Meeting C3 Stormwater Requirements with Silva Cells Case Study: Tech Company HQ in Menlo Park, CA

In 2011, CMG Landscape Architecture was approached by a tech company to design the courtyard of their new headquarters in Menlo Park, CA. The project was on a very fast track. In addition to the accelerated timeline, CMG’s proposed design for the site had the total impervious surface area increasing from 40 percent to 60 percent. This increase meant that they needed to find a way to meet California’s C3 requirements, which mandate that 85% of the storm runoff over a lifetime of the treatment facility be captured and treated.

The tech company was committed to creating a welcoming outdoor space where employees could work, meet, or simply gather socially. They were also committed to using green utilities wherever possible. CMG decided to use the Silva Cell to provide lightly compacted, ample amounts of soil for the trees, while also acting as an underground bio-retention areas under pedestrian pavements.In designing to meet C3, there were several primary areas the designers had to consider: major pollutants; geotechnical considerations and seasonal groundwater levels; distribution of treatment flows to the bio-retention area (as well as under drainage and overflow requirements); and pollutant removal rates.

Pollutants of Concern
The impervious sections treated by using the Silva Cell for bioretentionare pedestrian pavement areas. Considering the site uses, the most significant pollutant is Total Suspend Solids (TSS) associated with air borne particulates settling on the paving surfaces. Oil and grease are very limited, as is heavy metal contamination which is commonly associated with vehicle pavements.
Geotechnical Considerations and Seasonal Groundwater
The engineering and design of the Silva Cell system was developed with the project geotechnical engineer based on review of existing site soils and seasonal groundwater levels to ensure stability of the paving section and adjacent buildings.

Site borings indicated groundwater depths at 8.5 – 12 feet below existing grades, with seasonal fluctuations due to perched water from surface infiltration. Anecdotal evidence from project excavations in the project vicinity indicated that seasonal perched ground water co

uld rise to 5’ below existing grades. The bio-retention areas associated with the Silva Cell system have a maximum depth of 3.5 feet below existing grades. To address groundwater levels and heavy soils with limited infiltration capacity, the system includes an under drain system to allow treatment flows to drain through the bioretention soil before being collected and returned to the storm drain system.

To address groundwater levels and heavy soils with limited infiltration capacity, the system includes an under drain system to allow treatment flows to drain through the bioretention soil before being collected and returned to the storm drain system.

Distribution of Treatment Flows to the Tree Well Filter/Bio-Retention Area
Treatment flows are directed to the bioretention soils below the pavement via slot drains and perforated distribution pipes. A series of lateral pipes (15 – 20 feet on center) connect the slot drain to a double row of eight inch perforated distribution pipes, and the pipes are offset by 2’ – 3′. The distribution pipes run the length of the treatment area and distribute the treatment flows to the underlying bioretention soil. The frequency and length of the pipes provide a flow rate that exceeds the design requirement of 0.2 inches/hr of rainfall.

The flow factor in the system is the permeability of the bioretention soil. As required by San Mateo County Stormwater Technical Guidance, the bioretention soil is specified to have a minimum conductivity of 5 inches/hr. The distribution pipes are located to avoid
conflicts with proposed trees and light fixtures, and are also offset from the under drain system to avoid short circuiting from the distribution pipe to the under drain and allow the treatment flows to saturate the soil profile and filter through the 18” soil section.

All of the proposed Silva Cell bioretention areas exceeded the required treatment volume by 4 – 6 times, with the exception of one area that still was double. Calculations for that area confirmed the effectiveness of the distribution system relative to C3 requirements for flow-based treatment. (The sizing criteria are mandated by the Bay Area Municipal Regional Stormwater Permit, Order R2-2009-0074, which covers 76 permittees, including Menlo Park. The criteria can be found in Provision C.3.d, on page 30 of the permit). 

Treatment Efficacy and Pollutant Removal Rates
The pollutant removal mechanisms and hydrological principles of bioretention with Silva Cells are essentially the same as those in other

bioretention systems. (The Washington State DOE, for example, officially designated bioretention with Silva Cells as a functional equivalent of a rain garden.)

TSS removal will happen in the catchbasin, slot drain, and the distribution pipe. The cleanouts on the distribution pipe will allow for sediment to be cleaned out of the distribution pipe as needed. Finer sediment that gets through the pipe into the soil will be filtered out of the runoff by the soil medium.

We know tree roots grow in soil volumes where they can access adequate air and water, increasing and maintaining soil permeability and fostering biological activity in the soil. Additional studies have shown that street trees used in conjunction with under‐pavement bio‐retention areas are an effective stormwater treatment method. For example, Breen et al conclude that:

“Stormwater treatment in street‐tree systems is via a combination of physical, chemical and biological processes (Table1). Treatment also occurs over various temporal and spatial scales. Treatment occurs within the soil profile and during both runoff events and the inter‐event dry period. The wetting and drying phases within bio‐retention systems and street‐tree systems is important for both the effectiveness of the treatment processes and plant health… At this time the experimental results are consistent with the conceptual model of street‐tree stormwater treatment systems and suggest that it is feasible to use under‐pavement tree root zone soils as an element in a storm water treatment.”

–Breen, Peter; Denman, Liz; May, Peter, and Leinster, Shaun. 2004. Street Trees as Stormwater Treatment Measures. WSUD 2004.

The Silva Cell system uses similar biological uptake mechanisms and filtration associated with tree root zones and bio‐retention soils. Research indicates that the biological activity (micro‐organisms and fungi) associated with tree root zones and healthy soils can provide significant treatment function.

In conclusion, the Silva Cell system is consistent with the San Mateo Technical Guidelines and C3 requirements as noted by the City of Menlo Park. Not only does it provide stormwater treatment, but it provides the street trees with excellent growing conditions. Large, healthy trees will be a significant part of the campus experience for this company and its employees.

At this time, we unfortunately do not have permission to post finished photos of the site. As soon as we get it, we will be sure to update the post.

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