Wk4-5 - Broad Crested Weir - Lab

Question 1

What are the objectives of this lab? (2)

Answer 1

• To observe the flow patterns associated with this type of weir
• To examine the relationship between discharge and freeboard

Question 2

What is the formula for Q_Ideal? (Hook & point gauge method)

Answer 2

Q_Ideal = 1.705.b.H^1.5

Question 3

What is the volumetric method for calculating discharge?

Answer 3

Volume in m^3 / time taken in s

Question 4

How do you convert from litres, l, to metres cubed?

Answer 4

/1000

Question 5

What is the formula for Q_Real?

Answer 5

Q_Real = 1.705.Cd.b.H^1.5

Question 6

What is the formula for the coefficient of discharge, Cd?

Answer 6

Cd = Q / (1.705.b.H^1.5)

OR

Cd = Q_Real / Q_Ideal

Question 7

What is the freeboard in this lab?

Answer 7

The height of the water above the top of the weir, U/S of the weir

Question 8

What is the area directly D/S of the weir referred to as?

Answer 8

The nappe (spout)

Question 9

Describe the flow patterns moving right to left, assuming the flow over the weir is also moving right to left.

Answer 9

• sub-critical flow, pre weir
• super-critical flow, just after weir
• sub-critical flow again further D/S

Question 10

What big assumption must be stated in the calculation of discharge ‘Q’?

Answer 10

h ~ H

Because V^2/2g ~ 0, meaning H and h are effectively the same.