Pretreatment

Overland sheet flow

The following pretreatements are recommended where sheet flow is being directed to a BMP (typically a bioretention cell) from the edge of an impermeable surface such as an asphalt parking lot or a paved plaza:

• Gravel diaphragms
• Vegetated filter strips

A gravel diaphragm typically requires less setback between the catchment and the BMP. Where space permits, these two designs can be used in combination to even greater effect!

Concentrated overland flow

Where a curb cut, a disconnected downspout or a swale directs surface water to a point, increasing the flow rate, one or more of the following pretreatments should be considered:

• Forebays (well suited for bioretention cells)
• Sand filters (well suited for underground infiltration)
• Screens and racks

Concentrated underground flow

This type of pretreatement is dominated by proprietary systems, some of which have undergone verification under the Environmental Technology Verification (ETV) program.

Inline pretreatment options

Type	Description	ETV products	Other products (not verified)
OGS Baffles	Baffles slow flow, encouraging larger, heavier particles to drop out of the water column. A skimmer may also trap floating debris, oils and grease.	-	SAFL baffle
Hydrodynamic OGS	The shape causes rapidly flowing water to form a vortex which forces the larger, heavier particles are outwards and downwards into a lower chamber. Floating debris, oils and grease are trapped behind a baffle or in small tank area, towards the top of the main vortex chamber	CDS Downstream defender SDD	Stormceptor
Cartridge filters	Uses membrane technology adopted from water treatment plants.	-	Jellyfish filter
Catchbasin inserts	Great for retrofitting situations, to improve water quality	CB Shield	Snout

External resources

• CMHC OGS Effectiveness Study

This article is particularly heavy on proprietary systems and information. Please inform us of any omissions or broken links using the box below:

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.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice.

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

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.