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Line 1: *Geotextile fabric may be used to line the reservoir space. *A minimum of 150 mm reservoir gravel should be laid beneath the perforated pipe. *A minimum of 300 mm reservoir gravel should be laid over the perforated pipe. An exception would be where a small bioretention installation is being made in a stormwater planter and no compaction of the reservoir material is undertaken. Otherwise this material is required to protect the pipe from the compaction processes. *A layer of 75 - 100 mm pea gravel may then be included on top of the reservoir gravel to provide a smoother surface and reduce tearing to the geotextile. *A layer of geotextile is usually then used to prevent migration of fines into the underdrain/reservoir space.
{{:Geotextiles}}<h4>Pipes</h4> <h4>Reservoir gravel</h4>{{:Pea gravel}}
→Above ground
===Design===
Underdrains comprise a length of perforated [[pipe]] embedded into a layer of [[reservoir aggregate]]. They are an optional component of [[bioretention]] systems, [[stormwater planters]], [[swales]] and [[soil cells]] used to support urban [[trees]]. Their design varies according to the drainage requirements of the installation, and the available maintenance access.
<p>An underdrain comprises a length of perforated pipe embedded into a reservoir gravel layer. A pair of small wells are recommended to inspect and periodically flush accumulated sediment from the underdrain pipe.
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*To promote infiltration the base of the gravel reservoir and the underdrain pipe should be horizontal to optimize distribution of the water within.
==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 [[Bioretention: Internal_water_storage| 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, 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.
{| class="wikitable"
|+Clean out spacing<ref>Province of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332</ref>
|-
! 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. Be sure to check with manufacturers about the compatibility of their systems with [[trees]].
==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 deg bends, it is important that both ends of a perforated pipe be connected with a pair of 45 deg elbows/Y connectors instead.
===Above ground===
Where possible the underdrain pipe should be designed without any bends for easy inspection and maintenance. Otherwise see advice above regarding connectors.
*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%.
*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%.
===Materials===
==Maintenance and inspection==
<h4>Geotextile</h4>
[[File:45 degs.PNG|thumb|Schematic of pipes and connectors]]
[[File:Jet cleaning.jpg|thumb|Diagram of hydrojetting cleaning apparatus]]
*To permit access by cameras or cleaning apparatus, 90 degree connectors must not be used in subterranean underdrains. Instead 2 x 45 degree connectors, or preferably 3 x 30 degree connectors 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 Code<ref>Province of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332</ref>. 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===
{{:Pipes}}
{{:Pipes}}
===Reservoir gravel===
{{:Reservoir gravel}}
{{:Reservoir gravel}}
<h4>Pea gravel</h4>
===Choker layer===
{{:Choker layer}} Geotextiles may be used to separate the underdrain from native soils on the vertical faces.
==Alternative technology==
Smart Drain is a polymer ribbon-like material with capillary drains on the underside; it's use has recently been demonstrated in bioretention<ref>Redahegn 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 </ref>. It's low profile may make it particularly well suited to non-infiltrating practices, such as [[Stormwater planters]].
[[File:Smart drain.jpg|thumb|Ribbon-like drainage material]]
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