Stormwater Tree Trenches

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This is an image map of an extended tree pit, clicking on components will load the appropriate article.

Overview[edit]

Stormwater Tree Trenches are linear tree planting structures that feature supported impermeable or permeable pavements that promote healthy tree growth while also helping to manage runoff. They are often located behind the curb within the road right-of-way and consist of subsurface trenches filled with modular structures and growing medium, or structurally engineered soil medium, supporting an overlying sidewalk pavement. They improve tree health by providing access to soil, air and stormwater for irrigation, allowing them to survive longer in harsh urban conditions.

They also provide road and walkway drainage, contribute to stormwater pollutant removal and decrease the volume of urban runoff entering local waterways. They feature trees, soil, stormwater inlet and outlet structures, distribution and drainage pipes, and may include soil support structures, structural soil medium or structural concrete panels (as seen in the image map to the right). The tree planting pits and adjacent supported sidewalk pavements provide more soil volume for tree growth and water retention.


Take a look at the downloadable Stormwater Tree Trenches Fact Sheet below for a .pdf overview of this LID Best Management Practice:

Treetrench.png


Stormwater tree trench installations include:

  • Trees (tolerant to northern. urban conditions)
  • Planting soil
  • Structural soil
  • Stormwater inlet and outlet structures,
  • Distribution and drainage pipes
  • Mulch
  • Aggregate Base
  • Choker layer


Stomwater tree trenches are ideal for:

  • Sites with limited space for other surface stormwater BMPs that also possess primarily impermeable coverage (i.e. a municipality's "right-of-way", which includes the edge between private/public property, roadways, sidewalks and utility service land use)
  • Areas with limited greenspace
  • Projects with high traffic loads (pedestrian and vehicular), laneways, pedestrian plazas and walkways


Additional components may include:

  • an underdrain system
  • various configurations of soil cell additions
  • geotextile fabric around soil cell structures
  • impermeable geomembrane on sites where soil infiltration is not desired
  • monitoring wells and clean-outs
  • structural panels
  • permeable pavers

Planning considerations[edit]

A commonly held view is that a tree's root system will be similar to it's visible crown. For many trees, this is not the case, as roots will more often spread much more widely, but to a shallower depth [1]. For more detailed information on planning (site) considerations see Bioretention

Planting in slopes[edit]

Smooth slopes should be amended into localised terraces by the Landscape Architect when planting large trees into slopes > 5 %. [2]

Design[edit]

There are many design configuration options for including trees into stormwater management plans.

Soil cells[edit]

These are (usually plastic) supporting structures placed around the trees and beneath adjacent paved areas. They prevent compaction to the roots of the tree and prevent root damage to the paving. They are sometimes configured to receive stormwater and to enclose ponded water which can then infiltrate the soil surrounding the tree. Things to consider in design:

  • If the system is unlined it is hydraulically equivalent to a bioretention cell and provides similar water quality benefits.
  • If the system is lined and underdrained it is hydraulically similar to a large stormwater planter. Depending on the design detail it may retain significant stormwater within the planting soil volume and will provide water quality benefits.

Inlets[edit]

A surface inlet configuration featuring a depressed drain routing water collected from the street to an enclosed area infiltrating water to soil cells underneath.

Multiple methods for distribution and conveyance of runoff into the system are recommended for redundancy and conservative designs. Combinations may be made of:

  • tree well flow,
  • catchbasins and distribution pipes, and
  • direct infiltration from permeable paving.

See also Inlets and pretreatment

Species selection[edit]

For a detailed overview with in-depth information on species selection please visit our Plant lists wiki page and you can continue on to our further detailed tables showcasing how to develop planting plans that include selection of species for specific functions in several stormwater tree trenches and other LID practices.

Planting pit sizing[edit]

Bioretention: Sizing

Underdrain[edit]

Underdrain

Performance[edit]

Interception[edit]

Tree canopies intercept and store rainfall, thereby modifying stormwater runoff and reducing demands on urban stormwater infrastructure (Xiao et al., 1998; Xiao et al., 2000; Xiao and McPherson, 2002; Xiao et al., 2006). Canopy interception reduces both the actual runoff volumes, and delays the onset of peak flows (Davey Resource Group, 2008).

The extent of interception is influenced by a number of factors including tree architecture and it has been estimated that a typical medium-sized canopy tree can intercept as much as 9000 litres of rainfall year. (Crockford and Richardson, 2000).

A study of rainfall interception by street and park trees in Santa Monica, California found that interception rates varied by tree species and size, with broadleaf evergreen trees provided the most rainfall interception (Xiao and McPherson, 2002). Rainfall interception was found to range from 15.3% for a small jacaranda (Jacaranda mimosifolia) to 66.5% for a mature brush box (Tristania conferta now known as Lophostemon confertus). Over the city as a whole the trees intercepted 1.6% of annual precipitation and the researchers calculated that the annual value of avoided stormwater treatment and flood control costs associated with this reduced runoff was US$110,890 (US$3.60 per tree).

Transpiration[edit]

Trees suck! (Abstracted from Phyto, by K. Kennen)

Galleries[edit]

Open tree pits[edit]

Soil cells[edit]

External links[edit]

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.


  1. Crow, P. (2005). The Influence of Soils and Species on Tree Root Depth. Edinburgh. Retrieved from https://www.forestry.gov.uk/pdf/FCIN078.pdf/$FILE/FCIN078.pdf
  2. Wilkus A., 'Slope Style', Landscape Architecture Magazine, April 2018, accessed 21 December 2018, https://landscapearchitecturemagazine.org/2018/04/24/slope-style/