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May 2006 Issue

Problem Solving in Stormwater Bioretention Systems
Pitfalls in bioretention systems and how to avoid them.

By Barrett L. Kays, ASLA

Problem Solving in Stormwater Bioretention Systems Environmental Services, City of Portland, Oregon

Stormwater bioretention systems are invaluable elements of ecological design, as well as tools to help meet stormwater regulations. Unlike detention systems, which generally involve a piped stream discharge during storms, retention systems normally allow water to infiltrate, and thus do not require other means of discharge or overflow. In typical bioretention systems, stormwater flows through and is filtered by permeable soil until it reaches groundwater. The flow down through the soil is sufficiently slow—it may take months to reach groundwater—that pollutants are bioaccumulated, immobilized, or converted to harmless constituents by plants and microorganisms. Some newer bioretention systems, designed to overcome site and soil limitations, still biologically treat the stormwater but do not necessarily provide infiltration into the underlying soil.

Bioretention systems (which include infiltration basins, if the basins are designed for mature plant growth) hold great promise to reduce runoff, improve water quality, and, during dry periods, enhance low flows in adjacent streams due to their positive effect on groundwater flow. Unfortunately, criteria for siting, design, and maintenance of these systems are not well understood. Designing innovative stormwater retention systems has been challenging, in part because so many built systems have failed. To successfully employ bioretention to treat stormwater, landscape architects need to better understand the technology and why failures occur.

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