Difference between revisions of "Stormwater Tree Trenches"

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Image:Extended tree pit.png|thumb|500 px|This is an image map of an extended tree pit, clicking on components will load the appropriate article.
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File:SWTT Struct Pan High Perm Final crop.png|thumb|500 px|'''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. <span style="color:red">'''''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.''</span>
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rect 241 1143 296 1338 [[Soil cells]]
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rect 616 1143 669 1338 [[Soil cells]]
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rect 1632 3032 1696 3394 [[Overflow|Overflow to Underdrain]]
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rect 359 1148 517 1248 [[Bioretention: Filter media]]
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rect 1631 3392 1700 3461 [[Underdrain]]
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rect 361 1250 517 1277 [[Choking layer]]
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rect 1636 3463 1691 3632 [[Bioretention: Internal water storage|Internal Water Storage]]
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rect 299 1280 428 1337 [[Reservoir gravel]]
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rect 1261 3632 1638 3694 [[Soil groups|Uncompacted Subgrade Soil]]
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rect 540 1280 613 1337 [[Reservoir gravel]]
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rect 823 3636 1265 3694 [[Soil groups|Compacted Subgrade Soil]]
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rect 468 1280 514 1337 [[Reservoir gravel]]
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rect 1640 3636 2056 3694 [[Soil groups|Compacted Subgrade Soil]]
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rect 360 1124 411 1146 [[Forebays]]
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rect 1260 3036 1292 3540 [[Wells|Monitoring Well]]
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rect 510 1124 562 1146 [[Forebays]]
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rect 1265 3541 1289 3636 [[Digital technologies|Water Level Sensor]]
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rect 368 440 415 492 [[Forebays]]
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rect 821 3532 1172 3634 [[Aggregates|Aggregate]]
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rect 513 440 560 492 [[Forebays]]
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rect 2054 3636 1723 3534 [[Aggregates|Aggregate]]
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rect 560 1099 594 1123 [[Inlets|Slotted drain]]
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rect 821 3489 1116 3534 [[Choker layer]]
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rect 321 454 366 498 [[Inlets|Depressed drain]]
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rect 1785 3492 2049 3540 [[Choker layer]]
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circle 421 256 152 [[Trees]]
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poly 2057 3503 2027 3121 1936 3090 2002 3152 2002 3461 1842 3454 1851 3134 1931 3092 1820 3120 1800 3492 1942 3494 1967 3490 2014 3489 2038 3496 [[Soil cells: Gallery|Soil Cells]]
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poly 433 1078 447 1083 443 998 502 977 562 989 598 971 610 784 570 657 433 581 393 594 297 680 264 756 239 866 272 937 331 1000 432 1002 433 1076 [[Trees]]
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poly 852 3136 839 3494 1090 3490 1081 3461 1072 3139 990 3092 1040 3149 1043 3458 889 3460 881 3138 1012 3110 921 3094 [[Soil cells: Gallery|Soil Cells]]
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poly 362 1115 366 1091 406 1068 446 1087 466 1095 499 1078 552 1093 555 1111 530 1104 504 1120 504 1140 419 1141 419 1121 [[Perennials: List]]
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rect 1172 3038 1223 3163 [[Stormwater Tree Trenches|Cleanout Access]]
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circle 464 442 48 [[Perennials: List]]
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rect 1860 3027 1913 3167 [[Stormwater Tree Trenches|Cleanout Access]]
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circle 464 86 46 [[Perennials: List]]
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rect 1907 3121 723 3167 [[Pipes|Stormwater Distribution Pipe]]
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poly 518 1148 518 1348 2 1351 1 1369 524 1369 537 1356 539 1147 [[Overflow]]
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rect 654 3065 708 3114 [[Overflow|Outlet Distribution Pipe to Storm Sewer]]
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circle 530 107 18 [[Overflow]]
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poly 832 3490 839 3079 1147 3074 852 3079 830 3267 [[Geotextile|Geotextile Liner]]
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circle 447 1315 21 [[Underdrain]]
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poly 2051 3123 2047 3074 1756 3074 2049 3085 2051 3490 [[Geotextile| Geotextile Liner]]
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rect 854 3079 1080 3119 [[Stormwater Tree Trenches: Specifications|Concrete Footing]]
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rect 1800 3078 2058 3112 [[Stormwater Tree Trenches: Specifications|Concrete Footing]]
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rect 1751 3023 2056 3074 [[Stormwater Tree Trenches: Specifications|Structural Concrete Panel]]
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rect 849 3023 1156 3081 [[Stormwater Tree Trenches: Specifications|Structural Concrete Panel]]
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rect 1161 3045 1751 3076 [[Mulch]]
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rect 639 3025 774 3338 [[Pretreatment|Catch Basin]]
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rect 623 2813 758 2953 [[Pretreatment|Catch Basin]]
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rect 1152 3012 1329 3032 [[Curb extensions: Gallery|Tree Grate]]
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rect 1571 3014 1747 3030 [[Curb extensions: Gallery|Tree Grate]]
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rect 845 3081 2058 3636 [[Bioretention: Filter media|Filter Media]]
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rect 1054 1735 1818 2770 [[Trees: List|Trees]]
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rect 1149 582 1743 1142 [[Trees: List|Trees]]
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rect 1605 1273 1692 1350 [[Underdrain|Overflow to Underdrain]]
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rect 1258 1308 1301 1346 [[Wells|Monitoring Well]]
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rect 1158 1213 1227 1286 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1873 1211 1945 1282 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1154 547 1754 1370 [[Curb extensions: Gallery|Tree Grate]]
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rect 667 1197 803 1319 [[Pretreatment|Catch Basin]]
 
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==Overview==
 
==Overview==
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Trees can be incorporated into [[bioretention]] cells with other plant types, or otherwise into their own planting pits.  
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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.
 +
 
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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:
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{{Clickable button|[[File:Treetrench.png|200 px|link=https://wiki.sustainabletechnologies.ca/images/7/77/Tree_trenches_2022.pdf]]}}
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'''Stormwater tree trench installations include:'''
 +
* Overlying impermeable or [[permeable pavements]]
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* Trees (tolerant to northern urban conditions)
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* Planting soil
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* Modular soil support or "soil cell" structures (optional)
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* Structural soil (optional)
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* Structural concrete panels (optional)
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* Stormwater inlet and outlet structures
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* Distribution and drainage pipes
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* [[Choker layer]] (optional)
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* Geogrid and geotextile (optional)
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* [[Reservoir aggregate|Aggregate base]]
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 +
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{{textbox|Stomwater tree trenches are ideal for:
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*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)
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*Areas with limited greenspace
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*Projects with high traffic loads (pedestrian and vehicular), laneways, pedestrian plazas and walkways}}
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 +
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'''Additional components may include:'''
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* [[Bioretention: Internal water storage|Internal water storage]] layer
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* Distribution and [[underdrain]] pipe access and clean-out features
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* [[Wells|Monitoring well]] screened within internal water storage layer
 +
* Root barriers in locations where tree rooting is not desired
  
 
==Planning considerations==
 
==Planning considerations==
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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 <ref>Crow, P. (2005). The Influence of Soils and Species on Tree Root Depth. Edinburgh. Retrieved from https://www.forestry.gov.uk/pdf/FCIN078.pdf/$FILE/FCIN078.pdf</ref>. For more detailed information on planning (site) considerations  see [[Bioretention]]
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A commonly held view is that a tree's root system will be similar to it's visible crown. For many trees, this is not the case, as roots will more often spread much more widely, but to a shallower depth <ref>Crow, P. (2005). The Influence of Soils and Species on Tree Root Depth. Edinburgh. Retrieved from https://www.forestry.gov.uk/pdf/FCIN078.pdf/$FILE/FCIN078.pdf</ref>.  
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 +
===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===
 
