Enhanced swales: Specifications

Specifications for Stormwater Tree Trenches

Material	Specification	Quantity
Site Layout	Enhanced swales typically treat drainage areas of two hectares or less	Swale total width should be 2 metres or greater and bottom width between 0.75 and 3.0 metres. Swale length between check dams should be ≥ 5 m. Side slopes should be no steeper than 1:3 (33%) for mowing maintenance. Gentler slopes (e.g. 1:4 or 25%) are encouraged where runoff enters the swale as sheet flow. A maximum flow depth of 0.1 m is recommended during the design storm event
Inlets	For concentrated overland flow: Catch basins or other inlet structures should be located at all sag points in the gutter grade and immediately upgrade of median breaks, crosswalks and street intersections. Inlet types include curb openings (modified curbs, spillways), side inlet catch basins, trench drains or other pre-fabricated inlet structures. Spillways aid in turning flow 30, 45 or 90 degrees into the practice. Incorporate concrete aprons at curb opening or spillway locations to increase inflow effectiveness. If the inlet structure itself does not provide sedimentation or filtration pre-treatment, incorporate a pre-treatment feature at curb opening or spillway location to isolate sediment, trash and debris for ease of removal. Provide a 75 to 150 mm drop in elevation between the inlet invert and grass or mulch surface, pre-treatment feature or concrete apron.	Spillways turn flow into enhanced swale at 30, 45 or 90 degrees A 75 to 150 mm drop in elevation between the inlet invert and grass or mulch surface, pre-treatment feature or concrete apron.
Pre-treatment	Level spreader: A shallow trench structure (with concrete, metal or wood lip), graded to be level and installed parallel to the pavement edge or flush curb. Geotextile and stone filter inlets: Square or rectangular curb openings located directly over the practice, filled with clean aggregate, covered with a layer of geotextile filter fabric and stone, graded level or gently sloped and installed at concentrated overland flow inlets (e.g. curb cuts). Catch basin, manhole, or other inlet structure sumps in combination with a shield, baffle, trap, or filter insert device, or goss trap are used to pre-treat concentrated overland flow. They can be designed to retain both coarse and fine particulate sediments in the sump, and floatables (hydrocarbons, trash and debris). A variety of proprietary pre-treatment devices are available. Forebay: Constructed to an appropriated length to width ratio and sized to accommodate appropraite ponding volume of the surface ponding storage requirement. To be used with concentrated overland flow inlets.	Recommended sizing for Level spreader: 1.4 m of length for every 0.01 m³/s of inflow during the design storm event, width of 300 mm or 3 times inflow pipe diameter, depth of 200 mm or half the inflow pipe diameter. Used with overland sheet flow inlets. Elevation change of 75 to 100 mm from pavement to top of the stone cover. Stone cover may be 50 to 150 mm diameter crushed angular stone, river rock/beach stone or rip rap. Forebay: 2:1 length to width ratio and sized to accommodate ponding volume of 25%
Planting Soil/ Filter Media	Planting soil - use for enhanced grass swales Filter Media Blend A Filter Media Blend B Sand Topsoil Organic Material - Additives - Should be material low in available phosphorus such as leaf and yard waste compost, untreated woodchips, shredded paper or coir. Particle-size distribution Other Parameters - Phosphorus (Plant Available or Extractable)	For Planting Soil - hydraulic conductivity, saturated (ASTM D2434) at 85% maximum dry density (ASTM D698) should be of 15 - 300 mm/h. Filter Media Blend A: Drainage rate priority: Use when I:P ratio ≥15:1, 3 parts sand to 1 part organic material or additives, Porosity of 0.4, hydraulic conductivity, saturated (ASTM D2434) at 85% maximum dry density (ASTM D698) should be of 75 - 300 mm/h. Filter Media Blend B: Water quality treatment priority: Use when improved metals and phosphorus retention and/or more diverse planting options are desired, 3 parts sand to 2 parts topsoil to 1 part organic material or additives porosity of 0.35, hydraulic conductivity, saturated (ASTM D2434) at 85% maximum dry density (ASTM D698) should be 25 to 300 mm/h. Sand Should be coarse and have a fineness modulus index between 2.8 and 3.1 according to ASTM C33/C33M Topsoil Must contain at least 9%, and not greater than 36% clay-sized particles and have a sodium absorption ratio less than 15. Organic material Organic matter (ASTM F1647) should make up 3 to 10% of the filter media by dry weight. Additives: Typically 5 to 10% by volume of the filter media blend (follow product manufacturer instructions where applicable). Particle-size distribution (ASTM D7928): <25% silt-and clay-sized particles combined (smaller than 0.05 mm); 3 to 12% claysized particles (0.002 mm or smaller). Other parameters: Phosphorus (Plant Available or Extractable): should be between 10 and 40 ppm, and cation exchange capacity (ASTM D7503) >10 meq/100 g.
Check Dams	Low head dams to slow concentrated flow and promote settling and infiltration. Dam height depends on depth of ponded water that will infiltrate in the required drainage time. May be constructed of any resilient and waterproof material including concrete, metal and stone (typically <150mm rip rap) and may have spillways incorporated into their profile to direct water to the centre of the swale. Should include stone cover on the down-gradient side for erosion control. Check dam spacing should be based on the slope and desired ponding volume. They should be spaced far enough apart to allow access for maintenance equipment (e.g., mowers)	May be constructed of any resilient and waterproof material including concrete, metal and stone (typically <150mm rip rap)
Plants	Enhanced grass swales may be planted with sod or seed. If using seed, stabilize swale with erosion control blankets. Bioswale planting plans should feature a mixture of deeply rooting perennials adapted to both wet and dry conditions and local climate. If using a native seed mix, include a cover crop of oats, winter wheat, or rye to stabilize the swale in the short term. Road salt tolerance should be considered if facility will receive pavement runoff.	For Bioswale planting plans should feature a mixture of deeply rooting perennials adapted to both wet and dry conditions and local climate.

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.

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.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

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 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.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

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

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

The technique of removing pollutants from runoff as it infiltrates through the soil.

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.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

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.

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.

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 pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

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.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

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

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.

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)

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.

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).

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.

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

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

Penetration of water through the ground surface.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

Includes the protection of soil from dislocation by water, wind or other agents.

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.