Urban Plants & Soil as Stormwater Management Workhorses

Shoemaker Green at the University of Pennsylvania, designed by Andropogon Associates with stormwater engineering by Meliora Design. Photo credit Barrett Doherty
Shoemaker Green at the University of Pennsylvania, designed by Andropogon Associates with stormwater engineering by Meliora Design.
Photo credit Barrett Doherty

When high-intensity rainfall events roll through cities, particularly those with combined sewer systems, peak flows increasingly overwhelm grey infrastructure, compromise water quality, and induce sedimentation and erosion. New research suggests that engineered soil and purposely selected plants within green infrastructure may help offset these flows by offering more benefit than most stormwater engineering models and municipalities acknowledge.

A handful of progressive entities – like the Chesapeake Stormwater Network and the Commonwealth of Virginia – now award extra stormwater credit for management approaches that deploy high-performance engineered soils, dense and varied planting palettes, or an inter-connected series of green infrastructure elements. More research is needed, however, to mobilize engineers, designers, and policy makers to rely more heavily on the “green” in green infrastructure.

To quantify the contribution of engineered soil and plants in stormwater management, an ongoing, five-year monitoring study is being conducted in Philadelphia at Shoemaker Green – a 2.75-acre urban park built on top of a former tennis court complex at the University of Pennsylvania’s campus. Heading into its fourth year of investigation, this study has found that the site manages more than three times the stormwater volume than the municipally-required engineering models predicted. These results suggest that stormwater performance is attributed to multiple, interconnected factors:

  • Residence time: The site’s engineered soil layers and underlying sand storage bed allow rainwater and air conditioning condensate from adjacent buildings to remain in detention for an extended period. Since the majority of the system drains horizontally, like a green roof, rather than infiltrating into underlying subsoil layers, the water’s residence time was relatively malleable during design.
  • Moisture holding capacity & root depth: The sandy-loam soil was engineered to withstand the programmed uses and have a high moisture holding capacity, particularly within the root zone. Moisture throughout this zone encourages robust, deep root growth, thereby allowing plants to both withstand drought and transpire water from a thick, saturated soil profile.
  • Plant selection: The site’s native plant palette, inspired by the Delaware River Terrace and Piedmont Uplands ecoregions, proved surprisingly efficient at releasing water back into the atmosphere. Transpiration measurements of young vegetation, taken with a porometer to measure stomatal openings, demonstrated that native floodplain species, such as a young swamp white oak (Quercus bicolor), transpired up to 35 gallons of water per day per tree during the growing season.  The uncompacted lawn also proved to be a transpiration workhorse.
  • Stormwater storage & re-use: A 20,000 gallon cistern below the rain garden provides fodder for the site’s rainwater re-use irrigation system. The irrigation system allows for recirculation of water to maximize evapotranspiration and support the health of the plant material.
Shoemaker Green's water balance diagram, showing the site's urban water cycle. Photo credit: Andropogon Associates
Shoemaker Green’s water balance diagram, showing the site’s urban water cycle.
Photo credit: Andropogon Associates

Future studies that examine engineered soil and native, flood-tolerant vegetation’s ability to manage stormwater are paramount in arming designers, engineers, and policy makers with the knowledge that’s necessary to more effectively manage urban stormwater conditions. Few studies have monitored green infrastructure and best management practices over multiple years, particularly in urban settings.  Even fewer efforts exist that compare these types of projects to one another. Additional research around soil storage and transpiration rates of individual plant species is needed to foster a more robust understanding of stormwater performance of urban, green infrastructure. With this body of knowledge, we can be more effective advocates for regulating green infrastructure, incentivizing implementation, and increasing cost effectiveness of high-performance urban landscapes.

More about this project:
Andropogon Research
Meliora Design

Additional Research:
Bartens, J., Day, S. D., Harris, J. R., Wynn, T. M., & Dove, J. E. (2009). Transpiration and root development of urban trees in structural soil stormwater reservoirs. Environmental Management, 44(4), 646-657.

Christianson, R. D., Brown, G. O., Barfield, B. J., & Hayes, J. C. (2012). Development of a bioretention cell model and evaluation of input specificity on model accuracy. Transactions of the ASABE, 55(4), 1213-1221.

MacDonagh, P. (2015). The Urban Forest Is Broken: How We Can Enhance 1,000,000 Tree Initiatives to Meet Stormwater Goals. Low Impact Development Technology, 182.

Peters, E. B., Hiller, R. V., & McFadden, J. P. (2011). Seasonal contributions of vegetation types to suburban evapotranspiration. Journal of Geophysical Research. Biogeosciences, 116(1) doi:http://dx.doi.org/10.1029/2010JG001463.

