The Silva Cell is well known as a suspended pavement system that provides large soil volumes to aid tree growth and health. Less well known is the capacity of the Silva Cell system to provide onsite stormwater management.
Whereas traditional stormwater management has focused on collecting stormwater in piped networks and transporting it offsite as quickly as possible (thus incurring large infrastructure and environmental costs), this system utilizes green infrastructure, allowing soil and trees to act as on-site stormwater management. Capturing stormwater on-site prevents it from spreading pollution and overwhelming sewers and also reduces and in some cases eliminates, the need to use potable water for irrigation.
The flexibility of the Silva Cell system enables designers to adapt the system to meet the specific requirements of their site and project goals. This flexibility is especially important given that every city and state/province has different laws and requirements for stormwater management, as well as different climates and site conditions.
Keeping this in mind, the first step in developing a plan for using the Silva Cell system to manage stormwater is to determine your project goals:
- What treatment and storage capacity do you want to provide?
- What are the legal requirements in your city?
- How many trees?
- What is your 90% storm event?
- What kind of ground recharge do the base soils have?
It is also important to remember that the system is designed for small storms: rain events less than 2 inches (50.8 mm) can be easily handled. This is because the system is generally used in urban areas, which restricts the size of the footprint. The system can handle larger rain events, but you’ll need a larger footprint of Silva Cells.
Common Features in Silva Cell projects for Stormwater
Pretreatment: The mechanism that removes total suspended solids (TSS), prior to its entrance into the system.
Distribution: The mechanism/means that moves the water through the system. The water should be moved throughout the soil column for maximum treatment and to benefit the tree.
Outflow: Mechanism to drain water out of the system after it has passed through the soil and has been treated.
Overflow: The mechanism in place to serve as back up if the system reaches capacity.
Methods of Capture
The method of capture refers to how the water enters the system and the soil. The three most common methods for capturing stormwater that we see for the Silva Cell system are: porous pavement, sheet flow/curb cuts, and catch basins/sumps. Other methods are roof leader disconnections and trench drains.
Method of Capture: Porous (Permeable) Pavement
Porous pavement captures water right where it falls and channels it to the soil below. Typically, the pavement will extend across the full surface of the site above the Silva Cells, reflecting the footprint of the Silva Cell system below.
Pretreatment: The pavement functions as the pretreatment mechanism by stopping debris/TSS from entering the soil such that only water penetrates the pavement.
Distribution: No additional distribution is necessary because the water enters the system through the pavers covering the system: infiltration is achieved through the seeping of the water through the pavement. With adequate infiltration (for example, a sloped road that pulls all the water in through the pavers and into the soil, or one in which the full surface of the system is covered in porous pavement), there is no need for a pipe to move the water through the soil.
Outflow: Underdrains and/or groundwater recharge will be needed to allow water to leave the system. Check your geotechnical report to see what kind of ground recharge the base soils have.
Overflow: If the system saturates due to a larger than “design storm” event entering the Silva Cells then the excess water will sheet flow to the road or next catch basin.
Method of Capture: Sheet Flow/Curb Cut
Sheet flow and curb cuts are similar methods of capturing stormwater that use the curb as a mechanism to direct stormwater into the system. Sheet flow describes situations in which the water is evenly distributed on the pavement system, such as through “curbless” streets. Curb cuts are typically used in raised curb situations and allow the water to enter at specific points. When using either of these methods for a Silva Cell installation, a key design component is a splash pad, mulch, or other mechanism that helps to dissipate the energy of the water before it enters the system.
Pretreatment: The tree opening can be turned into ponding space where, for example, rock mulch can be used as both the energy dissipater as well as pretreatment area. Grates in the curb cuts can also serve to provide the pretreatment by stopping debris and TSS from entering the system.
Distribution: When the water in the tree pit ponding space fills up then it flows into a Distribution pipe which distributes the water through the soil column. The inlet for the Distribuiton pipe is at the back of the tree pit with a screened intake set just above the ponding level in the tree pit but below the grate.
