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→Geometry and Site Layout
*5:1 on low permeability soils, such as [[Soil groups|hydrologic soil group (HSG) C and D]],
*5:1 on low permeability soils, such as [[Soil groups|hydrologic soil group (HSG) C and D]],
*20:1 on high permeability soils [[Soil groups|hydrologic soil group (HSG) Aand D]].
*20:1 on high permeability soils [[Soil groups|hydrologic soil group (HSG) Aand D]].
==Pre-Treatment==
Pre-treatment captures sediment before it reaches the filter bed. It is typically necessary unless runoff sediment load is very low (e.g. roof drainage). Pre-treatment options include:
*level spreaders
*stone filter inlets with geotextile fabric and,
catch basins with sump.
==Planting Soil & Filter Media==
Planting soil or filter media should come pre-mixed from an approved vendor.
==Underdrains==
[[Underdrains]] are recommended for bioswales where native soil infiltration rate <15 mm/h (hydraulic conductivity < 1x10<sup>-6</sup> cm/s), and needed for non-infiltrating designs. They are comprised of a length of perforated pipe embedded near the top of the storage reservoir, with an overlying choker layer of medium-sized aggregate, and structures to provide inspection and maintenance access. Alternatively, the perforated pipe could be installed on the reservoir bottom and connected to an upturned pipe assembly or riser. Another option is to include a flow restrictor (e.g. orifice cap or valve) on the underdrain outlet pipe, to optimize infiltration while meeting the required drainage time.
==Perforated Pipe==
Continuously perforated pipe should be either smooth interior HDPE or PVC pipe with diameter ≥200 mm to reduce freezing risk and facilitate access by camera and cleaning equipment. Perforated pipe extends length of facility and solid pipe is used to connect to storm drain system.
==Access Structures==
The use of maintenance holes/vertical standpipes connected to the perforated pipe system are used for inspection and flushing, ensure couplings used for standpipe connections are 45° to facilitate pipe access by camera or cleaning equipment. (See below for example of couplings attaching to an underdrain pipe):
[[File:45 degs.PNG]]
==Access Structures==