What This Solves
Sizes a grass-lined or vegetated open channel (swale) to convey stormwater at non-erosive velocities using Manning's equation with vegetation-adjusted roughness.
Best Used When
- You are designing a vegetated stormwater conveyance channel for a parking lot, roadway, or low-impact development site
- You need to calculate flow depth, velocity, and residence time for a grass swale
- You want to verify that flow velocities are low enough to prevent erosion and allow pollutant settling
Do NOT Use When
- You are designing a hard-lined or riprap-lined channel instead of a vegetated swale — Use Manning's Channel Calculator
- You need detailed guidance on channel lining selection and permissible shear stress — Use Channel Lining Design Calculator
- You need a surface trench drain with grates rather than an open vegetated channel — Use Trench Drain Calculator
Key Assumptions
- Flow is uniform and steady along the swale length
- Vegetation is healthy and maintained at the specified height and density
- Manning's roughness is based on empirical correlations for grass retardance classes
- The cross-section (bottom width, side slopes) is constant along the swale
- Infiltration during flow is not accounted for (conservative for capacity)
Input Quality Notes
Vegetation type and grass height directly affect roughness. Use conservative (higher) roughness values for design, and verify that the selected vegetation can be established and maintained on-site.
Calculate Vegetated Swale Hydraulics
For educational purposes only. Not a substitute for professional engineering judgment.
Vegetated Swale Design Overview
Vegetated swales convey stormwater while providing filtration and infiltration. Design uses Manning's equation with variable roughness coefficients that depend on flow depth, velocity, and grass retardance.
- Normal Depth - Uniform flow depth for design flow
- Velocity Check - Must be below permissible velocity for erosion control
- Froude Number - Flow regime indicator (subcritical preferred)
- Residence Time - Contact time for water quality treatment
Vegetation Retardance Classes
| Vegetation Type | Class | Height Range |
|---|---|---|
| Bermuda Grass | C | 6-12 inches |
| Buffalo Grass | C | 3-6 inches |
| Kentucky Bluegrass | C | 4-10 inches |
| Tall Fescue | B | 8-18 inches |
| Native Grass Mix | B | 12-24 inches |
| Unmowed/Dense Grass | A | > 24 inches |
| Short Mowed Grass | D | 2-4 inches |
Source: Chow (1959) Open-Channel Hydraulics, Table 5-6
Manning's n Values by Retardance Class
| Class | Low VR | Medium VR | High VR | Description |
|---|---|---|---|---|
| A | 0.50 | 0.25 | 0.150 | Very high retardance - dense, tall vegetation |
| B | 0.35 | 0.15 | 0.080 | High retardance - tall grass (18-24 in) |
| C | 0.25 | 0.10 | 0.050 | Moderate retardance - medium grass (6-12 in) |
| D | 0.15 | 0.06 | 0.035 | Low retardance - short grass (2-6 in) |
| E | 0.08 | 0.04 | 0.025 | Very low retardance - very short grass |
VR = Velocity x Hydraulic Radius. Source: HEC-22 Table 7-6
Permissible Velocities (ft/s)
| Retardance Class | Slope 0-5% | Slope 5-10% | Slope > 10% |
|---|---|---|---|
| Class A | 8.0 | 7.0 | 6.0 |
| Class B | 7.0 | 6.0 | 5.0 |
| Class C | 6.0 | 5.0 | 4.0 |
| Class D | 5.0 | 4.0 | 3.0 |
| Class E | 4.0 | 3.0 | 2.5 |
For erosion-resistant soils. Source: HEC-22 Table 7-5
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Last verified: February 2026