Low permeability soils

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Introduction

The texture of native soils has a strong influence on the capacity of 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. practices to 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. volumes through infiltration. While sandy and 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. soils have a high capacity to infiltrate water, fine textured soils containing a high percentage of 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). may not be suitable for infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., or require design adaptations to promote sufficient infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

Post-to-predevelopment water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. matching

The amount of infiltration required on a given site is determined by comparing water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. estimates before and after development. Ideally, the volume of water infiltrated and evapotranspired prior to development would remain the same afterwards. In practice, increased imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. cover normally results in lower post development evapotranspiration. Best efforts should be made to match pre-developmentrefers to the characteristics and functions of a system prior to urban development. water balanceThe accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle. components. However, in some cases maintaining 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. volumes at predevelopment levels may require that more water is infiltrated after development than under the predevelopment condition.

Under natural conditions, sites with fine textured native soils will have lower infiltration volumes (and higher 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.) than those with coarse textured soils. On fine textured soils with very low permeability (hydrologic D type soils), the measured infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. may even approach zero. Under these conditions, the stormwater management approach should focus on 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. prevention and volume reduction through evapotranspirationThe quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration. or water reuse, rather than infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

Infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. measurement methods

A variety of methods are available for measuring and estimating infiltration rates. Selecting an appropriate method will depend on the size and scale of the area over which infiltration rates are being determined.

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. design adaptations on low permeability soils

Change in infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. observed in an infiltration trench in Caledon, ON, allowed to infiltrate over a 23 day dry period.

The rationale for variations in practice design for sites with fine textured soils is based on the relationship between hydraulic head and infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.. As the head pressure in the Caledon infiltration trench decreased from 1.5 m to below 1 m, infiltration rates dropped from 2.5 - 3.8 mm/h during the first two days to only 1 - 1.5 mm hour after six and half days.

Infiltration is enhanced by maintaining a hydraulic head above the point at which infiltration slows to negligible levels. This means:

  1. Allowing water to remain within the storage reservoir below the underdrainA perforated pipe used to assist the draining of soils. or outlet continuously, or at least for longer time periods than the typical 48 to 92 hour drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). requirements for other stormwater BMPs, and
  2. Designing the storage to be more vertically oriented to increase available hydraulic head. BMPs should have higher side wall to bottom ratios, and a portion of the total storage regarded as effectively permanent.

Another important element of infiltration practice design in the context of fine textured soils relates to the attraction of soil surfaces to water, which are strong in fine textured clays and silty clays and weaker in coarse textured sands or sandy loams. This attraction, referred to as the matric potential, allows water to move up from the groundwater tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body. into the soils. In fine textured soils, this distance can be in excess of one meter. Therefore if the base of the infiltration practice is only one meter above the seasonally high groundwater tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body., a direct connection between the practice and groundwater may form, bypassing the treatment properties of the soils. It is recommended, therefore that the groundwater tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body. be 1.5 m or lower when practices are installed on fine textured soils.

Performance studies on fine textured soils

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.

Facility wide infiltration rates for different 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. practices installed in the Greater Toronto Area

A number of field studies of 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. practices have been conducted in southern Ontario on fine textured soils. Several of these studies have yielded data that allow for calculation of the facility wide infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. during natural rain events of varying sizes:

  • Infiltration rates on silty 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 siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) and sandy siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) textured soils had a median value of 3.3 mm/h and a range between 0.3 and 17.8 mm/h.
  • Permeable pavements had lower measured rates, in part due to compaction of the subsoils to accommodate traffic loadingThe total mass of a pollutant entering a waterbody over a defined time period.The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration..
  • Stormwater 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 reductions varied between sites, primarily due to factors other than the native soilThe natural ground material characteristic of or existing by virtue of geographic origin. infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour.. For instance, the infiltration trenches and chambers had similar native soilThe natural ground material characteristic of or existing by virtue of geographic origin. infiltration rates (3.1 to 5.1 mm/h), but 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 values varying from 16 to 90%. This is attributed to differences in the I/P ratioThe ratio of the catchment (impervious area) to the footprint area of the receiving BMP (pervious area)., which ranged from 10:1 to 155:1.
  • The configuration of the outflow was also an important consideration. In systems where the outlet is elevated above the native soilThe natural ground material characteristic of or existing by virtue of geographic origin., 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 levels tend to be considerably higher than systems with underdrains located at the native soilThe natural ground material characteristic of or existing by virtue of geographic origin. interface. See Bioretention: Performance