===Planting in slopes===
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Smooth slopes should be amended into localised terraces by the Landscape Architect when planting large trees into slopes > 5 %. <ref>Wilkus A., 'Slope Style', Landscape Architecture Magazine, April 2018, accessed 21 December 2018, https://landscapearchitecturemagazine.org/2018/04/24/slope-style/</ref>
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Smooth slopes should be amended into localized terraces by the Landscape Architect when planting large trees into slopes > 5 %. <ref>Wilkus A., 'Slope Style', Landscape Architecture Magazine, April 2018, accessed 21 December 2018, https://landscapearchitecturemagazine.org/2018/04/24/slope-style/</ref>. 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===
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Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas.
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===Water Table===
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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.
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 +
===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]].
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 +
===Drainage Area===
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Typical contributing drainage areas are between 150 to 300 m<sup>2</sup> per tree, with a recommended maximum of 450 m<sup>2</sup> 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).
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===Setback from Buildings===
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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.
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 +
===Overhead Wires===
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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.
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 +
===Pollution Hot Spot Runoff===
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Tree trenches receiving road or parking lot runoff are not recommended in these areas.
 +
 
 +
===Proximity to Underground Utilities===
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Designers should consult local utility design guidance for the horizontal and vertical clearances required.
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 +
===[[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==
 
==Design==
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There are many design configuration options for including trees into stormwater management plans.  
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[[File:DepressedDrain_SoilCell.png|thumb|500px|A surface [[inlets|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]]
  
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==Soil cells==
 
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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:
 
Things to consider in design:
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*If the system is unlined it is hydraulically equivalent to a [[bioretention]] cell and provides similar water quality benefits.   
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*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.   
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*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.   
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*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.  
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*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===
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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.
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 +
===Inlets===
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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.
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 +
===Pre-Treatment===
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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.
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===Soil Volume===
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Each tree planted should have access to a minimum 30 m<sup>3</sup> 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. 
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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.
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 +
*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).
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*[[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.
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*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.
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 +
====Structural Soil Comparisons====
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{|class="wikitable"
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|+Specifications for Stormwater Tree Trenches using Structural Soils
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|-
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!Structural Soil Type
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!Median Stone size/range
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!Soil Texture
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!Tackifiying Agent
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!Approximate Porosity
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|-
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|'''[https://gailmaterials.net/wp-content/uploads/2019/08/cu-structural_soil_specifications.pdf CU-Soil™]'''
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|30mm (20-40mm)*
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|
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Gravel: <5%<br>
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Sand: 20-45%<br>
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Silt: 20-50%<br>
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Clay: 20-40%<br>
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Cation Exchange Capacity (CEC) >10<br>
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pH:  5.5 – 6.5<br>
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Organic Content: 2 – 5% by dry weight
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|[http://www.amereq.com/pages/12/index.htm Hydrogel (coated potassium propenoate-propenamide copolymer)]
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|26%
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|-
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|'''B.C Soil'''
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|75mm/60 – 80mm)
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|
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Sand:  45-55%<br>
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Silt: 25-35%<br>
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Clay: 0 – 10%<br>
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Silt + Clay: 25 – 45%<br>
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pH: 6.0 – 7.0<br>
 +
Organic Content: 15-20%**
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|[http://www.stabilizersolutions.com/products/stabilizer/ Stabilizer]
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|33%
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|-
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| colspan="5" style="text-align: left;" |<small>'''Note:'''<br>
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"*" = Larger or smaller stone sizes are accepted as long as they do not comprise more than 10% above or 10% below the indicated range.<br>
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"**" = Soil texture is the City of Vancouver specification for structural soils</small>
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|}
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 +
===Modular Soil Support Systems===
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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.
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[[File:Goss Trap.PNG|thumb|400px|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.<ref>City of Toronto. 2020. Construction Specifications and Drawings for Sewers and Watermains. https://www.toronto.ca/services-payments/building-construction/infrastructure-city-construction/construction-standards-permits/standards-for-designing-and-constructing-city-infrastructure/construction-specifications-for-sewers-and-watermains/</ref> This image was sourced from the [https://www.toronto.ca/services-payments/building-construction/infrastructure-city-construction/construction-standards-permits/standards-for-designing-and-constructing-city-infrastructure/construction-specifications-for-sewers-and-watermains/ City of Toronto's Construction Specifications and Drawings for Sewers and Watermains]. <ref>City of Toronto. 2020. Construction Specifications and Drawings for Sewers and Watermains. https://www.toronto.ca/services-payments/building-construction/infrastructure-city-construction/construction-standards-permits/standards-for-designing-and-constructing-city-infrastructure/construction-specifications-for-sewers-and-watermains/</ref>]]
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===Structural Soil Medium===
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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.
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 +
===Structural Concrete Panels===
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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).
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===Conveyance and Overflow===
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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.
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 +
===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.
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===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|underdrain]] pipes can be installed above the bottom of the trench and/or include flow control. Alternatively, the [[Underdrain|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.
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 +
===Variations of Stormwater Tree Trenches===
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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.
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<imagemap>
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File:SWTT Low Perm Soil Cells Final.png|thumb|left|400px|'''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. <span style="color:red">'''''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.''</span>
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rect 1494 1440 1584 1530 [[Overflow|Overflow to Underdrain]]
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rect 980 1335 1070 1434 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1860 1350 1946 1440 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1028 665 1545 1210 [[Trees: List|Tree]]
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rect 1143 1258 1410 1506 [[Shrubs: List|Shrubs]] 
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rect 1162 389 1389 610 [[Shrubs: List|Shrubs]] 
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rect 726 1320 886 1458 [[Pretreatment|Catch Basin]] 
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rect 961 303 1612 1563 [[Mulch]] 
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rect 963 1719 1595 2819 [[Trees: List|Tree]]
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rect 1160 2929 1411 3061 [[Shrubs: List|Shrubs]]   
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rect 982 3017 1044 3166 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1885 3043 1950 3159 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1092 3576 1116 3679 [[Digital technologies|Water Level Sensor]]
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rect 1606 3084 1724 3495 [[Soil cells: Gallery|Soil Cells]]
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rect 1738 3084 1856 3495 [[Soil cells: Gallery|Soil Cells]]
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rect 1867 3085 1986 3499 [[Soil cells: Gallery|Soil Cells]]
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rect 815 3109 1948 3159 [[Pipes|Stormwater Distribution Pipe]]
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rect 759 3074 802 3122 [[Overflow|Outlet Distribution Pipe to Storm Sewer]]
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rect 732 3034 877 3339 [[Pretreatment|Catch Basin]] 
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rect 1518 2997 1594 3573 [[Overflow|Overflow to Underdrain]]
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rect 1523 3577 1593 3644 [[Underdrain]]
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rect 950 3038 1610 3087 [[Mulch]]
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rect 958 3096 1988 3495 [[Bioretention: Filter media|Filter Media]]
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rect 938 3104 956 3510 [[Geotextile|Geotextile Liner]]
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rect 1986 3080 2003 3504 [[Geotextile|Geotextile Liner]]
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rect 1608 3078 1994 3093 [[Geotextile|Geotextile Liner]] 
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rect 949 3497 1997 3544 [[Choker layer]]
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rect 945 3543 1997 3682 [[Aggregates|Clear Stone Aggregate]]
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rect 950 3682 1588 3739 [[Soil groups|Uncompacted Subgrade Soil]]
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rect 1590 3682 2001 3739 [[Soil groups|Compacted Subgrade Soil]]
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</imagemap>
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<imagemap>
 +
File:SWTT High Perm Soil Cells Final.png|thumb|right|400px|'''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. <span style="color:red">'''''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.''</span>
 +
 
