Underdrains

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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 for exfiltrating practices

The pipe within the drain should be elevated from the base to promote infiltration of the water stored beneath. The depth of this internal water storage should be sized according to the desired drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). and the infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of the native soils below. An alternative design configuration permits the head of water to be stored by using an upturned outflow pipe.

  • At least one pair of vertical cleanout pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.. 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.
Clean out spacing[1]
Pipe internal diameter (mm) Maximum distance between cleanouts (m)
100 15
200 or greater 30

In a bioretention facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. has a void ratio of 0.4, whilst most bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter media may have a void ratio of 0.3 or lower. In some cases where the underdrainA perforated pipe used to assist the draining of soils. 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 infiltration chambers to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with trees.

Spacing drainage pipes to reduce groundwater mounding

The yellow box represents the recommended hydraulic conductivityA parameter that describes the capability of a medium to transmit water. of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

In most LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. underdrainA perforated pipe used to assist the draining of soils. applications, lateral drains should be spaced between 5 - 6 m apart.


This recommendation is supported by an analysis of Hooghoudt's equation [2][3][4] in relation to loamy or clayey native soils, where Kmedia>>Ksoil, finds the first term of the numerator negligible, so that the original equation\[Drain\ spacing=\sqrt{\frac{8K_{soil}H\left(D_{i}-D_{d}\right)\left(D_{d}-D_{w}\right)+4K_{soil}\left(D_{d}-D_{w}\right)^{2}}{q}}\] may be simplified to\[Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}\]

Where:

  • Kmedia is expressed in m/day
  • Dd is the depth to the drain pipe (m)
  • Dw is the minimum acceptable depth to the water tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body.Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone. 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 bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell receives concentrated flow from a catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m). The bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell comprises 0.6 m filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. (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 tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body.Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone. = 0 m.\[Drain\ spacing=\sqrt{\frac{4\times 2.4\ m/day\left(0.9\ m-0\ m\right)^{2}}{0.5\ m/day}}=6\ m\]

Underdrains for non-exfiltrating practices

Below ground

Where a stormwater planterA vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration. or biofiltration cell is contained within a concrete box or completely lined to prevent infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., the perforated pipe should be bedded on a thin layer of fine aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.. 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. A pair of vertical clean out pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.. 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.

Above ground

Where possible the underdrainA perforated pipe used to assist the draining of soils. pipe should be designed without any bends in order to facilitate easy maintenance. Otherwise see advice above regarding connectors.

  • To promote infiltration the base of the gravel reservoir and the underdrainA perforated pipe used to assist the draining of soils. 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 underdrainA perforated pipe used to assist the draining of soils. pipe may have a gradient of up to 1-2%.

Maintenance and inspection

Schematic of pipes and connectors
Diagram of hydrojetting cleaning apparatus

To permit access by cameras or cleaning apparatus, 90 degree connectors must not be used in subterranean underdrains. Instead 2 x 45 degree connectors, or preferably 3 x 30 degree connectors should be used instead.

For the same reason, dual walled perforated pipes with smooth internal walls are necessary to reduce the potential snagging of maintenance equipment.

Wells

  • Wells, of 100 - 150 mm diameter perforated pipe, should extend to the bottom of the facility.
  • The exposed tops of all wells should be fitted with lockable caps.

Material specifications

Pipes

Pipes are available with perforations on just one side, these should be situated on the lower half of the pipe. Pipes with 360° perforations should have a strip of geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. or membrane placed over the pipe to reduce the migration of finesSoil particles with a diameter less than 0.050 mm. from overlying media.

Perforated pipes are a common component of underdrains, infiltration trenches and exfiltration trenches.

Pipes should have been manufactured in conformity with the latest standards by the Canadian Standards Association (CSA) or ASTM International.

  • Perforated pipes should be continuously perforated, smooth interior HDPE (or equivalent material) with a minimum inside diameter of 100 mm.
    • Wherever possible, pipes should be ≥ 200 mm internal diameter.
    • Smooth interior facilitates inspection and maintenance activities; internal corrugations can cause cameras or hydrojetting apparatus to become snagged.
    • A perforated pipe with many rectangular slots has better drainage characteristics than a pipe with similar open area provided by fewer circular holes [5].
  • Non-perforated pipes should be used for conveyance to and away from the facility, including overflow. It is good practice to extend the non-perforated pipe approximately 300 mm within the reservoir or practice to reduce the chance of migration from native soils clogging the pipe at the interface.

See also: flow through perforated pipe


Reservoir gravel

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. to be used to store water for infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.. This is usually called 'Clear stone' at aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. yards.

To see an analysis of Ontario Standard Specifications for granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. materials, see OPSS aggregates.

For advice on decorative surface aggregatesA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. see Stone


Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a void ratio of 0.4[6].

The clean wash to prevent rapid accumulation of finesSoil particles with a diameter less than 0.050 mm. from the aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the void ratio). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles [7]. See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

  • Minimum durability index of 35
  • Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregatesA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. are maintained by ASTM D2940


Choker course

medium sized granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices., free from finesSoil particles with a diameter less than 0.050 mm.

