Exfiltration trenches

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Exfiltration trench under high flow and exfiltration conditions ExfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). trenches are similar to infiltration trenches but differ primarily in the the manner in which stormwater is delivered to the trench.
RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. enters infiltration trenches directly by infiltration from the surface. In exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems, surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. is collected by drainage inlets and delivered to the trench via subsurface perforated pipes.

Overview

ExfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems can be thought of as linear soakaways; designed for both conveyance and infiltration of excess stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events.. By attenuating runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volume, they reduce contaminant loads delivered to downstream BMPs, end-of-pipe facilities or receiving waterbodies.

ExfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). pipe systems are an ideal technology for:

  • Road retrofits where sewer lines are being replaced,
  • All new road/storm sewer constructions where no constraints to infiltration exist,
  • Tight urban spaces where no landscaped practices can be squeezed in, and there is a low risk tolerance for flooding.

The fundamental components of an exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). system are:

  • perforated pipes connected to manholes and catchabasins,
  • a gently sloping 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. reservoir.

An additional components is:

  • Geotextile to prevent migration of finesSoil particles with a diameter less than 0.050 mm. into the reservoir.

ExfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems can be used in place of conventional storm sewer pipes, where topography, 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. depth, and water quality conditions are suitable. They are suitable for treating runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from roofs, walkways, parking lots and low to medium traffic roads, with adequate pretreatment. Perforated catchbasins are a design variation, where the sump is perforated to allow runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. to infiltrate into the underlying soil. ExfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems can also be referred to as perforated pipe systems, pervious pipe systems, clean water collector systems and percolation drainage systems.

Planning

If properly located, designed and maintained, perforated pipe systems can greatly reduce runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volume while having little or no surface footprint, which helps to conserve highly valued developable land.

Systems should be located below shoulders of roadways, permeable paving or swales where they can be readily excavated for servicing. An adequate subsurface area outside of the 4 m setback from building foundations and suitable distance from other underground utilities must be available.

Site Topography

Systems cannot be located on natural slopes > 15 %.

Design

Geometry and Site Layout

  • Gravel beds in which exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems are installed are typically rectangular excavations with a bottom width between 0.6 and 2.4 m [1].
  • The gravel beds should have gentle slopes between 0.5 - 1 %.
  • Calculate maximum flow through perforated pipe

Drawings

Standard details for exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). trenches have been produced by City of Kitchener and City of Barrie. See Drawings

Gallery

Materials

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[2].

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 [3]. 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


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[4]
  • By reducing organic pollutant and nutrient concentrations [5]
  • 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 [6].

  • 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


Performance

Starting after TRIECA (end March) members of STEP will be undertaking a literature review on the performance of our most popular BMPs. The results will be combined with the information we have to date from the development of the Treatment Train Tool and agreed performance metrics established. Until then, please feel free to continue to ask questions via email or the feedback box below.

Ability of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems to meet SWMStormwater Management objectives

Water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. benefit Water quality improvement Erosion controlIncludes the protection of soil from dislocation by water, wind or other agents. benefit
Yes Yes Partial: depending on soil infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour.

Water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The degree to which water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. objectives are met will depend on the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. type on which the system is located. Several Ontario studies have assess the performance of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems in cold climates.

Volumetric runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction achieved by exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems
Practice Location Underlying soil type RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[7] Silty till 73 %
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[7] Sandy silty till 86 %
Perforated pipe Etobicoke, ON[8] Clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System)./clayey silty till over silty sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. 95 %
Perforated pipe North york, ON[8] Silty sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. 89 %

RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction estimate = 85 % on HSG A and B soils.
RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. reduction estimate = 45 % on HSG C and D soils.

