Changes

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 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-exfiltrating practices==

==Underdrains for non-infiltrating 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%.