Vertical separation

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In Ontario the required vertical separation between a practice and the 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. or bedrock is frequently cited as 1 meter. This comes from the 2003 Stormwater Management Planning and Design Manual.

"The depth to bedrock should be greater than or equal to 1 metre below the bottom of the perforated pipe storage media to ensure adequate drainageNatural or artificial means of intercepting and removing surface or subsurface water (usually by gravity)./hydraulic potential"[1]

"If a pervious pipe system is implemented in an area where the seasonally high 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. is higher than the obvert of the pipe, the pipe will drain the groundwater tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body.. In this scenario, depending on the native soilThe natural ground material characteristic of or existing by virtue of geographic origin. characteristics and whether the trench or pipe is wrapped in geotextileFilter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils. fabric, soil can be transported into the pipe system undermining the pipe foundation and leading to structural failure. Pervious pipe systems should not be implemented in areas where the seasonal high groundwater level is within 1 metre of the bottom of the storm sewer backfill to ensure this does not happen."[1]

Both of these considerations are equally relevant to the underdrain of any structural 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..


Whilst this is a great rule of thumb, like all aspects of LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting., this 1 meter figure might require amendment on a site-by-site basis. In areas where a 1.0 m separation cannot be provided, or where conditions dictate that an even greater separation may be warranted, additional discussion and/or analysis specific to the physical characteristics of the site and the proposed design should be completed. The design practitioner is advised to consult with approval agencies to understand their requirements and/or expectations prior to undertaking work, and to complete an appropriate level of analysis to support their conclusion. The requirement for additional investigation and/or documentation supporting a proposed design may be reduced in areas where ≥ 1.0 m separation is anticipated. Factors to consider include:

  • risks due to short periods of groundwater mounding and potentially unobserved seasonal fluctuations.
  • the potential for functional impacts associated with reduced percolation rates,
  • retaining an unsaturated zone beneath the practice maintains the physical and biochemical water quality treatment benefits provided within the vadose zone.

Other references of interest

The CSA standard W200, Design of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. systems, requires a minimum vertical separation of 0.6 m between the base of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. and the 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.[2]

The Ontario Building Code requires [3][4][5]

Rhode Island (largely located on glacial till with loamy sands) clarified their position on vertical separation in 2016 in relation to LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. including:

  • reduction to 0.6 m when infiltrating water from residential roofs and driveways,
  • flexibility when siting projects on brownfield (contaminated) land,
  • limited flexbility in retrofit scenarios, and
  • accounting for bioretention media as part of the overall separation depth from surface water to groundwaterThe water below the surface, and typically below the groundwater table..
[6][7][8] in turn citing [9]
  1. 1.0 1.1 https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design
  2. CSA Group. (n.d.). W200-18 Design of bioretention systems. Retrieved from https://store.csagroup.org/ccrz__ProductDetails?sku=2704497
  3. Province of Ontario. “O. Reg. 332/12: BUILDING CODE,” 2018. https://www.ontario.ca/laws/regulation/120332.
  4. Karathanasis, A D, T G Mueller, B Boone, and Y L Thompson. “Nutrient Removal from Septic Effluents as Affected by Soil Thickness and Texture.” Journal of Water and Health 4, no. 2 (June 2006): 177–95. http://www.ncbi.nlm.nih.gov/pubmed/16813011.
  5. Stall, Christopher, Aziz Amoozegar, David Lindbo, Alexandria Graves, and Diana Rashash. “Transport of E. Coli in a Sandy Soil as Impacted by Depth to Water Table.” Journal of Environmental Health 76, no. 6: 92–100. Accessed October 23, 2018. http://www.ncbi.nlm.nih.gov/pubmed/24645419.
  6. RI Department of Environmental Management. “Section 5.5.1 and 5.5.4 RISDISM Guidance -- Filtering Systems: Separation to Seasonal High Groundwater Table,” 2016.
  7. Ontario. “F-6-1 Procedures to Govern Separation of Sewers and Watermains | Ontario.Ca,” 2016. https://www.ontario.ca/page/f-6-1-procedures-govern-separation-sewers-and-watermains.
  8. Rathfelder, K., and M. Wei. “Underground Stormwater Infiltration: Best Practice for Protection of Groundwater Resources in British Columbia,” 2014. http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/library.html
  9. Washington St.ate Department of Ecology. “Guidance for UIC Wells That Manage Stormwater,” 2006.