Bioswale

Why this pattern exists

Conventional curb-and-gutter drainage moves runoff, and whatever pollutants it carries, directly into the storm system with no filtration; a bioswale intercepts that runoff at the source, reducing both the volume reaching the storm system and the pollutant load it carries. This pattern exists to document the design considerations that make a swale function as intended rather than as an underperforming ditch.¹

Key considerations

• Sizing must be based on the contributing drainage area and expected runoff volume; an undersized swale simply overflows during real storm events²
• Plant selection should tolerate both periodic inundation and dry periods, since swale conditions vary significantly between storm events
• Inlet design (curb cuts or similar) should prevent sediment and debris from clogging the swale’s inlet point over time

Related patterns

Flow-Through Planter (ELM-FTP-20), Pervious Strip (ELM-PVS-21), Pervious Pavement (ELM-PVP-22) (related green stormwater treatments); Curb Cut/Driveway Apron (ELM-CCA-04) (relevant where a curb cut serves as the swale’s inlet)

References

1. Legal / regulatory U.S. Environmental Protection Agency, Green Streets / Green Street Design Manual, 2021.
2. Precedent-based National Association of City Transportation Officials, Urban Street Stormwater Guide.

"Legal / regulatory" here refers to the EPA guidance that municipal NPDES stormwater permits are legally built around, not a single mandatory design spec like MUTCD or PROWAG.