4. Suitability for environmental conditions

Impact of soil type, frost susceptibility and erosion sensitivity of soil, slope, category of land use and catchment characteristics on the performance of the structure. Recommended location (Upstream/downstream, terrain location, distance to the water body).

The longitudinal slope of the swale should preferably be 1–3% and at most 5%. In steeper terrain, water should be retained, or the swale should be terraced to manage flow effectively. They should be aligned to avoid sharp bends, as these can cause erosion.

Depending on the soil type, implementation can also include infiltrating or filtering systems.

5. Groundwater protection considerations

Risks of groundwater contamination, suitable groundwater table level.

Unlined swales should not be used to treat runoff from areas with high contaminant loadings if the risk of groundwater pollution due to infiltration is unacceptably high. Where a liner is used to prevent infiltration, the seasonally high groundwater level should be below the level of the liner. If infiltration is allowed, the maximum likely groundwater level should be at least 1 m below the base of the swale.

6. Suitability for winter conditions

Impact of winter conditions, freezing, frost and snow load, on the performance of the structure.

Vegetated swales are typically located along streets or property edges, where they may be used for snow storage during winter. Those intended for snow storage should be designed wide enough so that snow does not need to be piled on the bottom, keeping the flow path open. In spring, swales should be cleared of snow. If the swales are also intended for stormwater infiltration, the bottom soil of the depression must have sufficient permeability. With sufficiently porous soil (permeability coefficient k~1*10⁻⁵), the bottom of the depression will not freeze completely, allowing infiltration to occur even at the beginning of the thawing season. Freezing can also be prevented with drainage.

7. Restricting factors

Factors that restrict the implementation of the structure.

Swales should not be located where extensive areas of trees or overhead structures will cause shade conditions that could limit growth of grass (or other vegetation).

The distance of the swale from buildings should be at least 3 meters.

8. Structure sizing and water management capacity

Land-take, space requirement (surface and underground), length-to-width ratio, size of the catchment area, water volumes to be treated. Water retention efficiency. Overflow structure requirement. Flood risk management allowance for climate change.

Vegetated swale is sized based on flow rate, channel shape, longitudinal slope, and surface material resistance. A sizing calculation is necessary for significant stormwater drainage pathways. The size of the upstream catchment area is recommended to be less than 2 hectares.

The width of the swale can vary significantly. Swales designed to channel small flows may be only about a meter wide, including side slopes, while larger swales intended for stormwater retention or directing water from extensive areas can have a bottom width of up to a couple of meters. Normal maximum swale depth is 400-600 mm.

For effective pollutant removal:

• • The depth of flow should be maintained below the height of vegetation (i.e., usually less than 100 mm).

  • The maximum flow velocity in the swale for such an event should be 0,3 m/s to ensure adequate runoff filtration.

  • The time of travel of runoff along the swale (residence time = length/velocity) should be at least 9 minutes (18 minutes from the top of the swale, if the swale has lateral inflows along its length).