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Sluice Gate - Volume Flow Measurements

Sluice gates can be used to control and measure volume flow rates in open channels and rivers, mainly in connection to hydro power plants.

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Sluice gate flow metering is often used to measure flow rate in open channels. Sluice gates are also often used to modulate flow.

Flow meter - sluice gate

The sluice gate flow rate measurement is based on the Bernoulli Equation and can be expressed as:

1/2 ρ v12 + ρ g h1 = 1/2 ρ v22 + ρ g h2                                    (1)

where

h = elevation height (m)

ρ = density (kg/m3)

v = flow velocity (m/s)

The pressure components in the equation are in general irrelevant since pressure upstream and downstream are the same (p1 - p2 = 0).

Assuming uniform upstream and downstream velocity profiles - the Continuity Equation gives:

q = v1 A1

    = v2 A2                             (2)

where

q = flow rate (m3/s)

A = flow area (m2)

(2) can be modified to:

q = v1 h1 b

    = v2 h2 b                            (3)

where

b = width of the sluice (m)

h1 = upstream height (m)

h2 = downstream height (m)

Combining (1) and (3), gives the "ideal" equation:

q = h2 b [2 g (h1 - h2) /(1 -(h2 / h1))]1/2                               (4)

Assuming h1 >> h2 (4) can be modified to:

q = h2 b [2 g h1]1/2                               (5)

This is approximately true when the depth ratio h1 / h2 is large, the kinetic energy upstream is negligible (v1 is small) and the fluid velocity after it has fallen the distance (h2 - h1) ≈ h1 - is:

v2 = [2 g h1]1/2                              (6)

The ideal equation (3) can be modified with a discharge coefficient:

q = cd h0 b [2 g h1]1/2                              (7)

where

cd = discharge coefficient

The discharge coefficient depends on different parameters - such as upstream and tail-water depths, gate opening, contraction coefficient of the gate and the flow condition.

In practice the typical discharge coefficient is approximately 0.61 for free flow conditions and depth ratios ho / h1 < 0.2. 

Sluice Gate Specifications

The most commonly used specification for sluice gates in water and wastewater treatment plants is ANSI/AWWA C560-00. This specification should be used as a guidance for gates selection and operating equipment and associated hardware.

Example - Flow Rate through a Sluice Gate

Water flows under a sluice gate with an opening height of 0.4 m. The width of the sluice is 3 m and the height from the water surface to the bottom of the sluice is 10 m.

Since h1 >> h2 and the depth ratio 0.4/10 < 0,2 - the contraction coefficient can be set to 0.61 - and equation (7) can be used for flow calculation:

q = 0.61 (0.4 m) (3 m) [2 (9.81 m/s2) (10 m)]1/2

    = 10.25 m3/s

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