Gates
Gates
• Gates are used to control the discharge
and also to stop the flow if required.
y1
w
y2
• The discharge can be evaluated according
to the condition of the water under the
gate. If the flow is supercritical i.e.
q2
w  Yc  3
g
• Then, the discharge can be calculated as
follows;
Q  Ce A 2gy1
• Where:
q is the discharge per unit width =Q/b
g acceleration of gravity = 9.81 m3/sec
Yc critical depth
Ce discharge coefficient (from graph)
b Channel width
A is the water section = b w
w is the opening height of the gate
Ce
• In case of subcritical flow, no jump, the
flow can be evaluated as follows;
Q  Cd A 2 g ( y1  y2 )
Where, Cd is discharge coefficient = 0.61
• Example
•
For a maximum discharge of 12 m3/sec in a
channel of width 14 m, upstream water depth of
2.2 m and downstream water depth 1.6 m,
design the gate opening?
Sol.
First assume subcritical flow,
Apply the equation
get A = 5.73 m2
then w = 5.73/14 = 0.41 m
Check for critical flow
Yc = 0.42 m
> w
Then assumption is wrong
Apply the equation for the supercritical
flow; assuming w = 0.41 m
Then y1/w = 5.4 and y2/w = 3.9
get Ce = 0.4
get A = 4.6 m
then w = 4.57/14 = 0.32 m
Then y1/w = 6.9 and y2/w = 5
get Ce = 0.38
get A = 4.81 m
then w = 4.81/14 = 0.34 m
Again Then y1/w = 6.5 and y2/w = 4.7
get Ce = 0.39
get A = 4.68 m
then w = 4.68/14 = 0.33 m
Close … then w = 0.33 m