Design infiltration rate

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This article discusses the determination of a suitable infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. to describe the native soils surrounding a proposed infiltration practice.
It does not relate to the hydraulic conductivityA parameter that describes the capability of a medium to transmit water. or permeability of filter mediaThe 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. or planting media used within vegetated BMPs.

The design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (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 conductivityA parameter that describes the capability of a medium to transmit water.' (Kfc), or 'saturated hydraulic conductivityA parameter that describes the capability of a medium to transmit water.' (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

Safety correction

The measured or calculated infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. must be divided by a value between 2 and 3 before use in design[1]
Selecting a safety factor
2 3
CatchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. < 100 m² CatchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. > 100 m²[2]
Permeameter or percolation test on site Double ring infiltrometer test on site, or grain size analysis used [3]
Loamy or sandy soil Clayey soil[4]

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:

  1. Surficial geology and underlying stratigraphy;
  2. Watercourses (perennial and intermittent), water bodies, wetlands and floodplains;
  3. Small headwaterReferring to the source of a stream or river. drainage features;
  4. Topography, slope, and drainage patterns;
  5. Existing land cover and land use;
  6. Natural heritage conservation areas; and
  7. 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:

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

Sampling

Infiltration testing should be carried out at relevant stages down through the profile of a test pit.

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

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. footprint (m²) Test pit Bore hole
< 50 1 1
50 – 900
(alternative)
1 2
2 0
> 900
(alternative)
1 every 450 m² 2 every 450 m²
2 every 450 m² 0
Linear 1 every 450 m² 1 every 50 m
  • Tests must be conducted within 10 m of the infiltration practice footprint area.
  • Where multiple test are carried out, they should be planned equidistant and mapped accordingly.

The recommendations above are guidelines

Additional tests should be conducted if local conditions indicate significant variability in soil types, geology, water tableThe 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. 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.[5]
  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.
  5. Christopher B. Burke Engineering LLC. (2009). Soil Infiltration Testing Protocol SOIL INFILTRATION TESTING PROTOCOL Purpose of this Protocol Who Should Conduct Testing. Retrieved from https://www.in.gov/ocra/files/Appendix%20F%20-%20Soil%20Infiltration%20Testing%20Protocol.pdf