Stormwater Tree Trenches

A Stormwater tree trench with structural concrete panels and soil cells. A structural concrete panel configuration features reinforced concrete panels supported by concrete footings and rows of modular soil support structures. Modular soil support structure rows are installed on layers of compacted aggregate material and filled with growing medium. Benefits of this configuration are improved adaptability around utilities, improved ease of utility access and repair tasks, and greater soil volume achieved per unit area compared to modular soil support- and structural soil medium-filled trench configurations. Preventing compaction of underlying native subgrade soil in the area between the aggregate base layer rows provides greater opportunity for drainage via infiltration. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

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 Trench Fact Sheet below for a .pdf overview of this LID Best Management Practice:

Stormwater tree trench installations include:

• Overlying impermeable or permeable pavements
• Trees (tolerant to northern urban conditions)
• Planting soil
• Modular soil support or "soil cell" structures (optional)
• Structural soil (optional)
• Structural concrete panels (optional)
• Stormwater inlet and outlet structures
• Distribution and drainage pipes
• Choker layer (optional)
• Geogrid and geotextile (optional)
• Aggregate base

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:

• Internal water storage layer
• Distribution and underdrain pipe access and clean-out features
• Monitoring well screened within internal water storage layer
• Root barriers in locations where tree rooting is not desired

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 .

Infiltration

For information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration.

Site Topography

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.

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.

Wellhead Protection

Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas.

Water Table

Maintaining a separation of 1 m between the elevations of the bottom of the trench and the seasonally high water table, or top of bedrock, is recommended. Lesser or greater values may be considered based on groundwater mounding analysis. See Groundwater for further guidance and spreadsheet tool.

Native Soil

Tree trenches can be constructed over any soil type, but hydrologic soil group A and B are best for achieving water balance objectives. Facilities designed to infiltrate water should be located on portions of the site with the highest infiltration rates. Native soil infiltration rate at the proposed location and depth should be confirmed through in-situ measurements of hydraulic conductivity under field saturated conditions. For guidance on infiltration testing and selecting a design infiltration rate see Design infiltration rate.

Drainage Area

Typical contributing drainage areas are between 150 to 300 m² per tree, with a recommended maximum of 450 m² per tree. For optimal performance recommended ratios of impervious drainage area to pervious facility footprint area (I:P area ratio) range from 5:1 on low permeability soils (HSG C and D) to 15:1 on high permeability soils (HSG A and B).

Setback from Buildings

Tree trenches should be set back from the building far enough to allow for the tree canopy to grow to a healthy, mature size, depending on the species selected. A minimum setback of 4 m from buildings is recommended.

Overhead Wires

Tree trenches should be implemented with caution under overhead wires. If overhead wires conflict with proposed tree trench locations, check the height of existing wires, and choose small form trees that will not grow tall enough to interfere with wires.

Pollution Hot Spot Runoff

Tree trenches receiving road or parking lot runoff are not recommended in these areas.

Proximity to Underground Utilities

Designers should consult local utility design guidance for the horizontal and vertical clearances required.

Karst

Tree trenches designed to drain primarily by infiltration are unsuitable in areas of known or implied karst topography.

For a table summarizing information on planning considerations and site constraints see Site considerations.

Design

A surface inlet configuration featuring a depressed drain routing water collected from the street to an enclosed area infiltrating water to soil cells underneath. Source: Emmons & Olivier Resources

Things to consider in design:

• If the trench is unlined it is hydraulically similar to a full- or partial-infiltration design bioretention cell and should provide similar water quality benefits.
• If the trench features an impermeable liner and underdrain it is hydraulically similar to a large stormwater planter or no-infiltration design bioretention cell and should provide similar water quality benefits.
• Depending on design details tree trenches may retain a significant volume of stormwater within the planting soil and internal water storage layer and provide runoff volume reduction benefit.

Geometry and Site Layout

Tree trenches are continuous, linear urban tree planting systems, often located behind the curb within the road right-of-way and feature sidewalk pavement and tree openings on top. Trench sections are connected hydrologically through sub-surface stormwater distribution and drainage pipes.

