Exfiltration trenches

Exfiltration trenches are similar to infiltration trenches but differ primarily in the the manner in which stormwater is delivered to the trench.
Runoff enters infiltration trenches directly by infiltration from the surface. In exfiltration systems, surface runoff is collected by drainage inlets and delivered to the trench via subsurface perforated pipes.

Overview

Exfiltration systems can be thought of as long infiltration trenches or linear soakaways that are designed for both conveyance and infiltration of stormwater runoff. They are underground stormwater conveyance systems designed to attenuate runoff volume and thereby, reduce contaminant loads to receiving waters. They are composed of perforated pipes installed in gently sloping granular stone beds that are lined with geotextile fabric that allow infiltration of runoff into the gravel bed and underlying native soil while it is being conveyed from source areas or other BMPs to an end-of-pipe facility or receiving waterbody.

Exfiltration systems can be used in place of conventional storm sewer pipes, where topography, water table depth, and runoff quality conditions are suitable. They are suitable for treating runoff from roofs, walkways, parking lots and low to medium traffic roads, with adequate pretreatment.

A design variation can include perforated catchbasins, where the catchbasin sump is perforated to allow runoff to infiltrate into the underlying native soil.

Perforated pipe systems can also be referred to as pervious pipe systems, exfiltration systems, clean water collector systems and percolation drainage systems.

Planning

If properly located, designed and maintained, perforated pipe systems can greatly reduce runoff 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 exfiltration systems are installed are typically rectangular excavations with a bottom width between 0.6 and 2.4 m .

The gravel beds should have gentle slopes between 0.5 - 1%.

Performance

Ability of exfiltration systems to meet SWM objectives

Water balance benefit	Water quality improvement	Erosion control benefit
Yes	Yes	Partial: depending on soil infiltration rate

Water balance

The degree to which water balance objectives are met will depend on the underlying native soil type on which the system is located. Several Ontario studies have assess the performance of exfiltration systems in cold climates.

Volumetric runoff reduction achieved by exfiltration systems

Practice	Location	Underlying soil type	Runoff reduction
Grass swale/perforated pipe	Nepean, ON	Silty till	73 %
Grass swale/perforated pipe	Nepean, ON	Sandy silty till	86 %
Perforated pipe	Etobicoke, ON	Clay/clayey silty till over silty sand	95 %
Perforated pipe	North york, ON	Silty sand	89 %

Runoff reduction estimate = 85 % on HSG A and B soils.
Runoff 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 . These types of practices provide effective removal for many pollutants as a result of sedimentation, filtering, and soil adsorption. It is also important to note that there is a relationship between the water balance and water quality functions. If an infiltration practice infiltrates and evaporates 100% of the runoff from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff continues to occur as it leaves the facility and moves through the native soil. The performance of perforated pipe systems would be expected to reduce pollutants in runoff in a manner similar to infiltration trenches.

Several studies of exfiltration 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 . Perforated pipe systems that incorporate grassed swales as pretreatment have been observed to reduce loads of suspended sediment, phosphorus, nitrogen, copper, lead and zinc in runoff flowing from the system between 75 to 90% in comparison to a similar catchment with conventional catchbasins and storm sewers . The Nepean systems were shown to release significantly less pollutants than the conventional sewer system, even after 20 years of operation.

Pollutant removal efficiencies of exfiltration systems

Practice	Location	Lead %	Copper %	Zinc %	Total suspended solids (TSS) %	Total Phosphorus %	Total Nitrogen (TKN) %
Soakaway	Valence, France	98	-	54 - 88	-	-	-
Infiltration trench	Various	70 - 90	70 - 90	70 - 90	70 - 90	50 - 70	40 - 70
Grass swale/perforated pipe	North York, ON	75	96	93	24	84	84
Grass swale/perforated pipe	Nepean, ON	- 99	66	0	81	81	72
Grass swale/perforated pipe	Nepean, ON	> 99	> 99	90	96	93	93

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External resources

https://www.chijournal.org/C390

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The 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).

The portion of water precipitated onto a catchment area, which then 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.

The 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).

The portion of water precipitated onto a catchment area, which then 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.

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

Gravel, 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.

The natural ground material characteristic of or existing by virtue of geographic origin.

The 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.

Stormwater Management

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

Includes the protection of soil from dislocation by water, wind or other agents.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

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

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, 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.

1. 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).

Mineral 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.

Deposition 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.

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

Soil, 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.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Total suspended solids

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native 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.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Toronto and Region Conservation Authority

Sustainable Technologies Evaluation Program

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

The water below the surface, and typically below the groundwater table.