VEB2122: Civil Engineering Laboratory II

Lab Report

Title: Streamflow Measurements

Lecturer: Dr. Husna Takaijudin

Group 3

No Name Student ID

1 Farah Najihah Azmani 19001679

2 Harith Ismail 18002507

3 Michael Tiong Shu Yu 18002455

4 Syameem Hakime bin Abdul Aziz 18002573

Introduction

The flow of water can be measured with multiple methods. For this experiment, the
flow was measured with a current meter and surface floaters. These two methods are widely
used in open channel flow. The current meter is a component with a propeller attached at the
bottom of a shaft. Then as the component is inserted into flowing water until it reaches the
base of the water body, the propeller will start to rotate from the water current. Based on the
rotation, the velocity of the water can be determined. The surface floater is a simple
experiment where it is mainly used to find the velocity of the water surface. The velocity
obtained is considered as mean velocity.

Objectives

The objective of this experiment is to determine the velocity of water using a current
meter and surface floaters. Besides that, the wetted cross-sectional area of the drain is to be
calculated.

Apparatus

1. 3 staffs
2. 1 propeller
3. Wires
4. Hydrometer
5. Measuring tape
6. Surface floaters
7. stopwatch
Procedures

Assembling a current meter

step description reference

1 The current meter was assembled by

connecting the three shafts and
attaching the propeller at the base of
the shaft. The height of propeller was
measured from the bottom of the shaft.

2 The joints and propeller of the current

meter were tightened.

3 The wire was screwed on the

propeller.
4 The other wire was connected at the
top of the shaft.

5 The other end of the wired was

connected to the sensor in the colour
coded sockets.

Measuring streamflow velocity using a current meter

1 The width of the drain was measured.

2 The width of the drain was divided into
3 sections. The first section of the drain
was 25cm from the drain. The depth of
the drain at the first section of the drain
was measured with the current meter

3 The readings of the hydrometer were

observed for each section of drain.

4 Step 2-3 was repeated with an interval

of 25cm of the drain until the last
section where the width is at the end of
the second section of the drain and the
end of the drain.
 Measuring the velocity of water using surface floaters.

1 The measuring tape was laid out along

the streamline with the length of 10m,

2 The surface floater was dropped at the

start of the measuring tape while the
stopwatch was initiated
simultaneously.

3 The surface floater was taken

immediately after it reaches at the end
of the measuring tape and the
stopwatch was stopped at the same
time. The time taken for the journey of
the surface floater was recorded.
 4 Step 2-3 was repeated and the
recordings were taken.

Results

1. Current meter

Width of drain = 94cm

Propeller depth from

Time
Width, B (m) Depth, Y (m) free surface of water Rotations (°)
(sec)
(m)

0 0 _ _ _

0.25 0.186 0.1116 190 30

0.5 0.226 0.1356 192 30

0.75 0.115 0.069 159 30

0.94 0 _ _ _
 Width of drain = 93cm

Propeller depth from

Time
Width, B (m) Depth, Y (m) free surface of water Rotations (°)
(sec)
(m)

0 0 _ _ _

0.25 0.186 0.1116 165 30

0.5 0.215 0.129 180 30

0.75 0.146 0.0876 160 30

0.93 0 _ _ _

2. Surface floater

Reading Length of section (m) Time (s)

1. 12 7.5

2. 12 7.48

3. 12 7.52
Calculation

Figure 1: Sketching of cross-section of river channel

1. Current meter

𝑟
𝑛 = 30 (n = Rotations per seconds ; r = The rotations for 30 seconds from the current meter)

𝑣𝑖 = 0.0123 + (0.2473 × 𝑛) if 0.00 < n < 1.74 (vi = Velocity of the individual depth)

𝑣𝑖 = −0.0042 + (0.2568 × 𝑛) if 1.74 < n < 10

Using Mean-Section Method,

1
𝑣𝑎𝑣𝑔 = 2 (𝑣𝑖 + 𝑣𝑖−1 ) (vi-1 = Velocity of the previous individual depth ; vavg = Average

velocity)

1
𝐴𝑖 = 2 × (𝐵𝑖 − 𝐵𝑖−1 ) × (𝑌𝑖−1 + 𝑌𝑖 ) (Ai = Area for each individual section ; B = Width of

channel ; Y = Depth of water) (For trapezoidal section)

1
𝐴𝑖 = 2 × (𝐵𝑖 − 𝐵𝑖−1 ) × 𝑌𝑖 (For triangular section)

𝑄 = 𝑣𝑎𝑣𝑔 × 𝐴𝑖 (Q = Discharge)
Width of drain = 94cm

n Velocity, vi Average Area, Ai (m2) Discharge, Q

(m/s) velocity, vavg (m3/s)
(m/s)

6.333 1.622 0.811 0.023 0.019

6.400 1.639 1.631 0.052 0.085

5.300 1.357 1.498 0.043 0.064

Total discharge = 0.019 + 0.085 + 0.064 = 0.168m3/s

Width of drain = 93cm

n Velocity, vi Average Area, Ai (m2) Discharge, Q

(m/s) velocity, vavg (m3/s)
(m/s)

