Regular readers may have followed our articles on the monitoring results of the two Silva Cell systems in Wilmington, North Carolina by Dr. Bill Hunt’s lab. Jonathan Page is an Extension Engineer in Dr. Hunt’s lab and was involved in the data collection, analysis, and writing for this project. He holds a B.S. degree in Civil Engineering and an M.S .in Biological and Agricultural Engineering, both from NCSU.
Earlier this month I had an opportunity to interview Jonathan and learn more about his views on using Tree Stormwater Control Measures (Tree SCMs) with suspended pavement. As a landscape architect primarily versed in soils, plants and landscapes, I was interested to hear an engineer’s perspective on Tree SCMs. Our conversation, which has been edited and condensed, is below. -Nathalie Shanstrom
What was the main purpose of the monitoring project with suspended pavement/Tree Stormwater Control Measures (SCMs) in Wilmington?
With this study, we wanted to show conceptually that you can use the soil-root matrix below a suspended pavement for stormwater control and treatment.
What did you expect the results to be?
We expected a high level of water quality improvement because the treatment mechanisms and processes are nearly the same as those observed in traditional bioretention. Using the soil beneath a suspended pavement as an SCM is not really that novel, we are just putting bioretention underground.
Did the results match your expectations?
Yes, for the most part.
Were there any surprises?
We expected to see more bypass. We saw about 20% of the annual runoff bypass the system, primarily due to the rainfall patterns in Wilmington, which are flashy, high intensity storms, so flow rates overwhelmed what could infiltrate into the system. However, volume benefits were greater than we expected. We also observed statistically significant mitigation of peak flowrates from inlet to outlet for storms that did not generate bypass.
We actually did not see any change in runoff volume, as expected with the liner. However, we did see statistically significant mitigation of peak discharges.
What are the main differences between traditional bioretention and suspended pavement SCMs with trees?
The primary difference is hydrologic function.
Traditional bioretention systems have a ponding area above the soil media. The primary function of this ponding area is peak discharge and runoff volume mitigation. Tree SCMs with suspended pavement typically do not have a ponding area. However, there are ways we can account for that. A typical Silva Cell system, like the one used in Wilmington, has 1 to 2 inches (2.5 to 5 cm) of ponding at the soil surface. By dropping the elevation of the soil down in the system profile, you could get up to 6 inches of ponding at the soil surface. This would likely yield a substantial improvement in runoff volume control and peak discharge mitigation.
What are the main similarities between traditional bioretention and suspended pavement SCMs with trees?
Suspended pavement SCMs with trees and traditional bioretention often use the same or similar soil media, the trees provide the same benefits in both, and the total stormwater benefits are the same except for that the limited ponding area within the suspended pavements.
You used a sandy bioretention soil in your monitoring study. How do you think the suspended pavement system you monitored would have performed with a loam soil instead?
We would expect more bypass with a loam soil since the infiltration rates are typically lower than the sandier mixes we used in North Carolina.
You are planning a follow up study to your thesis project. What will you be looking at?
We are going back to monitor the same two suspended pavement systems that we monitored 3.5 years ago to see if and how stormwater benefits have changed. We suspect nitrate removal may be greater now, for example, since nitrate removal depends on microbes and microbial populations are expected to increase with time. We will also be looking into how and what maintenance has been done on the Wilmington sites over the last few years.
As an Extension Engineer, do you often hear questions on suspended pavement SCMs with trees? Do most people know what they are?
It is definitely a new thing for most folks. On the landscape architecture side, trees and suspended pavements or structural soils are familiar to most practitioners. On the stormwater side (engineering and public works), people are trying to understand how it works, how they could use this type of green infrastructure, and where.
What are the most comment misconceptions you encounter about suspended pavement SCMs and trees?
People don’t realize how large a drainage area they can treat, especially where rainfall is not intense, like in the Pacific Northwest. A subsurface SCM with a small footprint can be quite successful at controlling and treating runoff from low intensity events.
If you could tell an engineer one thing about suspended pavement SCMs and trees, what would it be?
I tell them it’s just like bioretention but under pavement, because most stormwater engineers are familiar with and understand bioretention. The treatment mechanisms are the same.
What new developments or innovations do you believe could significantly improve the performance of urban tree SCMs?
Including additional storage above the soil media would help increase runoff volume control.
How do you envision the future of urban tree SCMs?
With many cities and municipalities moving toward green infrastructure, I see suspended pavement SCMs being very useful in dense urban areas, with no room for traditional bioretention. Suspended pavement systems combine pavement stability, urban tree growth, and stormwater treatment in a single installation.
Nathalie Shanstrom is a landscape architect with the Kestrel Design Group.