Aquaculture Engineering

Unit 11 - Water distribution and control structures

Computation of discharge through sluices
While water flows though a sluice or gate following three conditions are possible

• Free flow condition
• Submerged flow condition
• Partially submerged flow condition

When discharge from the gate at its outlet (downstream side) falls free into atmosphere it is called free flow condition. It is said to be submerged if out let of the gate discharges under the water.

Computation of discharge under free flow condition
Free discharge taking place through a monk sluice as shown in fig. can be computed by using the equation.

g = Acceleration due to gravity 9.81m/sec²
H = Height of water (on u/s) above the central axis of the pipe (m)
D = Internal diameter of the pipe (m)
C_d = Co-efficient of discharge.
The value of Cd for the corresponding length of the pipe in terms of its diameter (D) has been given below.

Where,
K_e = Entrance friction co-efficient value of which may be taken equal to 0.5
f = Friction factor value of which for a concrete pipe may be taken equal to 0.03
L = Length of the pipe

Free discharge through an open sluice as shown in Fig. can be computed by using the equation.

Where,
b= Bottom width of the sluice
H₁= Height of the water (on u/s side), above the top of the sluice opening or vent.
H₂ = Height of water (on the u/s side) above the bottom of the sluice.

Values of C_d under free flow condition for various conditions of sluice are given below.



Submerged flow condition
Discharge through a monk sluice whose outlet pipe is under water can be computed by using the equation.

Discharge under submerged condition taking place through an open sluice can be computed by using the equation.

Value of co-efficient of discharge C_d for a submerged flow condition may be taken one percent less than its value for the same sluice under the same effective head when discharging freely into the air (i.e., free flow condition). For sluice opening provided with cut waters and wing walls value of C_d is taken between 0.9 and 0.95. For head sluice this value is usually taken as 0.80.

Computation of discharge under partially submerged flow condition
Discharge through an open sluice, outlet side of which is partially under water can be computed by using the equation.

The value of C_d may be taken equal to 0.62. The value of C_d1 may be taken as given below.