Enhanced swales: Specifications

Specifications for Enhanced Swales
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)
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* 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.