Inlets

Water can enter the tree trench in a variety of ways: from the overlying sidewalk via sheet flow or curb cuts into tree openings, trench drains, or infiltration through permeable pavement; and from the road via distribution pipes connected to road or side inlet catch basins and curb cuts or depressed drains at tree openings. It is recommended that each tree trench have multiple inlets to keep the contributing drainage area relatively small, which provides redundancy to the system. Inlet structures and distribution pipes should be offset from tree root ball locations to avoid impact of de-icing salt laden runoff on newly planted trees during establishment.

Pre-Treatment

If water enters the trench via a catch basin, a removable pre-treatment device, like a Goss trap or proprietary catch basin insert device or filter should be included to help retain coarse sediment, debris and floatables and prevent it from entering the pipe or trench. Inlet structures should have a sump and curb cut inlets should include stone diaphragms or stone mulch to dissipate energy and spread flows. Pre-treatment features should be easy to access and clean out.

Soil Volume

Each tree planted should have access to a minimum 30 m³ of soil volume, including the growing medium within the tree pit and growing or structural soil medium below adjacent supported pavement. If more than one tree shares the same trench a minimum 20 m3 of soil per tree may be acceptable. It should be noted that structural soils are mostly filled with rock and will therefore have much lower soil volumes. However, trees have been found to grow reasonably well in these soils because roots only occupy a portion of the total soil medium.

Structural soils consist of 3 components, mixed in the following proportions by weight: a load bearing stone lattice, soil, and a tackifier. Soils may be clay loam or coarser textured soil if drainage is a priority. Common tackifiers include ‘hydrogel’, a coated potassium propenoate-propenamide copolymer) or ‘stabilizer’, a plant based organic product sourced from the US.

• Crushed stone (granite or limestone) should be narrowly graded, highly angular with no fines. Stone sizes may vary between 20 to 75 mm. In British Columbia, a larger 75 mm stone (range between 60 and 80 mm) is used because it was found to allow for larger soil volumes (up to 33% of the total soil medium volume).
• Geotextiles are used with structural soils to prevent migration of fines from the road or sidewalk base into the structural soils. In BC, a Nilex 4545 fabric is used for this purpose, but other fabrics may also be suitable.
• Compaction to 95% SPD is achieved in 1 m lifts to 95% SPD. Testing of compaction of levels is accomplished with a trolled nuclear densometer for larger rock size mixes.

Structural Soil Comparisons

Specifications for Stormwater Tree Trenches using Structural Soils

Structural Soil Type	Median Stone size/range	Soil Texture	Tackifiying Agent	Approximate Porosity
CU-Soil™	30mm (20-40mm)*	Gravel: <5% Sand: 20-45% Silt: 20-50% Clay: 20-40% Cation Exchange Capacity (CEC) >10 pH: 5.5 – 6.5 Organic Content: 2 – 5% by dry weight	Hydrogel (coated potassium propenoate-propenamide copolymer)	26%
B.C Soil	75mm/60 – 80mm)	Sand: 45-55% Silt: 25-35% Clay: 0 – 10% Silt + Clay: 25 – 45% pH: 6.0 – 7.0 Organic Content: 15-20%**	Stabilizer	33%
Note: "*" = Larger or smaller stone sizes are accepted as long as they do not comprise more than 10% above or 10% below the indicated range. "**" = Soil texture is the City of Vancouver specification for structural soils

Modular Soil Support Systems

Modular soil support systems (also referred to as “soil cells”) consist of plastic or concrete structures, available in a variety of shapes and sizes, that provide structural support for the overlying pavement while providing uncompacted planting soil within the tree root zone. They are installed adjacent to tree pits to provide room for roots to spread out under the supported pavement portion of the trench. Growing medium backfill typically has higher organic matter content than structural soil medium. The looser structure and higher nutrient content of the growing medium provides the most favorable environment for healthy tree growth in an urban setting. Critical to modular soil support system design is that each structure or layer of structures be independent of all adjacent ones, such that one or multiple layers can be removed to facilitate future utility installation or repair.

A profile view of a cast in place catch basin with sump, along with an attached Goss trap, used to trap and limit the amount of floatable materials, debris oils, hydrocarbons, etc. from entering the municipality's storm sewer system. This pre-treatment device attaches to the catch basin's outflow pipe. This image was sourced from the City of Toronto's Construction Specifications and Drawings for Sewers and Watermains.

