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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.
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==
==Spacing drainage pipes==
[[File:Drain spacing.jpg|thumb|The yellow box represents the recommended hydraulic conductivity of bioretention filter media]]
In most LID underdrain applications, lateral drains should be spaced between 5 - 6 m apart.
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This recommendation is supported by an analysis of Hooghoudt's equation <ref>H.P.Ritzema, 1994, Subsurface flow to drains. Chapter 8 in: H.P.Ritzema (ed.), Drainage Principles and Applications, Publ. 16, pp. 236-304, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90-70754-33-9</ref><ref>W.H. van der Molen en J.Wesseling, 1991. A solution in closed form and a series solution to replace the tables for the thickness of the equivalent layer in Hooghoudt's drain spacing equation. Agricultural Water Management 19, pp.1-16</ref><ref>van Beers, W.F.J. 1976, COMPUTING DRAIN SPACINGS: A generalized method with special reference to sensitivity analysis
and geo-hydrological investigations, International Institute for Land Reclamation and Improvement (ILRI) Wageningen, The Netherlands</ref> in relation to loamy or clayey native soils, where ''K<sub>media</sub>''>>''K<sub>soil</sub>'', finds the first term of the numerator negligible, so that the original equation:
<math>Drain\ spacing=\sqrt{\frac{8K_{soil}H\left(D_{i}-D_{d}\right)\left(D_{d}-D_{w}\right)+4K_{soil}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>
may be simplified to:
<math>Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>
Where:
''K<sub>media</sub> is expressed in m/day
''D<sub>d</sub>'' is the depth to the drain pipe (m)
''D<sub>w</sub>'' is the minimum acceptable depth to the water table during infiltration event
''q'' is the inflow volume expressed as a depth over the entire surface (m)
===Example===
During a 25 mm storm event, a bioretention cell receives concentrated flow from a catchment 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m).
The bioretention cell comprises 0.6 m filter media (K = 2.4 m/day), laid over 0.6 m clear, coarse reservoir gravel. The pipes are laid within the reservoir, 0.9 m below the surface.
The system is designed to fill entirely during the rainstorm event. i.e. Depth to water table = 0 m.:
<math>Drain\ spacing=\sqrt{\frac{4\times 2.4m/day\left(0.9m-0m\right)^{2}}{0.5m/day}}=6 m</math>
==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.