Surficial flow diverter

Surficial flow diverters are shallow dips, low bumps, or curb inlets that can redirect runoff from the impervious surface to a pretreatment or treatment BMP. Examples of these diverters are a shallow concrete valley gutter, a low water diversion bump, and a concrete curb cut inlet. While the diversion BMP used by itself does not reduce water quantity, its place in the treatment train is critical for collecting and diverting storm water to prevent direct discharge of pollutants into a water body.


These surficial flow diverters are very suitable for retrofitting existing PWA parking lots provided that grades are appropriate. Surficial flow diverters divert sheet flow into a bioretention basin (with pretreatment) and into parking lot islands or edges to prevent runoff from flowing down the boat launch ramp.

Cold Climate Suitability

During winter, surficial 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

Properly designed and installed surficial 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 surficial 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. Valley Gutter detail, Water Diversion Bump detail
  • Compliance with requirements of the Americans with Disabilities Act (ADA) for access around the diverter.


Surficial flow diverters are typically constructed of concrete or asphalt and built on a solid base to prevent settling.


Properly designed and constructed surficial flow diverters 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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