Overflow

Conceptual diagram of the excess routing alternatives: On the left, excess flow leaves the cell via an overflow; on the right, excess flow is diverted so that only the design volume enters the cell.

Routing

• Infiltration facilities can be designed to be inline or offline from the drainage system. See figure to the right for an illustration.
• Inline facilities accept all of the flow from a drainage area and convey larger event flows through an overflow outlet. The overflow must be sized to safely convey larger storm events out of the facility. The overflow must be situated at the maximum surface ponding elevation or furthest downgradient end of the facility to limit surface ponding during periods of flow in excess of the facility storage capacity.
• Offline facilities use flow splitters or bypass channels that only allow the design storm runoff storage volume to enter the facility. Higher flows are conveyed to a downstream storm sewer or other BMP by a flow splitting manhole weir or pipe, or when the maximum surface ponding depth has been reached, by by-passing the curb opening and flowing into a downstream catchbasin connected to a storm sewer.

Overflow Elevation

The invert of the overflow should be placed at the maximum water surface elevation of the practice (i.e. the maximum surface ponding level). A good starting point is 150 to 350 mm above the surface of the mulch cover. However, consideration should be given to public safety, whether or not an underdrain is included, the time required for ponded water to drain through the filter bed surface, and if no underdrain is present, into the underlying native soil (must drain within 48 hours). See Bioretention: Sizing and Stormwater planters for more details.

Freeboard

• In swales conveying flowing water a freeboard of 300 mm is generally accepted as a good starting point.
• In bioretention the freeboard is the difference between the invert elevation of the inlet and overflow structure. 150 mm will usually suffice, so long as the inlet will not become inundated during design storm conditions.
• In above grade stormwater planters, the equivalent dimension would be the difference between the invert elevation of the lip of the planter and the overflow structure (150 mm minimum).

Overflow Outlet Options

Metal grates are recommended (over plastic) in all situations.

Feature	Anti Vandalism/Robust	Lower Cost Option	Self cleaning
Dome grate			x
Flat grate		x
Catch basin	x
Ditch inlet catch basin	x		x
Curb cut	x	x	x

Gallery

• 

  Flat metal overflow with stone surround to reduce erosion around the cast concrete structure. Mississauga Road, ON

• 

  Domed, metal overflow grate. Being flush with the surface reduces potential infiltration of ponded water. Photo credit: Aaron Volkening

• 

  Overflow inlet for newly constructed stormwater bioretention areas in median of Bradley Road. Village of Brown Deer, Wisconsin. Bradley Road, east of 51st Street. Photo from October 2015. Constructed summer 2015.
  Photo credit: Aaron Volkening

• 

  Flat, metal overflow grate in Emeryville California Stormwater Curb Extension

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  Note the greyish colour change indicating UV degradation of this plastic overflow inlet in a bioretention cell; metal is recommended instead. Mississauga ON.

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

Refers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

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

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

Refers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

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

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

A perforated pipe used to assist the draining of soils.

The natural ground material characteristic of or existing by virtue of geographic origin.

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

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

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