 +
rect 1494 1440 1584 1530 [[Overflow|Overflow to Underdrain]]
 +
rect 1055 1464 1112 1508 [[Wells|Monitoring Well]]
 +
rect 980 1335 1070 1434 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1860 1350 1946 1440 [[Stormwater Tree Trenches|Cleanout Access]]
 +
rect 1028 665 1545 1210 [[Trees: List|Tree]]
 +
rect 1143 1258 1410 1506 [[Shrubs: List|Shrubs]] 
 +
rect 1162 389 1389 610 [[Shrubs: List|Shrubs]] 
 +
rect 726 1320 886 1458 [[Pretreatment|Catch Basin]] 
 +
rect 961 303 1612 1563 [[Mulch]] 
 +
rect 963 1719 1595 2819 [[Trees: List|Tree]]
 +
rect 1160 2929 1411 3061 [[Shrubs: List|Shrubs]]   
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rect 982 3017 1044 3166 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1885 3043 1950 3159 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1083 2988 1121 3570 [[Wells|Monitoring Well]]
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rect 1092 3576 1116 3679 [[Digital technologies|Water Level Sensor]]
 +
rect 1606 3084 1724 3495 [[Soil cells: Gallery|Soil Cells]]
 +
rect 1738 3084 1856 3495 [[Soil cells: Gallery|Soil Cells]]
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rect 1867 3085 1986 3499 [[Soil cells: Gallery|Soil Cells]]
 +
rect 815 3109 1948 3159 [[Pipes|Stormwater Distribution Pipe]]
 +
rect 759 3074 802 3122 [[Overflow|Outlet Distribution Pipe to Storm Sewer]]
 +
rect 732 3034 877 3339 [[Pretreatment|Catch Basin]] 
 +
rect 1516 2992 1591 3401 [[Overflow|Overflow to Underdrain]]
 +
rect 1516 3405 1595 3471 [[Underdrain]]
 +
rect 1531 3478 1579 3682 [[Bioretention: Internal water storage|Internal Water Storage]]
 +
rect 950 3038 1610 3087 [[Mulch]]
 +
rect 958 3096 1988 3495 [[Bioretention: Filter media|Filter Media]]
 +
rect 938 3104 956 3510 [[Geotextile|Geotextile Liner]]
 +
rect 1986 3080 2003 3504 [[Geotextile|Geotextile Liner]]
 +
rect 1608 3078 1994 3093 [[Geotextile|Geotextile Liner]] 
 +
rect 949 3497 1997 3544 [[Choker layer]]
 +
rect 945 3543 1997 3682 [[Aggregates|Clear Stone Aggregate]]
 +
rect 950 3682 1588 3739 [[Soil groups|Uncompacted Subgrade Soil]]
 +
rect 1590 3682 2001 3739 [[Soil groups|Compacted Subgrade Soil]]
 +
</imagemap>
 +
 
 +
 
 +
<imagemap>
 +
File:SWTT Struct Soil Med High Perm Final.png|thumb|center|400px|'''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.  <span style="color:red">'''''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.''</span>
 +
 
 +
rect 1605 3079 1983 3500 [[Stormwater Tree Trenches: Specifications|Structural Soil Medium]]
 +
rect 1494 1440 1584 1530 [[Overflow|Overflow to Underdrain]]
 +
rect 1055 1464 1112 1508 [[Wells|Monitoring Well]]
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rect 980 1335 1070 1434 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1860 1350 1946 1440 [[Stormwater Tree Trenches|Cleanout Access]]
 +
rect 1028 665 1545 1210 [[Trees: List|Tree]]
 +
rect 1143 1258 1410 1506 [[Shrubs: List|Shrubs]] 
 +
rect 1162 389 1389 610 [[Shrubs: List|Shrubs]] 
 +
rect 726 1320 886 1458 [[Pretreatment|Catch Basin]] 
 +
rect 961 303 1612 1563 [[Mulch]] 
 +
rect 963 1719 1595 2819 [[Trees: List|Tree]]
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rect 1160 2929 1411 3061 [[Shrubs: List|Shrubs]]   
 +
rect 982 3017 1044 3166 [[Stormwater Tree Trenches|Cleanout Access]]
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rect 1885 3043 1950 3159 [[Stormwater Tree Trenches|Cleanout Access]]
 +
rect 1083 2988 1121 3570 [[Wells|Monitoring Well]]
 +
rect 1092 3576 1116 3679 [[Digital technologies|Water Level Sensor]]
 +
rect 1606 3084 1724 3495 [[Soil cells: Gallery|Soil Cells]]
 +
rect 1738 3084 1856 3495 [[Soil cells: Gallery|Soil Cells]]
 +
rect 1867 3085 1986 3499 [[Soil cells: Gallery|Soil Cells]]
 +
rect 815 3109 1948 3159 [[Pipes|Stormwater Distribution Pipe]]
 +
rect 759 3074 802 3122 [[Overflow|Outlet Distribution Pipe to Storm Sewer]]
 +
rect 732 3034 877 3339 [[Pretreatment|Catch Basin]] 
 +
rect 1516 2992 1591 3401 [[Overflow|Overflow to Underdrain]]
 +
rect 1516 3405 1595 3471 [[Underdrain]]
 +
rect 1531 3478 1579 3682 [[Bioretention: Internal water storage|Internal Water Storage]]
 +
rect 950 3038 1610 3087 [[Mulch]]
 +
rect 958 3096 1988 3495 [[Bioretention: Filter media|Filter Media]]
 +
rect 938 3104 956 3510 [[Geotextile|Geotextile Liner]]
 +
rect 1986 3080 2003 3504 [[Geotextile|Geotextile Liner]]
 +
rect 1608 3078 1994 3093 [[Geotextile|Geotextile Liner]] 
 +
rect 949 3497 1997 3544 [[Choker layer]]
 +
rect 945 3543 1997 3682 [[Aggregates|Clear Stone Aggregate]]
 +
rect 950 3682 1588 3739 [[Soil groups|Uncompacted Subgrade Soil]]
 +
rect 1590 3682 2001 3739 [[Soil groups|Compacted Subgrade Soil]]
 +
</imagemap>
 +
 
 +
 
 +
 
 +
{{:Stormwater Tree Trenches: Specifications}}
 +
 
 +
==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.<ref>Berland, A., Shiflett, S.A., Shuster, W.D., Garmestani, A.S., Goddard, H.C., Herrmann, D.L. and Hopton, M.E. 2017. The role of trees in urban stormwater management. Landscape and urban planning, 162, pp.167-177. https://pdf.sciencedirectassets.com/271853/1-s2.0-S0169204617X00030/1-s2.0-S0169204617300464/Adam_Berland_green_infrastructure_2017.pdf</ref>, <ref>Kuehler, E., Hathaway, J. and Tirpak, A. 2017. Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network. Ecohydrology, 10(3), p.e1813. https://www.srs.fs.usda.gov/pubs/ja/2017/ja_2017_kuehler_001.pdf</ref> 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.
 +
 
 +
{{float right|{{#widget:YouTube|id=Un2yBgIAxYs}}}}
 +
{| class="wikitable"
 +
|+ 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===
 