Outflow: Underdrains and/or ground re-charge will be needed to drain the system. Check your geotechnical report to see what kind of ground recharge the base soils have
Overflow: If the system saturates it may back up the distribution pipe and the ponding space so that excess water will bypass the curb cut and run down the street to the catch basin.
Method of Capture: Catch Basin
A catch basin is a curbside drain that collects rainwater and traps debris and TSS in its sump so that it does not enter the drainage pipes. In the context of managing stormwater with the Silva Cells, catch basins are effective in slowing down the movement of water and thus are especially useful in contexts where the cleaning of the water is a project goal. Invert elevations are very important when using catch basins, and it is recommended to use the frames of the Silva Cells themselves to support the pipe that distributes water from the catch basin into the soil within the Silva Cell system. In many situations, it is easy to use a catch basin that is already present at the site.
Pretreatment: The curbside entry point for water entering the system may have a grate that helps to trap large debris before entering the system. The catch basin itself will also hold sediment or other solids in its sump, which will stop TSS from entering the Silva Cells.
Distribution: A pipe is required for bringing the water from the catch basin to the soil inside the Silva Cell system and also to distribute the water throughout the system. A simple perforated pipe will often suffice.
Outflow: Underdrains and/or ground recharge will be needed to drain the system. Check your geotechnical report to see what kind of ground recharge the base soils have. It is possible to raise the level of the underdrain to create more storage capacity in the system. You can also add an elbow at the outflow end of the drain line to slow up the drainage rate. By keeping the water in the system for a longer period of time you can increase the effectiveness of the cleaning action of the microbial colonies. In this manner you can “tune” the system to maximize the balance between rate of flow and capacity to clean the water of pollutants.
Overflow: When the system saturates it will back up the distribution pipe and the intake catch basin. The overflow from larger storm events is captured either by sheet flow down the street to the next catch basin or a higher overflow pipe inside the catchbasin can pick up the overflow water and send it directly to the stormwater system.
When considering (and planning for) the design of a Silva Cell system for managing stormwater, it is important to consider both the system’s maintenance (the infrastructure involved in bringing the water into the soil and treating it), as well as the soil maintenance.
The Cells themselves, properly installed, do not need any maintenance. TSS removal is the only maintenance the overall design needs; the specific maintenance protocol required depends on the method of capture and pretreatment. When designing the system, make sure to think about the level of maintenance that your client/the city is capable of. Don’t design something that the city doesn’t have the equipment/skills/resources to maintain.
Porous pavers: You will need to clean the pavers according to the schedule defined by the manufacturer as well as your location and site conditions. This will involve getting a machine that can pull out all of the sand, etc.
Sheet flow/Curb cuts: You will need to be able to pull up the tree grates, vacuum out the accumulated TSS, install new rock mulch, and clean the distribution pipe inlet filter. This can be more time consuming than other methods, depending on how much TSS is at your sites.
Catch basin: This method has the easiest maintenance of all, because all cities already have someone who routinely goes around to clean out the sumps.
All systems will have clean outs for both the distribution pipes and the drain lines that can be flushed on a set schedule.
When properly installed and maintained, no soil maintenance is necessary. Trees metabolize the nitrogen and phosphorous in the stormwater. Over time the trees grow radial roots, which keep the soil structure open through time. The radial growth of roots opens up soil channels and creates “water channels” underground. When these roots rot, this keeps the soil open.
For more details on each of the methods, including case studies and a comprehensive Q & A discussion involving curb cuts and bypass, recommended separation distances between utility lines and Silva Cells, working in areas with freeze/thaw cycles, and more, we recommend watching our recorded webinar: Techniques for Directing Stormwater into Silva Cells. Note that the Q & A starts at 41:42.
 This feature is referred to as “pre”treatment because, although there are some pollutants that attach to the TSS and thus are removed at this stage, the core pollutant removal happens in the soil column.
Top image: Century College Parking Lot in White Bear Lake, MN. Silva Cells were installed there in July 2009 (810 frames and 270 decks).
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