Street runoff is directed into the swale through curb cuts, trench drain, pipe, or as sheet flow in curbless conditions.
The slopes of graded sides should be determined based on the potential for erosion, ability to establish plants, maintainability, and ability for pedestrians and people using mobility devices to recover, should they enter bioretention facilities. Side slopes between 2.5–4H:1V (horizontal:vertical) are best suited to urban contexts, providing a balance between pedestrian comfort and landscape establishment. Vegetation and maintenance access are best supported by at least a 3H:1V or shallower slope.
Side slopes may be different on the sidewalk side and street side based on specific contextual requirements or concerns.
In areas where pedestrians may step off the sidewalk or curb into the bioretention swale, hardy vegetative ground covers can delineate the cell’s edge. Barriers such as short fencing can also help protect the facility from trampling, similar to that found on urban tree pits. Regular pedestrian access paths between the sidewalk and street can also discourage entry into swales.
In areas with significant on-street parking turnover, consider paving a 12- to 24-inch concrete strip at the back of the curb. In this instance, tailor the side slope to 3H:1V for the reduced level area in the step-out zone for pedestrian comfort.
Level areas at the edge of the bioretention swale should be compacted to support people walking on the surface as well as vehicles and people using bikes, strollers, or wheelchairs who may encroach on the swale from the adjacent roadway or sidewalk. Consider using undisturbed, firm, native soil for the edge zone. If placing soil in this zone, consider compaction to 95% density in the edge zone.
In constrained contexts, 2H:1V side slopes or steeper may be acceptable. These facilities are typically shallower in overall depth (generally 12 inches) and are more susceptible to soil erosion if the plants do not establish well and stabilize the soil.
Local stone boulders and cobbles can provide aesthetic enhancements, increased vertical slope, and erosion control if they are well-embedded and substantially anchored.
The infiltration gallery can be recessed under a vaulted sidewalk to increase detention capacity.
Where swales are planted with grass or lawn, a 3H:1V or 4H:1V side slope will ease mowing and maintenance access.
The bottom width may be uniform, or may be tapered if the cell slopes down, creating a more uniform ponding area across the cell (see page 103 for further detail).
Additional slope protection or reinforcement may be needed for stabilization if planters are deeper than 12 inches.