Structural Soil Medium

Structural soil is an engineered soil medium that can be compacted to support sidewalk or roadway pavement installation requirements while also permitting tree root growth. Structural soil medium filled trenches are installed adjacent to tree pits to provide room for tree roots to spread out under the supported pavement portion of the tree trench. The available soil for root growth ranges from 25 to 33% depending on the stone size. Larger stone sizes will typically allow for greater soil volume.

Structural Concrete Panels

Trenches where the overlying sidewalk pavement consists of reinforced structural concrete panels is another configuration. Panels are supported on each side by concrete footings and rows of modular soil support structures or structural soil medium, installed on aggregate bases. The benefit of this approach is that the native subgrade soil under the portions of the trench below tree pits and between rows of supports does not need to be highly compacted, allowing greater opportunity for drainage via infiltration. (see image map within the 'Overview' section of this page).

Conveyance and Overflow

Runoff is directed from overlying and adjacent pavements to the trench through such means as tree openings, perforated distribution pipes connected to catchbasins or trench drains, or curb cuts and depressed drains to tree openings. Runoff water percolates through the growing or structural soil medium to the underlying native subgrade soil. When runoff volume exceeds the trench water storage capacity, the perforated underdrain pipe directs excess filtered water to a downstream outlet storm sewer or other practice. During intense storm events, runoff in excess of the infiltration capacity of the growing or structural soil medium will overflow to the storm sewer either through an outlet pipe connection in the catch basin or via surface overflow standpipes or structures within tree openings.

Configuration

Modular soil support system and structural concrete panel trench configurations should provide a better growing environment for trees, and thereby improve tree longevity. Structural soil medium and structural concrete panel trench configurations provide the benefits of being more adaptable around utilities and existing trees and providing easier access to utilities when repairs are needed. Structural concrete panel trench configurations featuring rows of modular soil supports provide greater soil volume per unit area than those featuring structural soil medium.

Distribution and Underdrain pipes

To maximize the quantity of growing or structural soil medium irrigated, distribution pipes should be installed flat, just below modular soil support tops or at the top of the structural soil media layer and in both tree pit and supported pavement portions of the trench. Pipe perforations should be oriented to the sides and section ends should be sealed with a solid cap. To enhance runoff volume reduction underdrain pipes can be installed above the bottom of the trench and/or include flow control. Alternatively, the underdrain pipe may be installed on trench bottom and connected to a riser assembly in the outlet manhole. It is critical to include connections to outlet storm sewer pipes and multiple cleanout access points.

Variations of Stormwater Tree Trenches

Below, find three alternate stormwater tree trench design configurations that differ by native subgrade soil permeability, structural support systems used (modular soil cell systems vs. structural soil medium) and elevation of the underdrain perforated pipe in the cross-section based on subgrade soil permeability.

Tree trench with soil cells on low permeability subsoil - This tree trench configuration features an overflow outlet storm sewer pipe connection in the catch basin and underdrain to allow excess water to leave the practice. The underdrain perforated pipe is embedded in the aggregate base due to the slow drainage rate of the subsoil. Solid standpipes connected to the underdrain and distribution perforated pipes provide access for inspection and maintenance tasks over the lifespan of the facility. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Tree trench with soil cells on high permeability subsoil - This tree trench configuration features an overflow outlet storm sewer pipe connection in the catch basin and underdrain to allow excess water to leave the practice. The underdrain perforated pipe is embedded in the growing medium which factors in the fast drainage rate of the subsoil. A monitoring well screened within the aggregate base of the trench is included so drainage performance can be evaluated over its operating lifespan. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Tree Trench with structural soil medium on high permeability subsoil - This tree trench configuration features structural soil medium as pavement support, as an alternative to soil cells that improves adaptability around utilities. An overflow outlet storm sewer pipe connection in the catch basin and underdrain are included to allow excess water to leave the practice. The underdrain perforated pipe is embedded in the growing medium which factors in the fast drainage rate of the subsoil. A monitoring well screened within the aggregate base of the trench is included so drainage performance can be evaluated over its operating lifespan. Note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

