EXPERIMENT (6)
FLOW OVER WEIRS
1
By:
Eng. Motasem M. Abushaban.
Eng. Fedaa M. Fayyad.
INTRODUCTION

Weir is defined as a barrier over which the water
flows in an open channel
A
weir is a notch on a larger scale – usually found
in rivers.
 It
is used as both a flow measuring device and a
device to raise water levels.
2
INTRODUCTION

Crest
Is the edge or surface over which the water flows.

Nappe
Is the overflowing sheet of
water.
3
INTRODUCTION
Upstr
eam
level
h
H
dh
V=2gh
Nappe
W
eir
cr
est
Upstreamlevel
h
H
dh
V=2gh
Nappe
Weir
crest
P
Weir
Downstream
level
L4H
P
W
eir
Downstr
eam
level
L4H
4
INTRODUCTION

Usually named for the shape of the overflow opening

Rectangular

Triangular

Trapezoidal
5
RECTANGULAR WEIR
6
TRIANGULAR WEIRS
7
PURPOSE

To observe characteristics of flow over a weir.

To determine the head-discharge relationship of
two different shapes of weirs, and to compare the
experimental results with their corresponding
theoretical expressions.

Calculating the coefficient of discharge (Cd).
8
APPARATUS
9
THEORY
To determine an expression for the theoretical flow through a notch
we will consider a horizontal strip of width b and depth h below the
free surface, as shown in the figure
Velocity through the strip
V  2 gh
Discharge through the strip,
Q  AV  bh 2gh
Integrating from the free surface, h = 0, to the weir crest, h = H
gives the expression for the total theoretical discharge,
H
Q theoretical  2 g  bh 2 dh
1
0
10
THEORY
Rectangular Weir
For a rectangular weir the width does
not change with depth so there is no
relationship between b and depth h.
We have the equation, b = constant = B.
Substituting this with the general weir
equation gives:
H
Qtheoretical  B 2 g
 h 2 dh
1
O
3
2
B 2g H 2
3
To calculate the actual discharge we introduce a coefficient of
discharge, Cd, which accounts for losses at the edges of the weir

and contractions in the area of flow, giving :
Qactual  Cd
3
2
volume
B 2g H 2 
3
time
11
THEORY

In practice the flow through the notch will not be
normal to the plane of the weir. The 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
12
THEORY

The discharge from a rectangular notch will be
considerably less.
ln(Q act ) = ln(Cd
2
B 2g H 3/2 )
3
ln(Q act ) = ln(Cd
2
B 2g )  ln(H 3/2 )
3
ln(Q act ) = ln(Cd
2
3
2g )  ln(H )
3
2
y-axis = intercept  yaxis
int ercept  ln( Cd
2
B 2g )
3
eint ercept
Cd 
2
B 2g
3
13
EQUATIONS

For rectangular weir :
eint ercept
Cd 
2
B 2g
3

For Triangular weir :
Cd 
eint ercept
8

2 g tan( )
15
2
14
PROCEDURE
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.
 Place the specific weir plate which is to be tested
first and hold it using the five thumb nuts.
 Ensure that the square edge of the weir faces
upstream.
 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 Hzero.

15
PROCEDURE
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.
 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.
 Close the regulating valve, stop the pump and
then replace the weir with the next weir to be
tested. Repeat the test procedure

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RESULT AND CALCULATION
Record the results on a copy of the results sheet.
 Plot a graph of loge (Q) against loge (H) for each
weir.
 Measure the slopes and the intercepts.


From the intercept calculate the coefficients
of discharge and from the slopes of the graphs
confirm that the index is approximately 1.5 for
the rectangular weir and 2.5 for the triangular
weirs.
17
• Data & Results:
1
2
3
H (mm)
V (L)
T (sec)
Qact (m3/s)
ln (Qact)
ln(H) (m)
Qth (m3/s)
18
QUESTIONS
19