The studies, tabulated below, clearly indicate that significant volume reduction through infiltration is feasible on low permeability soils. If geotechnical investigations indicate that volume loss through infiltration is not possible, or would provide more limited benefits than found in these studies, the project should focus on reducing 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. through vegetative evapotranspirationThe quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.. See here for a list of options, and their relative potential to 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. through evapotranspirationThe quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/dayThe combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration..

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 reduction performance for selected monitoring studies of 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. practices or sites conducted over a period of a year or more
(Sortable, click headers)
BMPBest management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls. type Duration Site characteristics 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 (%)
Native soilThe natural ground material characteristic of or existing by virtue of geographic origin. I/P ratioThe ratio of the catchment (impervious area) to the footprint area of the receiving BMP (pervious area). Sump depth (m)*
Infiltration trench[1] 2 growing seasons Silty 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). 10:1 0.1; flow rate control 80
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[1] 2 growing seasons Silty 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). 10:1 0.1; flow rate control 90
Permeable Pavement[2] 5 growing seasons Silty 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). 1:1 0.1; flow rate control 45
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[3] 1 growing season Silty 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). 10:1 0.1; flow rate control 83
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. [4] 2 years Silty 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). 11:1 0.1; flow rate control 91
Infiltration chamber[5] 2 years Sandy siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) 20:1 1.2 (approx.) 90
Infiltration trench[5] 2 years Clayey siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) 64:1 2 36
Infiltration trench[5] 2 years Clayey siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) 155:1 2 16
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). trench[6] 2 years 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). to 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). siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) 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. till 7:1 (approx.) 0.65 >90
Infiltration trench[7] 2 years 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. with clayey siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) deposits 4:1 (approx.) 1 89
Permeable pavementAn alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil. + bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[8] 4 years Clayey siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) on siltSoil or media particles smaller than sand and larger than clay (3 to 60 m) till 6:1 (approx.) 0.2-0.3 (approx.) 80
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[9] 3 years Silty 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). 10:1 (approx.) 0.02 92
BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[10] 3 years Silty 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). fill over 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). till 30:1 0.025 72
Permeable pavementAn alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.[10] 3 years Silty 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). fill over 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). till 1:1 0.025 49

*Represents depth of sump below underdrain or overflow pipe. In some cases, a flow control device was installed to slow outflow rates and enhance infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.


  1. 1.01.1 Van Seters, T. and Young, D., 2015, Performance Comparison of Surface and Underground Stormwater Infiltration Practices, TRCAToronto Region Conservation Authority, Toronto, Ontario
  2. Van Seters, T. and Drake, J., 2015, Five year evaluation of Permeable Pavements, TRCAToronto Region Conservation Authority, Toronto, Ontario
  3. STEP study ongoing (2017)
  4. Van Seters T and Graham C, 2014, Performance Evaluation of a BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. System, TRCAToronto Region Conservation Authority, Toronto, Ontario
  5. 5.05.15.2 Young D, Van Seters T, Graham, C, 2013, Evaluation of Residential Lot Level Stormwater Practices – tech brief
  6. SWAMP, 2005. Performance Assessment of a Perforated Pipe Stormwater 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, Toronto, Ontario, TRCAToronto Region Conservation Authority, Toronto, Ontario
  7. SWAMP, 2002, Performance Assessment of a SwaleA 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. Perforated Pipe Stormwater Infiltration System, TRCAToronto Region Conservation Authority, Toronto Ontario
  8. Credit Valley Conservation, 2016, Elm Drive Low Impact Development Infrastructure Performance and Risk Assessment Technical Report, Mississauga Ontario
  9. Credit Valley Conservation, 2016, Lakeview Low Impact Development Infrastructure Performance and Risk Assessment Technical Report, Mississauga Ontario
  10. 10.010.1 Credit Valley Conservation, 2016, IMAX Low Impact Development Infrastructure Performance and Risk Assessment Technical Report, Mississauga Ontario