Underdrains

Underdrains comprise a length of perforated pipe embedded into a layer of reservoir aggregate. They are an optional component of bioretention systems, bioswales, permeable pavements and stormwater tree trenches, and a required component of stormwater planters. Underdrain perforated pipes should be located below the frost line to reduce the threat of ice clogging. Ontario provincial standard drawings of frost penetration depth are available from the Ministry of Transportation as OPSD 3090.100 for northern Ontario Ministry of Transportation. 2010. Foundation Frost Penetration Depths for Northern Ontario. OPSD 3090.100. Nov 2010. Rev.1. https://www.library.mto.gov.on.ca/SydneyPLUS/TechPubs/Portal/tp/opsViews.aspx and OPSD 3090.101 for southern Ontario Ministry of Transportation. 2010. Foundation Frost Penetration Depths for Southern Ontario. OPSD 3090.101. Nov 2010. Rev.1. https://www.library.mto.gov.on.ca/SydneyPLUS/TechPubs/Portal/tp/opsViews.aspx . Underdrains featuring perforated pipes require connections to standpipes or structures that provide access to the pipe for inspection and maintenance purposes. Underdrain designs vary according to whether or not the facility is designed to infiltrate stormwater.
==Underdrains for infiltrating practices==
The perforated pipe within the drain should be elevated from the base to promote infiltration of the water stored beneath. The depth of this internal water storage reservoir should be sized to capture and infiltrate the design storm event runoff volume based on the desired drainage time and the design infiltration rate of the native soils below. An alternative design configuration is to install the perforated pipe on the base of the practice and using an upturned outflow pipe to permit the required head of water to be stored. 
*Underdrain access structures/cleanouts, which may be maintenance holes or vertical standpipes connected to the perforated pipe, must be included in the design for periodic inspection and flushing of the perforated pipe. Negotiating 90 degree bends will be troublesome for most push camera and jet nozzle cleaning equipment, so it is preferable that 45 degree pipe couplings be used instead. 
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)Province of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332.
Clean out spacingProvince of Ontario. (2022). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332
Pipe internal diameter (mm)	Maximum distance between cleanouts (m)
100 	15
200 or greater 	30
In some cases where the underdrain layer has sufficient depth to accommodate it, a larger diameter perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity to a bioretention or a bioswale project. Ultimately this idea may result in the use of infiltration chambers or other void-forming structures to create significant reservoir storage beneath a bioretention filter media bed or permeable pavement. Be sure to check with manufacturers about the compatibility of their systems with trees.
See the Flow through perforated pipe for underdrain capacity equation with LID BMPs.
==Underdrains for non-infiltrating practices==
===Below ground===
Where a stormwater planter or biofiltration cell is contained within a concrete box or completely lined to prevent infiltration, the perforated pipe should be bedded on a thin layer of fine aggregate. This thin layer is to hold the pipe in place during construction, and to permit free ingress of accumulated water through holes on the underside of the pipe. As storage in a non-infiltrating practice is predominantly through soil/water tension, the depth of reservoir should be minimised to just accommodate the pipe. 
A pair of vertical clean out pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sediment. As most hydro-jetting apparatus used for this has some trouble accommodating narrow 90 degree bends, it is important that both ends of a perforated pipe be connected with a pair of 45 degree elbows or Y couplings instead.
===Above ground===
Where possible the underdrain pipe should be designed without any bends for easy inspection and maintenance. Otherwise see advice above regarding pipe couplings. 
*To optimize water distribution and promote infiltration the base of the storage reservoir and the underdrain pipe should be graded level. 
*Where drainage or conveyance to a downstream facility is a greater priority, the base of the reservoir and the underdrain pipe may have a gradient of up to 1-2%.
==Inspection and maintenance==
thumb|Schematic of pipes and connectors
*To permit access by cameras or cleaning apparatus, 90 degree pipe couplings must not be used in subterranean underdrains. Instead 2 x 45 degree couplings, or 3 x 30 degree couplings should be used (see figure to the right). 
*For the same reason, dual walled perforated pipes with smooth internal walls are highly recommended to reduce the potential snagging of maintenance equipment. 
*The recommended distances between clean outs is based on advice for filter beds in the Ontario Building CodeProvince of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332. However, the access capabilities of difference apparatus and contractors varies and designers are advised to take advice from maintenance operators in this matter. 
*Wells
==Material specifications==
===Pipes===
===Reservoir gravel===
===Choker layer===
 Geotextiles may be used to separate the underdrain from native soils on the vertical faces.
==Alternative technologies==
* Smart Drain is a polymer ribbon-like material with capillary drains on the underside; it's use has recently been demonstrated in bioretentionRedahegn Sileshi; Robert Pitt, P.E., M.ASCE; and Shirley Clark, P.E., M.ASCE Performance Evaluation of an Alternative Underdrain Material for Stormwater Biofiltration Systems, Journal of Sustainable Water in the Built Environment, 4(2), May 2018 https://doi.org/10.1061/JSWBAY.0000845 . It's low profile may make it particularly well suited to non-infiltrating practices, such as Stormwater planters. 
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