Hydraulics & drainage · Stormwater & WSUD
Kerb and channel flow spread and kerb inlets calculator
Flow in a kerb and channel with a depressed gutter: spread T from Izzard's equation with the composite-gutter ratio Eo (solved by bisection), depth at the kerb and d·V, then a side-entry inlet on grade (length for total interception LT, efficiency and bypass) and the capacity of the same inlet in a sag as a weir.
Standard: FHWA HEC-22 (3rd ed.) Ch 4 gutter flow (Izzard, composite gutter Eo) and kerb-opening inlets · QUDM Table 7.4.1 flow width and d·V limits
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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 |
|---|---|---|
| Gutter flow approaching the inlet | 60 | L/s |
| Longitudinal grade | 2 | % |
| Road cross-fall | 3 | % |
| Gutter (channel) width | 0.45 | m |
| Gutter depression | 25 | mm |
| Manning n | 0.016 | |
| Kerb inlet opening length | 2.4 | m |
| Kerb opening height | 150 | mm |
| Allowable flow width (QUDM: 1 lane clear) | 2.5 | m |
| Allowable d·V | 0.4 | m²/s |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Spread T | 1.82 | m |
| Ratio of flow in the gutter Eo | 0.629 | |
| Depth at the kerb | 80 | mm |
| Mean velocity / d·V | 1.09 m/s / 0.086 m²/s | |
| Equivalent cross slope Se | 6.49 | % |
| Length for total interception LT | 4.78 | m |
| Inlet efficiency (on grade) | 71 | % |
| Intercepted / bypass flow | 42.9 / 17.1 | L/s |
| Capacity of the same inlet in a sag | 90 | L/s |
- ✓ Spread ≤ allowable — 1.82 m
- ✓ d·V ≤ allowable
- ✗ Bypass ≤ 15 % (carry-over to the next pit) — 29 %
Method, formulas and assumptions
- Composite gutter (HEC-22 Eq 4-4): Q = Qs/(1 − Eo), Qs = (Ku/n)Sx^(5/3)SL^(1/2)(T − W)^(8/3), Eo = 1/{1 + (Sw/Sx)/[(1 + (Sw/Sx)/(T/W − 1))^(8/3) − 1]}, Ku = 0.376 (SI). T solved by bisection.
- Kerb inlet on grade: LT = 0.817Q^0.42·SL^0.3·(1/nSe)^0.6, Se = Sx + Sw′·Eo; efficiency E = 1 − (1 − L/LT)^1.8.
- Sag: weir Q = Cw(L + 1.8W)d^1.5 (Cw = 1.25) while d ≤ h, orifice beyond. QUDM pit capacity charts for proprietary pits (with blockage factors) govern in Queensland.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.