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

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.).

Safety correction

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

Selecting a safety factor

2	3
Catchment < 100 m²	Catchment > 100 m²

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.

Referring to the source of a stream or river.

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

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

Penetration of water through the ground surface.