Overflow

From LID SWM Planning and Design Guide
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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[edit]

  • 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 far end of the facility to prevent any localised ponding to cause bypassing of the infiltration facility.
  • 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.

Overflow elevation[edit]

The invert of the overflow should be placed at the maximum water surface elevation of the practice. i.e. the maximum ponding depth. A good starting point is around 300 mm over the surface of the practice. However, consideration should be given to public safety and drainage time for the ponded water to drain. See Bioretention and Stormwater planters

Freeboard[edit]

  • 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).

Options[edit]

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[edit]