Urban Plants & Soil as Stormwater Management Workhorses
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.
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.
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