===Species selection===
−
[[File:DepressedDrain_SoilCell.png|thumb|400px|Sometimes configured to enclose ponded water which can then infiltrate the soil surrounding the tree, for instance in a depressed drain [[inlets|inlet]] design.]]
+
For an overview of key considerations affecting planting plans and tree species selection please visit our [[Planting design| planting design]], [[Plant lists|plant selection]] and [[Trees: List|tree species]] wiki pages that provide guidance to help you develop functional and attractive planting plans.
−
[[Trees: List]]
+
 
−
===Planting pit sizing===
+
===Tree planting best practices===
−
[[Bioretention: Sizing]]
+
An extensive compendium of recommended standard tree planting details and specifications are available from [http://www.jamesurban.net/specifications 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 [http://www.jamesurban.net/up-by-roots#:~:text=Up%20By%20Roots%2C%20written%20by,trees%20in%20the%20built%20environment "Up by Roots"] <ref>Urban, J. 2008. Up By Roots: Healthy soils and trees in the built environment. International Society of Arboriculture. http://www.jamesurban.net/up-by-roots#:~:text=Up%20By%20Roots%2C%20written%20by,trees%20in%20the%20built%20environment.</ref> <br>
 +
</br>
 +
 
 +
[[File:UpbyRoots JU.png|750px]]<br>
 +
</br>
 +
 
 +
 
 +
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: [https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf Urban Soil and Site Assessment Presentation] <ref>Urban, J. 2014. Urban Soil and Site Assessment [Presentation]. University of Washington Botanic Gardens. Seattle, WA https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf.</ref>
 +
 
 +
==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.
  
−
===Inlets===
+
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.
−
Multiple methods for distribution and conveyance of runoff into the system are recommended for redundancy and conservative designs.  
+
 
−
Combinations may be made of:
+
See further details here: [[Stormwater Tree Trenches: Maintenance]]
−
*tree well flow,
+
 
−
*catchbasins and distribution pipes, and
+
<br>
−
*direct infiltration from permeable paving.
+
 
−
See also [[Inlets]] and [[pretreatment]]
+
Also take a look at the [[Inspection and Maintenance: Bioretention & Bioswales]] page by clicking below for further details about proper inspection and maintenance practices:
  
−
===Underdrain===
+
{{Clickable button|[[File:Cover Photo.PNG|150 px|link=https://wiki.sustainabletechnologies.ca/wiki/Inspection_and_Maintenance:_Bioretention_%26_Bioswales]]}}
−
[[Underdrain]]
 
  
 
==Performance==
 
==Performance==
−
===Interception===
+
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.<ref> Page, J.L., Winston, R.J., Hunt, W.F. 2015. Soils beneath suspended pavements: An opportunity for stormwater control and treatment. Ecological Engineering. v.82. pp.40-48. https://www.sciencedirect.com/science/article/abs/pii/S0925857415001706 </ref>  To read about the use of stormwater tree trenches featuring soil cells in the Greater Toronto Area see the STEP [https://sustainabletechnologies.ca/app/uploads/2020/09/Soil-cells-tech-brief-FINAL.pdf technical brief] and [https://sustainabletechnologies.ca/app/uploads/2018/10/Queensway-Case-Study_FINAL.pdf 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. <ref> STEP. 2018. The Queensway Sustainable Sidewalk Project https://sustainabletechnologies.ca/app/uploads/2018/10/Queensway-Case-Study_FINAL.pdf </ref> <ref> STEP. 2020. Assessing the Health of Toronto Street Trees Irrigated by Stormwater. https://sustainabletechnologies.ca/app/uploads/2020/09/Soil-cells-tech-brief-FINAL.pdf </ref> 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. <ref> Li, J., Alinaghaian, S., Joksimovic, D., Chen, L. An Integrated Hydraulic and Hydrologic Modeling Approach for Roadside Bio-Retention Facilities. Water. 2020, 12, 1248 https://www.mdpi.com/2073-4441/12/5/1248 </ref>  Also see Credit Valley Conservation [https://cvc.ca/wp-content/uploads/2016/06/CaseStudy_CPW_Final.pdf Central Parkway LID case study] and [https://cvc.ca/wp-content/uploads//2021/07/TechReport_CPW_Final.pdf 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%. <ref>Credit Valley Conservation. 2016. Central Parkway: Road Right-of-Way Retrofits - Case Study. https://cvc.ca/wp-content/uploads/2016/06/CaseStudy_CPW_Final.pdf</ref> <ref>Credit Valley Conservation. 2016. Central Parkway: Low Impact Development Infrastructure
−
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).  
+
Performance and Risk Assessment - Technical Report. https://cvc.ca/wp-content/uploads//2021/07/TechReport_CPW_Final.pdf</ref>
 +
 
