Problem Solving in Stormwater Bioretention Systems
Pitfalls in bioretention systems and how to avoid them.
By Barrett L. Kays, ASLA
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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