ton hydraulic crane and the stormwater units took two months.

Safety issues limited the number of holes the installation crew could have open at once. The site design called for placing 13 units in 10 holes, and only five holes could be open at any one time. This meant completing the installation in two phases and scheduling around the logistics of other activities on the building site. Because of the depth of the holes, safety also dictated using the crane to place the pre-cast concrete vaults, Hall said. The construction team completed the first installation of six units in one day. A month later, it placed seven more in five holes. "The advanced planning and technical assistance from CONTECH made both installations go quickly and smoothly," said Hall. "We just timed our installations around the other construction at the site."

Operation and maintenance

Runoff flows off the domed, Birdair fabric roof and down to the ground. Several storm drains collect it along with the parking area runoff and move the flow away from the building site into the hydrodynamic separators. Located at 10 points around the property, the 13 units clean the runoff before it enters the Bethany channel.

By combining three of the stormwater systems with a grassy strip south of the stadium and bordering the channel, developers provided a cost-effective alternative that meets the needs of the state, developers, and the Cardinals. "The team made it clear they want every facet of the stadium to provide fans with the best possible game-day experience, and that includes the parking areas, which are very well landscaped to reduce the amount of asphalt," said Jones.

During a rainstorm, runoff tangentially enters the swirl chamber of the systems at an angle and induces a gentle swirling flow pattern that enhances gravitational separation. As pollutants sink, they stay in the swirl chamber while floating pollutants are stopped at the baffle wall. Flow rates are controlled through the system by a flow-control wall that uses an orifice and a Cippoletti weir.

In a heavy rainstorm, the water level rises above the low-flow control and begins to stream through the high-flow control. The layer of floating pollutants is elevated above the influent pipe, preventing re-entrainment. The swirling action increases with the intensity of the storm and helps maintain sediment removal rates. When the storm drain flows at peak capacity, the water surface in the system approaches the top of the high-flow control. The stormwater management system was designed so that previously captured pollutants are retained in the system even during these infrequent storms.

As a storm subsides, the treated water level drains at a controlled rate to the dry-weather level equal to the pipe invert, or about 3 feet. Besides facilitating inspection and cleaning, the low water level significantly reduces maintenance costs by reducing pump-out volume.

Regular inspection will keep the stadium's stormwater solution working effectively. Maintenance is easily performed through access manholes and keeps the system operating as designed. Stormwater consultants recommend cleaning the units whenever an inspection shows the sediment depth is within 6 inches of the dry-weather water surface level. Maintenance personnel can determine the levels by taking two measurements with a stadia rod or similar measuring device. They first measure from the manhole opening to the top of the sediment pile, and then from the manhole opening to the water surface. If the difference between the two measurements is 6 inches or less, either manual laborers or a vacuum service truck can clean it. Depending on the sediment and pollutant loading, it might be as much as a year or two between cleanings.

Because national, state, and local regulators increasingly look at stormwater as a cause of non-point source pollution, stadium designers worked to make sure the project exceeded federally mandated best management practice guidelines. The underground stormwater treatment system made it possible for the stadium designers to meet state environmental requirements, maximize the 120-acre site for on-site parking, and include two large grassy areas with more than 1,200 shade trees where fans can picnic.

Jones said, "The Cardinals' Stadium stormwater system meets environmental requirements, provides more land for tailgating and other stadium activities, and saved the project about $1 million."

A.J. Margetis is a stormwater consultant with CONTECH Stormwater Solutions in San Diego. He can be contacted at margetisa@contech-cpi.com. W. Scott Gorneau, P.E., CPSWQ, is an applications engineer with CONTECH Stormwater Solutions in Scarborough, Maine, and can be contacted at gorneaus@contech-cpi.com.

Turf tray

The Cardinals' University of Phoenix Stadium features a natural-grass playing field contained in a 234-foot-wide by 403-foot-long, rollout tray. During most of the year, the 17 million-pound tray remains outside of the stadium, where it receives sunlight and watering. For football games, the tray is moved into the dome stadium (retractable dome) in about 45 to 60 minutes, driven by electric motors on steel wheels that ride on tracks embedded in the concrete stadium floor. The movable playing field allows for multi-purpose use of the facility, including basketball games, trade and consumer shows, conventions, concerts, motor sports, and rodeos.


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