Bioretention Basin Calculator

A bioretention basin (also called a rain garden or bioretention cell) is a landscaped depression that captures stormwater runoff, allows it to pond temporarily, and infiltrates it through an engineered soil mix into the ground. The basin removes pollutants, reduces peak runoff rates, and recharges groundwater. They are commonly used on residential and commercial properties as part of green infrastructure stormwater management.

The required basin area is typically sized to capture and infiltrate runoff from a design storm (often the 1-inch or 1.5-inch rainfall event). The design method from the EPA and most state stormwater manuals is: Basin Area = (Drainage area x Runoff coefficient x Rainfall depth) / (Ponding depth + Infiltration rate x Drain time). Many states use a simplified rule of 5-15% of the impervious drainage area.

The maximum ponding depth for a bioretention basin is typically 6-12 inches (0.5-1.0 foot). Deeper ponding can stress or kill plants, create anaerobic conditions that reduce pollutant removal, and cause nuisance ponding. The basin must drain completely within 24-72 hours after the design storm to maintain aerobic conditions and plant health.

The recommended bioretention soil mix (BSM) is typically 85-88% sand, 8-12% fines (silt and clay), and 3-5% organic matter (compost) by volume. This mix provides a balance of drainage (minimum 1-2 inches per hour infiltration rate), water retention for plants, and nutrient/pollutant sorption capacity. The EPA recommends a minimum soil depth of 24-48 inches depending on target pollutants.

Bioretention basins may or may not include an underdrain depending on site conditions. Basins in areas with poorly draining soils (percolation rates below 0.5 inches per hour) typically require a perforated pipe underdrain to prevent extended ponding. Basins with an underdrain act as a filter but do not significantly recharge groundwater. Sites with high groundwater tables need underdrains to maintain adequate separation.