Difference between revisions of "Design infiltration rate"

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The number of test pits or soil borings varies depending on site conditions and the proposed development plan. General guidelines are as follows:
 
The number of test pits or soil borings varies depending on site conditions and the proposed development plan. General guidelines are as follows:
  
 
Result
 
 
{| class="wikitable"
 
{| class="wikitable"
 
! BMP Footprint (m²)
 
! BMP Footprint (m²)
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| < 50 || 1 || 1
 
| < 50 || 1 || 1
 
|-
 
|-
| 50 – 900 || 1 || 2
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|rowspan = 2| 50 – 900 || 1 || 2
 
|-
 
|-
 
|  || 2 || 0
 
|  || 2 || 0
 
|-
 
|-
| > 900 || 1 every 450 m² || 2 every 450 m2
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| rowspan = 2|> 900 || 1 every 450 m² || 2 every 450 m2
 
|-
 
|-
 
|  || 2 every 450 m² || 0
 
|  || 2 every 450 m² || 0

Revision as of 21:55, 18 March 2018

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

Safety correction[edit]

The measured or calculated infiltration rate must be divided by a value between 2 and 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 analysis used [3]
Loamy or sandy soil Clayey soil[4]

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.).

Sampling[edit]

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. [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 F - Soil Infiltration Testing Protocol.pdf