Swales

Typical design

Swales are vegetated (usually with grass) ditches with gentle side slopes that are commonly placed alongside streets but can be used in other areas as well. They usually have a triangular and trapezoidal cross section.

The width of the swale and the slope of the sides varies. Deeper swales with steeper sides can carry and store more water overall, but shallower swales with gentler side slopes tend to lead to lower flow velocities. This also allows for more infiltration and trapping of sediments on the side slope, especially during less intense rains. This has a positive effect on water quality. In these cases, the side slope may also be called a buffer strip.

Short grass

Long grass

Filter material

For intense rains the swale will transport water along the length of the swale. Swales reduce downstream peak flows due to the lower flow velocity compared to pipe systems. If the longitudinal slope (i.e. in the main flow direction) is higher than 2-4%, there is a risk that the flow velocities will become high which reduced the swales treatment potential and could lead to erosion. In this case, small dams may be installed across the swale at regular intervals to create some ponding and slow down the flow. This can also increase infiltration.

Swales are vegetated at least with grass. This lowers the flow velocities, supports trapping of sediment and reduces the risk of erosion. In some cases, taller and more dense vegetation (and sometimes also boulders) is added into the swale to further reduce flow velocities and allow increased removal of pollutants. Such a swale may be referred to as a treatment swale or wetland swale.

For even more treatment, swales can also be fitted with some type of filter media below, like what might be used for biofilters (see XX next step). Such a system may be called a bioswale or biofilter swale.

Aims in stormwater management

In its simplest form, a swale is designed only to handle stormwater flows and reduce and delay runoff. The underlying soil will determine how much runoff volume can be decreased by infiltrating part of the water into the soil. There can also be some effect on stormwater quality, but this is often not the primary purpose.

In more complex forms, with lower slopes and increased vegetation decreasing flow velocities, swales also serve to remove some pollutants from stormwater. This will mainly happen through sedimentation and so it will mainly affect particles and pollutants tied to the particles. There is no or little treatment of dissolved pollutants.

Implementation in catchments

Swales are most commonly placed alongside streets so that gutters, gully pots and pipes are not needed. The width of the swale should ideally be 0.5 m – 3 m in the bottom. The wider the bottom of the swale is the lower the flow velocities will be, which improves detention and water treatment through sedimentation. The sides normally have a slope of between >1:5 or steeper. Together with the depth of the swale this determines how wide the swale is and thus how much space it requires.

In areas with substantial snowfall during winter swales are also used to store snow that has been removed by snow ploughs. In this case, swales can be designed even bigger to accommodate this snow storage.

Swales are mostly placed throughout the catchment, typically alongside roads but this is not required: swales can also through green areas to transport water. In the latter case the side slopes can be relatively steep as there will be none or very little inflow from the sides.

It is less common to place swales at the outlet of the catchment (end of pipe solution), although sometimes part of the pipe can be replaced with a swale.

Additional benefits

• Swales can provide some amenity value.
• Treatment swales or biofilter swale (that is, more vegetation than just short grass) can contribute to biodiversity.