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Line 69: − There are many design configuration options for including trees into stormwater management plans. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
→Design
==Design==
==Design==
===Geometry and Site Layout===
Tree trenches are often modular systems connected hydrologically through sub-surface stormwater distribution and drainage pipes. Road runoff may be directed to the trench via curb cuts or depressed drains located at tree openings.
===Inlets===
Water can enter the tree trench in a variety of ways: from the overlying sidewalk via sheet flow or curb cuts into tree openings, trench drains or infiltration through permeable pavement; and from the road via distribution pipes connected to road or side inlet catch basins and curb cuts or depressed drains at tree openings. It is recommended that each tree trench have multiple inlets to keep the contributing drainage area relatively small, which provides redundancy to the system. Inlet structures and distribution pipes should be offset from tree root ball locations to avoid impact of de-icing salt laden runoff on newly planted trees during establishment.
===Pre-Treatment===
If water enters the trench via a catch basin, a removable pre-treatment device, like a Goss trap or proprietary catch basin insert device or filter should be included to help retain coarse sediment, debris and floatables and prevent it from entering the pipe or trench. Inlet structures should have a sump and curb cut inlets should include stone diaphragms or stone mulch to dissipate energy and spread flows. Pre-treatment features should be easy to access and clean.
===Soil Volume===
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 m<sup>3</sup> of soil per tree may be acceptable.
===Modular Soil Support Systems===
Modular soil support systems (also referred to as “soil cells”) consist of plastic or concrete structures, available in a variety of shapes and sizes, that provide structural support for the overlying pavement while providing uncompacted planting soil within the tree root zone. They are installed adjacent to tree pits to provide room for roots to spread out under the supported pavement portion of the trench. Growing medium backfill typically has higher organic matter content than structural soil medium. The looser structure and higher nutrient content of the growing medium provides the most favorable environment for healthy tree growth in an urban setting.
===Structural Soil Medium===
Structural soil is an engineered soil medium that can be compacted to support sidewalk or roadway pavement installation requirements while also permitting tree root growth. Structural soil medium filled trenches are installed adjacent to tree pits to provide room for tree roots to spread out under the supported pavement portion of the tree trench.
===Structural Concrete Panels===
Trenches where the overlying pavement consists of structural concrete panels supported on each side by concrete footings and rows of modular soil support structures installed on aggregate bases is another configuration. The benefit of this approach is that the native subgrade soil under the portions of the trench below tree pits and between rows of supports does not need to be highly compacted, allowing greater opportunity for drainage via infiltration (see image map within the 'Overview' section of this page).
===Conveyance and Overflow===
Runoff is directed from overlying and adjacent pavements to the trench through such means as tree openings, perforated distribution pipes connected to catchbasins or trench drains, or curb cuts and depressed drains to tree openings. Runoff water percolates through the growing or structural soil medium to the underlying native subgrade soil. When runoff volume exceeds the trench water storage capacity, the perforated underdrain pipe directs excess filtered water to a downstream outlet storm sewer or other practice. During intense storm events, runoff in excess of the infiltration capacity of the growing or structural soil medium will overflow to the storm sewer either through an outlet pipe connection in the catch basin or via surface overflow standpipes or structures within tree openings.
===Configuration===
Structural concrete panel and modular soil support system 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 to being fit around utilities or existing trees and provide easier access to utilities when repairs are needed.
===Distribution and Underdrain pipes===
To maximize the quantity of growing or structural soil medium irrigated, distribution pipes should be installed flat, just below modular soil support tops or at the top of the structural soil media layer and in both tree pit and supported pavement portions of the trench. Pipe perforations should be oriented to the sides and section ends should be sealed with a solid cap. To enhance runoff volume reduction underdrain pipes can be installed above the bottom of the trench and/or include flow control. Alternatively, the underdrain pipe may be installed on trench bottom and connected to a riser assembly in the outlet manhole. It is critical to include connections to outlet storm sewer pipes and multiple cleanout access points.
===General Specifications===
====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%.
* Should have a pH value between 6.0 and 8.0.
* Percent organic matter shall be 3-5%, by dry weight.
* Soluble salt level shall be less than 2 mmhos/cm.
* Growing medium should be compacted to 80-90% below the tree root ball to prevent settling.
* Bioretention filter media (see [[LID BMP Fact Sheets|Bioretention Fact Sheet]]) may be suitable for use as growing medium, depending on climate and tree species.
====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 shall be structurally independent of all adjacent structures, such that one or multiple layers can be removed after the completion of the installation 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%).
** 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 to5% 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.
* Total moisture at mixing should be 10% as per AASHTO T-99 optimum moisture.
* 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.
* The entire pile is turned and mixed until a uniform blend is produced. The structural soil is then installed and compacted in 150 mm lifts.
==Soil cells==
==Soil cells==