Stormwater Management Guidelines for Three Units or More

For three-unit developments or larger, it is a requirement of the City’s Subdivision and Development Control Bylaw that all Stormwater Management Plans be designed and certified by an appropriated qualified Professional Engineer.  Guidelines for source control design are provided in the link below.

For these types of developments it is required that all runoff (volume) originating from newly constructed impervious surfaces be mitigated by source controls.  In addition, any drainage collected along the newly constructed frontage of these types of developments shall also be managed for water quality and volume.  The volume of runoff is based on a total rainfall depth of 58 mm (equivalent to 90% of the annual average rainfall).

At this intensity of development, the City allows that source controls for the development site can be implemented in the road right-of-way provided that it doesn’t interfere with proposed use.  This typically involves construction of below-grade soil cells (e.g. silva cells) or rain gardens along the frontage to manage both rainfall volume and water quality.  Detention-infiltration cells, or bioswales  would also be appropriate for the road right-of-way and has been more successful town home and lower rise residential developments.

Management of 58 mm of rainfall over 24 hours is often difficult on densely developed sites, and is often best achieved starting with capture at the roof level with either a blue or green roof.  In the event that a stormwater retention approach is used, the target release rate is 0.25 l/s/ha an equivalent value to that of forested lands.  Use of roof based detention is a requirement under the Subdivision and Development Control Bylaw for all flat roofs, which is a very effective approach for rainwater management.  In the event that source controls cannot be provided on site, the City typically works with developers to meet equivalent impervious area mitigation elsewhere in the City, or to contribute an equivalent fee (based on volume of water) to City delivered source control projects.

For development with a floor space ratio (FSR) of 1.0 or higher, it is a requirement for developments to monitor the effectiveness of source controls for a period equivalent to the off-site contribution warranty period. Guidelines and typical details are provided in the related links below.

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