 +
{|class="wikitable"
 +
|+Ability for Stormwater Tree Trenches to Meet Stormwater Management Objectives
 +
|-
 +
!BMP
 +
!Water Balance
 +
!Water Quality
 +
!Erosion Control
 +
|-
 +
|'''Stormwater Tree Trench'''
 +
|
 +
</br>Partial-based on on native soil infiltration rate and if flow restrictor is used <br>
 +
</br>
 +
|
 +
</br>Yes-size for water quality storage requirement<br>
 +
</br>
 +
|
 +
</br>Partial-based on native soil infiltration rate, available storage and if flow restrictor is used <br>
 +
</br>
 +
|}
 +
<br>
 +
</br>
 +
{|class="wikitable"
 +
|+Volumetric runoff reduction from Stormwater Tree Trench/Bioretention
 +
|-
 +
!'''LID Practice'''
 +
!'''Location'''
 +
!'''<u><span title="Note: Runoff reduction estimates are based on differences in runoff volume between the practice and a conventional impervious surface over the period of monitoring." >Runoff Reduction*</span></u>'''
 +
!'''Reference'''
 +
|-
 +
|rowspan="4" style="text-align: center;" | Bioretention without underdrain
 +
|style="text-align: center;" |China
 +
|style="text-align: center;" |'''<u><span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and  expressway service area.">85 to 100%*</span></u>'''
 +
|style="text-align: center;" |Gao, ''et al.'' (2018)<ref>Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.</ref>
 +
|-
 +
|style="text-align: center;" |Connecticut
 +
|style="text-align: center;" |99%
 +
|style="text-align: center;" |Dietz and Clausen (2005) <ref>Dietz, M.E. and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. Water Air and Soil Pollution. Vol. 167. No. 2. pp. 201-208. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.365.9417&rep=rep1&type=pdf</ref>
 +
|-
 +
|style="text-align: center;" |Pennsylvania
 +
|style="text-align: center;" |80%
 +
|style="text-align: center;" |Ermilio (2005)<ref>Ermilio, J.F., 2005. Characterization study of a bio-infiltration stormwater BMP (Doctoral dissertation, Villanova University). https://www1.villanova.edu/content/dam/villanova/engineering/vcase/vusp/Ermilio-Thesis06.pdf</ref>
 +
|-
 +
|style="text-align: center;" |Pennsylvania
 +
|style="text-align: center;" |70%
 +
|style="text-align: center;" |Emerson and Traver (2004)<ref>Emerson, C., Traver, R. 2004. The Villanova Bio-infiltration Traffic Island: Project Overview. Proceedings of 2004 World Water and Environmental Resources Congress (EWRI/ASCE). Salt Lake City, Utah, June 22 – July 1, 2004. https://ascelibrary.org/doi/book/10.1061/9780784407370</ref>
 +
|-
 +
|rowspan="8" style="text-align: center;" | Bioretention with underdrain
 +
|-
 +
|style="text-align: center;" |Texas
 +
|style="text-align: center;" |'''<u><span title="Note: Runoff reduction estimates are based on differences in runoff volume between the practice and a conventional impervious surface over the period of monitoring.">82%*</span></u>'''
 +
|style="text-align: center;" |Mahmoud, ''et al.'' (2019)<ref>Mahmoud, A., Alam, T., Rahman, M.Y.A., Sanchez, A., Guerrero, J. and Jones, K.D. 2019. Evaluation of field-scale stormwater bioretention structure flow and pollutant load reductions in a semi-arid coastal climate. Ecological Engineering, 142, p.100007. https://www.sciencedirect.com/science/article/pii/S2590290319300070</ref>
 +
|-
 +
|style="text-align: center;" |China
 +
|style="text-align: center;" |'''<u><span title="Note: Runoff reduction estimates are based on SWMM and RECARGA models applied to generate the runoff reduction percentages of a bioretention installation near one of China's and  expressway service area.">35 to 75%*</span></u>'''
 +
|style="text-align: center;" |Gao, ''et al.'' (2018)<ref>Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.</ref>
 +
|-
 +
|style="text-align: center;" |Ohio
 +
|style="text-align: center;" |36 to 59%
 +
|style="text-align: center;" |Winston ''et al.'' (2016). <ref>Winston, R.J., Dorsey, J.D. and Hunt, W.F. 2016. Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio. Science of the Total Environment, 553, pp.83-95.</ref>
 +
|-
 +
|style="text-align: center;" |Virginia
 +
|style="text-align: center;" |97 to 99%
 +
|style="text-align: center;" |DeBusk and Wynn (2011)<ref>DeBusk, K.M. and Wynn, T.M., 2011. Storm-water bioretention for runoff quality and quantity mitigation. Journal of Environmental Engineering, 137(9), pp.800-808. https://www.webpages.uidaho.edu/ce431/Articles/DeBusk-ASCE-2011.pdf</ref>
 +
|-
 +
|style="text-align: center;" |Maryland and North Carolina
 +
|style="text-align: center;" |20 to 50%
 +
|style="text-align: center;" |Li ''et al.'' (2009). <ref>Li, H., Sharkey, L.J., Hunt, W.F., and Davis, A.P. 2009. Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland. Journal of Hydrologic Engineering. Vol. 14. No. 4. pp. 407-415.</ref>
 +
|-
 +
|style="text-align: center;" |North Carolina
 +
|style="text-align: center;" |40 to 60%
 +
|style="text-align: center;" |Smith and Hunt (2007)<ref>Smith, R and W. Hunt. 2007. Pollutant removals in bioretention cells with grass cover. Proceedings 2nd National Low Impact Development Conference. Wilmington, NC. March 13-15, 2007.</ref>
 +
|-
 +
|style="text-align: center;" |North Carolina
 +
|style="text-align: center;" |33 to 50%
 +
|style="text-align: center;" |Hunt and Lord (2006). <ref>Hunt, W.F. and Lord, W.G. 2006. Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-5. North Carolina State University. Raleigh, NC.</ref>
 +
|-
 +
|rowspan="5" style="text-align: center;" | Bioretention with underdrain & liner
 +
|-
 +
|style="text-align: center;" |Ontario
 +
|style="text-align: center;" |15 to 34%
 +
|style="text-align: center;" |<span class="plainlinks">[https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf STEP (2019)]</span> <ref>STEP. 2019. Comparative Performance Assessment of Bioretention in Ontari0. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf.</ref>
 +
|-
 +
|style="text-align: center;" |Queensland, Australia
 +
|style="text-align: center;" |33 to 84%
 +
|style="text-align: center;" |Lucke and Nichols (2015). <ref>Lucke, T., & Nichols, P. W. B. 2015. The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation. Science of The Total Environment, 536, 784-792. doi:http://dx.doi.org/10.1016/j.scitotenv.2015.07.142</ref>
 +
|-
 +
|style="text-align: center;" |Victoria, Australia
 +
|style="text-align: center;" |15 to 83%
 +
|style="text-align: center;" |Hatt ''et al.'' (2009). <ref>Hatt, B. E., Fletcher, T. D., & Deletic, A. 2009. Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale. Journal of Hydrology, 365(3), 310-321. doi:http://dx.doi.org/10.1016/j.jhydrol.2008.12.001</ref>
 +
|-
 +
|style="text-align: center;" |Maryland
 +
|style="text-align: center;" |49 to 58%
 +
|style="text-align: center;" |Davis (2008). <ref>Davis, A.P. 2008. Field performance of bioretention: Hydrology impacts. Journal of hydrologic engineering, 13(2), pp.90-95. https://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90)</ref>
 +
|-
 +
| colspan="2" style="text-align: center;" |'''<u><span title="Note: This estimate is provided only for the purpose of initial screening of LID practices suitable for achieving stormwater management objectives and targets.  Performance of individual facilities will vary depending on site specific contexts and facility design parameters and should be estimated as part of the design process and submitted with other documentation for review by the approval authority." >Runoff Reduction Estimate*</span></u>'''
 +
|colspan="2" style="text-align: center;" |'''85% without underdrain;'''
 +
'''45% with underdrain''' | '''30% with underdrain and liner'''
 +
|-
 +
|}
  
−
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).  
+
===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.<ref> Berland, A., Shiflett, S.A., Shuster, W.D., Garmestani, A.S., Goddard, H.C., Herrmann, D.L., Hopton, M.E. The role of trees in urban stormwater management. Landscape and Urban Planning. v.162. pp.167-177. https://www.sciencedirect.com/science/article/abs/pii/S0169204617300464?via%3Dihub </ref>  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. <ref> Xiao, Q., McPherson, E.G.. 2016. Surface water storage capacity of twenty tree species in Davis, California. Journal of Environmental Quality. v.45. pp. 188-198. https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/jeq2015.02.0092 </ref>  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. <ref> Scharenbroch, B.C., Morgenroth, J., Maule, B. 2016. Tree species suitability to bioswales and impact on the urban water budget. Journal of Environmental Quality. v.45. pp. 199-206. https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/jeq2015.01.0060 </ref>
  
−
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).
+
[[File:TreeTranspiration.png|thumb|Trees suck! Comparison of transpiration rates of various tree species and ages (Abstracted from Phyto, by K. Kennen)]]
  
