Outflow from the surface layer of the enhanced swale LID unit can either be surface runoff when the unit is unconfined (as in a rain garden) or be flow through an overflow structure for a confined unit (such as a street planter). For surface runoff, the Manning equation can be used in the same fashion that VO currently employs it for overland surface flow:

n = surface roughness coefficient,
S = surface slope (m/m),
A = flow area (flow width times flow depth) (), and
R = hydraulic radius of the flow area (m)