Infiltration

From LID SWM Planning and Design Guide
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Infiltration of stormwater into underlying soils is one the priority level 1. mechanisms to achieve MOECC Runoff Volume Control targets.

Infiltration is promoted through a number of LID BMPs:

Constraints[edit]

Infiltration practices are not recommended:

  1. Where the bedrock is within 1 m below the bottom of the proposed BMP,
  2. Where the seasonal high groundwater level comes within 1 m below the bottom of the proposed BMP,
    Or in areas where increased infiltration will result in elevated groundwater levels, which can be demonstrated to damage critical utilities or private property,
  3. Over swelling clays or unstable sub-soils,
  4. Over contaminated soils or sites with high risk of contamination from onsite activities,
  5. In flood prone areas where the wastewater system is sensitive to groundwater conditions causing sewer backups, and where LID BMPs have been found to be ineffective,
  6. In areas where the ecology and natural hydrology are dependent upon surface water,
  7. Where prohibitions and/or restrictions exist per approved Source Protection Plans (see MOECC guidance).

Designing within constraints[edit]

Infiltration can still be optimized over 'tight' soils with infiltration rates ≤ 15 mm/hr:

Where infiltration is impossible, LID design alternatives exist which can still capture and retain stormwater:

Planning[edit]

Soil infiltration rate

Infiltration tests must be undertaken at the location, depth and with a head of water that replicates the proposed design.

Suggested safety factors to be applied to measured percolation rate when designing infiltration facilities
Catchment area Consequence of failure
No damage or inconvenience Minor damage or inconvenience to external structures (e.g. ponding in parking lot) Significant damage to buildings or infrastructure (e.g. flooding damage)
<100 m2 1.5 2 10
100 - 1000 m2 1.5 3 10

Groundwater

The bottom of the infiltration BMP must ≥ 1 m vertically separated from the seasonally high water table, and underlying bedrock. This reduces the risk of contaminating groundwater and helps to ensure reliable infiltration rates throughout the year.

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