Infiltration

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:

• Infiltration chambers (underground)
• Infiltration trenches (underground)
• Dry ponds (surface)
• Bioretention cells (surface)
• Bioswales (surface)

Constraints

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

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

• Bioretention with internal water storage
• Partially infiltrating bioretention

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

• Bioretention planters
• Rainwater harvesting
• Green roofs

Planning

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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The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

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.

Land owned by private individuals or companies.

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

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

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

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.