Stormwater treatment retrofit

October 2011 » Web Exclusive » PROJECT CASE STUDY
Redeveloped shopping center treats stormwater with horizontal-flow biofiltration system
The upscale Golden Cove Shopping Center in Rancho Palos Verdes, Calif., offers panoramic views of the Pacific Ocean, Catalina Island, and surrounding beaches.

With panoramic views of the Pacific Ocean, Catalina Island, and surrounding beaches, the Golden Cove Shopping Center is a top-notch destination for the community of Rancho Palos Verdes, Calif. However, originally built in 1965, the shopping center recently needed a facelift. In this affluent suburb of Los Angeles, the city is highly selective about the development projects it allows.

Project
Golden Cove Shopping Center stormwater treatment, Rancho Palos Verdes, Calif.
Civil engineer
John Peters & Associates
Product application
Modular Wetland Systems – Linear underground stormwater treatment units provide biofiltration in a restricted space.

The center hosts an array of restaurants, retailers, and professional office space. The planning commission granted approval in December 2008 for a $4 million Trader Joe’s Market and upgrades to the center. The scope of the work included demolition of an existing restaurant and construction of a new Trader Joe’s grocery store with 11,000 square feet of retail area. Other aspects included restriping and reconfiguring the existing park lot, providing two-way drive aisles, a courtyard area along the front of the existing two-story building, relocation of a trash enclosure, and general upgrades.

Just six months later, the design team at Modular Wetlands and the civil engineer, John Peters & Associates, began discussion of forthcoming challenges associated with this project.

Four Modular Wetland Systems - Linear, 22-foot Underground Grate Types were installed strategically to collect water flows within the project site.

The challenge
All significant redevelopment projects mean working within particular constraints; this project was no exception. Because of the size and addition of more than 5,000 square feet of impervious surface, this project falls under federal, state, and local water quality requirements. The overall size of the project is approximately 6.3 acres, with more than 90,000 square feet of restaurant, retail, and office space.

An existing site with established drainage infrastructure posed a challenge requiring the engineers and design team to work with current slopes of the parking lot, including existing pipe inverts and downstream connection points. The current storm drainage system is an underground French drain system. The runoff from the site flows from the east side to the west, and then discharges to the street gutter via several gutter drains.

An internal bypass is crucial to success of the project. Limited fall from finish surface to pipe invert is always a critical factor with filtration systems, especially biofiltration systems that require a specified amount of fall to operate properly.

These biofiltration systems were designed for a custom shallow height to work with existing pipe inverts.

Among other concerns is the increase in vehicular traffic and demand for parking. The city is concerned with sufficient parking and required more spaces to be added. In allowing for additional parking, landscape areas were diminished, thereby eliminating the space for traditional non-proprietary biofiltration systems. Because of increased intensity of land use, pollutants become a greater concern, especially since the receiving body of water is the Pacific Ocean. The pollutants of concern associated with the site are trash and debris, oil and grease, sediments, nutrients, pesticides, and oxygen-demanding substances.

The solution
The design and plans were finalized in the fall of 2008 by John Peters & Associates and the Modular Wetland engineering team. Four Modular Wetland Systems (MWS) - Linear, 22-foot Underground Grate Types were installed strategically to collect water flows within the project site. The systems were placed in parking spaces and driveways toward the west end of the site. Each unit was installed to treat just over 1 cubic foot per second of stormwater, which is equal to 85 percent of all runoff, while it continues to remove 100 percent of the flows’ trash and sediments.

These systems were designed for a custom shallow height to work with existing pipe inverts. The MWS - Linear was chosen because it is the only biofiltration system that can be placed in hardscape areas, as no landscape areas existed. These particular systems consist of a solid concrete lid rated for indirect traffic. Plants often are used with the MWS - Linear but were eliminated for this project according to the engineer’s request for placement and space constraints.

Solid concrete lids on the Modular Wetland Systems - Linear units are rated for indirect traffic.

Runoff flows within the site and then collects and filters through the MWS - Linear systems prior to entering the discharge point along Palos Verde Drive. The MWS - Linear incorporates a high-flow bypass along with its four stages of treatment: screening, hydrodynamic separation, media filtration, and biofiltration.

The Result
The systems have been in ground for one year. Choosing a biofiltration system that can be placed in a hardscape area was a huge advantage compared with the other similar systems used in the project. Based on land and use calculation, the city required additional parking spots; 275 spaces to 296 spaces. Installing the MWS - Linear underground system allowed the city’s parking requirements to be met.

This article was contributed by Bio Clean Environmental


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