Changes

Underdrains comprise a length of perforated pipe embedded into a layer of [[reservoir gravel]]. They are a key component and vary hugely according to the drainage requirements of the installation, and the available maintenance access.  

Underdrains comprise a length of perforated pipe embedded into a layer of [[reservoir gravel]]. They are a key component and vary hugely according to the drainage requirements of the installation, and the available maintenance access.  

===Infiltrating practices below ground===

==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 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.  

*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.  

*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.  

{| class="wikitable"

{| class="wikitable"

|+Clean out spacing

|+Clean out spacing

| 30

| 30

|}

|}

In a [[bioretention]] facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir gravel has a void ratio of 0.4, whilst most bioretention [[filter media]] may have a void ratio of 0.3 or lower.  

In a [[bioretention]] facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir gravel has a void ratio of 0.4, whilst most bioretention [[filter media]] may have a void ratio of 0.3 or lower.  

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 retention chambers to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatability 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 retention chambers to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatability of their systems with [[trees]].

===Non-infiltrating practices below ground===

==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 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.  

===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.  

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.  

===Non-infiltrating practices 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 in order to facilitate easy maintenance. Otherwise see advice above regarding connectors.