WK 4

Question 1

Where do critical conditions occur in open channels?

Answer 1

1) Transition from tranquil (subcritical) flow -> rapid (supercritical flow)

E.G. Hydraulic jumps
reverse transition from rapid flow to tranquil flow

2) Entrance to a steep slope from a reservoir of dam

3) At a free outfall from a mild slope

4) At a change of bed level or width of a channel

Question 2

Describe:
Transition from tranquil (subcritical) flow -> rapid (supercritical flow)

where does critical condition occur?

Answer 2

At tranquil flow -> slope of bed is mild
S<Sc

Steep flow -> slope of bed is steep
S>Sc

Critical conditions occur just downstream of change in slope.

Question 3

Where do hydraulic jumps occur? (terms of bed slope)

Purpose?

Answer 3

The hydraulic jump is formed in the region around the change in bed slope.

Deliberately engineered in order to dissipate excess energy.

E.G. design of a spillway over a dam -> the energy of the fast flowing stream is partially dissipated to prevent erosion of the bed downstream, which could ultimately lead to failure of the dam.

Question 4

Describe: Entrance to a steep slope from a reservoir of dam

where does critical condition occur?

Answer 4

The reservoir/dam will act like that of a mild slope and behave as if tranquil flow is present. Therefore, critical conditions will occur at the entrance to the slope.

Bed slope is steep -> rapid flow occurs
depth of flow < critical depth.

Question 5

Describe: At a free outfall from a mild slope

where does critical condition occur?

Answer 5

Depth of the flow at outfall is ~ 0.7dc with critical conditions located between 3 and 10dc upstream of outfall.

Tranquil to Rapid
(S<Sc) (S>Sc)

Question 6

At a change of bed level or width of a channel

where does critical condition occur?

Answer 6

• Critical conditions may occur in a channel if the bed level is raised as a hump (broad-crested weir) or the width of the channel is suddenly reduced and then enlarged (Venturi effect).

Question 7

define broad-crested weir

Answer 7

If the weir is designed with a flat upper surface or crest sufficiently broad in the direction of flow for the surface of the liquid to become parallel to the crest

Question 8

Define Venturi Flume

Answer 8

A lateral contraction followed by an expansion can be used for flow measurement

Question 9

Answer 9

When the width of the channel is reduced (in the narrow bit),
the discharge per unit width, q must increase.
(q = Q/w, Q is constant, w↓ , so q must ↑)

Depends on flow type:

For tranquil (sub-critical) flows, the depth will decrease as
the channel narrows whilst for rapid (super-critical) flows the
depth will increase as the channel narrows