Specifications for Stormwater Tree Trenches

Material	Specification
Growing Medium	Should be Canadian Soil Classification System sandy loam with combined silt- and clay-sized content between 18-35%; and sand- to fine gravel-sized content (0.074 to 5 mm dia.) between 65-82%. pH value (6.0 - 8.0). Salt level < 2 mmhos/cm. Percent organic matter shall be 3-5%, by dry weight. Growing medium compacted to 80-90% below the tree root ball to prevent settling. Bioretention filter media may be suitable for use as growing medium, depending on climate and tree species (see Bioretention: Filter media).
Modular Soil Support System	Structures are designed to be filled with growing medium for tree rooting and support a vehicle loaded pavement up to and including AASHTO H-20 and Ontario Building Code standards for sidewalks. Critical to modular soil support system design is that each structure or layer of structures be independent of all adjacent ones, such that one or multiple layers can be removed to facilitate future utility installation or repair.
Structural Soil Medium	Structural soils are installed in the trench adjacent to tree planting pits under permeable or impermeable pavements. Structural soils consist of 3 components, mixed in the following proportions by weight: crushed stone (79.07%), clay loam soil (20%), and hydrogel tackifier (0.03%). Total moisture at mixing should be 10% as per AASHTO T-99 optimum moisture. Crushed stone (granite or limestone) should be narrowly graded from 20 to 40 mm diameter, highly angular with no fines. The clay loam soil should conform to the Canadian soil classification system (gravel <5%, sand 25-30%, silt 20-40%, clay 25-40%). Organic matter should range between 2 to 5% by dry weight. The hydrogel, a potassium propenoate-propenamide copolymer, is added in a small amount to act as a tackifier, preventing separation of the stone and soil during mixing and installation. Mixing can be done on a paved surface using front end loaders. Typically the stone is spread in a layer, the dry hydrogel is spread evenly on top and the screened moist clay loam soil is the top layer. Them entire pile is turned and mixed until a uniform blend is produced. The structural soil is then installed and compacted in 150 mm lifts.
Structural Concrete Panel	Structural concrete panel is 250 mm thick, contains rebar reinforcements and sits on equal-sized concrete footing supports on rows of modular soil support structures or structural soil medium, which are supported by a minimum 150 mm base of compacted granular material. Decompact native subgrade soil under tree openings and between granular bases of modular soil support structure rows during installation for better infiltration drainage performance.
Aggregate Base	Aggregates are used in modular soil support systems below the structures as the trench base layer, and sometimes, on top of the structures, as the pavement base layer. Specifications for aggregate base materials determined by the designing Engineer based on varying levels of structural loading and hydraulic requirements.
Geotextile & Geogrid	Geotextile, geogrid or combinations are typically used on top of modular soil support structures and along the sides of the trench to separate growing or structural soil mediums from native soil or aggregate backfill. Geotextile and geogrid should not be installed on sides adjacent to pervious landscaped areas to provide opportunities for tree roots to grow outside the trench in these locations. Geotextile material specifications should conform to Ontario Provincial Standard Specification (OPSS) 1860 for Class II geotextile fabrics. Geotextile, geogrid or combination products in contact with modular soil support system structures should be according to manufacturer’s specifications. Geotextile installed on tree trench sides and around perforated distribution and underdrain pipes should be woven monofilament or non-woven needle punched fabrics. Woven slit film and non-woven heat bonded fabrics should not be used as they are prone to clogging. Where a root barrier is needed to prevent the migration of roots out of the tree trench, use impermeable ribbed barrier material with a thickness of 1-2 mm.
Underdrain	Should be minimum 150 mm dia. perforated HDPE or equivalent material, smooth interior wall and continuously perforated with geotextile sock. A solid standpipe connected to the underdrain pipe and extending to the growing medium or pavement surface can be used for inspection and maintenance access. The top of the standpipe should be covered with a sealable cap or plug and secured with a vandal-proof fastener.
Stormwater Distribution Pipe	Perforated pipe should be minimum 150 mm dia. rigid, smooth interior wall HDPE or PVC with perforations on sides, wrapped with geotextile sock, with capacity and perforation specifications confirmed by the designing Engineer based on hydraulic requirements. Solid pipe from inlet structures should transition to perforated pipe once 300 mm inside the trench.

Benefits of trees

Stormwater tree trenches help support healthy street trees in urban settings where conventional plantings have limited space for root establishment. Trees play a critical role in stormwater management from reducing runoff through canopy interception, evapotranspiration, filtering out pollutants, and increasing infiltration capacity of soils, to retaining runoff., Trees also provide a myriad other environmental benefits, from shading impervious surfaces and thereby reducing urban heat island effects, to providing wildlife habitat and improving the aesthetics of streets and neighbourhoods. Research has shown that healthy trees increase property values, retail spending and contribute to a sense of community pride and safety.

