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→Design
==Design==
==Design==
All swales should be designed to meet the following criteria:
All swales should be designed to meet the following criteria:
*Treat drainage areas of <u><</u>2 hectares
*Minimum residence time of 5 minutes.
*Minimum residence time of 5 minutes.
*Maximum flow velocity 0.3 m/s
*Maximum flow velocity 0.3 m/s
*Minimum length 30 m
*Minimum length 30 m
*Minimum length between checkdams <u>></u> 5m
*Minimum length between checkdams <u>></u> 5m
*Maximum depth of flow should be 50% height of grass for regularly mown swales, to maximum of 75 mm, or 33% height of vegetation for infrequently mown swales.
*Maximum depth of flow should be 50% height of grass for regularly mown swales, to a maximum of 100 mm, or 33% height of vegetation for infrequently mown swales
*Cross-section shape may be parabolic or trapezoidal, but parabolic is preferable for aesthetics, maintenance and hydraulics.
[[File:GrassSwale.bmp parabolic.png|thumb|500px|Cross-section view of a parabolic designed swale. Source: Horry County, Southern California.<ref>Horry County Government. 2022. Stormwater Engineers - Plans and the Construction Process. https://www.horrycounty.org/Departments/Stormwater/engineers</ref>.]]
==Geometry and Site Layout==
Minimum planting soil or filter media bed footprint area is based on the design storm runoff volume and effective surface ponding depth behind check dams. Recommended impervious drainage area to pervious facility footprint area ratios (I:P ratios) range from:
*5:1 on low permeability soils, such as [[Soil groups|hydrologic soil group (HSG) C and D]],
*20:1 on high permeability soils [[Soil groups|hydrologic soil group (HSG) Aand D]].