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→Design
===Design===
===Design===
<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.
*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%.
*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.
===Materials===
===Materials===