Treatment MeasuresSwales : Design Details

More Information

Comprehensive guidelines for the design of swales can be found in Chapter 8 "Swales and buffer strips" in the WSUD Engineering Procedures available for purchase from CSIRO publishing. A number of 'rules of thumb', along with design techniques for sizing swales, are presented below:

The area of a swale should be around 1% the area that drains to the swale.
Swales should not be used in areas where maximum flow rates exceed 0.3 m/s (Horner et al 1994) for a 1-year ARI (average recurrence interval) event, or 1.0 m/s for a 100-year ARI event.
The depth of stormwater should not exceed the height of the grass even in 100-year ARI events, this translates to a Manning's 'n' value of between 0.15 and 0.2. In a 100-year ARI event, the value will be lower and can be assumed to be 0.03.
Swale Longitudinal slope: Between 2 and 4 per cent in order to promote uniform flow conditions across the swale. Slopes outside this range ie. <2% slope - use filter media below the swale to increase filtration rates, >5% slope - use checks dams to spread flow and reduce flow velocity through the swale.
Swale width: Should not exceed 2.5m, unless check-dams or other structural measures are employed to ensure a uniform spread of flow.
Geometry: Parabolic or trapezoidal shapes in order to minimise corners, with slopes at maximum of 3:1 (h:v).

Steps Involved in Swale Design

Estimate the design flow for the design storm event and confirm it falls within the requirements above.
Determine the slope of the filter strip. (MUSIC: 'Bed Slope [%]')
Set the design flow depth (MUSIC: 'Depth')
Solve Manning's Equation to determine the width of flow. (Mowed grass 0.2, natural or infrequently mowed grass 0.24).
Determine the flow area. (Using the flow width and established depth).
Calculate the resultant flow velocity. Reduce the flow, increase the flow width or reduce the depth of flow if the velocity exceeds 0.3m/s.
Calculate the flow length using the resulting velocity, to achieve a residence time of nine (9) minutes. (Absolute minimum residence should be 5 minutes to keep in accordance with Best Practice) (MUSIC: 'Length')

Design Factors That May Impede Performance

Only limited removal of fine sediments and dissolved pollutants;
High flows and high flow depths reduce effectiveness;
Limited application in shaded areas;
Only suitable for gentle slopes (less that 5%); for steeper slopes check dams are required.
Regular inspections required
Attempts to maintain sheet flow (even depth of flow over the area) over the length of the swale may require further engineering to input flow paths.

Trapping Performance

Grass swales can achieve high removal rates, although limited Australian data exists. An example of data obtained by Horner et al (1994) is below:

Pollutant	Retention (%)	Pollutant	Retention (%)
Suspended Solids	83	Lead	67
Oil & Grease	75	Total Phosphorous	29
Iron	72	Total Nitrogen	Negligible
Pollutant Retention rates for filter strips

Design Considerations

Swales adjacent to roads may be compacted by vehicular traffic, which may cause reduced infiltration rates.
Higher than design flows can cause erosion to the swale in addition to scouring. For such events a bypass system should be installed.

Computer Evaluation of Water Quality Treatment Effectiveness

Model for Urban Stormwater Improvement Conceptualisation (MUSIC)

MUSIC is a computer aid to decision making. It enables users to evaluate conceptual designs of stormwater management systems to ensure they are appropriate for their catchments. (MUSIC User manual Version. 1)

Previous methods of sizing of a swale were based purely on hydraulic requirements and did not take into account the subsequent water quality effects. The parameters from the hydraulic calculations can be directly transferred to MUSIC to determine the water quality effect that this swale will have in the treatment train.

References

Horner, R.R., Skupien, J.J., Linvingston, E.H. and Shaver, H.E., 1994, Fundementals of Urban Runoff Management.