Bioswales are vegetated drainage ways that provide an alternative to buried storm sewer pipes. Bioswales may be cheaper to install and provide water quality benefits that storm sewers can't provide. Bioswales are designed to infiltrate runoff of from frequent, small rains and convey large runoff events to receiving streams. A bioswale has a perforated subdrain, a sandy soil mix, and strategically spaced berms to facilitate infiltration of small runoff events. The berms - usually about 1 foot in height - do not impede flows of large runoff events.
Bioswales will infiltrate about 80-85% of runoff events, which will significantly reduce the delivery of pollutants to receiving streams. Bioswales will also reduce the flashiness of urban stream flows, which will reduce stream corridor erosion - a major source of sediment in Iowa surface waters. Bioswales convey runoff from large storms in a non-erosive manner and are competitive in cost with expensive large storm sewer installation.