Below is a list of trees that are known to tolerate conditions in northern (Zone 3) urban stormwater tree trenches.

Tree Species Known to Tolerate Conditions in Northern (Zone 3) Urban Stormwater Tree Trenches

Latin Name	Common Name
Ulmus americana	American Elm
Acer x freemanii	Freeman’s Maple
Alnus incana	White Alder
Celtis occidentalis	Hackberry
Gleditsia triacanthos var. inermis	Thornless Honeylocust
Gymnocladus dioicus	Kentucky Coffeetree
Quercus bicolor	White Oak
Quercus macrocarpa	Bur Oak
Quercus rubra	Red Oak

Species selection

For an overview of key considerations affecting planting plans and tree species selection please visit our planting design, plant selection and tree species wiki pages that provide guidance to help you develop functional and attractive planting plans.

Tree planting best practices

An extensive compendium of recommended standard tree planting details and specifications are available from James Urban. See the figure below that depicts the relationship between soil volume, water storage volume provided by the soil volume, and tree size from James Urban's (2008) book, entitled "Up by Roots"

You can also review Urban's presentation he gave at the University of Washington in 2014 about some of the lessons learned in his book here: Urban Soil and Site Assessment Presentation

Inspection and Maintenance

Tree trenches have fewer maintenance requirements than bioretention cells or bioswales, but maintenance is still critical to their success. The most critical maintenance task is the removal of trash, sediment and debris accumulated in inlet structure sumps, gravel diaphragms and tree openings at curb cuts. This should be done at least once per year, however the frequency will depend on pavement uses, traffic volumes and tree canopy size. Inspect new trenches closely during the first two years of operation to measure the rate of accumulation and set an optimal maintenance frequency.

Underdrains and distribution pipes within the tree trench must be designed for ease of maintenance. Pipe couplings should be no greater than 45 degrees to allow inspection and cleaning equipment to access it, with enough cleanout access standpipes or structures to access the full length of the pipe.

Tree care is also an important part of tree trench maintenance. Provide regular irrigation and weed control in the tree openings until newly planted trees are fully established. Prune trees as needed once established to prevent safety hazards to pedestrians, overhead utility lines, and adjacent buildings. Monitor trees for damage by insects and other pests and replace trees that are in decline. A tree trench containing a diseased or dying tree is not a fully functional practice.

See further details here: Stormwater Tree Trenches: Maintenance

Also take a look at the Inspection and Maintenance: Bioretention & Bioswales page by clicking below for further details about proper inspection and maintenance practices:

Performance

In a proof-of-concept study of two stormwater tree trenches in Wilmington, North Carolina, Page et al. (2015) found that the soil-root matrix beneath the supported pavements can be used for stormwater control to achieve runoff volume reduction (80% over a yearlong evaluation period), pollutant retention, pavement stability and urban forestry goals. To read about the use of stormwater tree trenches featuring soil cells in the Greater Toronto Area see the STEP technical brief and case study on the Queensway Sustainable Sidewalk Pilot Project in the City of Toronto. Evaluations of the project found that stormwater tree trenches are able to increase the urban street tree canopy coverage while requiring minimal surface area below, and provide stormwater benefits associated with TSS and heavy metal contaminant removal and runoff volume reduction, with lower routine maintenance costs than other surface practices like bioretention. In a hydrologic study of the Queensway Sustainable Sidewalk project, Li et al. (2020) highlight the importance of inlet hydraulics and spatial distribution of inflow along the stormwater tree trench and propose an integrated modelling approach to simulate overall runoff control performance. Also see Credit Valley Conservation Central Parkway LID case study and technical report that summarize findings from evaluation of a stormwater tree trench featuring soil cells located in the median of a high-traffic road in Mississauga, Ontario. Monitoring showed the stormwater tree trench performed well over the eight storm events monitored with an average runoff volume reduction of 97%, and peak flow reduction of 96%.

