Enhanced grass swales

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance which promotes infiltration, see Exfiltration trenches.
For conveyance along planted channels, on both surface and underground, see Bioswales.

Overview

The fundamental components of an enhanced grassed swale are:

• Graded channel
• Resilient turf grass
• Check dams, to facilitate short term ponding

Additional components may include:

• Amended soil, to increase infiltration to soils below
• Impermeable membrane, to prevent infiltration to soils below
• Turf reinforcement, to prevent scour

Design

• Minimum residence time of 5 minutes.
• Maximum flow velocity 0.3 m/s
• Bottom width between 0.6 - 2.4 m
• Minimum length 30 m

Slope variations

< 2 %	Use underdrain to distribute water for infiltration
2 - 5 %	Design swale for surface flow, consider check dams
5 %	Check dams required to reduce effective slope to 2 – 3 %

Maximum depth of flow

1/3 height of vegetation for infrequently mown swales. ½ height of grass for regularly mown swales, to maximum of 75 mm.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A perforated pipe used to assist the draining of soils.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.