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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 reservoir gravel layer.
*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.
*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.
===Spacing drain pipe===
<math>Drain\ spacing=\sqrt{\frac{8K_{soil}\left(D_{i}-D_{d}\right)\left(D_{d}-D_{w}\right)+4K_{soil}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>