1.

0 OBJECTIVE

To determine manning coefficient’s, n for open channel flow

2.0 LEARNING OUTCOME

It’s expected by completing the experiment, the students will be able:

a. applies correct method and procedure of hydraulic solution towards practical problem.

b. acquires appropriate knowledge in mirror loss in pipe and uniform flow in open channel

3.0 INTRODUCTION
The Manning formula has been widely used to determine the roughness coefficient for
open channels. The n value is a measure for surface roughness that affects the flow resistance.
Small errors of this value can also give effect to the calculations. Although the n value has been
practiced in hydraulic and hydrology analysis, the determination is a challenge for engineers
because the values cannot be computed equally for all open channels.

4.0 THEORY
Uniform flow in a channel is such that the channel width and depth of water is the same
at any section along the flow. Hence, the velocity, v can be determined by using the Manning’s
equation:-

v = Am2/3i1/2N
Where,
m = mean hydraulic = A/P (m)
A = cross-sectional (m2)
P = wetter perimeter (m) 
b = base (m)
d =depth of water (m)
i = bed slope
n = Manning’s roughness coefficient

Manning’s roughness coefficient, n shows the degree of roughness of a surface channel.Although

these values vary with depth, but it is not so obvious and can be regarded as a constant for the
channel of the same material.
The general features of a channel are thus:
5.0 EQUIPMENTS AND DESCRIPTION

NO. EQUIPMENTS DESCRIPTIONS

1. Open channel demonstration apparatus

2. Caliper

3. Steel rule

4. Weir plate
1. channel element 2.5 m
2. flow channel element
3. groove for overflow weir
4. outflow segment
5. tank with outflow valve
6. flow meter
7. shut-off valve.
8. bearing pedestal with fixed centrifugal pump and switch box
9. pressure line
10. outflow valve with measuring glands
11. inclination adjustment device

6.0 PROCEDURE
Set 1 : Fix Gradient

1. The channel was  made sure to be horizontal.

2. The channel width was measured at mid-length of the channel.
3. The main supply was powered-up and the pump was started.
4. The inlet valve was opened-up slowly to obtain a flow (started with a small flow).
5. The inclination adjustment device was used to fix the channel gradient to 2.5%.
6. The flow was left to equilibrate for 30 seconds.
7. The flow was set to about 4 m3/hr, this was the first reading value.
8. The flow was left to equilibrate for 30 seconds and the water level was measured using
the gauge at mid-channel length.
9. Step 8 was repeated with consecutive increments of 1 m3/hr flow, and step 9 until 5
readings were obtained.

Set 2 : Fix Flow

10. For the next set of experimental readings, the flow was fixed to 7 m3/hr
11. The channel elevation was set to 0.5%. The flow was left to equilibrate for 30 seconds.
12. The water level was measured using the gauge at mid-channel length.
13. Step 12 was repeated with consecutive increments of 0.5% channel elevation, and step
until 5 readings were obtained.
 14. The inlet valve was closed down fully, and switched off the pump first followed by the
starter switch.
15. The main was switched off. All items were returned to their respective places.

7.0 RESULTS
7.1 General readings:

Channel width, b, : 80 mm = 0.08 m

 7.2 Experimental data

 1. Fixed slope, i, at 2.5%

Tes Flow depth, d (mm) Flow rate, Q (m3/hr)

t

1 18 4

2 20 5

3 22 6

4 26 7

5 29 8

2. Fixed flow rate, Q, at 7 m3/hr

Tes Flow depth, d (m) Bed slope, i (%)

t

1 41 0.5

2 34 1.0

3 29 1.5

4 27 2.0

5 25 2.5

7.3 Standardization of experimental data

1. Fixed slope, i, at 2.5%

 Test Flow depth, d (m)  Flow rate, Q    (m3/s)

1 0.018 0.0011111111111

2 0.021 0.0013888888888

3 0.024 0.0016666666666

4 0.027 0.0019444444444

5 0.029 0.0022222222222

2. Fixed flow rate, Q,  at 7 m3/s

Tes Flow depth, d (m) Bed slope, i (%)

t

1 0.041 0.5

2 0.034 1.0

3 0.029 1.5

4 0.027 2.0

5 0.025 2.5

7.4 Computation of experimental data

Fixed slope, i, at 2.5%

 Height Area of flow,A Wet Mean Flow rate,Q  Velocity,V Manning’s
of (m2) perimeter,P hydraulics (m3/s) (m/s) coefficient
water (m) (M)

0.018 1.53 ×10−3 0.121 0.0126 0.001111111 0.72621568 0.0372844

0.021 1.70 ×10−3 0.125 0.0136 0.001388889 0.81699411 0.0110266

0.022 1.87 ×10−3 0.129 0.0145 0.001666667 0.89126737 0.0105487

0.026 2.21 ×10−3 0.137 0.0161 0.001944444 0.87983710 0.0114585

0.029 2.465 ×10−3 0.143 0.0172 0.002222222 0.90150912 0.0116866

Fixed flow rate, Q at 7 m3/h

Heigh Area of flow,A Wet Mean Flow rate,Q  Velocity,V Manning’s

t of (m2) perimeter,P hydraulics (m3/s) (m/s) coefficient
water (m) (M)

0.041 3.485 ×10−3 0.167 0.0208 0.00194444 0.55794548 0.0358654

0.034 2.89 ×10−3 0.153 0.0189 0.00194444 0.67281660 0.0279020

0.029 2.465 ×10−3 0.143 0.0172 0.00194444 0.78881947 0.0223494

0.027 2.295 ×10−3 0.139 0.0165 0.00194444 0.84725054 0.0202396

0.025 2.125 ×10−3 0.135 0.0157 0.00194444 0.91503058 0.0181296

8.0 CONCLUSION

Finally, it was discovered that a flatter slope had a reduced effect on the Manning roughness
coefficient. Normally, risk assessment channel n values vary between 0.025 and 0.075. Overbank
n values might vary from 0.05 to 0.15. The Manning's roughness coefficient, n, has a range that
includes our value.

9.0 REFERENCE
1. https://www.hec.usace.army.mil/confluence/rasdocs/ras1dtechref/6.2/performing-a-dam-break-
study-with-hec-ras/mannings-roughness-coefficients#:~:text=In%20general%2C%20channel
%20n%20values,range%20between%200.05%20and%200.15.
2. https://publisher.unimas.my/ojs/index.php/JCEST/article/view/124/99#:~:text=So%2C%20the
%20roughness%20coefficient%20for,slope%20(1%3A900).