Design infiltration rate

This article discusses the determination of a suitable infiltration rate to describe the native soils surrounding a proposed infiltration practice.
It does not relate to the hydraulic conductivity or permeability of filter media or planting media used within vegetated BMPs.

The design infiltration rate (f') is used in all sizing calculations for bioretention and bioswales, dry ponds, infiltration trenches and chambers, and exfiltration trenches. It is related to, but not synonymous with 'permeability', 'field hydraulic conductivity' (K_fc), or 'saturated hydraulic conductivity' (K_sat). Before application a safety correction factor (SCF) is incorporated into the figure, depending upon the source of the raw data, and the risk of failure of the finished practice.

It is highly recommended that testing be conducted on site.

Sources of data

On site testing methods	Off site analysis methods	Unacceptable methods
Permeameter Double ring infiltrometer Cased borehole (percolation test)	Grain size analysis (Hazen)	Single ring infiltrometer Soil textural classification

Safety correction

The measured or calculated infiltration rate must be divided by a value between 2 and 3 before use in design

Selecting a safety factor

2	3
Catchment < 100 m²	Catchment > 100 m²
Permeameter or percolation test on site	Double ring infiltrometer test on site, or grain size analysis used
Loamy or sandy soil	Clayey soil

Planning for testing

Background Evaluation

Prior to performing testing and developing a detailed site plan, existing site conditions should be inventoried and mapped including, but not limited to:

• Surficial geology and underlying stratigraphy;
• Watercourses (perennial and intermittent), water bodies, wetlands and floodplains;
• Small headwater drainage features;
• Topography, slope, and drainage patterns;
• Existing land cover and land use;
• Natural heritage conservation areas; and
• Other man-made features or conditions that may impact design such as existing nearby structures (buildings, infrastructure, etc.).

A sketch plan or preliminary layout plan for the proposed development should be evaluated, including:

• The preliminary grading plan and areas of cut and fill;
• The location and water surface elevation of all existing, and location of proposed water supply sources and wells;
• The location of all existing and proposed on-site wastewater (septic) systems;
• The location of other features of note such as utility rights-of-way, water and sewer lines, etc.;
• Existing data from borehole, well and geophysical testing; and
• Proposed location of development features (buildings, roads, utilities, etc.).

Sampling

The number of test pits or soil borings varies depending on site conditions and the proposed development plan. General guidelines are as follows:

BMP Footprint (m²)	test pit	Bore hole
< 50	1	1
50 – 900	1	2
		2	0
> 900	1 every 450 m²	2 every 450 m2
		2 every 450 m²	0
Linear	1 every 450 m²	1 every 50 m

• For infiltration BMPs with footprint surface areas from 50 - 900 m², a minimum of two test pits or one test pit and two soil borings are required at, or within 10 m of the proposed location to determine the suitability and distribution of soil types present;
• For infiltration BMPs with footprint surface areas > 900 m², a minimum of one test should be conducted for each 450 m² of footprint area. Tests should be conducted equidistant from each other to provide adequate characterization of the area;
• For linear infiltration BMPs (e.g. bioswales, enhanced grass swales, infiltration trenches, exfiltration trenches) a minimum of one test should be conducted within each soil mapping unit present along the proposed BMP location. Soil borings should be conducted every 50 m and a test pit should be conducted every 450 m; and
• For sites with multiple infiltration BMPs, each with footprint surface area < 50 m², a minimum of one test pit is required and one soil boring per infiltration BMP location is recommended.

• The recommendations above are guidelines. Additional tests should be conducted if local conditions indicate significant variability in soil types, geology, water table levels, depth and type of bedrock, topography, etc.
• Similarly, uniform site conditions may indicate that fewer test pits are required. Excessive testing and disturbance of the site prior to construction is not recommended.

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

A parameter that describes the capability of a medium to transmit water.

The 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.

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

Referring to the source of a stream or river.

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 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.

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

Penetration of water through the ground surface.

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

Penetration of water through the ground surface.