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LID SWM Planning and Design Guide β


Revision as of 19:07, 2 November 2017 by Jenny Hill (talk | contribs)
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.
Domed, metal overflow/outlet grate
Photo credit:Aaron Volkening


  • Infiltration facilities can be designed to be inline or offline from the drainage system.
  • 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 invert of the overflow should be placed at the maximum water surface elevation of the bioretention area, (typically 150 - 250 mm above the surface).
  • Offline facilities use flow splitters or bypass channels that only allow the required water quality storage volume to enter the facility.
Higher flows are diverted and do not enter the infiltration practice. A pipe can by used for this, but a weir or curb cut minimizes clogging and reduces the maintenance frequency.


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