Hydraulics & drainage · Stormwater & WSUD
Grassed swale and bioretention basin calculator
A trapezoidal grassed swale conveying the treatment flow and the major flow to a bioretention basin. Normal depths and velocities by Manning (vegetation n), scour and depth limits, then the bioretention filter: area against the usual 2 % of catchment, the maximum filtration rate through the media under the extended detention head (Darcy), and the drain-down time of the detention depth.
Standard: Water by Design (SEQ) Bioretention Technical Design Guidelines 2014 · QUDM Sec 9 · Manning open-channel flow
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Catchment area | 1.5 | ha |
| Runoff coefficient | 0.75 | |
| Treatment intensity (≈ 3-month ARI) | 20 | mm/h |
| Major-event intensity (1 % AEP, tc) | 140 | mm/h |
| Swale base width | 2.5 | m |
| Side slopes 1 : z | 4 | |
| Swale grade | 1 | % |
| Manning n, flow within vegetation | 0.15 | |
| Manning n, major flow above vegetation | 0.045 | |
| Swale depth available | 0.5 | m |
| Bioretention filter area | 300 | m² |
| Filter media saturated conductivity | 200 | mm/h |
| Extended detention depth | 0.3 | m |
| Filter media depth | 0.5 | m |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Treatment flow Qt | 62.5 | L/s |
| Major flow Qm | 438 | L/s |
| Treatment depth / velocity | 133 mm / 0.16 m/s | |
| Major depth / velocity | 202 mm / 0.66 m/s | |
| Depth × velocity (major) | 0.13 | m²/s |
| Filter area as % of catchment | 2.00 | % |
| Maximum filtration rate | 26.7 | L/s |
| Extended detention drain-down | 0.9 | h |
- ✓ Treatment flow stays within vegetation (depth ≤ 0.15 m) — 133 mm
- ✓ Treatment velocity ≤ 0.5 m/s
- ✓ Major flow fits in the swale — 202 mm
- ✓ Major velocity ≤ 2.0 m/s (scour)
- ✓ Filter area ≈ 1.5–3 % of catchment
- ✓ Drain-down ≤ 12 h (vegetation health)
Method, formulas and assumptions
- Q = C·I·A/360; normal depth by bisection on Manning Q = (1/n)A·R^(2/3)·S^(1/2) for the trapezoid. High n (0.15–0.3) applies while flow is below the grass height, 0.03–0.05 once flow overtops it.
- Bioretention: Qmax = ks·Af·(h + d)/d (Darcy with the extended-detention head); drain-down ≈ h/(ks(h + d)/d).
- Media: loamy sand, ks 100–300 mm/h (SEQ guideline), filter depth 0.4–0.6 m, extended detention 0.1–0.3 m. Confirm treatment performance with MUSIC.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.