Overflow: Installation

Overflows are features of inline facilities and convey larger storm events out of the LID feature.

Construction Steps:

1. Connect the overflow drain to the underdrain.
2. Fit a metal, domed grate to the overflow.
3. Install overflow pit to grade at the location specified in the contract documents.

Key Inspection Points:

• Overflow drain matches design specifications.
• Overflow pit is positioned at the maximum water surface elevation of the practice, as per the contract documents.
• Sufficient freeboard is provided between the overflow and inlet such that the inlet is not inundated by design storm flows.
• Overflow grate matches design specifications.
• Overflow grates located in high-traffic areas are screwed on or equipped with locks.

Mistakes to Avoid:

• Backflows – ensure that backflows are avoided by confirming that inlet and overflow elevations match design specifications with surveys.
• Incorrect sequencing – install overflow drain prior to backfilling with granular material and engineered soil.
• Incorrect grate material – avoid using plastic overflow grates due to breakages and UV degradation.

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  Overflow pipe installed to an underdrain T-connection. (Photo Source: CVC, 2010)

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  Overflow installed with a plastic cap. Note that the use of plastic grates is discouraged. (Photo Source: CVC)

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

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 perforated pipe used to assist the draining of soils.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Credit Valley Conservation