5.500 1.408 0.704 0.023 0.016

6.000 1.537 1.473 0.050 0.074

5.333 1.365 1.451 0.045 0.065

Total discharge = 0.016 + 0.074 + 0.065 = 0.155m3/s

2. Surface Floater

𝑆
𝑣𝑖 = 𝑡 (vi = Velocity of the individual depth; S = Length of section; t = Time)

Width of drain = 94cm

Velocity, vi (m/s)

1.600

1.604

1.596

(1.600+1.604+1.596)
Average velocity, vAvg = = 1.6𝑚/𝑠
3
Total area, AT = 0.023 + 0.052 + 0.043 = 0.118m2

𝑄 = 𝐴 𝑇 × 𝑣𝑎𝑣𝑔 = 1.6 × 0.118 = 0.19𝑚3 /𝑠

Width of drain = 93cm

Velocity, vi (m/s)

1.600

1.604

1.596

(1.600+1.604+1.596)
Average velocity, vAvg = = 1.6𝑚/𝑠
3

Total area, AT = 0.023 + 0.050 + 0.045 = 0.118m2

𝑄 = 𝐴 𝑇 × 𝑣𝑎𝑣𝑔 = 1.6 × 0.118 = 0.19𝑚3 /𝑠

Comparison between velocities measured by the current meter and surface floaters

(𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 − 𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙)
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑒𝑟𝑟𝑜𝑟(%) = × 100
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙

Velocity Velocity Velocity Percentage error Percentage error

measured by measured by measured by between between
current meter current meter surface floaters velocity velocity
for 94cm width for 93cm width for both 94cm measured by measured by
of drain (m/s) of drain (m/s) and 93cm current meter current meter
width(m/s) with velocity with velocity
measured by measured by
surface floaters surface floaters
for 94cm width for 93cm width
of drain (%) of drain (%)

1.622 1.408 1.600 1.36 13.64

1.639 1.537 1.604 2.14 4.36

1.357 1.365 1.596 17.61 16.92

Discussion
The current meter is commonly used to measure the streamflow especially in water
bodies such as rivers. The propellers of the current meter were placed underwater and the
rotations and time of rotation were recorded to be used to find the velocity and discharge value
of the stream. By taking a constant time of 30 seconds, from this experiment, we found out that
the number of rotations between each 0.25 meters intervals was 190, 192, and 159 rotations
respectively for the 94cm wide drain and 165, 180, and 160 for the 93cm wide drain. Then, we
found out the rotation per second, n values for each of the points by dividing it by 30 seconds.
The velocity was then calculated using the formula 𝑣𝑖 = 0.0123 + (0.2473 × 𝑛) if n is
between 0 and 1.74, or 𝑣𝑖 = −0.0042 + (0.2568 × 𝑛) if n is between 1.74 and 10. The
notation vi refers to the velocity of the individual depth.

We then sketched the cross-section of the drain as Figure 1 to help visualize to help
with the calculations. Next, we used the mean-section method to obtain the discharge, Q at
each section using 𝑄 = 𝑣𝑎𝑣𝑔 × 𝐴𝑖 where average velocity, vavg was calculated using 𝑣𝑎𝑣𝑔 =
1
(𝑣𝑖 + 𝑣𝑖−1 ). Note that vi-1 was the velocity of the previous individual depth. Ai was the area
2
1
for each individual section. Therefore, we used the equation 𝐴𝑖 = 2 × (𝐵𝑖 − 𝐵𝑖−1 ) × (𝑌𝑖−1 +

𝑌𝑖 ) where B was the width of the channel and Y was the depth of water, as the sections were in
trapezoidal shapes. From the tests conducted, the total discharge for the 94cm and 93cm drain,
were 0.168m3/s and 0.155m3/s respectively.

For the surface floater test, we obtained the velocity of flow by recording the time taken
for the ping pong ball to move from one point to another and calculate the length of the section
divided by time. This process was repeated to get the average velocity. For the 94cm and 93cm
drains, the average velocity was both 1.6m/s. The discharge, Q value was calculated using the
equation Q=Av, where A was the cross-sectional area of the drain and v was the average
velocity. Since the total area for both 94cm and 93cm drain was 0.118m2, therefore the Q values
were 0.19m3/s.

The percentage errors between the velocity measured by the current meter and surface
floaters were then calculated. Based on the calculation, we found that the percentage errors
were relatively small. These errors may be caused by parallax errors when using the
instruments where the eyes were not perpendicular to the scale when taking the values. To
reduce this error, we should have taken more than 3 readings and take the average value to
obtain more accurate data.

Conclusion

In conclusion, there are two methods that can be used to measure the velocity of water
in open channels. The two methods are current meter methods and surface floaters methods.
The streamflow information gathered is commonly used for infrastructure designs such as
dams, reservoirs, water treatment plants, and more. It is also important to the flood control
system and aquatic habitat as they need to know the amount of water released. Therefore,
largely flowing rivers can receive pollution discharges and be little affected compared to the
small streams which have less capacity to dilute and degrade wastes. The objective of this
experiment is achieved.

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