Stormwater Tree Trenches

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

Overview

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:

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

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 . For more detailed information on planning (site) considerations see Bioretention.

Planting in slopes

Smooth slopes should be amended into localized terraces by the Landscape Architect when planting large trees into slopes > 5 %. . Contributing slopes should be between 1-5%. The bottom of the trench and distribution pipes should be graded flat to allow water to spread out.

Design

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

Soil cells

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

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

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.

• Trees: List
• Shrubs: List
• Perennials: List
• Graminoids: List

Planting pit sizing

Bioretention: Sizing

Underdrain

Underdrain

Performance

Interception

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).

• http://www.mdpi.com/2072-4292/9/11/1202/pdf
• http://lfs-mlws.sites.olt.ubc.ca/files/2014/10/an_analytical_model_of_rainfall_interception_by_urban_trees.pdf
• https://www.nrcan.gc.ca/earth-sciences/land-surface-vegetation/biophysical-parameters/9162
• https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999WR900003

Transpiration

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

• http://www.wastormwatercenter.org/tree-resources/
• http://www.itreetools.org/eco/international.php
• http://treesandstormwater.org/
• http://cvc.ca/wp-content/uploads/2016/06/CaseStudy_CPW_Final.pdf
• https://cvc.ca/wp-content/uploads/2016/06/TechReport_CPW_Final.pdf

Galleries

• 
• 

  Birch trees in parking lot bioretention 'IMAX site' Mississauga, ON

• 

  Trees in parking lot bioretention 'IMAX site' Mississauga, ON

Open tree pits

• 

  Rainwater harvesting cistern, which discharges to tree pits during dry conditions. Image credit Mississippi WMO

• 

  Extended tree pit planting in USA
  Photo credit: USEPA

• 

  A right of way bioswale installed by the New York City Department of Environmental Protection on Dean Street, Brooklyn

Soil cells

• 

  Soil cells under construction.

• 

  Soil cell installation on Central Parkway in Mississauga.

• 

  Built in 2014, this stormwater planter on Central Parkway in Mississauga uses supported soil cells and is functioning well.

• 

  Soil Cell installation along the Moynes and Berl Avenues on north side of The Queensway in Toronto. The picture depicts stormwater distribution pipes through the system, used to help provide water to the trees that will be later planted in the BMP feature.
  Photo credit: City of Toronto
  

• 

  Silva Cell “top” deck prior to organic layer and screenings for pavers. Photo credit: City of Toronto

• 

  Soil Cells being installed along Edgewater Drive for Waterfront Toronto.
  Photo credit: DeepRoot
  

• 

  Stormwater Tree Trenches at East Bayfront Promenade, Toronto.
  Photo credit: DeepRoot
  

• 

  Seawall Soil Cells (Tree Trenches) located in Vancouver.
  Photo credit:City of Vancouver
  

• 

  Stormwater tree trenches designed with a structural concrete pad over top of the installation.
  Photo credit:DeepRoot
  

External links

• http://gievidencebase.botanicgardens.sa.gov.au/contents/7-water-management#Canopy-Interception
• standard details and specifications from James Urban

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.

• CityGreen - Stratavault
• Contech - Filterra
• Cupolex
• Deeproot - Silva Cell
• GreenBlue Urban - RootSpace
• Storm-Tree

---

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Land owned by the City Toronto or any other level of government or public agency (such as Toronto and Region Conservation Authority, Toronto Transit Commission, school boards).

A perforated pipe used to assist the draining of soils.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A perforated pipe used to assist the draining of soils.

The interception, storage and eventual evaporation of rainfall from vegetation canopies.

The interception, storage and eventual evaporation of rainfall from vegetation canopies.

Any form of rain or snow.

Tank used to store rainwater (typically roof runoff) for later use.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.