University of Duhok
College of Engineering
Department of Water Resources
2nd Stage
Fluid Mechanics
Lab Manual
(Fluid Static)
Prepared By: Alaa Ismail Ahmed
Experiment No.2
Group: A
Testing Date: 17/2/2021
Submitting Data: /2/2021
Discharge over a Broad Crested Weir
2.1Introduction
Weirs are a small overflow-type dams commonly used to raise the level of a
river or stream and cause a large change of water level behind them. The use
of portable instrument like kinds of weirs, flumes, floats, and volumetric
tank are common. Discharges measured range from a trickle in ditch to a
flood on the Amazon. Many researchers have studied the head discharge
relations for flows over sharp-crested weirs and broad-crested weirs with a
simple cross section shape, such as rectangular, triangular, trapezoidal,
truncated triangular. The laboratory flume, or flow channel, is one of the
most important tools available to the hydraulics engineer whether engaged in
teaching or research. Frequently, so-called indirect methods of discharge,
measurement are the only practicable means of obtaining the magnitude
of a peak flood flow past a given site. These determinations are based
on the water surface profile, usually defined from high-water marks,
and upon the geometry and hydraulic.
2.2 Objectives
The objectives of the Experiment are:
To show the measurement of flow rate.
To determine the relationship between upstream head and flow rate for
water flowing over a Broad crested weir.
To calculate the discharge coefficient Cd.
2.3Apaaratuse
1-Rota meter
4-Broad weir
2-Stop watch
5-Gage reading
3-Motor
6-Flow mete
1
2.4Description of Equipment
Figure 2.1 Broad Crested Weir
2.5 Procedures
1-Ensure the flume is level, with no stop logs installed at the
of the channel.
discharge end
2-Measure and record the actual breath (b) and the height (P) of the broad
crested weir. Then, the weir plate was placed and fixed carefully
perpendicular to the sides and bottom of the flume and leveled on all axes by
a carpenter’s level Ensure that the weir is secured using a mounting hook
through the bed of the flume. For accurate results the gaps between the weir
and the channel should be sealed on the upstream side using Plasticine.
3-A series of different flow rates were overtopped over the weir and the
corresponding heads above the weir crest were recorded after the zero on the
2
point gauge must correspond to the level of the weir crest or the apex of the
weir. Take enough care not damage the weir and the point gauge.
4-For each flow rate, wait until steady condition is attained then measure and
record the head (H) some way upstream from the weir point gauge.
5-Repeat the above procedure for different flow rates by adjusting the inlet
valve opening and tabulate the readings.
For each step measure the flow
rate Q, the upstream depth of flow H and the depth of flow over the weir h
(where flow becomes parallel to the weir). The flow rate Q can be
determined using the direct reading flow meter or the volumetric tank with a
stopwatch.
6-Complete the tabulation and find the mean value of Cd.
7-Draw the necessary graphs and calibrate the notch.
2.6Results and Calculations
 While the flow rates passing over the weir were calculated from the
flow equation of broad crest weir. Shown in fig. 2.
√
(
)……………………………………2.1
Where: 𝑄𝑡ℎ=Theoretical flow rate,
H =upstream head of water over the weir crest, and
h = depth of the flow over the weir
b = width of the channel
 For each flow rate (actual discharge), measure and record the volumes
in the collecting tank and the time required to collect that volume by
stopwatch.
………………………………………………………..2.2
Where: 𝑄𝑎𝑐𝑡=actual flow rate,
V = volume of the collecting tank (liter),
3
t = time taken to rise volume (sec.).
Figure2.2: Cross section of Broad Crested Weir
 The coefficient of discharge Cd is defined as the ratio of actual
discharge obtained experimentally to the theoretical discharge. i.e,
…………………………………………………..2.3
Where: Cd = Coefficient of discharge is a dimensionless discharge
coefficient which takes account the effects of neglecting losses and the
contraction of the jet as it passes over the notch in Qth
Tabulate your readings and calculations as follows:
Breadth of weir (b) = 7.3 cm
73mm
Height of weir (P) = 10 cm
Specific gravity =9.81*1000=9810
√
(
)
4
5.24 ⁄
=
= 2.36
Table2.1:Result with Broad Crested Weir
No
Volume
l/sec
Time
sec
H (cm)
h (cm)
Qact
l/sec
Qth
l/sec
Cd
1
2
3
4
5
10
15
15
20
25
4.24
7.88
9.7
18.93
34.15
15.8
14.98
14.26
12.92
11.54
4.91
4.08
3.5
3.11
2.3
2.36
1.904
1.55
1.1
0.7321
5.24
4.36
4
3.15
2.261
0.4504
0.44
0.39
0.35
0.32
2.7Sketch
Sketch2.1: Relationship between Qact and H
1000
y = 125.15x0.2562
115.4129.2
149.8 158
142.6
H mm
100
Series1
Power (Series1)
10
1
0.1
1
10
Qact
5
2.8 Discussion
the discharge coefficient (cd) of rectangular broad-crested weir can be
written as a function of the width of the channel (b), total energy head
upstream of the weir (h),)mean flow velocity in the main channel (v), length
of broad-crested weir (l), and acceleration due to gravity (g), dimensional
analysis based on Buckingham’s theorem was used to find non-dimensional
variables in the present study).There is no doubt that the theoretical results
are different from actual results and this is because in theory we neglect
many factors which effect on the results and we always take an ideal case,
but in actual we saw the influence of all these factors on our results. In our
results the Qtheoretical and Qactual were not close to each other in value, maybe
because we didn’t place the calibrated scale on the center of the weir during
taking the value of (h), maybe we read the values of calibrated scale in a
wrong way because the scale was accurate to (mm), or maybe student
careless when taking the time during the Weight-Time method. in order to
estimate the outflow over a rectangular broad-crested weir, the discharge
coefficient in the weir equation needs to be known
2.9 Conclusion
The experiment was so useful to us because by it we can find the discharge
of any stream, channel, and also it useful when we want to rise the level of
the water to make branch from dam or stream at specific discharge, and thus
the importance of this experiment had been clarified to us, and special
in large hydraulic constructions. In this study, laboratory measurements were
carried out on rectangular broad-crested weir with different geometries
located on a straight rectangular main channel to investigate the new
equation for discharge coefficient. As a result of dimensional analysis, the
results indicate that the dimensionless parameter of h 1/B should not
be ignored in equations determining the discharge coefficient of the
rectangular broad-crested weir. Multiple regression analysis equations based
on the dimensional analysis concept were developed for computing the
discharge coefficient of a rectangular broad-crested weir; and discharge
coefficient equation was used for computing the discharge over rectangular
broad-crested weir.
6