−
*http://www.mdpi.com/2072-4292/9/11/1202/pdf
+
For other recent research on the water management benefits of urban trees, and modelling approaches see the following articles and projects.
−
*http://lfs-mlws.sites.olt.ubc.ca/files/2014/10/an_analytical_model_of_rainfall_interception_by_urban_trees.pdf
+
* '''[https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub Stormwater runoff volume reduction benefits of urban street tree canopy (Selbig et al., 2022)]''' <ref> Selbig, W.R., Loheide II, S.P., Shuster, W., Scharenbroch, B.C., Coville, R.C., Kruegler, J., Avery, W., Haefner, R., Nowak, D. Quantifying stormwater runoff volume reduction benefit of urban street tree canopy. Science of the Total Environment. v.806 (2022) 151296. https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub </ref>
−
*https://www.nrcan.gc.ca/earth-sciences/land-surface-vegetation/biophysical-parameters/9162
+
** 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.
−
*https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999WR900003
+
* '''[https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 Stormwater tree trench and bioswale performance in Vancouver, BC (Vega 2019)]''' <ref> Vega, O.M. Green infrastructure in the City of Vancouver: performance monitoring of stormwater tree trenches and bioswales. UBC Theses and Dissertations. https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 </ref>
 +
** 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.
 +
* '''[https://www.sciencedirect.com/science/article/abs/pii/S0925857417306365 Stormwater infiltration capacity of street tree pits in New York City (Elliott et al. 2018)]''' <ref> Elliott, R.M., Adkins, E.R., Culligan, P.J, Palmer, M.I., Stormwater infiltration capacity of street tree pits: Quantifying the influence of different design and management strategies in New York City. Ecological Engineering. v.111. pp. 157-166. https://www.sciencedirect.com/science/article/abs/pii/S0925857417306365</ref>
 +
** 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.
 +
* '''[https://www.sciencedirect.com/science/article/abs/pii/S0022169418306346?via%3Dihub Tree pit hydrology in Melbourne, Australia (Grey et al. 2018)]''' <ref>Grey, V., Livesley, S.J., Fletcher, T.D. and Szota, C. 2018. Tree pits to help mitigate runoff in dense urban areas. Journal of Hydrology, 565, pp.400-410. https://www.sciencedirect.com/science/article/abs/pii/S0022169418306346?via%3Dihub</ref>
 +
** Grey ''et al''. (2018), conducted a streetscape experiment to determine the runoff retention rate of tree pits in heavy [[Soil groups|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.
 +
* '''[https://ascelibrary.org/doi/10.1061/JSWBAY.0000865 Health of trees in bioretention (Tirpak et al. 2018)]'''<ref>Tirpak, R.A., Hathaway, J.M., Franklin, J.A. and Khojandi, A. 2018. The health of trees in bioretention: A survey and analysis of influential variables. Journal of Sustainable Water in the Built Environment, 4(4), p.04018011. https://ascelibrary.org/doi/10.1061/JSWBAY.0000865</ref>
 +
**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.
 +
* '''[https://onlinelibrary.wiley.com/doi/10.1002/eco.1813 Review of stormwater benefits of urban trees (Kuehler et al. 2017)]'''<ref>Kuehler, E., Hathaway, J. and Tirpak, A. 2017. Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network. Ecohydrology, 10(3), p.e1813. https://www.srs.fs.usda.gov/pubs/ja/2017/ja_2017_kuehler_001.pdf</ref>
 +
** 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.
 +
* '''[https://www.mdpi.com/2072-4292/9/11/1202 Estimating tree leaf area density with LIDAR (Li et al. 2017)]<ref>Li, S., Dai, L., Wang, H., Wang, Y., He, Z., & Lin, S. (2017). Estimating leaf area density of individual trees using the point cloud segmentation of terrestrial LiDAR data and a voxel-based model. Remote sensing, 9(11), 1202. https://www.mdpi.com/2072-4292/9/11/1202/pdf'''</ref>
 +
** 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).
 +
* '''[https://www.tandfonline.com/doi/full/10.1080/07011784.2017.1375865 Modelling rainfall interception by urban trees (Huang et al. 2017)]'''<ref>Huang, J.Y., Black, T.A., Jassal, R.S. and Lavkulich, L.L. 2017. Modelling rainfall interception by urban trees. Canadian Water Resources Journal/Revue canadienne des ressources hydriques, 42(4), pp.336-348. https://www.researchgate.net/profile/LesLavkulich/publication/320085997_Modelling_rainfall_interception_by_urban_trees/links/59fc87bf0f7e9b9968bdc715/Modelling-rainfall-interception-by-urban-trees.pdf</ref>
 +
** 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.
 +
* '''[https://www.nrcan.gc.ca/earth-sciences/land-surface-vegetation/biophysical-parameters/9162 Optical Leaf Area Index In-situ Measurement (Leblanc 2011)]<ref>Abuelgasim, A. A., & Leblanc, S. G. (2011). Leaf area index mapping in northern Canada. International journal of remote sensing, 32(18), 5059-5076. https://www.academia.edu/download/55035075/Leaf_area_index_mapping_in_northern_Canada.pdf'''</ref>
 +
** 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.
 +
* '''[https://www.wastormwatercenter.org/project/tree-project/ Washington Stormwater Center Tree Project]<ref>Washington Stormwater Center. 2022. Tree Project. https://www.wastormwatercenter.org/project/tree-project/'''</ref>
 +
**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.
  
−
===Transpiration===
+
===Modelling tools===
−
[[File:TreeTranspiration.png|thumb|Trees suck! (Abstracted from Phyto, by K. Kennen)]]
+
[https://www.itreetools.org/ 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.
 +
* [https://www.itreetools.org/about About iTree Tools]
 +
* [https://www.itreetools.org/tools iTree Tools Overview]
 +
* [https://www.itreetools.org/documents/552/International_iTree_Tools_Summary_17Jan2019.pdf iTree Tools international users summary]
 +
* [http://www.itreetools.org/eco/international.php iTree Eco International]
  
−
*http://www.wastormwatercenter.org/tree-resources/
+
==External links==
−
*http://www.itreetools.org/eco/international.php
+
{{:Disclaimer}}
−
*http://treesandstormwater.org/
+
 
−
*http://cvc.ca/wp-content/uploads/2016/06/CaseStudy_CPW_Final.pdf
+
*[https://citygreen.com/stratavault/ CityGreen - Stratavault]
−
*https://cvc.ca/wp-content/uploads/2016/06/TechReport_CPW_Final.pdf
+
*[http://cupolex.ca/ Cupolex]
 +
*[http://www.deeproot.com/index.php Deeproot - Silva Cell]
 +
*[https://greenblue.com/na/products/rootspace/ GreenBlue Urban - RootSpace]
 +
*[https://www.imbriumsystems.com/stormwater-treatment-solutions/filterra Imbrium Systems - Filterra]
 +
*[https://www.storm-tree.com Storm-Tree]
  
−
==Galleries==
+
==Gallery==
−
{{:Trees: Gallery}}
 
 
===Open tree pits===
 
===Open tree pits===
 
{{:Extended tree pits: Gallery}}
 
{{:Extended tree pits: Gallery}}
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{{:Soil cells: Gallery}}
 
{{:Soil cells: Gallery}}
  
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==External links==
+
==References==
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{{float right|{{#widget:YouTube|id=Un2yBgIAxYs}}}}
+
Also see references as direct web page links above.
−
*http://gievidencebase.botanicgardens.sa.gov.au/contents/7-water-management#Canopy-Interception
 
−
*[http://www.jamesurban.net/specifications standard details and specifications from James Urban]
 
−
 
 
−
{{:Disclaimer}}
 
−
*[http://cupolex.ca/ Cupolex]
 
−
*[http://www.deeproot.com/index.php Deeproot]
 
−
*[http://www.conteches.com/Products/Stormwater-Management/Biofiltration-Bioretention/Filterra Filterra]
 
−
*[http://www.greenblue.com/na/ GreenBlue]
 
−
*[http://http://www.storm-tree.com/index Storm-Tree]
 
−
*[http://citygreen.com/products/stratacell/ Stratacell]
 
  
 
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−
[[Category:Green infrastructure]]
+
[[Category:Infiltration]]
 +
[[Category: Green infrastructure]]

Latest revision as of 14:16, 17 May 2024

Overflow to UnderdrainUnderdrainInternal Water StorageUncompacted Subgrade SoilCompacted Subgrade SoilCompacted Subgrade SoilMonitoring WellWater Level SensorAggregateAggregateChoker layerChoker layerSoil CellsSoil CellsCleanout AccessCleanout AccessStormwater Distribution PipeOutlet Distribution Pipe to Storm SewerGeotextile LinerGeotextile LinerConcrete FootingConcrete FootingStructural Concrete PanelStructural Concrete PanelMulchCatch BasinCatch BasinTree GrateTree GrateFilter MediaTreesTreesOverflow to UnderdrainMonitoring WellCleanout AccessCleanout AccessTree GrateCatch Basin
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[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 Trench Fact Sheet below for a .pdf overview of this LID Best Management Practice:

Treetrench.png


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:

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].