Ability for Stormwater Tree Trenches to Meet Stormwater Management Objectives

BMP	Water Balance	Water Quality	Erosion Control
Stormwater Tree Trench	Partial-based on on native soil infiltration rate and if flow restrictor is used	Yes-size for water quality storage requirement	Partial-based on native soil infiltration rate, available storage and if flow restrictor is used

Volumetric runoff reduction from Stormwater Tree Trench/Bioretention

LID Practice	Location	Runoff Reduction*	Reference
Bioretention without underdrain	China	85 to 100%*	Gao, et al. (2018)
	Connecticut	99%	Dietz and Clausen (2005)
	Pennsylvania	80%	Ermilio (2005)
	Pennsylvania	70%	Emerson and Traver (2004)
Bioretention with underdrain
	Texas	82%*	Mahmoud, et al. (2019)
	China	35 to 75%*	Gao, et al. (2018)
	Ohio	36 to 59%	Winston et al. (2016).
	Virginia	97 to 99%	DeBusk and Wynn (2011)
	Maryland and North Carolina	20 to 50%	Li et al. (2009).
	North Carolina	40 to 60%	Smith and Hunt (2007)
	North Carolina	33 to 50%	Hunt and Lord (2006).
Bioretention with underdrain & liner
	Ontario	15 to 34%	STEP (2019)
	Queensland, Australia	33 to 84%	Lucke and Nichols (2015).
	Victoria, Australia	15 to 83%	Hatt et al. (2009).
	Maryland	49 to 58%	Davis (2008).
Runoff Reduction Estimate*		85% without underdrain; 45% with underdrain | 30% with underdrain and liner

Performance research

Tree canopies influence various components of the urban hydrologic cycle. Water losses occur via canopy interception and evaporation, transpiration, improved soil infiltration and percolation along root channels, and water table management, thereby attenuating stormwater runoff and reducing demands on drainage infrastructure. Canopy interception loss is relevant during and immediately after a storm event, while transpiration plays a role in managing soil moisture over the days and weeks between events. Canopy interception contributes to runoff volume reduction, delays the onset of peak flows and helps protect water quality. Urban tree canopy interception and evaporation rates vary according to canopy type (e.g., closed vs. open), tree species attributes, season, and storm characteristics (e.g., rainfall intensity, duration and time between events). Berland et al. (2017), call for greater consideration of arboriculture as a stormwater control measure in their literature review, noting that trees are compatible with various types of LID facilities and may improve the function of these installations through evapotranspiration and maintaining or improving drainage performance. In a study of twenty tree species in Davis, California, Xiao and McPherson (2016) found that conifers generally stored more water than broadleaf deciduous species and that leaf surfaces have larger capacities to store rainfall than stem surfaces. Tree species with large mature sizes and high stomatal conductance (e.g., Quercus macrocarpa, bur oak) were shown to markedly improve the function of parking lot bioretention swales in the City of Chicago, Illinois.

Trees suck! Comparison of transpiration rates of various tree species and ages (Abstracted from Phyto, by K. Kennen)

For other recent research on the water management benefits of urban trees, and modelling approaches see the following articles and projects.

