In This Question The First Step Is To Get Fr Value I Don’T Really Understand Why We Need To Get This Value

Exercise 6
Water is flowing at a velocity of 3 m/s and a depth of 3 m in a channel of rectangular
cross-section. Find the maximum allowable size of upward step for the upstream flow
to remain as specified.
Solution
Fr = –
3
V9.81 x3
= 0.55 – The flow regime is subcritical.
The unit discharge is:
q=v*y=9m’ Is
Specific Energy in the cross-section is:
32
E=y+
2g
=3+
2×9.81
-= 3.459m
The maximum size of the step which will not change the depth and velocity upstream
will be one on which the critical depth will be formed.
The critical depth is:
Fr’ =1
9.81x ye
yo = 2.02 m
The Specific Energy on the step is:
E. = =y. = =x2.021 = 3.033 m
Therefore the step size is:
Az = E-E. =3.459-3.032
Az = 0.427 m
If the step is above 0.427 m, the specific energy in front of the step (i.e. the depth) will
increase while the critical depth will form on the step.Engineering Technology