Schueler, T. & Lane, C. (2012). Recommendations of the Expert Panel to Define Removal Rates for New State Stormwater

Performance Standards. Retrieved from http://chesapeakestormwater.net/bay-stormwater/urban-stormwater-workgroup/performance-standards/ Whitlow, T.H. and N.L. Bassuk. 1988. Ecophysiology of urban trees and their management –The North American experience. HortScience 23:542– 546. Whitlow, T.H., N.L. Bassuk, and D.L. Reichert. 1992. A 3-year study of water relations of urban street trees. J. Appl. Ecol. 29:436–450.

Virginia DEQ. (2011).  Stormwater Design Specification No. 9. Retrieved from http://www.vwrrc.vt.edu/swc/NonPBMPSpecsMarch11/VASWMBMPSpec9BIORETENTION.html

José Almiñana, FASLA, PLA, LEED AP, AlminanaJ@andropogon.com
Emily McCoy, ASLA, PLA, MccoyE@andropogon.com
Lauren Mandel, ASLA PLA, MandelL@andropogon.com

5 thoughts on “Urban Plants & Soil as Stormwater Management Workhorses

  1. Lauren Mandel July 21, 2016 / 2:38 pm

    Thank you, Jim! We think this topic is relevant to urbanites and urban stormwater policy alike.

  2. Trevor Kimball October 7, 2016 / 9:01 pm

    I’m not sure if this is the right place to post this, but I wanted to get some feedback on issues related to stormwater infiltration/management in planting beds along a roadway, especially in the Intermountain West region. I figured this could be a good forum to possibly get some answers.

    1. What is the right way to deal with high clay content in soil for infiltration areas? What are appropriate procedures for amending the existing soil in these types of situations?
    2. How do you deal with chemicals from roadway runoff, especially high salt concentrations from winter road maintenance on a busy street? If irrigation is in place (for use when needed), is there procedure for “flushing” these types of contaminants through the soil? Or is it simply handled by selecting the appropriate plants for the situation, with the expectation that they will be resilient enough to withstand such chemicals?

    I apologize if these seem to be simple or naïve questions. I have read a lot of information concerning green infrastructure, but don’t have a lot of experience with it and am trying to become more educated so I can have the correct approach when working on these types of projects. I would appreciate anyone’s insight who may have expertise in this area.


    • Jim Urban October 26, 2016 / 5:35 pm

      Trevor On clay soils, There is emerging research that suggest that fracturing these soils to a depth of about 24 inches and then tilling the top 8″ really increases the permeability and reduces runoff of these soils. in your area I would also apply compost when tilling. If you send me an email to jimtree123@gmail.com I will send you the papers. On salt, your biggest issue is low rainfall means less flushing, and flushing along with good soil in the bed is critical. Be sure to include under drainage lines in all beds is the best recommendation along with using plants that have a good track record of salt tolerance.

  3. Jim Urban November 28, 2016 / 4:17 pm

    Trevor: Sorry it took me so long to respond. Here is my take on your questions.
    1. Clay soil: Simply fracturing, deep tilling, ripping, lofting what ever we should be calling it will make clay soils excellent soils for infiltration. My method id to use a back hoe and dig into the soil to about 24 inches and then drop the soil back into place. There is an excellent paper on using clay soil that was presented at the ASLA conference in Chicago “Amending Site Soil to Enhance Infiltration on Compacted Urban Sites” The actual paper reference is “Controlling Storm water Quality and Quantity by amending soils for enhanced infiltration on construction sites in North Carolina Richard McLaughlin et all April 28, 2014. This study only looked at runoff and they were not looking at plant growth. I would add a 3” layer of compost to the fractured soil and surface till it to improve plant growth if the soil is low in organic matter (less than 2% in my areas but in the intermountain areas a much lower SOM may be fine. A second study Below Ground Matters: Urban Soils rehabilitation increases tree canopy and speeds establishment, Lyman et all Urban Forestry & Urban greening, 16 (2016) 25-35, looks at a similar soil fracturing concept on tree growth. This paper did not look at infiltration but there is a strong link between infiltration and plant growth. If you get one you will get the other. There are several other research projects that examine reuse of existing soils on other soil types.
    The easiest way to get the papers is to send me an email to Jimtree123@gmail.com and i will send you copies ( and anyone else who responds to this post).
    2. Chemicals from road ways particularly salt. There is a section in my book “Up by roots” on salt (pare 64-65 and 314) that should guide your thinking on a wide range of salt issues. The most simplistic answer I can offer is that you need to identify the extent of the problem. I find for example that in dense retail environments where normal road salt and sidewalk salt are combined with additional salt spread by business owners the problem is the most severe and a multi range approach is needed. Road soil by itself is not too severer if the soil is well draining an not compacted. This can most often be felt with by better plant choices. But road salt that becomes airborn and drifts can impact a wide range of plants.

    Hope this helps.
    Jim Urban

    • Trevor Kimball January 11, 2017 / 6:09 pm

      Thanks for your reply, I appreciate your input! I will send you an email to get the papers you mentioned, that should be helpful.

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