What this calculator does
A bioswale is a planted channel that slows, filters and infiltrates stormwater on its way through the site. This calculator gives you the trapezoidal cross-section, the volume the channel stores, the extra storage in an engineered media bed, and how long that volume takes to drain.
How to use it
- Enter the bottom width and design depth of the swale; 0.6–1.0 m bottoms are typical.
- Set the side slopes — 3H:1V to 5H:1V for a stable vegetated channel.
- Enter the length, which usually runs the length of the drainage path.
- Add a media bed length and porosity to gain extra storage, then check the drain time against the storm interval.
Governing formula
A = (b + z·d)·d ; V = A·L ; t = V / (k · P·L) Where:
-
b= Bottom width [m] -
z= Side slope [–] -
d= Design depth [m] -
L= Swale length [m]
Derived Equations:
T = b + 2d√(1+z²) Worked example — A 40 m swale, 0.8 m bottom, 0.3 m deep at 3:1
The figures below were produced by running this page's own engine with the example inputs (Design inflow (m³/s), Bottom width (m), Side slope (H:V) (–), Design depth (m), Swale length (m), Media porosity (%), Media bed length (m), Infiltration rate of the bed (mm/h)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Swale storage volume | 20.4 m³ |
|---|---|
| Cross-sectional area | 0.51 m² |
| Water surface width | 2.6 m |
| Additional media storage | 0 m³ |
| Drain time | 7563 h |
| Flow velocity | 0.2 |
| Assessment | 40 m swale stores ≈ 20.4 m³ (media adds ≈ 0.0 m³ of void hold) and drains in ≈ 7563 h at 25 mm/h. |
- The cross-section is (0.8 + 3 × 0.3) × 0.3 = 0.51 m².
- Over 40 m that stores about 20 m³ of stormwater.
- At 25 mm/h infiltration the volume drains in roughly 20 hours.
Assumptions & limitations
Engineering calculation. Trapezoidal swale geometry with a porosity-storage term and Darcy drawdown. No longitudinal slope effects, no inlet/outlet control structures.
- No longitudinal slope, so the real storage is reduced by the bed gradient along the channel.
- No inlet or outlet control — a real swale needs a forebay and an underdrain or overflow level.
- Vegetation density and sediment deposition reduce storage over time and need a maintenance plan.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- FHWA — urban stormwater treatment technical guidance for bioswales.
- Local water sensitive urban design manuals.