Freeboard

• In swales conveying flowing water a freeboard of 300 mm is generally accepted as a good starting point.
• In bioretention the freeboard is the difference between the invert elevation of the overflow structure and the inlet. 150 mm will suffice, so long as the inlet will not become inundated during design storm conditions.
• In above grade stormwater planters, the equivalent dimension would be the depth between the invert of the overflow and the lip of the planter (150 mm minimum)