Urban site development often results in compacted soil, increased impervious surfaces, and decreased vegetative cover. These changes result in more stormwater runoff, which degrades water quality and stream ecosystems.
An interdisciplinary collaboration of researchers from Virginia Tech, Cornell University, and the University of California at Davis is working to protect water resources by integrating trees into urban stormwater management systems.
The group, led by Susan Day, assistant professor of urban forestry in the departments of forest resources and environmental conservation in Virginia Tech's College of Natural Resources and horticulture in the College of Agriculture and Life Sciences, has released the manual Managing Stormwater for Urban Sustainability Using Trees and Structural Soils. The manual outlines four years of research on a unique system that uses structural soils, which are engineered tree soils, to both detain stormwater and allow tree root growth in confined urban spaces. By utilizing tree root and canopy growth to manage stormwater, this new approach more closely resembles the natural water cycle than traditional stormwater management techniques.
Soil cannot serve its natural function of absorbing and filtering stormwater when it is paved over or made impervious through soil compaction. As water flows over land, it collects pollutants such as sediment, oil, and metals. If water is not absorbed by soil, the contaminated runoff eventually makes its way to water bodies that are used for drinking or recreational activities.
In addition to contributing to runoff, compacted soils also limit tree root health and growth, especially for sidewalk and parking lot tree plantings. This amplifies the problem because trees assist in stormwater management in many ways, such as:
The stormwater management system described in the manual directs water under pavement to a structural-soil reservoir, where it is held until it either infiltrates into the underlying soil or is absorbed via tree transpiration. Structural soils are designed to support the weight of pavement and vehicles while allowing for adequate root growth, and to facilitate the infiltration, storage, and recharge of stormwater.
These engineered soils can be used beneath either pervious surfaces (such as turf or permeable paving) or impervious ones (concrete, blacktop, for instance) to increase the available area for tree roots to grow and to act as a stormwater reservoir and recharge zone.
“The end result? Stormwater remains on site and trees have greater rooting volume, which means they can attain greater size and intercept more stormwater,” Day said. “This is the first system that I am aware of that directly integrates full-size trees into a stormwater management facility.”
In order to test the stormwater management system and evaluate its efficiency, the team set up demonstration sites at the three participating universities. There are two demonstration sites in Blacksburg:
As proof of the system’s effectiveness, the demonstration sites have had zero runoff. Not only does this system more closely resemble the natural water cycle, it takes up less space and is safer than traditional pond systems.
This project was made possible in part by a grant from the United States Department of Agriculture Forest Service Urban and Community Forestry Program on the recommendation of the National Urban and Community Forestry Advisory Council.
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