Several bioswales now remove sediment and other pollutants before road runoff goes into Monoosnoc Brook in Leominster. These runoff remedies are linear bio-retention areas that both filter and convey stormwater. Similar to rain gardens, bioswales briefly store the “first flush” of runoff (i.e., first inch of rain). The plants and mulch remove pollutants, and a sandy soil used to fill the swale allows stormwater to soak into the ground for more filtering. During larger storms, runoff with less pollutants will either bypass or overflow to nearby drainage system, depending on design of the bio-swale.
As shown by the photo, runoff from a half acre of road enters the catch basin grate. A six-inch diameter pipe then brings the stormwater into the bio-swale where it fills the two storage cells and infiltrates to the ground. At the “beehive” riser seen on the far end, runoff goes to an underground leaching chamber where stormwater seeps through sandy filter media into native soils beneath the swale. The cleansing by plants and infiltration removes 80 – 90% of the pollutants in the first inch of storm runoff.
Annual maintenance includes cleanout of the catch basin inlet as well as replacement of plants and mulch as needed. This low-cost upkeep enables the swale to filter pollutants for many years.
Examples of Leominster bioswales will be part of the October 19 tour being hosted by the Ecological Landscaping Association (see blog below). For more details about bio-swales, visit the MWC website www.commonwaters.org or email firstname.lastname@example.org.