Infiltration[edit]

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

Site Topography[edit]

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[edit]

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

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

Water Table[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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

Proximity to Underground Utilities[edit]

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

Karst[edit]

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[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. 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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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.[3] This image was sourced from the City of Toronto's Construction Specifications and Drawings for Sewers and Watermains. [4]

Structural Soil Medium[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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.


Overflow to UnderdrainCleanout AccessCleanout AccessTreeShrubsShrubsCatch BasinMulchTreeShrubsCleanout AccessCleanout AccessWater Level SensorSoil CellsSoil CellsSoil CellsStormwater Distribution PipeOutlet Distribution Pipe to Storm SewerCatch BasinOverflow to UnderdrainUnderdrainMulchFilter MediaGeotextile LinerGeotextile LinerGeotextile LinerChoker layerClear Stone AggregateUncompacted Subgrade SoilCompacted Subgrade Soil
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.
Overflow to UnderdrainMonitoring WellCleanout AccessCleanout AccessTreeShrubsShrubsCatch BasinMulchTreeShrubsCleanout AccessCleanout AccessMonitoring WellWater Level SensorSoil CellsSoil CellsSoil CellsStormwater Distribution PipeOutlet Distribution Pipe to Storm SewerCatch BasinOverflow to UnderdrainUnderdrainInternal Water StorageMulchFilter MediaGeotextile LinerGeotextile LinerGeotextile LinerChoker layerClear Stone AggregateUncompacted Subgrade SoilCompacted Subgrade Soil
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.


Structural Soil MediumOverflow to UnderdrainMonitoring WellCleanout AccessCleanout AccessTreeShrubsShrubsCatch BasinMulchTreeShrubsCleanout AccessCleanout AccessMonitoring WellWater Level SensorSoil CellsSoil CellsSoil CellsStormwater Distribution PipeOutlet Distribution Pipe to Storm SewerCatch BasinOverflow to UnderdrainUnderdrainInternal Water StorageMulchFilter MediaGeotextile LinerGeotextile LinerGeotextile LinerChoker layerClear Stone AggregateUncompacted Subgrade SoilCompacted Subgrade Soil
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[edit]

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.[5], [6] 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[edit]

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[edit]

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" [7]

UpbyRoots JU.png


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 [8]

Inspection and Maintenance[edit]

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:

Cover Photo.PNG

Performance[edit]

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.[9] 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. [10] [11] 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. [12] 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%. [13] [14]

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)[15]
Connecticut 99% Dietz and Clausen (2005) [16]
Pennsylvania 80% Ermilio (2005)[17]
Pennsylvania 70% Emerson and Traver (2004)[18]
Bioretention with underdrain
Texas 82%* Mahmoud, et al. (2019)[19]
China 35 to 75%* Gao, et al. (2018)[20]
Ohio 36 to 59% Winston et al. (2016). [21]
Virginia 97 to 99% DeBusk and Wynn (2011)[22]
Maryland and North Carolina 20 to 50% Li et al. (2009). [23]
North Carolina 40 to 60% Smith and Hunt (2007)[24]
North Carolina 33 to 50% Hunt and Lord (2006). [25]
Bioretention with underdrain & liner
Ontario 15 to 34% STEP (2019) [26]
Queensland, Australia 33 to 84% Lucke and Nichols (2015). [27]
Victoria, Australia 15 to 83% Hatt et al. (2009). [28]
Maryland 49 to 58% Davis (2008). [29]
Runoff Reduction Estimate* 85% without underdrain;

45% with underdrain | 30% with underdrain and liner

Performance research[edit]

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.[30] 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. [31] 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. [32]

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) [33]
    • 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) [34]
    • 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) [35]
    • 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) [36]
    • 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)[37]
    • 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)[38]
    • 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)[39]
    • 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)[40]
    • 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)[41]
    • 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[42]
    • 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[edit]

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.

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.

Gallery[edit]

Open tree pits[edit]

  • 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[edit]

  • 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

References[edit]

Also see references as direct web page links above.

