Bioretention and Bioswales
Bioretention cells and bioswales are treatment devices for stormwater, which generally occur in public spaces. They infiltrate stormwater but this is not their primary purpose. Moisture levels fluctuate widely in these features, since the flow of water they receive is concentrated, sometimes up to 40 times what a given area would receive naturally.
WIth tight clay subsoils in our region, these features normally include a subdrain system that ultimately ties to the storm sewer system. Water passes through the soil where physical, chemical and biological processes break down, filter, or hold contaminants. Water that reaches the subdrain is far cleaner, its temperature is moderated, and the flow rate and volume are attenuated. Suspended pavements which receive stormwater inputs function in the same way.
There is a lot of confusion over the terminology and overlap between bioretention, biofilter, bioswale, and rain garden. The ALIDP takes the approach that the term bioretention should be used to indicate the highly engineered feature, generally occuring in the public realm, where treatment is the primary function. The term rain garden should be used when describing depressions, typically on residential lots where downspouts are directed into them, receiving about four times the normal amount of moisture, where volume attenuation is the primary function.