• Bio-Retention
• Grassed Swale
• Infiltration Basin
• Infiltration Trench
• Modular Pavement
• Rain Garden

Bio-Retention

Figure 1. Bio-Retention in a Residential Area--Seattle

Figure 2. Bio-Retention in a Residential Area--Seattle

Description

Shallow, landscaped depressions that channel and collect runoff. To increase infiltration, the soil bed is sometimes amended, such as with mulch.

Function

Stormwater flows in, ponds on the surface, and gradually infiltrates into the soil bed. Pollutants are removed by adsorption, filtration, volatilization, ion exchange, and decomposition. Vegetation takes up nutrients, and stored runoff is reduced through evapotranspiration.

Common Uses

Bio-retention is commonly located in parking lot islands, or within small pockets in residential areas. Bio-retention is primarily designed to remove sediment, nutrients, metals, and oil and grease. Secondary benefits include flow attenuation, volume reduction, and removal of floatables, fecal coliform, and BOD.

When Not to Use

- On large drainage areas

- Without pre-treatment to remove excessive or fine sediment

- Where space is limited (can take up to 5% of a drainage area)

Grassed Swale

Figure 1. Water Quality Swale Adjacent to a Residential Street

Description

A series of vegetated open channels designed specifically to treat and attenuate runoff for a specified water quality volume.

Function

Filtering by vegetation in the channel, filtering through a subsoil matrix, and infiltration into underlying soil.

Common Uses

Because they are primarily linear in shape, grassed swales are often use adjacent to roads and in residential areas.

When Not to Use

- Large drainage areas

- Highly urban areas, where space is at a premium (swales take up a lot of space)

- Stormwater "hot spots", areas with the potential to generate highly contaminated runoff, such as gas stations (where infiltration can be a threat to groundwater)

Infiltration Basin

Figure 1. Construction of an Infiltration Basin (Foreground) at the End of a Drainage Channel. Overflow riser visible in near foreground.

Description

Stormwater runoff impoundment designed to capture, hold, and infiltrate into the ground a specific runoff volume.

Function

Removes fine sediment, nutrients, trace metals, and organics through filtration by surface vegetation, and through infiltration through the subsurface soil. Deep-rooted vegetation can increase infiltration capacity by creating small conduits for water flow.

Common Uses

Typically installed as an off-line BMP, with the runoff design volume directed to the infiltration basin. Pretreatment required to remove any coarse particulates, oil and grease, and soluble organics.

When Not to Use

- Runoff with high concentration of sediment or suspended solids (pretreatment may be required)

- Highly urban areas (they require a lot of space. One option in urban areas is to design infiltration basins for multiple uses, as in parks or playgrounds)

- Stormwater "hot spots", which are areas with the potential to generate higly contaminated runoff (including soluble pollutants), such as gas stations (where infiltration can be a threat to groundwater)

Infiltration Trench

Figure 1. Infiltration Trench

Description

Shallow excavations, with filter-fabric-lined sides, filled with stone to create underground reservoirs for stormwater runoff from a specific design storm. Contained runoff infiltrates through the bottom of the trench into the underlying soil.

Function

Controls total runoff volume. Removes suspended solids, nutrients, and trace metals. Some peak flow attenuation. Some removal of dissolved solids up to the design storm volume.

Common Uses

Appropriate for small watersheds of two acres or less. Because they are narrow, infiltration trenches can be used in areas with limited space.

When Not to Use

- Sites with the potential for release of large amounts or high concentrations of pollutants, such as industrial or commercial sites.

- Areas with the potential for groundwater contamination, such as in close proximity to drinking water wells.

Modular Pavement

Figure 1. Parking Area Covered with Modular Pavement (Background), Adjacent to a Traditionally Paved Driveway

Figure 2. Parking Area Covered with Modular Pavement

Figure 3. Close-Up of Modular Pavement, Showing Vegetation Growing in the Voids

Figure 4. Modular Paving Blocks, Removed and Stacked, Prior to Re-Installation

Description

Modular paving blocks or grids, or cast-in-place concrete grids.

Function
Voids in the blocks or grids minimize the amount of impervious area, allow vegetation to grow, and allow runoff to infilrate into the subsurface soil. Reduces runoff volumes. Surface vegetation will trap some sediment and nutrient from runoff.

Common Uses
Roadside right-of-ways, emergency access lanes, delivery access routes, overflow parking areas.

When Not to Use

- Heavy-traffic areas

- Areas that require handicap accessibility

Rain Garden

Figure 1. Landscaping, Foot Bridge, and Rock-Lined Channel in a Rain Garden

Figure 2. Landscaped Area in a Rain Garden

Description

Low areas in gardens, landscaped with native vegetation. To increase infiltration, the soil bed is sometimes amended, as with mulch. The rain garden is a form of bio-retention.

Function

Filtration of polluted stormwater by vegetation. Uptake of nutrients and excess stormwater by vegetation. Evapotranspiration. Infiltration of stored stormwater through the soil.

Common Uses

Suitable in residential, commercial, and light industrial areas. Consider conversion of existing gardens to rain gardens.

When Not to Use

- On large drainage areas

- Without pre-treatment to remove excessive or fine sediment

- Where space is limited (can take up to 5% of a drainage area)