12/19/2001 Env Bd Packet - Laserfiche WebLink

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Exhibit �{ Infiltration Basins Description An infiltration basin is stormwater runoff impoundment designed to capture a stormwater runoff volume, hold this volume and infiltrate it into the ground over a period of days. It does not retain a perma- nent pool of water. Infiltration basins are typically off -line, end -of- pipe BMPs. A flow splitter or weir is usually used to divert runoff from a storm sewer system into the infiltration basin. Infiltration basins in this BMP Section refer to end -of -pipe infiltra- tion systems that treat stormwater runoff from a few lots or proper- ties as opposed to rainwater gardens which are primarily used for a single lot application (see the On -Lot Infiltration BMP Section for information on this type of BMP). A key feature of an infiltration basin is its vegetation. It is important to vegetate the bottom of the basin with deep- rooted plants to increase the infiltration capacity of the basin. Roots create small conduits for water to infiltrate. The root penetration and thatch formation of the vegetation maintains and may enhance the original infiltration capacity. Dense vegetation will also impede soil erosion and scouring of the basin floor. Infiltration basins are not appropriate for areas that contribute high concentrations of sediment, or suspended solids, without adequate pretreatment. Excessive sediment can clog the basin and take up storage volume. Infiltration basins require pretreatment of stormwater in order to remove as many of the suspended solids from the runoff as possible before the water enters the basin. Pretreatment, such as grit cham- bers, swales with check dams, filter strips, or a sedimentation basin should be a fundamental component of any BMP system relying on infiltration. Good housekeeping measures should also be investi- gated (e.g., street sweeping, reduction of sanding or salting practices, etc.). Public education with respect to street and driveway sediments should be provided in areas where an infiltration basin is proposed. Metropolitan Council / Barr Engineering Co. Purpose Flow attenuation Water Quantity u Runoff volume reduction Pollution prevention Soil erosion A Sediment control A Nutrient loading A Water Quality Pollutant removal Total suspended sediment (TSS) Total phosphorus (P) Nitrogen (N) Heavy metals Floatables Oil and grease Other Fecal coliform Biochemical oxygen demand N/ N/ N/ • • • 11 11 11 11 ■giG14 design benefit EiSecondary design benefit Little or no design benefit 3-155