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

Oil and grit seperators (OGS)

OGS are devices which use physical principles to improve water quality. Typically these are underground chambers installed in catch basins or manholes. Many commercial and custom configurations exist for both new installations and retrofit scenarios.

One method is to incorporate baffles within a tank or chamber:

• The first baffle slows the incoming water flow, encouraging any larger, heavier particles to drop out of the water,
• A second baffle skims the surface of the water, preventing floating debris, oils and grease from passing.

Proprietary examples of baffle systems (not verified): SAFL baffle

Another option is a hydrodynamic device, which forces the incoming water flow into a circular vortex:

• The vortex motion 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.

Proprietary examples of verified hydrodynamic systems :

• CDS
• Downstream defender
• SDD

Proprietary examples of other hydrodynamic systems (not verified): Stormceptor

Inline filters

Using technology adopted from water treatment plants.

• Jellyfish

Catchbasin inserts

Great for retrofitting situations, to improve water quality:

Proprietary examples of verified inserts:

• CB Shield

Proprietary examples of other inserts (not verified):

• Snout

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 button below:

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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.

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.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

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

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