Velocity of water through sluice gate

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Velocity of water through sluice gate

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The model:
I have modeled a rectangular channel, width 1,5 meter and height 4,5 meter. Length 25 meter and outlet is 0,25 meter lower than inlet.
A pool is placed at the outlet of the channel, and the outlet of the pool is placed so that the surface of the water in the pool is 0,2 above the outlet of the channel.

A sluice gate is placed 4 meter downstream the inlet of the channel and at the same level as the inlet. I have defined the sluice gate opening to 0,175 meter, this allows the upstream water level to stay constant with steady flow data set to 1,4 m3/s. I want the sluice gate to allow a steady flow of Q = 1,4 m3/s to pass, in other word the same as the steady flow date flow rate.

The problem:
When viewing the results I get an velocity of 1,75 m/s at the sluice gate. When controlling the velocity with a hand calculation (Q=V*A) I get 5,3 m/s.
Input: Q = 1,4 m3/s, A = 1,5m x 0,175m = 0,2625 m2
Result: V = Q x A = 1,4 m3/s x 0,2625 m2 = 5,3 m/s

In other words, the equation of continuity does not apply.

Is it wrong to use this kind of control? What do you think the problem/error is?