California Department of Transportation
 

LID Sidewalk Stormwater Tree Trench

 

Tree Trench
Tree Trench.  Photo Source: Architectural Record
                                                                               
Stormwater Tree Trench
Tree Trench Schematic.  Source: Philadelphia Water Department, Green Stormwater Infrastructure Tools

Sidewalk Stormwater Tree Trench:

Another space saving LID solution to manage street stormwater runoff is the sidewalk stormwater tree trench.  A stormwater tree trench runs underneath the sidewalk parallel to the curb.  On the surface it looks like an ordinary sidewalk with trees planted in tree wells.  Below the surface, structural soils, gravel, and/or modular structural cells form a continuous trench that stores and infiltrates stormwater. Landscape Architects must work closely with their Project development Team to identify opportunities to include sidewalk stormwater tree trenches.

Stormwater from the street is diverted to the trench through curb-cuts.  Runoff from sidewalks can be diverted to the trench through permeable pavers between tree wells.  If the capacity of the trench is exceeded, runoff can be directed to bypass the trench and flow into an existing storm drainage system.

In addition to detaining and infiltrating stormwater runoff below grade, the stormwater tree trench provides much better growing conditions for street trees than conventional tree wells.  Tree roots have access to air and water stored in open pore spaces in the soil and gravel in the continuous trench.   The canopies of street trees reduce stormwater flow intensities by intercepting and slowing down rainfall.  Evapotranspiration (water evaporated through leaf surfaces) further reduces the volume of stormwater stored below grade.

Below are links to design guidance and construction details for sidewalk stormwater tree trenches. 

Philadelphia has design guidance for stormwater tree trenches, Philadelphia Water Department, Green Stormwater Infrastructure Tools, Stormwater Tree Trench- http://www.phillywatersheds.org/what_were_doing/green_infrastructure/tools/stormwater_tree_trench

Modular structural cells, such as Silva Cells from Deep Root or Shasta Cells from Citigreen, provide structural support for the pavement system over the trench.
http://www.deeproot.com/products/silva-cell/applications.html

http://www.citygreen.com/products/structural-cells/

Urban Horticultural Institute, Structural Soils:
http://www.hort.cornell.edu/uhi/outreach/csc/article.html

          

Deep Root
Tree Trench Schematic by Deep Root Partners, LP

 

Modular Structural Cells
Modular Structural Cells by CityGreen