Drainage Swales - Residential

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Swale: a shallow, sloped ditch which is often a wet tract of land; used to direct surface runoff towards a City right-of-way (lane or street).

Minimum slopes for drainage swales:

At least + 1.5% for a grass drainage swale and + 0.75% for a concrete drainage swale

Drainage Swale Requirements

Internal drainage swales are required for locations where a common side property swale cannot be constructed due to existing grading or unapproved grading on an abutting property. Minimum slope of 1.5%, a minimum depth of 10 cm and a width of 15 cm is required.

Common property swales (Detail A) are possible where common adjacent properties are suitably graded.

Retaining walls must be constructed to support any elevation difference between a Infill Housing Development property and an adjacent private property (Detail B)

Final Grade Swale
Drainage swale using a retaining wall
During Heavy Rain
After Light Rain
After Average Rain
Final Grade Swale

Zero Lot Line Drainage

Zero Lot Line properties developed in subdivisions have an internal swale in between the homes along with an Easement registered on title and design elevations as per Lot Grading Plan.

Example of an Engineered Subdivision Zero Lot Line Grading Plan

Zero Lot Line swale on topsoil with string line
Swale on sod with downspouts tied to storm service

Common Property Drainage

Lots that drain from rear to front

For lots that drain from rear to front, a swale is located in the back yard where the forward slope of the lot meets the rearward slope of the foundation grading. The rear internal swale directs surface drainage to one, or both, of the side lot swales.

Split drainage lots

For Split Drainage Lots, swales are necessary to convey surface drainage from the highest points to the nearest street or lane.

Common property drainage

Common property swales (Detail A) are possible where common adjacent properties are suitably graded. Grading must provide a minimum of 15 cm of unobstructed width within each property.

If decorative rocks or wood chips are used, the rough grade must be raised to final grade to maintain the function of surface drainage.

Typical Drainage Swales between lots
Typical drainage Swales between lots
Typical rear to front drainage
Split Surface conveys water to the front and back
Maintaining at least 15 cm of unobstructed width
Placing decorative rock on the clay (rough grade)
Improper grading generates a chance for ponding
Raising base to final grade before placing rock
Acceptable final grade base

Right-of-ways and Easements for Drainage Swales

Major drainage swales are often constructed on private properties in an easement or a right-of-way. These are indicated on a Lot Grading Plan and are Registered on Title.

Example of an Engineered Subdivision Lot Grading Plan with a concrete swale.

Example of an Engineered Subdivision Lot Grading Plan with a grass swale.