Outlet Works Capacity Calculator

Calculator Inputs

Units SI (m, m³/s) US (ft, cfs)

All computations run internally in metric.

Outlet type Gate / Orifice Conduit (with losses)

Use conduit mode for long barrels and loss modeling.

Flow condition Free (unsubmerged) Submerged (tailwater controls)

Submerged uses head difference Hu − Hd.

Upstream head, Hu (m)

Head above opening or conduit centerline.

Downstream head, Hd (m)

Used only for submerged discharge cases.

Discharge coefficient, Cd

Typical range: 0.6–0.9 (project-specific).

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Gate / Orifice Geometry

Opening shape Rectangular Circular

Choose based on gate/orifice type.

Width, b (m)

Opening height, a (m)

Diameter (m)

Conduit Geometry and Losses

Diameter, D (m)

Length, L (m)

Minor loss sum, K

Include entrance, bends, valves, exit, trash rack, etc.

Friction method Direct friction factor (f) Swamee-Jain (ε, ν)

Swamee-Jain iterates using Reynolds number.

Friction factor, f

Typical: 0.015–0.03 for many pipes.

Roughness, ε (m)

Kinematic viscosity, ν (m²/s)

Water near 20°C: about 1.0×10⁻⁶ m²/s.

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Velocity warning threshold (m/s)

Used for a simple advisory note.

Rounding (decimals) 2 3 4 5 6 7 8

Calculate Capacity

These examples show typical inputs and approximate outputs. Always verify coefficients and loss assumptions for your project conditions.

Formulas Used

1) Gate / Orifice (free discharge)

Q = C_d · A · √(2 g H)

2) Gate / Orifice (submerged discharge)

Q = C_d · A · √(2 g (H_u − H_d))

3) Conduit with losses

H = (v² / (2 g)) · (K + f · L / D)

v = √(2 g H / (K + f · L / D)),   Q = A · v

4) Swamee-Jain friction factor (iterated)

f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^0.9)]²

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• H is effective head driving the flow.
• A is flow area of the opening or conduit.
• K is total minor loss coefficient sum.
• f is Darcy friction factor.

Outlet Works Capacity Planning Notes

1) Purpose and typical use cases

Outlet works convey controlled releases from reservoirs, detention basins, and canals. Capacity checks support drawdown planning, environmental releases, construction bypasses, and emergency operations while confirming downstream conveyance is not overloaded. Include seasonal operations, sediment management, and maintenance drawdowns in the capacity envelope. Always check allowable downstream stage rise and erosion protection near the outfall.

2) Data you should compile before sizing

Collect upstream water level, outlet centerline elevation, and expected tailwater for normal and flood conditions. Document gate opening limits, pipe diameter, barrel length, bends, valves, transitions, and trash rack geometry. Because discharge varies with the square root of head, even a 0.5 m head change can materially alter capacity.

3) Coefficients and loss assumptions

For gates and orifices, C_d commonly ranges from 0.60 to 0.90 depending on entrance shape and contraction. In conduit mode, represent fittings with a total minor-loss coefficient K; values from 1 to 10 are typical when entrances, bends, valves, trash racks, and exits are present. For friction, either input a Darcy factor f (often 0.015–0.03 for many lined pipes) or use Swamee-Jain with roughness ε and kinematic viscosity ν (water near 20°C has ν ≈ 1.0×10⁻⁶ m²/s).

4) Reading results and operational checks

Review discharge against target releases and check velocity for abrasion and cavitation risk. Many projects flag sustained velocities above roughly 3–8 m/s for closer review of lining, aeration, and energy dissipation. For submerged flow, the driving head is H = H_u − H_d, so higher tailwater can significantly reduce capacity.

5) Reporting and design documentation

For design records, note the head datum, selected coefficients, how K was assembled, friction method, and the final discharge and velocity. Use the CSV and PDF exports to keep a consistent audit trail when comparing multiple scenarios and supporting review comments.

FAQs

1) What does “effective head” mean in this tool?

Effective head is the energy driving flow. For free flow, it is H = Hu. For submerged flow, it is H = Hu − Hd to account for tailwater backpressure.

2) When should I choose Gate/Orifice vs Conduit mode?

Use Gate/Orifice for short openings where losses are represented mainly by Cd. Use Conduit mode for long barrels where friction and fittings dominate capacity and velocity.

3) What is a reasonable Cd value?

Cd varies with entrance geometry and gate details. Many practical cases fall around 0.60–0.90. If you lack test data, justify Cd using guidance, manufacturer data, or calibrated project experience.

4) How do I estimate the total minor-loss coefficient K?

Sum K values for entrance, bends, valves, transitions, trash racks, and exit. If the layout is uncertain, run sensitivity cases (for example K = 2, 5, and 10) to bracket capacity.

5) Why does Swamee-Jain show iterations?

Swamee-Jain uses Reynolds number, which depends on velocity. The calculator iterates until velocity and friction factor are consistent with the head, roughness, viscosity, and conduit size.

6) What velocity warning threshold should I set?

Use your project criteria. Many teams flag sustained velocities above 3–8 m/s for closer review of cavitation, abrasion, vibration, and downstream energy dissipation details.

7) Are the example values suitable for final design?

No. The example table is illustrative. Always confirm head references, geometry, coefficients, and loss assumptions using drawings, standards, and project-specific hydraulic criteria.

How to Use This Calculator

1. Select your units and choose an outlet type.
2. Pick free flow for un-submerged discharge to air.
3. Pick submerged flow if tailwater controls the outlet.
4. Enter Hu and (if submerged) Hd as head above centerline.
5. For Gate/Orifice, select shape and provide dimensions.
6. For Conduit, set D, L, K, and friction method inputs.
7. Press Calculate Capacity to show results above the form.
8. Use Download CSV or Download PDF to export your last case.