The LID Philosophy

Traditional stormwater management collects runoff quickly with curb-and-gutter systems, conveys it through storm drain pipes, and discharges it to natural waterways — concentrating peak flows and increasing downstream flooding and erosion. Low Impact Development (LID) takes the opposite approach: manage rainfall where it falls, using distributed practices that infiltrate, evapotranspire, or harvest runoff before it becomes a concentrated flow problem.

LID is now required or strongly incentivized by many municipal stormwater ordinances, the EPA's NPDES Construction and Municipal programs, and green building rating systems (LEED, Envision). Understanding LID practices is essential for modern civil site engineers.

Bioretention Cells (Rain Gardens)

Bioretention cells — commonly called rain gardens at the residential scale — are vegetated depressions filled with engineered soil media (typically a sand/compost mix with high infiltration rate) and planted with native plants adapted to both wet and dry conditions. Runoff flows into the cell, ponds briefly, infiltrates through the media, and is either absorbed by plants and soil or drains through an underdrain to the storm system.

A properly designed bioretention cell can capture and treat the "water quality volume" — typically the first 0.5–1 inch of runoff from the contributing area — removing suspended solids, nutrients, metals, and hydrocarbons. Sizing is typically 5–10% of the contributing drainage area for a 1-inch design storm.

Permeable Pavement

Permeable pavement allows rainwater to infiltrate through the pavement surface into a stone reservoir base, where it infiltrates to the subgrade or drains through an underdrain. Types include:

  • Pervious concrete — open-graded concrete with no fine aggregate; infiltration rate 50–800 in/hr when new
  • Porous asphalt — similar open-graded design in asphalt
  • Permeable interlocking concrete pavers (PICP) — solid units with open joints filled with aggregate
  • Grid pavers — plastic or concrete grids with grass or gravel fill (suitable for overflow parking)

Maintenance (annual vacuuming to prevent clogging) is critical to long-term performance. Permeable pavement is not suitable for heavy truck traffic or areas with clay soils and high water tables.

Vegetated Swales

Vegetated swales (bioswales) are linear channels with dense grass or wetland vegetation that convey stormwater at reduced velocities, promoting infiltration and settling of sediment and pollutants. They replace traditional curb-and-gutter drainage in low-density developments and can be designed to provide both conveyance and water quality treatment in a single facility.

Green Roofs

Green roofs (vegetated roofs) replace conventional roofing with growing media and plants. Extensive green roofs (2–6 inches of media, sedums and grasses) retain 40–80% of annual rainfall through evapotranspiration, significantly reducing roof runoff volumes and peak flows. They also reduce urban heat island effect and extend roof membrane life. Structural loading requirements (typically 12–50 psf saturated) must be accommodated in the building design.

LID Sizing and Regulatory Compliance

Most municipal LID ordinances require that post-development hydrology match pre-development hydrology for specified design storms. LID practices are credited with volume reduction (not just peak flow attenuation), allowing smaller or eliminated conventional pipe systems. The EPA's SWMM model and the NRCS TR-55 method can both model LID practices for regulatory compliance calculations.