Sluice Gate Flow Calculator
Discharge under a sharp-edged vertical sluice gate from gate opening, width, and upstream depth, with the contraction-based discharge coefficient and a free-versus-submerged outflow check against the tailwater using the conjugate depth of the issuing jet.
Valid for a rectangular sharp-edged vertical lift gate discharging to an open channel with a/y1 ≲ 0.5. When the tailwater exceeds the jet's conjugate depth the tool reports SUBMERGED — the free-flow Q shown is then an upper bound, and a submerged-flow analysis (Henry's diagram or momentum solution) is required for the actual rating. Radial gates and pressurized outlets use different coefficients.
Contraction coefficients
| Gate lip | Cc |
|---|---|
| Sharp-edged (standard) | 0.59–0.62 (use 0.61) |
| Rounded lip | 0.70–0.80 |
| 45° bevel, downstream face | ≈ 0.70 |
Worked example
Example — canal head gate rating point
References: Henderson, F.M. (1966). Open Channel Flow, Macmillan, Ch. 6. Rajaratnam, N. & Subramanya, K. (1967). "Flow Equation for the Sluice Gate." J. Irrigation & Drainage Div., ASCE 93(3). Henry, H.R. (1950). Discussion of "Diffusion of Submerged Jets." Trans. ASCE 115. Chow, V.T., Open-Channel Hydraulics.
Common mistakes
- Using the free-flow equation with high tailwater. If tailwater exceeds the jet’s conjugate depth, the gate is submerged and passes less flow than this tool reports. The tool flags it, but the result shown is then only an upper bound.
- Measuring y1 too close to the gate. Upstream depth should be measured where the water surface is level, typically a few gate openings upstream, not in the drawdown right at the gate.
- Applying the vertical-gate Cc to a radial (Tainter) gate. Radial gates have a contraction coefficient that depends on the lip angle. USACE and USBR publish separate curves.
- Rating large openings. Above about a/y1 = 0.5 the gate starts acting more like a weir, and the free-flow equation loses accuracy.
When this calculator doesn’t apply
The equations cover sharp-lipped vertical lift gates in rectangular channels with free outflow. Submerged gates need Henry’s diagram or a momentum solution. Radial gates, gates in pressure conduits and gates near the crest of an ogee spillway use different coefficients. For a dam spillway or regulated outlet, a physical-model or agency rating curve should govern.
Related tools
- Orifice flow — pressurized low-level outlets and tank drains
- Hydraulic jump & stilling basin — dissipate the jet downstream
- Broad-crested weir — overflow rating for the same structure
- Ogee spillway — gated spillway crest rating
- Manning's equation — the downstream channel's normal depth (tailwater)
- Orifice Cd table
- Critical depth & specific energy