Pollutant removal capacity

Performance results from a limited number of field studies indicate that subsurface stormwater infiltration practices are effective BMPs for pollutant removal [9]. These types of practices provide effective removal for many pollutants as a result of sedimentationDeposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.Settling-out or deposition of particulate matter suspended in runoff., filtering, and soil adsorptionThe attachment of gas, vapour or dissolved matter onto the surface of solid materials.. It is also important to note that there is a relationship between the water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. and water quality functions. If an infiltration practice infiltrates and evaporates 100% of the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from a site, then there is essentially no pollution leaving the site in surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.. Furthermore, treatment of infiltrated runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. continues to occur as it leaves the facility and moves through the native soilThe natural ground material characteristic of or existing by virtue of geographic origin.. The performance of perforated pipe systems would be expected to reduce pollutants in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. in a manner similar to infiltration trenches.

Several studies of exfiltrationThe downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed). systems in Ontario have examined their water quality benefits. Seasonal contaminant load reductions in the order of 80% were observed for most constituents, with the exception of chloride, in the study of the system installed in a low density residential neighbourhood in Etobicoke [10][8]. Perforated pipe systems that incorporate grassed swales as pretreatment have been observed to reduce loads of suspended 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., phosphorus, nitrogen, copper, lead and zinc in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. flowing from the system between 75 to 90% in comparison to a similar catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. with conventional catchbasins and storm sewers [11]. The Nepean systems were shown to release significantly less pollutants than the conventional sewer system, even after 20 years of operation[7].

Pollutant removal efficiencies of vegetated filter strips
Practice Location Lead % Copper % Zinc % Total suspended solids (TSSTotal suspended solids) % Total Phosphorus % Total Nitrogen (TKN) %
SoakawayA pit into which liquids may flow and then percolate slowly into the subsoil.An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits. Valence, France[12] 98 - 54 - 88 - - -
Infiltration trench Various[13] 70 - 90 70 - 90 70 - 90 70 - 90 50 - 70 40 - 70
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe North York, ON[10] 75 96 93 24 84 84
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[7] - 99 66 0 81 81 72
Grass swaleVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales./perforated pipe Nepean, ON[7] > 99 > 99 90 96 93 93

See also

STEP and partners research and reports on exfiltration systems

External resources


  1. Greater Vancouver Regional District. 2005. Stormwater Source Control Design Guidelines 2005. Prepared by Lanarc Consultants Limited, Kerr Wood Leidal Associates Limited and Goya Ngan
  2. Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017
  3. 3.0 3.1 3.2 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
  4. 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.
  5. 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.
  6. 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
  7. 7.0 7.1 7.2 7.3 7.4 J.F. Sabourin and Associates Incorporated. 2008a. 20 Year Performance Evaluation of Grassed Swale and Perforated Pipe Drainage Systems. Project No. 524(02). Prepared for the Infrastructure Management Division of the City of Ottawa. Ottawa, Ontario.
  8. 8.0 8.1 8.2 Stormwater Assessment Monitoring and Performance (SWAMP) Program. 2005. Synthesis of Monitoring Studies Conducted Under the Stormwater Assessment Monitoring and Performance Program.. Toronto and Region Conservation Authority, Toronto, Ontario.
  9. Toronto and Region Conservation (TRCA). 2009. Review of the Science and Practice of Stormwater Infiltration in Cold Climates. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.
  10. 10.0 10.1 Stormwater Assessment Monitoring and Performance (SWAMP) Program. 2002. Performance Assessment of a Swale/Perforated Pipe Stormwater Infiltration System – Toronto, Ontario. Toronto and Region Conservation Authority, Toronto, Ontario.
  11. J.F. Sabourin and Associates Incorporated. 1999. Research Project for the Updated Investigation of the Performance Evaluation of Grass Swales and Perforated Pipe Drainage Systems. Executive Summary. Prepared for the Infrastructure Management Division of the City of Ottawa. Ottawa, Ontario.
  12. Barraud, S., Gautier, A., Bardin, J.P., Riou, V. 1999. The Impact of Intentional Stormwater Infiltration on Soil and Groundwater. Water Science and Technology. Vol. 39. No. 2. pp. 185-192.
  13. ASCE (2000)Pollutant removal efficiencies are reported as ranges because they are based on a synthesis of several performance monitoring studies that were available as of 2000.