Hydraulically separated or connected with an underdrain, these are often quite small units of 5 - 50 m2 each. In urban settings the bioretention may be bounded entirely with hardscape, restricting options for pretreatment and sheet flow. Amenity and safety can be enhanced on sidewalks with a short (~ 45 cm wall) for seating, although the finished grade is usually only slightly lower than surroundings. Where underground space permits, shade trees are common feature of these facilities, enhancing the streetscape experience and optimizing transpiration. This type of bioretention is often designed offline, with bypass overflow.
An excellent opportunity for integrating more bioretention into the street is through careful design of curb extensions for traffic control.
Bioretention on residential street, with covered drain inlet (grate in the gutter) and iron grate covered overflow in the centre of the cell, Hinsdale, IL Photo credit: CNT
Curb extension on Queen Lane, in Philidelphia
Photo credit: PWD
Extended tree pit planting in USA
Photo credit: USEPA