Test: Weirs & Notches - Civil Engineering (CE) MCQ

Weirs:
Weirs are sharp-crested, overflow structures that are built across open canals. They are easy to construct and can measure the discharge accurately when correctly installed. It is important that the water level downstream is always below the weir crest, otherwise the discharge reading will be incorrect.
According to the discharge conditions the weirs are classified as follows:

Freely discharging weir:
If the water level on the downstream of a weir is well below the weir crest , then the nappe emerges freely in the atmosphere and thus it is called as freely discharging weir.

Submerged weir:
If the water level downstream of a weir is above the crest level of the weir, then the weir is known as submerged weir. 

Concept:
Discharge through a triangular notch,
Q = 8/15 C_d √2 g tan θ/2 × H^5/2
Where Q = discharge, C_d = Co-efficient of discharge, H = Head above the bottom of the notch, and θ = enclosed angle of the vee.
Differentiating equation w.r.t. H,
we have
dQ / dH = 8/15 C_d √2 g tanθ/2  × 5 /2 H^{3 / 2}
dQ = 8/15 C_d √ 2 g tanθ/2  × 5 /2 H^3/2 × d H
∴ dQ/Q = 5/2 dH/H = 2.5 dH/H
Calculation:
Given:
dH/H X 100 = 4%
The percentage error in computing discharge
=  2.5 dH/H = 2.5 x 4 = 10%

Sutro weir:

• The Sutro weirs are special sharp-crested weirs.
• These weirs are widely used in industry and irrigation because they are easily controlled by float-regulated dosing devices.
  

Parshall flume:

• Parshall flumes are calibrated devices for the measurement of water in open channels.
• The flume consists of a converging section with a level floor, a throat section with a downward sloping floor, and a diverging section with an upward sloping floor.
  

Weir and notch:

• A notch may be defined as an opening provided on one side of a tank or reservoir with an upstream liquid level below the top edge of the opening.
• A weir may be defined as a structure constructed across a river or a canal to store water on the upstream side.
  

Concept:
Cippoletti weir

• The Cippoletti weir is a trapezoidal weir, having side slopes 1 horizontal to 4 vertical.
• The purpose of the slope, on the sides, is to obtain an increased discharge through the triangular portions of the weir, which otherwise would have been decreased due to end contractions in the case of rectangular weirs.
• A type of contracted weir that is related to the rectangular sharp-crested weir is Cippoletti weir, which has a trapezoidal cross-section with side slopes 1: 4 (H: V).
  

Statement 4: False Ventilation of triangular weir is not necessary.

Weir:
A weir is a concrete or masonary structure, placed in an open channel over which the flow occurs. It is generally in the form of vertical wall, with a sharp edge at the top, running all the way across the open channel. The notch is of small size while the weir is of a bigger size. The notch is generally made of metallic plate while weir is made of concrete or masonary structure.

Triangular weir:
The expression for the discharge over a triangular weir or notch is

From this expression flow through the weir depend upon the vertex angle (θ) and coefficient of discharge (Cd) is constant for all the heads, the value of Cd is nearly equal to 0.6.

Rectangular weir:
The expression for the discharge over a rectangular weir or notch is

Advantage of triangular weir over rectangular weir:
A triangular weir is preferred to a rectangular weir due to following reason.
(1) The expression for discharge for a right angled V- notch or weir is very simple.
(2) For measuring low discharge, a triangular weir gives more accurate results than a rectangular weir.
(3) In case of triangular weir, only one reading, (H) is required for the computation of discharge.
(4) Ventilation of triangular weir is not necessary.

The MoM (Material of Manufacture) of notch is Metals.

Circular notch is not a way of classifying based on the shape of opening.

Trapezoidal crested weir is not the way of classifying weir based on their shape of crest.

Trapezoidal weir is also called Cipolletti weir.

This is not the way of classifying weir based on the emerging nappe.