How do the stormwater benefits of Silva Cell suspended pavement tree/soil systems compare to the stormwater benefits of traditional bioretention systems? Since the mechanisms by which the tree and soil provide the stormwater benefits in these systems are the same as those in traditional bioretention, we expected the benefits to be comparable as well. The final results from the Silva Cell stormwater treatment performance monitoring study in Wilmington, NC show that tree/soil systems with Silva Cells can indeed provide stormwater benefits equal to (or better) than traditional bioretention systems.
While the Silva Cells themselves do not directly provide stormwater benefits, the Silva Cells make it possible for the “workhorses” – the trees and rootable soil – to provide stormwater benefits even in ultra urban environments. They protect soil under paving, up to loads of up to HS20, from compaction – thus allowing the soil can to grow healthy plants, and both the soil and the plants to provide on-site stormwater benefits. The study, which was conducted by William Hunt, Jonathan Page, and Ryan Winston, all with North Carolina State University, shows that soil and roots under pavement that are protected from compaction by Silva Cells can indeed provide comparable stormwater benefits equal to traditional bioretention systems.
Before I share the final results, here is a recap of the experimental setup from previous blogs sharing preliminary results:
- The systems were installed on two adjacent residential streets (see map of study site shown below):
- Two tree/soil/Silva Cell systems are installed with slightly different soil media: one has 3% organic matter by weight, while the other one has 6% organic matter by weight:
Table 1: Fill media summary (Page et al., 2013)
|Media Gradationa||Orange Street Site||Ann Street Site|
a - Gravel, sand, silt and clay gradations are by volume; organic content is by weight
- Each system has 700 cubic feet of soil and a 2 cell deep Silva Cell system: 68 Silva Cell frames plus 34 decks.
- Both systems are lined with a pond liner
- Runoff from the street is directed into the systems via a catch basin with a sump into a distribution pipe into the Silva Cell systems (see A on the image below)
- Underdrains with upturned elbows slow water and then direct runoff into the City’s existing storm sewer system (see B on the image below).
- Wilmington has an average annual precipitation of 57.61 inches, and a mean temperature of 64.0 degrees F (data from NOAA; Period used to compute averages and normals: 1981-2010)
Contributing drainage areas to each Silva Cell/Tree/Soil system are summarized below:
Drainage area to each of these Silva Cell/Tree/Soil systems is significantly greater than for a typical installation. In a typical installation, there would be more than just one tree capturing runoff from this watershed. If, for example trees were spaced 30’ apart, there would be 10 trees capturing the runoff that is captured by only one tree at these sites. If the trees were 30’ apart, and the street was 22’ wide from crown to curb, the watershed for each tree would be 660 s.f. per tree, almost 10 times smaller than the watersheds here (5231 s.f. and 5663 s.f., respectively for Orange and Ann Streets).
Table 2 shows water quality results from the Wilmington Silva Cell installations compared to the mean from bioretention systems in peer reviewed literature per Page et al (2013).
For all of the pollutants monitored, the Silva Cell systems performed better or about the same as the mean for bioretention systems in peer reviewed literature. Of particular note, the Silva Cell systems continued to show good nutrient removal. This is especially encouraging because, while most bioretention systems show excellent TSS and metals removal, some bioretention systems have shown leaching of nutrients out of the compost in the bioretention soil. Controlling nutrients in bioretention effluent is especially important to protect water quality of receiving water bodies: phosphorus is typically the limiting nutrient in fresh water systems, and nitrogen is typically the limiting nutrient in salt water bodies.
Whereas the mean total phosphorus (TP) removal rates for the Ann Street and Orange Street Silva Cell Systems were 72% and 74%, respectively, the mean from the peer reviewed literature for traditional bioretention systems in Page et al (2013) was a 70% increase in TP! The Ann Street and Orange Street systems also had 70% and 82% phosphate-removal rates respectively (no peer reviewed mean concentration given in Page et al (2013)).
The mean nitrate removal rates for the Ann Street and Orange Street Silva Cell systems were 35% and 60% respectively, while the mean from the peer reviewed literature for traditional bioretention systems in Page et al (2013) was a 14% increase in nitrates!
The exceptionally high nitrate removal is likely due to the upturned elbow and impermeable liner. This is because the extended saturated conditions are thought to allow anaerobic conditions to occur, leading to denitrification which transforms nitrite/nitrate to nitrogen gas – therefore total nitrite/nitrate and total nitrogen concentrations in the effluent are decreased.
Even though the Ann St. soil had twice as much organic matter as the Orange St. soil, there was no significant difference between the effluent nitrate concentration of the two sites. While compost is generally thought to leach nitrogen (and phosphorus), the pine bark used in the Wilmington soil mixes does not appear to be leaching nitrogen or phosphorus.
A typical Silva Cell system provides volume benefits through infiltration and peak reduction, as well as interception and evapotranspiration by the tree. The Silva Cell installation at Wilmington is lined with an impermeable liner, so no infiltration is possible. Because of the sandy underlying soils, large infiltration benefits would be expected at this site without the liner. However, if all the water from these systems infiltrated directly into the underlying soils, it would not be possible to collect water quality samples. Therefore, even though huge volume benefits would be attainable without the liner, the researchers chose to line the system, eliminating infiltration, in order to ensure it would be possible to monitor water quality benefits. Tree interception and evapotranspiration are minimal for small, newly planted trees, but increase dramatically as the tree ages. As a result, the only volume benefit expected from the newly planted Wilmington Silva Cell installations is peak reduction. Peak reduction for storms that did not generate bypass, peak flow reduction at the Ann Street site was significant: 62%.
In conclusion, monitoring results from two tree/soil/Silva Cell systems in Wilmington, North Carolina, show that the Silva Cell systems performed better or about the same as the mean for bioretention systems in peer reviewed literature for total suspended solids [TSS] and heavy metals. Unlike some bioretention systems, which leach nutrients, these two tree/soil/Silva Cell systems also provided nutrient removal. Tree/soil/Silva Cell systems have now been shown to be a viable option to provide sustainable stormwater management in ultra urban areas, by providing tree rooting volume under paved areas with loads up to HS20 loading, where space does not allow for traditional bioretention systems.
Page, J.L., R.J. Winston, and W.F. Hunt, III. 2013. Draft Field Monitoring of Two Silva Cell™ Installations in Wilmington, North Carolina: Preliminary Monitoring Report.
Top image Flickr credit: Matt N. Charlotte