Oct 02, 2019

Public-Private Partnership Diverts Urban Runoff Discharges

This article originally appeared in the October 2019 print issue as "Up & Running" in a Runoff Control Special Report 

The portion in red represents the area tributary.
The portion in red represents the area tributary.

In an effort to combat the complexity of storm water quality treatment and compliance with local, state and federal regulations,  Culver City, Wash., is implementing the Washington Boulevard Stormwater and Urban Runoff Diversion through a public-private partnership (P3) with the warehouse-club store Costco Wholesale. The project will address storm water runoff discharges from the city within the Marina del Rey Watershed. 

The project started in June 2017 and is on track to be completed by 2020 with a goal to reduce the quantity of pollutants, such as oil, trash, sediment, metals, nutrients and bacteria, from reaching the Marina Del Rey Harbor and Santa Monica Bay through the discharge of wet-weather and dry-weather runoff. This project ultimately captures storm water and urban runoff and conveys it to the sewer system, preventing untreated flows from being released to downstream receiving waters. 

These goals align with regulatory requirements established by the National Pollutant Discharge Elimination System (NPDES) and Municipal Separate Storm and Sewer System (MS4) Permit for Los Angeles County. The city and Costco solicited CWE, a civil engineering firm located in Fullerton, Calif., for the design, and J.C. Chang & Associates, a multi-disciplined engineering and architect firm located in Torrance, Calif., for electrical engineering.

 

Impervious Area Replacement

In 2016, Costco planned to demolish a former neighboring grocery store building and expand the size of its parking lot. This redevelopment meant water quality components would need to be included as part of the reconstruction project under Los Angeles County’s MS4 Permit, as projects replacing impervious areas must construct storm water elements to capture and treat the associated runoff. This conundrum could not have come at a more convenient time for the city, which had been looking for its own MS4 projects to comply with the Marina del Rey Enhanced Watershed Management Program (EWMP). EWMP seeks and investigates opportunities throughout Los Angeles County to construct multi-benefit projects that will improve storm water quality, water supply and flood control. 

The  city and Costco worked together to develop a Memorandum of Understanding to share the burden of challenges. As the official owner, the city will maintain the quality of the project, relinquishing Costco’s responsibility of running tests and arranging repairs of the project on its property at the cost of an annual fee to the city. This arrangement between public and private sectors to form a P3 benefits the city by meeting Marina del Rey EWMP requirements and receiving $2.14 million in funding from Costco out of the $8 million needed to implement the project. It is mutually beneficial to Costco by having another entity focus on the water quality challenges tied to the redevelopment and expansion of the parking lot. 

The project even benefits local taxpayers by conserving funds for use on other projects throughout the city.  In addition to the funds obtained from Costco, the city received funds from Proposition A, administered by Los Angeles County Regional Park and Open Space District. The remaining cost will be covered by the city’s Measure CW funding source, the Clean Water, Clean Beach parcel tax, which was passed by the city in 2016 for storm water projects. 

The Marina del Rey Watershed is comprised of 1,409 acres. The city makes up only 3% of this watershed, as 42 of these acres are within the city’s jurisdiction. The largest parcel within those 42 acres is associated with Costco, which is within a 16-acre parcel at the westernmost edge of the city near Walnut Avenue. The red area shown in the above image represents the area tributary to the project, which is 56.4 acres in total, while the city boundary indicated by the white and black dashed line shows the split between Culver City and the city of Los Angeles. Thinner blue lines represent local storm drains, the light blue circles are the catch basins connected to the storm drain lines, and the thick dark blue line represents the Marina del Rey Watershed. The city of Culver City can capture all flows from the city that are within that watershed as part of this project, which allows them to meet regulatory requirements for this area.

 

How It Works

First, four surface diversions along Washington Boulevard and a diversion off of Costco’s 42-in. reinforced concrete pipe (RCP) storm drain within the public right-of-way were designed to capture storm water during the rainy season and urban runoff during the dry season before it enters the storm drain system, which conveys flows to downstream receiving waters. The diversion design does not result in any modifications to infrastructure owned and operated by the Los Angeles County Flood Control District (LACFCD).  This approach avoids LACFCD permitting and maintains a shallower system; both benefits result in reduced cost and schedule.

Next, a pretreatment unit will be installed downstream of the surface and Costco diversions to remove trash and debris before being pumped into the subsurface storage system. A high-flow variable frequency drive (VFD) diversion pump will then be placed in a wet well downstream of the diversions and pretreatment, which will only operate during storm events, as the sewer is not able to receive non-sewer flows at that time. 

Pumped storm water will be stored in a concrete subsurface storage system running approximately 700 ln ft along Washington Boulevard, which is capable of holding up to 3 acre-ft of water, requiring an excavation depth of up to 30 ft. Flows retained in the storage system will be held during the rain event and for an additional 48 hours before being slowly released to the sanitary sewer. A portion of the water collected will be used to offset irrigation within five proposed medians, offsetting up to 50,000 gal of potable water per year. Opportunities to infiltrate were evaluated and determined to be infeasible due to high groundwater levels and poor soil conditions that are not conducive for infiltration.

Finally, flows will be conveyed from the subsurface storage system via gravity flow back to a wet well in close proximity to the diversion pump. Flows will be pumped using a low-flow VFD pump to the sanitary sewer system within the City of Los Angeles, which will convey flows to the Hyperion Water Treatment Plant next to Deckweiler State Beach on Santa Monica Bay. The pump to the sewer will operate at a rate not to exceed 280 gal per minute (gpm), as this rate was approved by the City of Los Angeles Bureau of Sanitation based on the existing capacity of the sewer between the point of connection and the treatment plant. 

During the dry season, flows will be diverted, pretreated and conveyed directly to the sewer discharge pump, skipping the subsurface storage system. Flow directions are controlled with automatic valves that operate based on a project-specific rain guage and water levels within the wet wells and subsurface storage system. The piping between the pumps, subsurface storage system and sewer will all be installed using trenchless installation methods, adding up to about 1,870 total ln ft in total, including diameter sizes of 4 in., 8 in. and 12 in. It is anticipated that jack-and-bore, directional drilling or alternative installation methods will be used by the awarded contractor.

 

Challenges

The subsurface storage system will be placed below the roadways, avoiding the front of the store in an effort to maintain access during construction. The storage system placement was a challenge, as it was important to minimize impacts to existing traffic on Washington Boulevard.

 The system placement was designed precisely to accommodate the excavation and construction equipment, and to maximize travel lanes. Ultimately, 800 ft of parking and 36 ft of width from the existing travel way will be lost during the construction. This means traffic control is going to be extensive, boasting a significant challenge during the project’s implementation.

During the peak hour of Costco’s business day, approximately 2,500 vehicles travel in two lanes going in either direction. The flow will be limited to one lane of travel toward the west and two lanes toward the east, restricting turning movements and increasing the wait of daily commuters and traffic getting in and out of the parking lot on Washington Boulevard. 

To help alleviate the congestion where possible, media outlets will report detailed information on the location and timeframe of the construction, and traffic control message signs will be placed several miles prior to the construction zones to encourage alternate routes. Costco offered to utilize social media channels to alert members about other options for obtaining Costco-brand goods, such as recommending delivery services.

Construction is scheduled to begin in January 2020 and will be completed by November 2020, with the subsurface storage system representing the majority of work and disturbance. The city plans to implement extended construction hours, which has been encouraged by the local community, in an effort to reduce the duration of disturbance along Washington Boulevard.     

About the author

Katie Harrel, P.E., is special projects manager for CWE Corp. Harrel can be reached at [email protected].

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