Hybrid cells are used in low to moderate density, low-traffic contexts, and are typically placed in the planting strip on neighborhood or residential streets. Planting strips are often not wide enough to accommodate fully graded side slope bioretention swales, but are wide enough for hybrid cells.
The hybrid bioretention planter can reconcile demands for space with a strategy for stormwater management, and can fit into a commonly available space on existing streets.
Graded side slopes are less complex to adjust or modify after installation, such as to retrofit for future utilities or service installation, compared with a fixed, difficult-to-modify vertical wall.
The availability of space between the sidewalk and the curb may affect decisions about hybrid bioretention planter design. If space is constrained, locating the vertical wall on the sidewalk side may require less overall width for the facility footprint.
Design hybrid bioretention planters to accommodate street trees. The graded side slope is usually the best place for street trees, preventing the vertical wall from hindering tree growth. Additionally, avoid conflicts between location of subsurface utilities and planned root space for street trees (see page 86 for more information on root space).
Consider whether the desired tree canopy should shade the street or the sidewalk when deciding whether to site the planter with graded side slopes on the street side or sidewalk side. Plan for mature tree height and branching clearance—develop clear criteria for vertical clearance desired over bicycle facilities (typically 8 to 14 feet), pedestrian paths, or travel lanes.