Changes

Underdrains comprise a length of perforated [[pipe]] embedded into a layer of [[reservoir aggregate]]. They are an optional component of [[bioretention]] systems, [[stormwater planters]], [[swales]] and [[soil cells]] used to support urban [[trees]]. Their design varies according to the drainage requirements of the installation, and the available maintenance access.

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.

*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 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.  

==Spacing drainage pipes==

===Spacing drainage pipes to reduce groundwater mounding===

In most LID underdrain applications, drains should be spaced between 5 - 6 m apart.

In most LID underdrain applications, lateral drains should be spaced between 5 - 6 m apart.

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This limits the flow path through the granular material and 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

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_media''>>''K_soil'', finds the first term of the numerator negligible, so that the original equation:

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_media''>>''K_soil'', 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>

<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:

may be simplified to:

<math>Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>

<math>Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}</math>

Where:

{{Plainlist|1=Where:

''K_media is expressed in m/day

*''K_media is expressed in m/day

''D_d'' is the depth to the drain pipe (m)

*''D_d'' is the depth to the drain pipe (m)

''D_w'' is the minimum acceptable depth to the water table during infiltration event

*''D_w'' 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)

*''q'' is the inflow volume expressed as a depth over the entire surface (m)}}

 

===Example===

===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).  

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 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.:     

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>

<math>Drain\ spacing=\sqrt{\frac{4\times 2.4\ m/day\left(0.9\ m-0\ m\right)^{2}}{0.5\ m/day}}=6\ m</math>

 

 

 

 

==Recommendations for material specifications==

===Wells===

 

==Material specifications==

===Pipes===

===Pipes===

{{:Pipes}}

{{:Pipes}}

===Reservoir gravel===

===Reservoir gravel===

{{:Reservoir gravel}}

{{:Reservoir gravel}}

===Pea gravel===

===Choker course===

{{:Pea gravel}}

{{:Choker layer}}

 

===Geotextiles===

===Geotextiles===

{{:Geotextiles}}

{{:Geotextiles}}