Experiment (6): Flow over weirs -...

Hydraulics Lab (ECIV 3122) Islamic University – Gaza (IUG)

Instructors : Dr. Khalil M. Alastal Eng. Mohammed Y. Mousa 1

Experiment (6): Flow over weirs

Introduction:

In open channel hydraulics, weirs are commonly used to either regulate or to measure the

volumetric flow rate, they are of particular use in large scale situations such as irrigation schemes,

canals and rivers. For small scale applications, weirs are often referred to as notches and invariably

are sharp edged and manufactured from thin plate material.

Purpose:

To investigate the discharge-head characteristics of a rectangular and triangular weirs.

Apparatus:

1. Rectangular and triangular notches (Figure 2).

2. Hydraulic bench.

3. Basket of glass spheres.

Figure 1: Flow over weirs apparatus



Figure 2: Rectangular and triangular notches

Equipment set up:

1. Place the flow stilling basket of glass spheres into the left end of the weir channel and attach

the hose from the bench regulating valve to the inlet connection into the stilling basket.

2. Remove the five thumb nuts which hold the standard weir in place, remove the standard weir

and replace it with the specific weir plate which is to be tested first. Ensure that the square

edge of the weir faces upstream.

A) Flow measurement : The discharge from the weir may be measured using either the rotameter

(if fitted) or by using the volumetric measuring tank and taking the time required to collect a

quantity of water. The quantity should be chosen so that the time to collect the water is at least

120 seconds to obtain a sufficiently accurate result.

B) Measuring the weir datum : Fill the weir channel with water up to the weir plate crest level

and use the hook gauge to measure the level of the water. This will be the zero or datum level

for the weir.

C) Measuring the head : The surface of the water as it approaches the weir will fall, this is

particularly noticeable at high rates of discharge caused by high heads. To obtain an accurate

measure of the undisturbed water level above the crest of the weir it is necessary to place the

hook gauge at a distance at least three times the head.



Figure 3: Rectangular and triangular weirs

Theory:

1. Flow through a rectangular notch

A rectangular notch in a thin square edged weir plate installed in a weir channel as shown in figure

4.

Figure 4: Rectangular notch

Consider the flow in an element of height at a depth below the surface. Assuming that the flow

is everywhere normal to the plane of the weir and that the free surface remains horizontal up to the

plane of the weir, then

Velocity through element

Theoretical discharge through element

Integrating to obtain the total discharge between and



Total theoretical discharge

In practice the flow through the notch will not be parallel and therefore will not be normal to the

plane of the weir. The free surface is not horizontal and viscosity and surface tension will have an

effect. There will be a considerable change in the shape of the nappe as it passes through the notch

with curvature of the stream lines in both vertical and horizontal planes as indicated in figure 5, in

particular the width of the nappe is reduced by the contractions at each end.

Figure 5: Shape of a nappe

The discharge from a rectangular notch will be considerably less, approximately 60% of the

theoretical analysis due to these curvature effects. A coefficient of discharge is therefore

introduced so that:

2. Flow through a triangular notch

A sharp edged triangular notch with an included angle of is shown in figure 6.



Figure 6: Triangular notch

Again consider an element of height at a depth

Breadth of element

Hence area of element

Velocity through element

Discharge through element

Integrating to obtain the total discharge between and

Again, a coefficient of discharge has to be introduced.

The triangular notch has advantages over the rectangular notch since the shape of the nappe does

not change with head so that the coefficient of discharge does not vary so much. A triangular notch

can also accommodate a wide range of flow rates.



Procedures:

1. Start the pump and slowly open the bench regulating valve until the water level reaches the

crest of the weir and measure the water level to determine the datum level.

2. Adjust the bench regulating valve to give the first required head level of approximately 10mm.

Measure the flow rate using the volumetric tank or the rotameter. Observe the shape of the

nappe.

3. Increase the flow by opening the bench regulating valve to set up heads above the datum level

in steps of approximately 10mm until the regulating valve is fully open. At each condition

measure the flow rate and observe the shape of the nappe.

4. Close the regulating valve, stop the pump and then replace the weir with the next weir to be

tested. Repeat the test procedure.

Results:

1. Record the results on a copy of the results sheet. Record any observations of the shape and type

of nappe paying particular attention to whether the nappe was clinging or sprung clear, and of

the end contraction and general change in shape.

2. Plot a graph of against for each weir. Measure the slopes and the intercepts.

Calculate the coefficient of discharge from the intercept and confirm that the slope is

approximately 2.5 for the rectangular notch and 1.5 for the triangular notch.

Trials 1 2 3

(mm)

(L)

(sec)

(m3/s)

Experiment (6): Flow over weirs -...

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