In bioretention systems a choking layer of ≥ 100 mm is the recommended method to prevent migration of finer filter media into an underlying reservoir of coarse aggregate. These same mid sized granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. materials are recommended for use in Stormwater planter underdrains and may be useful in the fine grading of foundations courses for permeable paving.

Suitable materials include:

High performance bedding (HPB)
Clean, angular aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. screened to between 6 - 10 mm. Widely available and designed specifically for drainage applications. Free from finesSoil particles with a diameter less than 0.050 mm. by definition.
HL 6
Is a clean, angular aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. screened between 10 - 20 mm. Free from finesSoil particles with a diameter less than 0.050 mm. by definition.
Pea Gravel
Rounded natural aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations., screened between 5 - 15 mm, and washed free from finesSoil particles with a diameter less than 0.050 mm..

In most scenarios, a geotextile layer is unnecessary and have been associated with rapid decline and clogging in some circumstances.

Geotextiles

The properties of geotextiles vary widely.

See Clogging for notes on their application in LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. structures.

Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregatesA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations., and slump of heavier particles into finer underlying courses. The formation of biofilm on geotextiles has also been shown to improve water quality:

  • By degrading petroleum hydrocarbons[8]
  • By reducing organic pollutant and nutrient concentrations [9]
  • When installing geotextiles an overlap of 150 - 300 mm should be used.

Material specifications should conform to OPSS 1860 for Class II geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. fabrics [10].

  • Fabrics should be woven monofilament or non-woven needle punched.
  • Woven slit film and non-woven heat bonded fabrics should not be used, as they are prone to clogging.

In choosing a product, consider:

  1. The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
  2. The load bearing ratio of the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. (i.e. is the geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. needed to prevent downward migration of aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. into the native soilThe natural ground material characteristic of or existing by virtue of geographic origin.?),
  3. The texture (i.e., grain size distribution) of the overlying and underlying materials, and
  4. The suitable apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, to maintain water flow even with sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls. and microbial film build-up.
Recommended criteria for selection of geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. fabric
Percent soil/filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. passing 0.075 mm (#200 sieve) Non-woven fabric apparent opening size (AOS, mm) Woven fabric percent open area (POA, %) Permittivity (sec-1)
>85 ≤ 0.3 - 0.1
50 - 85 ≤ 0.3 ≥ 4 0.1
15 - 50 ≤ 0.6 ≥ 4 0.2
5 - 15 ≤ 0.6 ≥ 4 0.5
≤ 5 ≤ 0.6 ≥ 10 0.5

Performance research

http://www.mdpi.com/2073-4441/7/4/1595/htm


Alternative Technology

Smart drain is a polymer ribbon-like material with capillary drains on the underside; it's use has recently been demonstrated in bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[11]. It's low profile may make it particularly well suited to non-infiltrating practices, such as Stormwater planters.

Ribbon-like drainage material

  1. Province of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332
  2. 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
  3. 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
  4. 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
  5. Hazenberg, G., and U. S. Panu (1991), Theoretical analysis of flow rate into perforated drain tubes, Water Resour. Res., 27(7), 1411–1418, doi:10.1029/91WR00779.
  6. Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017
  7. 7.0 7.1 7.2 Judge, Aaron, "Measurement of the Hydraulic Conductivity of Gravels Using a Laboratory Permeameter and Silty Sands Using Field Testing with Observation Wells" (2013). Dissertations. 746. http://scholarworks.umass.edu/open_access_dissertations/746
  8. Newman AP, Coupe SJ, Spicer GE, Lynch D, Robinson K. MAINTENANCE OF OIL-DEGRADING PERMEABLE PAVEMENTS: MICROBES, NUTRIENTS AND LONG-TERM WATER QUALITY PROVISION. https://www.icpi.org/sites/default/files/techpapers/1309.pdf. Accessed July 17, 2017.
  9. Paul P, Tota-Maharaj K. Laboratory Studies on Granular Filters and Their Relationship to Geotextiles for Stormwater Pollutant Reduction. Water. 2015;7(4):1595-1609. doi:10.3390/w7041595.
  10. ONTARIO PROVINCIAL STANDARD SPECIFICATION METRIC OPSS 1860 MATERIAL SPECIFICATION FOR GEOTEXTILES. 2012. http://www.raqsb.mto.gov.on.ca/techpubs/OPS.nsf/0/2ccb9847eb6c56738525808200628de1/$FILE/OPSS%201860%20Apr12.pdf. Accessed July 17, 2017
  11. Redahegn Sileshi; Robert Pitt, P.E., M.ASCE; and Shirley Clark, P.E., M.ASCE Performance Evaluation of an Alternative Underdrain Material for Stormwater Biofiltration Systems, Journal of Sustainable Water in the Built Environment, 4(2), May 2018 https://doi.org/10.1061/JSWBAY.0000845