From LID SWM Planning and Design Guide
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Where possible, it is recommended that infiltration BMPs maintain a 1.0 m vertical separation above the seasonally high groundwater tableThe upper surface of the zone of saturation, except where the surface is formed by an impermeable body..
Retention of this unsaturated zone:

  • Minimizes the potential for functional impacts associated with reduced percolation rates,
  • Maintains the physical and biochemical water quality treatment benefits provided within the vadose zone,
  • Minimizes the potential for localized groundwater mounding to affect nearby buildings or infrastructure, and/or
  • Provides a factor-of-safety against potential uncertainties related to the elevation of the seasonally high groundwater elevation in areas of limited monitoring.

The requirement for additional investigation and/or documentation supporting a proposed design may be reduced in areas where ≥ 1.0 m separation is anticipated.
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.