  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/
  3. ↑ City of Toronto. 2020. Construction Specifications and Drawings for Sewers and Watermains. https://www.toronto.ca/services-payments/building-construction/infrastructure-city-construction/construction-standards-permits/standards-for-designing-and-constructing-city-infrastructure/construction-specifications-for-sewers-and-watermains/
  4. ↑ City of Toronto. 2020. Construction Specifications and Drawings for Sewers and Watermains. https://www.toronto.ca/services-payments/building-construction/infrastructure-city-construction/construction-standards-permits/standards-for-designing-and-constructing-city-infrastructure/construction-specifications-for-sewers-and-watermains/
  5. ↑ Berland, A., Shiflett, S.A., Shuster, W.D., Garmestani, A.S., Goddard, H.C., Herrmann, D.L. and Hopton, M.E. 2017. The role of trees in urban stormwater management. Landscape and urban planning, 162, pp.167-177. https://pdf.sciencedirectassets.com/271853/1-s2.0-S0169204617X00030/1-s2.0-S0169204617300464/Adam_Berland_green_infrastructure_2017.pdf
  6. ↑ Kuehler, E., Hathaway, J. and Tirpak, A. 2017. Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network. Ecohydrology, 10(3), p.e1813. https://www.srs.fs.usda.gov/pubs/ja/2017/ja_2017_kuehler_001.pdf
  7. ↑ Urban, J. 2008. Up By Roots: Healthy soils and trees in the built environment. International Society of Arboriculture. http://www.jamesurban.net/up-by-roots#:~:text=Up%20By%20Roots%2C%20written%20by,trees%20in%20the%20built%20environment.
  8. ↑ Urban, J. 2014. Urban Soil and Site Assessment [Presentation]. University of Washington Botanic Gardens. Seattle, WA https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf.
  9. ↑ Page, J.L., Winston, R.J., Hunt, W.F. 2015. Soils beneath suspended pavements: An opportunity for stormwater control and treatment. Ecological Engineering. v.82. pp.40-48. https://www.sciencedirect.com/science/article/abs/pii/S0925857415001706
  10. ↑ STEP. 2018. The Queensway Sustainable Sidewalk Project https://sustainabletechnologies.ca/app/uploads/2018/10/Queensway-Case-Study_FINAL.pdf
  11. ↑ STEP. 2020. Assessing the Health of Toronto Street Trees Irrigated by Stormwater. https://sustainabletechnologies.ca/app/uploads/2020/09/Soil-cells-tech-brief-FINAL.pdf
  12. ↑ Li, J., Alinaghaian, S., Joksimovic, D., Chen, L. An Integrated Hydraulic and Hydrologic Modeling Approach for Roadside Bio-Retention Facilities. Water. 2020, 12, 1248 https://www.mdpi.com/2073-4441/12/5/1248
  13. ↑ Credit Valley Conservation. 2016. Central Parkway: Road Right-of-Way Retrofits - Case Study. https://cvc.ca/wp-content/uploads/2016/06/CaseStudy_CPW_Final.pdf
  14. ↑ Credit Valley Conservation. 2016. Central Parkway: Low Impact Development Infrastructure Performance and Risk Assessment - Technical Report. https://cvc.ca/wp-content/uploads//2021/07/TechReport_CPW_Final.pdf
  15. ↑ Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.
  16. ↑ Dietz, M.E. and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. Water Air and Soil Pollution. Vol. 167. No. 2. pp. 201-208. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.365.9417&rep=rep1&type=pdf
  17. ↑ Ermilio, J.F., 2005. Characterization study of a bio-infiltration stormwater BMP (Doctoral dissertation, Villanova University). https://www1.villanova.edu/content/dam/villanova/engineering/vcase/vusp/Ermilio-Thesis06.pdf
  18. ↑ Emerson, C., Traver, R. 2004. The Villanova Bio-infiltration Traffic Island: Project Overview. Proceedings of 2004 World Water and Environmental Resources Congress (EWRI/ASCE). Salt Lake City, Utah, June 22 – July 1, 2004. https://ascelibrary.org/doi/book/10.1061/9780784407370
  19. ↑ Mahmoud, A., Alam, T., Rahman, M.Y.A., Sanchez, A., Guerrero, J. and Jones, K.D. 2019. Evaluation of field-scale stormwater bioretention structure flow and pollutant load reductions in a semi-arid coastal climate. Ecological Engineering, 142, p.100007. https://www.sciencedirect.com/science/article/pii/S2590290319300070
  20. ↑ Gao, J., Pan, J., Hu, N. and Xie, C., 2018. Hydrologic performance of bioretention in an expressway service area. Water Science and Technology, 77(7), pp.1829-1837.
  21. ↑ Winston, R.J., Dorsey, J.D. and Hunt, W.F. 2016. Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio. Science of the Total Environment, 553, pp.83-95.
  22. ↑ DeBusk, K.M. and Wynn, T.M., 2011. Storm-water bioretention for runoff quality and quantity mitigation. Journal of Environmental Engineering, 137(9), pp.800-808. https://www.webpages.uidaho.edu/ce431/Articles/DeBusk-ASCE-2011.pdf
  23. ↑ Li, H., Sharkey, L.J., Hunt, W.F., and Davis, A.P. 2009. Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland. Journal of Hydrologic Engineering. Vol. 14. No. 4. pp. 407-415.
  24. ↑ Smith, R and W. Hunt. 2007. Pollutant removals in bioretention cells with grass cover. Proceedings 2nd National Low Impact Development Conference. Wilmington, NC. March 13-15, 2007.
  25. ↑ Hunt, W.F. and Lord, W.G. 2006. Bioretention Performance, Design, Construction, and Maintenance. North Carolina Cooperative Extension Service Bulletin. Urban Waterways Series. AG-588-5. North Carolina State University. Raleigh, NC.
  26. ↑ STEP. 2019. Comparative Performance Assessment of Bioretention in Ontari0. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.pdf.
  27. ↑ Lucke, T., & Nichols, P. W. B. 2015. The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation. Science of The Total Environment, 536, 784-792. doi:http://dx.doi.org/10.1016/j.scitotenv.2015.07.142
  28. ↑ Hatt, B. E., Fletcher, T. D., & Deletic, A. 2009. Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale. Journal of Hydrology, 365(3), 310-321. doi:http://dx.doi.org/10.1016/j.jhydrol.2008.12.001
  29. ↑ Davis, A.P. 2008. Field performance of bioretention: Hydrology impacts. Journal of hydrologic engineering, 13(2), pp.90-95. https://ascelibrary.org/doi/abs/10.1061/(ASCE)1084-0699(2008)13:2(90)
  30. ↑ Berland, A., Shiflett, S.A., Shuster, W.D., Garmestani, A.S., Goddard, H.C., Herrmann, D.L., Hopton, M.E. The role of trees in urban stormwater management. Landscape and Urban Planning. v.162. pp.167-177. https://www.sciencedirect.com/science/article/abs/pii/S0169204617300464?via%3Dihub
  31. ↑ Xiao, Q., McPherson, E.G.. 2016. Surface water storage capacity of twenty tree species in Davis, California. Journal of Environmental Quality. v.45. pp. 188-198. https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/jeq2015.02.0092
  32. ↑ Scharenbroch, B.C., Morgenroth, J., Maule, B. 2016. Tree species suitability to bioswales and impact on the urban water budget. Journal of Environmental Quality. v.45. pp. 199-206. https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/jeq2015.01.0060
  33. ↑ Selbig, W.R., Loheide II, S.P., Shuster, W., Scharenbroch, B.C., Coville, R.C., Kruegler, J., Avery, W., Haefner, R., Nowak, D. Quantifying stormwater runoff volume reduction benefit of urban street tree canopy. Science of the Total Environment. v.806 (2022) 151296. https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub
  34. ↑ Vega, O.M. Green infrastructure in the City of Vancouver: performance monitoring of stormwater tree trenches and bioswales. UBC Theses and Dissertations. https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388
  35. ↑ Elliott, R.M., Adkins, E.R., Culligan, P.J, Palmer, M.I., Stormwater infiltration capacity of street tree pits: Quantifying the influence of different design and management strategies in New York City. Ecological Engineering. v.111. pp. 157-166. https://www.sciencedirect.com/science/article/abs/pii/S0925857417306365
  36. ↑ Grey, V., Livesley, S.J., Fletcher, T.D. and Szota, C. 2018. Tree pits to help mitigate runoff in dense urban areas. Journal of Hydrology, 565, pp.400-410. https://www.sciencedirect.com/science/article/abs/pii/S0022169418306346?via%3Dihub
  37. ↑ Tirpak, R.A., Hathaway, J.M., Franklin, J.A. and Khojandi, A. 2018. The health of trees in bioretention: A survey and analysis of influential variables. Journal of Sustainable Water in the Built Environment, 4(4), p.04018011. https://ascelibrary.org/doi/10.1061/JSWBAY.0000865
  38. ↑ Kuehler, E., Hathaway, J. and Tirpak, A. 2017. Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network. Ecohydrology, 10(3), p.e1813. https://www.srs.fs.usda.gov/pubs/ja/2017/ja_2017_kuehler_001.pdf
  39. ↑ Li, S., Dai, L., Wang, H., Wang, Y., He, Z., & Lin, S. (2017). Estimating leaf area density of individual trees using the point cloud segmentation of terrestrial LiDAR data and a voxel-based model. Remote sensing, 9(11), 1202. https://www.mdpi.com/2072-4292/9/11/1202/pdf
  40. ↑ Huang, J.Y., Black, T.A., Jassal, R.S. and Lavkulich, L.L. 2017. Modelling rainfall interception by urban trees. Canadian Water Resources Journal/Revue canadienne des ressources hydriques, 42(4), pp.336-348. https://www.researchgate.net/profile/LesLavkulich/publication/320085997_Modelling_rainfall_interception_by_urban_trees/links/59fc87bf0f7e9b9968bdc715/Modelling-rainfall-interception-by-urban-trees.pdf
  41. ↑ Abuelgasim, A. A., & Leblanc, S. G. (2011). Leaf area index mapping in northern Canada. International journal of remote sensing, 32(18), 5059-5076. https://www.academia.edu/download/55035075/Leaf_area_index_mapping_in_northern_Canada.pdf
  42. ↑ Washington Stormwater Center. 2022. Tree Project. https://www.wastormwatercenter.org/project/tree-project/

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.

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