Circular and box culverts under full-flow pressure conditions. Computes both inlet control and outlet control capacity — the governing (lower) flow rate is the design value. Supports sizing (Find D), headwater estimation (Find HW), and capacity check (Find Q).
Open Channel Flow · Hydraulic StructuresSolve for:
Culvert shape:
Inlet type:
HW measured from inlet invert (pipe invert elevation = 0)
Tailwater depth TW above outlet invert (0 = free outfall):
Inlet control — orifice equation:
Outlet control — full-flow pressure equation:
Barrel runs full (pressure flow). Capacity limited by friction and tailwater. Entrance loss Ke accounts for flow contraction at the inlet.
Design flow:
Inlet type and loss coefficients:
| Inlet type | Cd | Ke |
|---|---|---|
| Projecting (no headwall) | 0.52 | 0.9 |
| Square-edge with headwall | 0.6 | 0.5 |
| Beveled 30°–45° with headwall | 0.7 | 0.2 |
| Groove-end with headwall | 0.65 | 0.4 |
| Rounded entrance (r/D ≈ 0.15) | 0.75 | 0.15 |
| Mitered to embankment slope | 0.65 | 0.7 |
HW/D ratio guidance:
| HW/D | Implication |
|---|---|
| < 1.0 | Culvert not flowing full — pressure-flow equations may overestimate Q |
| 1.0–1.5 | Normal full-flow range for most culvert design standards |
| 1.5–2.0 | High head — check road overtopping and downstream scour |
| > 2.0 | Excessive — upsize culvert or provide parallel barrel |