LIDs are modeled as several interconnected, fully mixed layers representing the surface, pavement, soil, storage, and underdrain portions of a LID unit. Infiltration, drainage, and overflow control the storage in each of the layers dynamically. In VO, soakaway pit, underground chamber, pavement, rain garden, green roof, bioretention cell and enhanced swale could be explicitly modeled. The infiltration rate in the soil layers is provided by the user in VO single event and modeled with the Green-Ampt equation in VO continuous simulations. Outflow from the storage zone into the native soil is provided by the rating curve described by the discharge (i.e. outflow) and storage relationship. Underdrain flow is represented by an orifice-type equation. The presence and absence of each layer type depends on the type of LID. For example, a soakaway pit only has the storage layer, whereas bioretention systems have surface, soil, storage, and underdrain layers.
