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VARIATION OF CHANNEL GEOMETRY
&
THE CHOKE CONCEPT
A. CHANNEL BOTTOM LEVEL CHANGE
1. Bottom Rise (Upward Step) ‘ + z ’.
i) Sub-critical Flow Regime
ii) Super critical Flow Regime
*** CHOKE MAY OCCUR***
2- Bottom Drop (Downward Step) ‘ - z ’.
i) Sub-critical Flow Regime
ii) Super critical Flow Regime
B. CHANNEL BOTTOM WIDTH CHANGE
1- Sudden Contraction ‘ B2 < B1 ’
i) Sub-critical Flow Regime
ii) Super critical Flow Regime
2- Sudden Enlargement ‘ B2 > B1 ’
i) Sub-critical Flow Regime
ii) Super critical Flow Regime
*** CHOKE MAY OCCUR***
A. CHANNEL BOTTOM LEVEL CHANGE
1- Bottom Rise (Upward Step) ‘ + z ’.
*** CHOKE MAY OCCUR***
i. Sub-critical Flow Regime
When the flow regime is Sub-critical and there is an upward obstacle along the
channel width, the flow will try to jump over it by trying to approach to critical flow
depth.
To determine the flow depth over the step, the total energy just before the step
value has to be compared with the minimum energy Emin over the step.
For this calculation, the same discharge should be considered. Energy of ycr
occuring over the step and hence the minimum energy Emin (i.e. Emin= 3/2ycr for
rectangular channel cross-section) should be compared with the existing energy
and the maximum step height +zmax.
If the given step height +z < +zmax then; the flow depth over the step drops.
If the given step height +z < +zmax then; the flow depth over the step drops.
If the given step height +z = +zmax then; the flow over the step is passing on critical depth ycr.
If the given step height +z > +zmax then; the flow depth can not pass over the
step hence, choke occurs just before the step so that the water level height
increases and the energy level increases as well such that, the flow passes over
the step at critical depth ycr. (i.e. at Emin). But immediately after the step
Hydraulic Jump occurs so that, the gained energy due to choke will be absorbed,
based on the uniform flow depth of the second reach. (if the slope is Mild slope
after the step).
Note that depending on the channel parameters FREE or SUBMERGED jumps
may occur.
EXAMPLE 11.1:
A rectangular channel cross-section of bottom width b = 9.0 m
has a uniform depth of water y1 = 2.0 m when the flow rate
(discharge) Q = 27.0 m3/s. If along the reach, there is an upward
step (rise) of:
i. z = 30 cm,
ii. z = 310 cm;
a.
b.
c.
d.
e.
determine the possibility of choke occurrence;
discuss the possibility of Hydraulic Jump occurrence;
depth just before the step, over the step and just after the step;
draw the longitudinal variation the flow profile;
draw the specific energy versus flow depth curve and show all
the relevant data due to the effect of the step.
310 cm
310 cm
EXAMPLE 11.2:
A rectangular channel cross-section of bottom width b = 3.05 m
has a uniform depth of water y1 = 1.53 m when the flow rate
(discharge) Q = 9.91 m3/s with an equivalent Mannings roughness
coefficient n=0.02. If along the reach, there is an upward step (rise)
of z = 0.45 m at the middle portion of the reach;
a.
b.
c.
d.
e.
f.
determine the Energies E1 and E min;
discuss the possibility of choke occurrence;
discuss the possibility of Hydraulic Jump occurrence;
depth just before the step, over the step and just after the step;
draw the longitudinal variation the flow profile;
draw the specific energy versus flow depth curve and show all
the relevant data due to the effect of the step
g. estimate the length of the H.J.
h. Estimate the lengths of the occuring curves.
EXAMPLE 11.3: (old exam question)
2.28
0.001
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