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, swales or forested conservation areas.
The length of the level spreader should be determined by the type of filter area and the design flow:
- 1.4 m of level spreader length per every 0.01 m³/s of inflow for discharges to a filter strip or grassed area;
- 4.3 m of level spreader length per every 0.01 m³/s of inflow when the spreader discharges 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, with a well anchored footer, or other accepted rigid, non-erodible material.
- The ends of the level spreader section should be tied back into the slope to avoid overflow scouring or erosion around the ends of the spreader.
- Where relevant, the width of the level spreader channel on the up-stream side of the level lip should be three times the diameter of the inflow pipe, and the depth should be 20 cm or one-half the culvert diameter, whichever is greater.
- Hathaway, J. M., & Hunt, W. F. (2006). Level Spreaders : Overview , Design , and Maintenance.