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Underdrains comprise a length of perforated pipe embedded into a reservoir gravel layer.
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===
According to the storage and infiltration rate of the native soils below, the pipe within the drain should be elevated from the base to promote infiltration of the water stored beneath. An alternative design configuration permits the head of water to be stored by using an upturned outflow 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.
*To promote infiltration the base of the gravel reservoir and the underdrain pipe should be horizontal to optimize distribution of the water within.
*To promote infiltration the base of the gravel reservoir and the underdrain pipe should be horizontal to optimize distribution of the water within.
*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%.
*A minimum of 150 mm reservoir gravel should be laid beneath the perforated pipe.
*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 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 geotextile is usually then used to prevent migration of fines into the underdrain/reservoir space.
*A layer of geotextile is usually then used to prevent migration of fines into the underdrain/reservoir space.
A pair of small wells are recommended to inspect and periodically flush accumulated sediment from the underdrain pipe.
A pair of small wells are recommended to inspect and periodically flush accumulated sediment from the underdrain pipe.