- Infiltration facilities can be designed to be inline or offline from the drainage system. See Inlets
- 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.
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.
- 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 overflow structure and the inlet. 150 mm will 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 overflow structure and the lip of the planter (150 mm minimum)
Overflow outlet options
Metal grates are recommended (over plastic) in all situations.
|Feature||Anti Vandalism/Robust||Lower Cost Option||Self cleaning|
|Ditch inlet catch basin||x||x|
Flat metal overflow with stone surround to reduce erosion around the cast concrete structure. Mississauga Road, ON
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