Types of swales

Design Variants

(Adapted from Georgia Stormwater Manual and Center for Watershed Protection)

As filtration becomes a more common tool in stormwater management, and as the number of design variants increases, so does the number of names for each of these variants. For example:

• Sand filters are also referred to as filtration basins, filter systems, first-flush filtration, or media filtration systems.
• Grass channels are also referred to as biofilters. (Seattle METRO, 1992 from CWP)
• Dry swales are also referred to as grassed or vegetated swales.

The following types of filtration systems are appropriate for Minnesota, depending upon project scale and site conditions:

Media Filters

Surface Sand Filter

For a surface sand filter, a flow splitter is used to divert runoff into an off-line sedimentation chamber. The chamber may be either wet or dry, and is generally used for pre-treatment. Runoff is then distributed into the second chamber, which consists of a sand filter bed (~18”) and temporary Runoff storage above the bed. Pollutants are trapped or strained out at the surface of the filter bed. The filter bed surface may have a sand or grass cover. A series of perforated pipes located in a gravel bed collect the Runoff passing through the filter bed, and return it to the stream or channel at a downstream point. If underlying soils are permeable, and ground water contamination unlikely, the bottom of the filter bed may have no lining, and the filtered runoff may be allowed to infiltrate. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Underground Sand Filter

The underground sand filter was adapted for sites where space is at a premium. In this design, the sand filter is placed in a three chamber underground vault accessible by manholes or grate openings. The vault can be either on-line or off-line in the storm drain system. The first chamber is used for pre-treatment and relies on a wet pool as well as temporary runoff storage. It is connected to the second sand filter chamber by an inverted elbow, which keeps the filter surface free from trash and oil. The filter bed is 18 inches in depth and may have a protective screen of gravel or permeable geotextile to limit clogging. During a storm, the water quality volume is temporarily stored in both the first and second chambers. Flows in excess of the filter’s capacity are diverted through an overflow weir. Filtered runoff is collected, using perforated under-drains that extend into the third “overflow” chamber. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Perimeter Sand Filter

Schematic of Delaware sand filter

The perimeter sand filter consists of two parallel trench-like chambers that are typically installed along the perimeter of a parking lot (See Schematic of Delaware sand filter to right). Parking lot runoff enters the first chamber, which has a shallow permanent pool of water. The first trench provides pre-treatment before the runoff spills into the second trench, which consists of a sand layer (12”-18”). During a storm event, runoff is temporarily ponded above the normal pool and sand layer, respectively. When both chambers fill up to capacity, excess parking lot runoff is routed to a bypass drop inlet. The remaining runoff is filtered through the sand, and collected by under-drains and delivered to a protected outflow point. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.