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HYDRAULIC LABORATORYECWOPEN-ENDED LABFEB 2020 – JUNE 2020TITLE OF EXPERIMENT : FLOW OVER BROAD-CRESTED WEIRDATE OF EXPERIMENT : 23 APRIL 2020GROUP : 3B2AGROUP MEMBERSLECTURER :LEVEL OF OPENESS : 1MARKS COMMENTSINTRODUCTIONBASIC CONCEPTSMETHODOLOGYRESULTS&ANALYSIS 1 2 3 4 5DISCUSSION 1 2 3 4 ...

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HYDRAULIC LABORATORY ECW437 OPEN-ENDED LAB FEB 2020 – JUNE 2020 TITLE OF EXPERIMENT

: FLOW OVER BROAD-CRESTED WEIR

DATE OF EXPERIMENT

: 23 APRIL 2020

GROUP

: 3B2A

GROUP MEMBERS LECTURER

:

LEVEL OF OPENESS

:1

MARKS

COMMENTS

INTRODUCTION BASIC CONCEPTS METHODOLOGY RESULTS&ANALYSIS

1

2

3

4

5

DISCUSSION

1

2

3

4

5

CONCLUSION

1

2

3

4

5

ORGANIZATION

1

2

3

4

5

TOTAL MARKS

Table of Content

1.1

Introduction

3

2.0

Basic concept

4

3.0

Methodology

5

4.0

Result and Analysis

6

5.0

Discussion

9

6.0

Conclusion

10

Reference

11

Organization

12

INTRODUCTION

The broad crested weirs are a hydraulic structures widely used for depth control and flow measurement in field and laboratory canals .The geometry described as a flatcrested structure with a length (L) of crest large enough compared to the flow thickness over the crest of the weir. The crest is broad when the streamlines of the flow are parallel to the crest and the pressure distribution is hydrostatic. For instance, weirs as a type of hydraulic structure are consisting of some obstruction to increase water level and commonly used to measure discharge. The relationship between the flow rate and water depth above the weir can be derived by applying the Bernoulli’s equation and by making some assumptions with regard to head loss and pressure distribution of the flow passing over the weir. A coefficient of discharge needs to be determined experimentally for each weir to account for errors in estimating the flow rate that is due to these assumptions.

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1.0 BASIC CONCEPT

Whenever an obstruction such as a broad-crested weir is placed in the channel, flow is disrupted, and the flow is no longer uniform. If the flow is subcritical, the water will flow over the weir and a drop in the weir level occurs over the weir. If the weir height exceeds the critical value of E – Eₘᵢₙ . Where E and Eₘᵢₙ are the specific energy and minimum specific energy in the channel respectively, the flow over the weir becomes critical flow and the depth is equal to the critical depth, yc. The critical depth, yc for a rectangular channel of width, B discharging at a rate, Q can be obtained from the equation

= {� 2 } ⅓ gB²

(i)

Rearranging the equation yields the discharge in the channel as � = � √(���³)

(ii)

But the relationship between Emin and yc for a rectangular channel is given by =2 (�ₘᵢₙ) 3

(iii)

Hence combining (ii) and (iii) yieds = ��√(2){�ₘᵢₙ} ³ 3

(iv)

Upon simplifying Eq. (iv) � = 1.705��ₘᵢₙ³/₂ This can be further approximated by = 1.705�� ³/₂

(v)

Where H is the depth of water behind weir, measure from the top of the weir. If the height of the weir is known and the depth just upstream of the weir is measured, the discharge in the channel is easily determined by Eq. (v). The broad-crested weir can be used to measure the discharge in an open channel and can also be used to raise the depth of flow in a small stream. 4

2.0 Methodology

2.1 Apparatus i. ii. iii.

Open Channel Flume (hydraulic laboratory) Broad-crested weir Point gauge

2.2 Procedures i. ii. iii. iv. v. vi. vii. viii. ix.

The width of channel, B was recorded. The channel slope was adjusted to a mild slope. The height, p_w and the length, L_w of the broad-crested weir were measured. Broad-crested weir was installed with the rounded top edge upstream. Let the water flow throughout the weir. Make sure that the choking occurs and the depth of water behind the weir increases. Flow rate in the channel were recorded by using flow meter. The depth behind the weir was measured and the value of H was deduced. The procedures were repeated by using different flow rates until 9 different flow rates, Q and depths, H were obtained.

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3.0 Result and Discussion And Calculation Record the following dimensions:  Width of the channel, B: 0.30m  Height of the broad-crested weir, pw: 0.15m  Length of the broad-crested weir, Lw: 0.30m Data Table: Experiment Data No 1 2 3 4 5 6 7 8 9 10

Upstream Depth, y1 (m) 0.172 0.175 0.179 0.183 0.187 0.190 0.195 0.198 0.204 0.213 Table 1: Result

x.

6

Experiment Data No 1 2 3 4 5 6 7 8 9 10

Width of the channel, B (m) 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300

Height of the broad-crested weir,pw (m) 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 Table 2: Result

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Upstream Depth, y1 (m) 0.172 0.175 0.179 0.183 0.187 0.190 0.195 0.198 0.204 0.213

Depth of water behind weir, H (m) 0.022 0.025 0.029 0.033 0.037 0.040 0.045 0.048 0.054 0.063

Discharge, Q (m^3/s) 0.002 0.002 0.003 0.003 0.004 0.004 0.005 0.005 0.006 0.008

Depth of water behind weir, H (m) = y1 -pw = 0.172 - 0.150 = 0.022 m

Discharge, Q (m^3/s) = 1.705 x B x H^(3/2) = 1.705 X 0.300 x (0.022)^(3/2) = 0.002 m^3/s

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4.0 Discussion

Broad crested weir is a strong structure that usually made of reinforced concrete and cover the entire width of the channel. The use of this broad crested weir is to measure the flow rate and this method is very simple to use and has no edge that will produce smooth flow. The broad crested weir has the advantage that it operates effectively with higher downstream water levels compare to a sharp crested weir. This is because of the weir is broad enough for the flow to pass through the critical depth that somewhere near to its downstream edge.

From the observation in this experiment, the discharge coefficient is variation because of the inconsistent in taking flow rate. If the flow rate taken is increase, this will affect the discharge coefficient to also be increase.

The error that may happen in this experiment is reaction time error when using the stopwatch and due to parallax error in taking data from Vernier scale or tank. This error can be minimized with taking the average reading. Then, it also possible that flow may not been fully stabilize when the reading is taken. This make the data is not fully accurate.

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5.0 Conclusion

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 h1/B should not be ignored inequations 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.

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Reference

1. https://uta.pressbooks.pub/appliedfluidmechanics/chapter/experiment-4/ 2. http://rpitt.eng.ua.edu/Class/Water%20Resources%20Engineering/2009%20spring %20lab%20new/Broad_crested_weir_module-3_.pdf 3. Fluid Mechanics Fundamental And Applications, Yunus A. Cengel, John M. Cimbala

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