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[edit]
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
Planning considerations[edit]
Best cross sections[edit]
Enhanced swales aim to both reduce the flow rate and retain a portion of the conveyed water. For these purposes the best x-section is that which maximizes the wetted perimeter for a given area. For a given width and depth, the difference between a triangular and trapzoidal section is small. As shown in the diagrams, under low flow conditions the trapezoidal has greater wetted perimeter, and at higher flows the triangular profile does.
Design[edit]
All swales should be designed to meet the following criteria:
- 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
Adding check dams[edit]
To calculate the spacing and sizing of check dams, see Check dams: Sizing
Maximum depth of flow
- 50% height of grass for regularly mown swales, to maximum of 75 mm.
- 33% height of vegetation for infrequently mown swales.
Modeling
It is recommended that grass and enhanced grass swales be modelled using the 'Swale' element in the TTT. A 'swale' has to connect two existing elements within the TTT Bioswales or dry swales, which have amended filter media, should be modelled as bioretention cells. The alternative is to use the 'enhanced swale' within the LID toolbox, but this incorporates fewer design parameters (and doesn't account for infiltration).
| General Info | |
|---|---|
| Upstream Node | Name of node on the inlet end of the swale (higher elevation) |
| Downstream Node | Name of node on the outlet end of the swale (lower elevation) |
| Manning's Roughness | Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06 [1] |
| Upstream Invert (m) | Depth of swale invert above node invert at inlet end of the swale |
| Downstream Invert (m) | Depth or elevation of the swale invert above the node invert at the outlet end of the swale |
| Cross section | |
| Maximum Depth (m) | Depth of the swale |
| Bottom Width (m) | Bottom width of the trapezoidal swale For a triangular channel, enter 0 |
| Left Side Slope (m/m) | Left side slope (run/rise). Suggested value of 3 or 4 if design permits. |
| Right Side Slope (m/m) | Right side slope (run/rise). Suggested value of 3 or 4 if design permits. |
| Seepage (mm/hour) | Infiltration rate of native (or amended) soil |
| Surface | |
|---|---|
| Berm height (mm) | This is the height of the curb which constrains the overland sheet flow of water. Where the bottom of the slope discharges directly into another LID facility without impedance, the value is 0. |
| Surface roughness (Manning’s n) | Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06 [1] |
| Surface slope (%) | If the slope > 3%, use Check dams to create temporary ponding, increase infiltration, and slow flow to reduce erosion. |
| Swale side slopes (run/rise) | Suggested value of 3 or 4 if design permits. |
Materials
- ↑ 1.0 1.1 Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233.https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.