Gravel Bed Stormwater Detention System for Growing Trees in Extra-urban Areas

The purpose of this demonstration project was to show the potential of an inexpensive, gravel-based stormwater detention system to store water for use by trees in ultra-urban areas without sacrificing needed parking space.

Typically in smaller communities parking lots take priority over tree canopy cover. Reduced tree canopy cover increases stormwater runoff and may not be very attractive to visitors. Providing adequate soil volume for tree growth can be expensive in highly impervious areas. The purpose of this demonstration project was to show the potential of an inexpensive, gravel-based stormwater detention system to store water for use by trees in ultra-urban areas without sacrificing needed parking space. This Green Stormwater Infrastructure system can maximize water-storage capacity by using gravel, which has greater pore space than soil (compacted or non-compacted), while providing increased belowground rooting volume for trees. Storing water in this way allows for greater infiltration into the existing soil layer helping to improve water quality downstream and has the potential for creating a more natural hydrologic runoff pattern in extra-urban areas.

Larger trees have greater leaf area which drives environmental services (i.e. energy conservation, reduced urban heat island, air pollution removal, stormwater runoff mitigation, etc.). Trees need an adequate volume of mineral soil to reach their full growth potential. Tree roots grow where there is water, oxygen, and nutrients. Stormwater runoff typically has essential nutrients in it for trees to utilize (i.e. N and P). It has yet to be determined how this gravel system impacts tree growth. Even if trees planted in this type of system are smaller than trees growing in a larger mineral soil bed, having some canopy cover is better than nothing to provide environmental benefits and attract potential visitors for the local economy.

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