Low permeability soils

Introduction

The texture of native soils has a strong influence on the capacity of LID practices to reduce runoff volumes through infiltration. While sandy and silty sand soils have a high capacity to infiltrate water, fine textured soils containing a high percentage of clay may not be suitable for infiltration, or require design adaptations to promote sufficient infiltration.

Post-to-predevelopment water balance matching

The amount of infiltration required on a given site is determined by comparing water balance estimates before and after development. Ideally, the volume of water infiltrated and evapotranspired prior to development would remain the same afterwards. In practice, increased impervious cover normally results in lower post development evapotranspiration. Best efforts should be made to match pre-development water balance components. However, in some cases maintaining runoff 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 runoff) than those with coarse textured soils. On fine textured soils with very low permeability (hydrologic D type soils), the measured infiltration rate may even approach zero. Under these conditions, the stormwater management approach should focus on runoff prevention and volume reduction through evapotranspiration or water reuse, rather than infiltration.

Infiltration rate 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.

LID design adaptations on low permeability soils

The rationale for variations in practice design for sites with fine textured soils is based on the relationship between hydraulic head and infiltration.

Figure xx shows this relationship for an infiltration trench in Caledon that was filled to the outflow elevation during an intense rain event, and then allowed to naturally infiltrate over a 23 day dry period (ref tech brief). As the head 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.

< Insert figure 8 from tech brief >

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 underdrain or outlet continuously, or at least for longer time periods than the typical 48 to 92 hour drawdown time 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 table 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 table, a direct connection between the practice and groundwater may form, bypassing the treatment properties of the soils. It is recommended, therefore that the groundwater table be 1.5 m or lower when practices are installed on fine textured soils.

Performance studies on fine textured soils

A number of field studies of LID 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 rate during natural rain events of varying sizes. These are summarized in Figure xx. Infiltration rates on silty clay, clayey silt and sandy silt 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 values in part due to compaction of the subsoils to accommodate traffic loading.

Figure xx: Facility wide infiltration rates for different LID practices installed in the Greater Toronto Area

Stormwater runoff volume reductions varied from site to site, primarily due to factors other than the native soil infiltration rate. For instance, the infiltration trenches and chambers shown in the Figure xx had similar native soil infiltration rates (3.1 to 5.1 mm/h), but runoff reduction values varying from 16 to 90%, chiefly due to site to site differences in the I:P ratio (reference definition), 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 soil, runoff reduction levels tend to be considerably higher than systems with underdrains located at the native soil interface, even if outflow rates from the non-elevated drains are controlled by orifices or flow control valves.

The studies 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 runoff through vegetative evapotranspiration. See here for a list of options, and their relative potential to reduce runoff through evapotranspiration.

Runoff volume reduction performance for selected monitoring studies of LID practices or sites conducted over a period of a year or more

BMP type	Duration	Site characteristics			Runoff reduction
		Native soil	I/P ratio	Sump depth*
Infiltration trench	Two growing seasons	Silty clay	10:1	<10 cm; flow rate control	80%
Bioretention	Five growing seasons	Silty clay	1:1	<10 cm; flow rate control	45%
Bioretention	One growing season	Silty clay	10:1	<10 cm; flow rate control	83%
Bioretention	Two years	Silty clay	11:1	<10 cm; flow rate control	91%
Infiltration chamber	Two years	Sandy silt	20:1	Approx.: 1.2 m	90%
Infiltration trench	Two years	Clayey silt	64:1	2 m	36%
Infiltration trench	Two years	Clayey silt	155:1	2 m	16%
Infiltration trench	Two years	Clay to clay silt till over silty sand till	Approx: 7:1	0.65 m	>90%
Infiltration trench	Two years	Silty sand with clayey silt deposits	Approx: 4:1	1 m	89%
Permeable pavement + bioretention	Four years	Clayey silt on silt till	Approx: 6:1	?	80%
Bioretention	Four years	Silty clay	Approx.: 10:1	?	92%
Bioretention	Four years	Silty clay fill over clay till	30:1	?	72%
Permeable pavement	Four years	Silty clay fill over clay till	1:1	?	?

*Represents depth of sump below underdrain or outflow pipe. In some cases, a flow control device was installed to slow outflow rates and enhance infiltration 2. Also known as the Etobicoke Exfiltration system

Low 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.

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 slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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.

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).

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

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The 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/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

refers to the characteristics and functions of a system prior to urban development.

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

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

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A perforated pipe used to assist the draining of soils.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

The 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.

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

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.

Best 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.

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

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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).

An 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.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

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).

Toronto and Region Conservation Authority