Level spreaders

From LID SWM Planning and Design Guide
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This clever design incorporates a level spreading device after a curb cut has narrowed the flow path. This kind of treatment train approach would provide an opportunity to provide pretreatment at the point of concentration. Photo credit: MPCA
Diagram from Wikimedia commons. Credit: U.S. Army Corps of Engineers, Construction Engineering Research Laboratory

Level spreaders should conform to the following design criteria in order to ensure non-erosive sheet flow into vegetated areas such as vegetated filter strips, bioretention, swales or forested conservation areas[1].

The length of the level spreader should be determined by the design storm inflow rate and type of landscape downstream:

  • 1.4 m of level spreader length per every 0.01 m³/s of inflow when draining to a vegetated filter strip, bioretention, swale or grassed area;
  • 4.3 m of level spreader length per every 0.01 m³/s of inflow when draining to a forested conservation area.
  • The minimum level spreader length is 4 m and the maximum is 40 m.
  • The level spreader lip should be concrete, wood or pre-fabricated metal.
  • The ends of the level spreader section should be tied back into the slope to avoid overflow scouring or erosion around the ends.
  • The width of the level spreader should be the greater of 300 mm or three times the diameter of the inflow pipe.
  • The depth should be the greater of 200 mm or half the inflow pipe diameter.

  1. Hathaway, J. M., & Hunt, W. F. (2006). Level Spreaders : Overview , Design , and Maintenance.