Hydraulics & drainage · Stormwater & WSUD
Stormwater pipe network (QUDM rational method) calculator
Minor drainage network laid out as pits draining to a downstream pit (0 = outlet), listed upstream first. For each pipe the tool accumulates the contributing C·A, finds the critical time of concentration (inlet time or upstream time plus pipe travel), reads the intensity from your IFD table (log–log interpolation), computes Q = Cy·I·A/360 and selects the smallest standard RCP that flows full at the pipe grade, with velocity checks.
Standard: QUDM 2017 Sec 4 rational method Q = Cy·I·A/360 · ARR 2019 IFD · Manning full-pipe capacity, RCP class sizes (AS 4058)
Open the live calculator → All hydraulics & drainage calculators
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 |
|---|---|---|
| Pits: id, to (0 = outlet), area (ha), C10, inlet time (min), pipe length (m), grade (%) | 1, 3, 0.35, 0.80, 7, 45, 1.0 2, 3, 0.28, 0.75, 6, 40, 1.2 3, 5, 0.30, 0.80, 7, 60, 0.8 4, 5, 0.42, 0.70, 8, 55, 1.0 5, 6, 0.25, 0.85, 5, 50, 0.7 6, 0, 0.20, 0.85, 5, 30, 0.6 | |
| IFD for the design AEP: duration (min), intensity (mm/h) | 5, 175 10, 138 15, 117 20, 103 30, 85 45, 69 60, 59 90, 46 | |
| Frequency factor Fy (10 % AEP = 1.0, 1 % = 1.2) | 1 | |
| Manning n (concrete) | 0.013 | |
| Minimum full-pipe velocity | 0.7 | m/s |
| Maximum velocity | 6 | m/s |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Pipe 1→3: ΣCA 0.280 · tc 7.0 min · I 156 mm/h | Q 121 L/s → 375 mm (cap 175, 1.59 m/s) | |
| Pipe 2→3: ΣCA 0.210 · tc 6.0 min · I 164 mm/h | Q 96 L/s → 375 mm (cap 192, 1.74 m/s) | |
| Pipe 3→5: ΣCA 0.730 · tc 7.5 min · I 152 mm/h | Q 309 L/s → 525 mm (cap 385, 1.78 m/s) | |
| Pipe 4→5: ΣCA 0.294 · tc 8.0 min · I 149 mm/h | Q 122 L/s → 375 mm (cap 175, 1.59 m/s) | |
| Pipe 5→6: ΣCA 1.237 · tc 8.6 min · I 145 mm/h | Q 500 L/s → 600 mm (cap 514, 1.82 m/s) | |
| Pipe 6→outlet: ΣCA 1.407 · tc 9.0 min · I 143 mm/h | Q 558 L/s → 675 mm (cap 651, 1.82 m/s) | |
| Outlet flow | 558 | L/s |
| Total pipe length | 280 | m |
- ✓ Every pipe flows within full capacity
- ✓ Full-pipe velocity ≥ 0.7 m/s (self-cleansing) — 1.59 m/s min
- ✓ Velocity ≤ 6 m/s
- ✓ Pipe sizes do not reduce downstream
Method, formulas and assumptions
- Q = Cy·I·A/360 (m³/s, A in ha, I in mm/h) with Cy = Fy·C10 ≤ 1 (QUDM Table 4.5.3 frequency factors). Partial-area effects are not tested.
- Critical tc at each pit = max(inlet time, upstream tc + upstream pipe travel); travel time uses the full-pipe velocity.
- Required diameter from Manning full-pipe flow D = (Q·n·4^(5/3)/(π√S))^(3/8); the next standard RCP size is chosen. HGL and pit losses are not calculated here: run a full HGL analysis (e.g. DRAINS) for design.
- Replace the IFD with the Bureau of Meteorology 2016 IFD for the site and design AEP.
Related hydraulics & drainage calculators
- Grassed swale and bioretention basin — A trapezoidal grassed swale conveying the treatment flow and the major flow to a bioretention basin. Normal…
- Kerb and channel flow spread and kerb inlets — Flow in a kerb and channel with a depressed gutter: spread T from Izzard's equation with the…
- Rational method peak flow — Time of concentration (Bransby-Williams), peak flow for a design intensity and catchment.
- On-site detention storage (triangular hydrographs) — Storage from post-development inflow and permissible site discharge using the triangular-hydrograph…
- Open channel flow (Manning) — Discharge, velocity and Froude number for a trapezoidal channel at a given depth.
- Culvert outlet control headwater — Headwater for a full-flowing culvert in outlet control: HW = H + ho − L S with entrance, friction and exit…
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.