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Both of these considerations are equally relevant to the [[underdrain]] of any structural BMP.
Both of these considerations are equally relevant to the [[underdrain]] of any structural BMP.
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Whilst this is a great rule of thumb, like all aspects of LID, 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 bioretention systems, requires a minimum vertical separation of 0.6 m between the base of bioretention and the water table<ref>CSA Group. (n.d.). W200-18 Design of bioretention systems. Retrieved from https://store.csagroup.org/ccrz__ProductDetails?sku=2704497</ref>
The Ontario Building Code requires
<ref>Province of Ontario. “O. Reg. 332/12: BUILDING CODE,” 2018. https://www.ontario.ca/laws/regulation/120332.</ref>
<ref>Province of Ontario. “O. Reg. 332/12: BUILDING CODE,” 2018. https://www.ontario.ca/laws/regulation/120332.</ref>
<ref>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.</ref>
<ref>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.</ref>
Rhode Island (largely located on glacial till with loamy sands) clarified their position on vertical separation in 2016 in relation to LID 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 groundwater.
<ref>RI Department of Environmental Management. “Section 5.5.1 and 5.5.4 RISDISM Guidance -- Filtering Systems: Separation to Seasonal High Groundwater Table,” 2016.</ref>
<ref>RI Department of Environmental Management. “Section 5.5.1 and 5.5.4 RISDISM Guidance -- Filtering Systems: Separation to Seasonal High Groundwater Table,” 2016.</ref>
<ref>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</ref> in turn citing <ref>Washington St.ate Department of Ecology. “Guidance for UIC Wells That Manage Stormwater,” 2006.</ref>
<ref>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</ref> in turn citing <ref>Washington St.ate Department of Ecology. “Guidance for UIC Wells That Manage Stormwater,” 2006.</ref>
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