Vertical separation

In Ontario the required vertical separation between a practice and the water table 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 drainage/hydraulic potential"

"If a pervious pipe system is implemented in an area where the seasonally high water table is higher than the obvert of the pipe, the pipe will drain the groundwater table. In this scenario, depending on the native soil characteristics and whether the trench or pipe is wrapped in geotextile 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."

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

The Ontario Building Code requires

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.

in turn citing

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The 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.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

The natural ground material characteristic of or existing by virtue of geographic origin.

Filter 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.

Best 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.

Low 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.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The water below the surface, and typically below the groundwater table.

A strain of bacteria found in the gut and waste products of animals. Some strains of this bacterium can be harmful to humans or animals if consumed. Food or water can become contaminated with E.Coli where unsanitary conditions exist.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.