Design infiltration rate

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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' (Kfc), or 'saturated hydraulic conductivity' (Ksat). 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
  • Single ring infiltrometer
  • Soil textural classification

Planning for testing[edit]

Background Evaluation[edit]

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[edit]

The measured or calculated infiltration rate must be divided by a value 2 or 3 before use in design[1]

Selecting a safety factor
2 3
Catchment < 100 m² Catchment > 100 m²[2]
Permeameter or Percolation test on site Double ring infiltrometer test on site, or Grain size used [3]
Loamy or sandy soil Clayey soil[4]

  1. Design criteria for infiltration. (2018, February 14). Minnesota Stormwater Manual, . Retrieved 00:16, March 14, 2018 from https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_infiltration&oldid=35583.
  2. Ballard, B. W., Wilson, S., Udale-Clarke, H., Illman, S., Scott, T., Ashley, R., & Kellagher, R. (2015). The SuDS Manual. London.
  3. San Francisco Public Utilities Commission. (2017). Determination of Design Infiltration Rates for the Sizing of Infiltration‐based Green Infrastructure Facilities. Retrieved from http://sfwater.org/modules/showdocument.aspx?documentid=9681
  4. CRC for Water Sensitive Cities. (2015). Adoption Guidelines for Stormwater Biofiltration Systems: Appendix C - Guidelines for filter media in stormwater biofiltration systems.