• Stormwater runoff volume reduction benefits of urban street tree canopy (Selbig et al., 2022)
  • In a paired-catchment study design involving medium density residential areas in Wisconsin, with removal of 29 mature green ash and Norway maple street trees as the treatment, tree removal resulted in a 4% increase in runoff volume over the evaluation period, while peak discharge was generally not affected. Runoff volume reduction benefit of the street tree canopy was estimated at 6376 L per tree, which is similar to values reported in previous studies based largely on simulation.
• Stormwater tree trench and bioswale performance in Vancouver, BC (Vega 2019)
  • A study of a stormwater tree trench featuring structural soil medium and two bioswales in Vancouver, British Columbia found that these practices are effective in treating heavy metals, suspended solids and other typical stormwater pollutants, and are effective tools for reducing runoff volume by promoting infiltration to native soils.
• Stormwater infiltration capacity of street tree pits in New York City (Elliott et al. 2018)
  • In a study of forty tree pits representing typical varieties of physical conditions in New York City, Elliott et al. found the most significant factor influencing infiltration rate was the presence of fencing or guard rails, with guarded tree pits having higher infiltration rates. Additionally, higher infiltration rates were associated with larger tree pit areas, built-up surface elevations and the combined presence of ground cover plantings and mulch.
• Tree pit hydrology in Melbourne, Australia (Grey et al. 2018)
  • Grey et al. (2018), conducted a streetscape experiment to determine the runoff retention rate of tree pits in heavy clay soil with low exfiltration rates. Their research found that runoff retention is possible in even very dense urban streetscapes, and that sizing needs to be between 2.5% to 8% of the impervious catchment area (dependent upon tree pit exfiltration rates) to achieve 90% reduction in annual runoff.
• Health of trees in bioretention (Tirpak et al. 2018)
  • Tirpak et al. (2018), conducted a study on tree health in bioretention systems in southeastern U.S. Of the 6 species studied, only 1 showed greater health when grown in bioretention media compared to urban trees not planted in bioretention systems. Results show that species selection should be based on bioretention filter media analysis and compatability with the growing conditions found in bioretention systems.
• Review of stormwater benefits of urban trees (Kuehler et al. 2017)
  • Kuehler, et al. (2017) in their literature review found that urban trees can retain sizable amounts of annual rainfall in their crowns, delay the flow of stormwater runoff, substantially increase the infiltration capacity of urban soils, and provide transpiration of sequestered runoff. Tree canopy effectiveness is highest during short, low‐intensity storms and lower as rainfall volume and intensity increases.
• Estimating tree leaf area density with LIDAR (Li et al. 2017)
  • Li, et al. (2017), determine an effective means for leaf area density (LAD) estimation of a canopy of magnolia trees using high-resolution LiDAR data and ground measured leaf area index (LAI).
• Modelling rainfall interception by urban trees (Huang et al. 2017)
  • Huang, et al. (2017), developed an analytical model to compare rainfall interception rates between four deciduous tree species (white oak, Norway maple, green ash and cherry). The ratio of evaporation rate to rainfall rate was the most dynamic differing parameter amongst the trees selected. The study was able to provide some information on improved tree selection in urban environments.
• Optical Leaf Area Index In-situ Measurement (Leblanc 2011)
  • Abuelgasim, A. and Leblanc, S. G. (2011), discuss how NRCan have developed methods to measure the leaf density in vegetation canopies with minimum destructive sampling. The measured quantity, Leaf Area Index (LAI), is used in estimates of carbon absorption by plants.
• Washington Stormwater Center Tree Project
  • The Washington Stormwater Center, conducts their own research on the effectiveness of LID installations, assists homeowners, businesses and organizations with permit assistance for stormwater management and pollution prevention installations, discuss emerging SWM technologies and provide Technology Assessment Protocol - Ecology (TAPE) certification for Washington State.

Modelling tools

i-Tree is a free software suite developed by the USDA Forest Service and partners that assesses tree and forest structure, ecosystem services, and value of a community’s tree resources. See external links below for further details and tool downloads.

• About iTree Tools
• iTree Tools Overview
• iTree Tools international users summary
• iTree Eco International

External links

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
• Cupolex
• Deeproot - Silva Cell
• GreenBlue Urban - RootSpace
• Imbrium Systems - Filterra
• Storm-Tree

Gallery

Open tree pits

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

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  Extended tree pit planting in USA
  Photo credit: USEPA

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

Soil cells

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  Soil cells under construction.

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  Soil cell installation on Central Parkway in Mississauga.

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

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  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
  

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

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

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

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

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

References

Also see references as direct web page links above.

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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.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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.

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.

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).

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The natural ground material characteristic of or existing by virtue of geographic origin.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A parameter that describes the capability of a medium to transmit water.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

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.

A perforated pipe used to assist the draining of soils.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

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

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Soil particles with a diameter less than 0.050 mm.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

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.

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.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

The natural ground material characteristic of or existing by virtue of geographic origin.

A perforated pipe used to assist the draining of soils.

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

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

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.

Sustainable Technologies Evaluation Program

Total suspended solids

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.

The circulation of water from the atmosphere to the earth and back, through precipitation, runoff, infiltration, groundwater flow and evapotranspiration.

Abiotic transfer of water vapour to the atmosphere.

The rate of rainfall in millimeters per hour.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The greatest volume of stream flow occurring during a storm event.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

Stormwater Management

A biological community, including humans and their natural environment.

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.

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

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 lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

The total mass of a pollutant entering a waterbody over a defined time period.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The mathematical expression of the water balance.

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.