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Line 17: In a [[bioretention]] facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir aggregate has a void ratio of 0.4, whilst most bioretention [[filter media]] may have a void ratio of 0.3 or lower.
===Example===During a 25 mm storm event, a bioretention cell receives concentrated flow from a catchment 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m). The bioretention cell comprises 0.6 m filter media (K = 2.4 m/day), laid over 0.6 m clear, coarse reservoir gravel. The pipes are laid within the reservoir, 0.9 m below the surface. The system is designed to fill entirely during the rainstorm event. i.e. Depth to water table = 0 m.: <math>Drain\ spacing=\sqrt{\frac{4\times 2.4\ m/day\left(0.9\ m-0\ m\right)^{2}}{0.5\ m/day}}=6\ m</math> Line 32:
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Line 44: ===Geotextiles==={{:Geotextiles}}
→Above ground
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==Underdrains for exfiltrating practices==
==Underdrains for infiltrating practices==
The 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 should be sized according to the desired drainage time and the infiltration rate of the native soils below. An alternative design configuration permits the head of water to be stored by using an upturned outflow pipe.
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.
*At least one pair of vertical cleanout 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.
*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"
{| 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>
|+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>
| 30
| 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]].
In some cases where the underdrain layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity. Ultimately this idea may result in the use of [[infiltration chambers]] to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with [[trees]].
==Underdrains for non-infiltrating practices==
==Underdrains for non-exfiltrating practices==
===Below ground===
===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.
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.
===Above ground===
===Above ground===
Where possible the underdrain pipe should be designed without any bends in order to facilitate easy maintenance. Otherwise see advice above regarding connectors.
Where possible the underdrain pipe should be designed without any bends for easy inspection and maintenance. Otherwise see advice above regarding connectors.
*To promote infiltration the base of the gravel reservoir and the underdrain pipe should be horizontal to optimize distribution of the water within.
*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%.
[[File:45 degs.PNG|thumb|Schematic of pipes and connectors]]
[[File:45 degs.PNG|thumb|Schematic of pipes and connectors]]
[[File:Jet cleaning.jpg|thumb|Diagram of hydrojetting cleaning apparatus]]
[[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 instead.
*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.
*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.
*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.
===Reservoir gravel===
===Reservoir gravel===
{{:Reservoir gravel}}
{{:Reservoir gravel}}
===Choker course===
===Choker layer===
{{:Choker layer}}
{{:Choker layer}} Geotextiles may be used to separate the underdrain from native soils on the vertical faces.
==Alternative Technology==
==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]].
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]]
[[File:Smart drain.jpg|thumb|Ribbon-like drainage material]]
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