Subsurface flow diverter

Subsurface flow diverters are structures installed below the surface that redirect runoff from an impervious surface to a pretreatment or treatment BMP. Examples of subsurface flow diverters are trench drains and catch basins with a pipe outlet. While the diversion BMP used by itself does not provide water quantity reduction or water quality benefits, its place in the treatment train is critical for collecting and diverting storm water to prevent direct discharge of pollutants into a water body.


Subsurface flow diverters, often used at the top of boat launch ramp areas, divert sheet flow into a bioretention basin (with pretreatment) to prevent runoff from flowing down the boat launch ramp. While there are various brands of trench drains, a slotted vane drain functions more effectively than a conventional grate by capturing more volume from the surface and conveying it into the drain. The curved shape of the leading edge of the vane provides a surface for the water to adhere to, and the flat face of the second vane directs the flow downwards. Efficiency of the vane grate is 100% for typical design flows.

Subsurface flow diverters are suitable for retrofitting in existing PWA parking lots provided that grades are appropriate.

Cold Climate Suitability

During the winter season, subsurface flow diverters may be affected by snow and ice accumulation, causing runoff to find an alternate flow path, possibly flowing over or past the diverter.

Water Quantity and Water Quality Benefits

When properly designed and installed, subsurface flow diverters direct storm water runoff to pretreatment and treatment BMPs. While the diverter by itself does not reduce water quantity or improve water quality, it is part of the treatment train critical for collection and diversion of storm water into treatment BMPs.

Design Considerations

Critical components for an effective subsurface flow diverter design:

  • Contributing drainage area size: affects the cumulative volume and depth of storm water to be diverted.
  • Slope of contributing drainage area: affects the slope and angles used in the design of the diverter.
  • Ramp width: affects the slope and angles used in the design of the diverter.
  • Reinforcement of the outflow area: reinforce from the end of the diverter to the pretreatment or treatment BMP with either a concrete apron or a turf reinforcement mat. Trench drain outlet detail
  • Compliance with requirements of the Americans with Disabilities Act (ADA) for access around the diverter.


Subsurface flow diverters are typically constructed of PVC pipe, concrete pipe, poured concrete, and/or cast iron and built on a solid base to prevent settling.


When properly designed and constructed, surficial flow diversions are relatively self-cleaning as a result of the periodic flows of storm water runoff. However, the sediment in the storm water runoff will accumulate in the receiving pretreatment and treatment BMPs, so periodic removal of sediment accumulation will be needed in those locations.







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