For both vertical walled and graded bioretention cells, a single cell’s or system of cells’ ability to infiltrate runoff effectively relies upon three criteria:
- Cell wetted area footprint;
- Ponding depth; and
- Infiltration rate of underlying and engineered soils.
Bioretention Cell Wetted Area provides the infiltration footprint area of the facility; the wetted area is the surface area at maximum ponding depth of a facility. A larger wetted area maximizes the infiltration area used in sizing the facility. As the bottom area increases, the wetted area increases to maximize the cell’s storage capacity.
Engineered soils can increase the bioretention capacity of facilities, providing additional storage.
For bottom width, 4 feet is a typical preferred minimum for bioretention planters with walled sides; narrower planters may be possible, but increase the challenge of maintaining healthy plants and are usually less cost-effective to implement, given construction and maintenance costs and performance. For bioretention swales the minimum recommend bottom width is 1 foot.
Cell length, or the length of a bioretention swale or planter along the curb, can range from 10 feet to the length of an entire block with intermittent berms. Besides stormwater storage and infiltration capacity, cell length is also affected by longitudinal slope and design infiltration rate of bioretention soil media and native soils.
Aside from managing stormwater runoff efficiently, the length of an individual cell is driven by all-mode curbside access needs. Where parking is permitted, provide for sidewalk access from the curb about every 40 feet, or approximately the length of two parking stalls. Cell crossings can be provided at sidewalk level or may slope down to street level, but must be above the ponding depth of the cell.
The full bottom length and width of an individual bioretention cell should be used for temporary stormwater storage; where possible, cell design should maximize temporary storage capacity.
A level bottom area accommodates relatively simple maintenance access for routine cleaning and plant care.
On streets with longitudinal slope, install elevated berms or check dams to allow runoff to pond and infiltrate downward through the entire cell bottom area, rather than only flow downstream to the end of the cell.
Ponding depth provides for temporary storage of the stormwater before it filters downward through the bioretention facility. The temporary ponding depth for bioretention facilities ranges from 2 inches (for mitigating sidewalk runoff alone, or in fast draining soils) to up to 12 inches (for mitigating roadway runoff, or in slower draining soils). In areas with moderate to high pedestrian activity (e.g. commercial streets and business districts, and near busy community facilities), limit the ponding depth to 6 inches or install short fencing around the bioretention facility for depths greater than 6 inches.
Vector control issues make drawdown time and maintenance critical from a health and safety perspective. Typically, cells should drain out (have no surface ponding) within 72 hours after the rain event has ended to prevent insect-borne diseases (e.g. mosquitoes breeding cycle). However, a 12- or 24-hour drawdown period is often preferable, especially in contexts with regular precipitation (to accomodate the next storm), high pedestrian volumes, or other influencing factors. If fast-infiltrating native soils are present, faster drawdown is also realistic.
Freeboard depth, measured from the maximum ponding depth to the top of the facility’s overflow elevation, provides a buffer during larger storm events when water overflows the facility and flows into either a storm collection structure or back into the street gutter system through an inlet or curb cut. Freeboard depth is affected primarily by the street grade, and should range from 2–6 inches or greater, depending upon site context, potential for overflow occurrence and its impacts, capacity of the downstream conveyance, the volume of water that the facility is managing (runoff from one block versus multiple blocks), and other engineering or design judgements. Set a deeper freeboard (usually with higher walls) at locations with more frequent overflows or greater potential impacts.